Saccade

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Saccade 

Trace of saccades of the human eye on a face while scanning

A saccade (/səˈkɑːd/ sə-KAHD; French: [sakad]; French for 'jerk') is a quick, simultaneous movement of both eyes between two or more phases of fixation in the same direction. In contrast, in smooth-pursuit movements, the eyes move smoothly instead of in jumps. The phenomenon can be associated with a shift in frequency of an emitted signal or a movement of a body part or device. Controlled cortically by the frontal eye fields (FEF), or subcortically by the superior colliculus, saccades serve as a mechanism for fixation, rapid eye movement, and the fast phase of optokinetic nystagmus. The word appears to have been coined in the 1880s by French ophthalmologist Émile Javal, who used a mirror on one side of a page to observe eye movement in silent reading, and found that it involves a succession of discontinuous individual movements.

Function

Humans and many animals do not look at a scene in fixed steadiness; instead, the eyes move around, locating interesting parts of the scene and building up a mental, three-dimensional 'map' corresponding to the scene (as opposed to the graphical map of avians, that often relies upon detection of angular movement on the retina).

When scanning immediate surroundings or reading, human eyes make saccadic movements and stop several times, moving very quickly between each stop. The speed of movement during each saccade cannot be controlled; the eyes move as fast as they are able. One reason for the saccadic movement of the human eye is that the central part of the retina—known as the fovea—which provides the high-resolution portion of vision is very small in humans, only about 1–2 degrees of vision, but it plays a critical role in resolving objects. By moving the eye so that small parts of a scene can be sensed with greater resolution, body resources can be used more efficiently.

Timing and kinematics

Saccades are one of the fastest movements produced by the human eye (blinks may reach even higher peak velocities). The peak angular speed of the eye during a saccade reaches up to 700°/s in humans for great saccades (25° of visual angle); in some monkeys, peak speed can reach 1000°/s. Saccades to an unexpected stimulus normally take about 200 milliseconds (ms) to initiate, and then last from about 20–200 ms, depending on their amplitude (20–30 ms is typical in language reading). Under certain laboratory circumstances, the latency of, or reaction time to, saccade production can be cut nearly in half (express saccades). These saccades are generated by a neuronal mechanism that bypasses time-consuming circuits and activates the eye muscles more directly. Specific pre-target oscillatory (alpha rhythms) and transient activities occurring in posterior-lateral parietal cortex and occipital cortex also characterise express saccades.

Saccadic mainsequence, showing single saccades from a participant performing a visually-guided saccade task

The amplitude of a saccade is the angular distance the eye travels during the movement. For amplitudes up to 15 or 20°, the velocity of a saccade linearly depends on the amplitude (the so-called saccadic main sequence, a term borrowed from astrophysics; see Figure). For amplitudes larger than 20°, the peak velocity starts to plateau (nonlinearly) toward the maximum velocity attainable by the eye at around 60°. For instance, a 10° amplitude is associated with a velocity of 300°/s, and 30° is associated with 500°/s. Therefore, for larger amplitude ranges, the main sequence can best be modeled by an inverse power law function.

The high peak velocities and the main sequence relationship can also be used to distinguish micro-/saccades from other eye movements (like ocular tremor, ocular drift, and smooth pursuit). Velocity-based algorithms are a common approach for saccade detection in eye tracking. Although, depending on the demands on timing accuracy, acceleration-based methods are more precise.

Saccades may rotate the eyes in any direction to relocate gaze direction (the direction of sight that corresponds to the fovea), but normally saccades do not rotate the eyes torsionally. (Torsion is clockwise or counterclockwise rotation around the line of sight when the eye is at its central primary position; defined this way, Listing's law says that, when the head is motionless, torsion is kept at zero.)

Head-fixed saccades can have amplitudes of up to 90° (from one edge of the oculomotor range to the other), but in normal conditions saccades are far smaller, and any shift of gaze larger than about 20° is accompanied by a head movement. During such gaze saccades, first, the eye produces a saccade to get gaze on target, whereas the head follows more slowly and the vestibulo-ocular reflex (VOR) causes the eyes to roll back in the head to keep gaze on the target. Since the VOR can actually rotate the eyes around the line of sight, combined eye and head movements do not always obey Listing's law.

Types

Saccades can be categorized by intended goal in four ways:

1. In a visually guided saccade, the eyes move toward a visual transient, or stimulus. The parameters of visually guided saccades (amplitude, latency, peak velocity, and duration) are frequently measured as a baseline when measuring other types of saccades. Visually guided saccades can be further subcategorized:

   • A reflexive saccade is triggered exogenously by the appearance of a peripheral stimulus, or by the disappearance of a fixation stimulus.
   • A scanning saccade is triggered endogenously for the purpose of exploring the visual environment.

2. In an antisaccade, the eyes move away from the visual onset. They are more delayed than visually guided saccades, and observers often make erroneous saccades in the wrong direction. A successful antisaccade requires inhibiting a reflexive saccade to the onset location, and voluntarily moving the eye in the other direction.
3. In a memory guided saccade, the eyes move toward a remembered point, with no visual stimulus.
4. In a sequence of predictive saccades, the eyes are kept on an object moving in a temporally and/or spatially predictive manner. In this instance, saccades often coincide with (or anticipate) the predictable movement of an object.

As referenced to above, it is also useful to categorize saccades by latency (time between go-signal and movement onset). In this case the categorization is binary: Either a given saccade is an express saccade or it is not. The latency cut-off is approximately ~200 ms; any longer than this is outside the express saccade range.

Microsaccades are a related type of fixational eye movement that are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during visual fixation, not only in humans, but also in animals with foveal vision (primates, cats, etc.). Microsaccade amplitudes vary from 2 to 120 arcminutes.

In depth

When exploring the visual environment with the gaze, humans make two to three fixations a second. Each fixation involves binocularly coordinated movements of the eyes to acquire the new target in three dimensions: horizontal and vertical, but also in-depth. In literature it has been shown how an upward or a vertical saccade is generally accompanied by a divergence of the eyes, while a downward saccade is accompanied by a convergence. The amount of this intra-saccadic vergence has a strong functional significance for the effectiveness of binocular vision. When making an upward saccade, the eyes diverged to be aligned with the most probable uncrossed disparity in that part of the visual field. On the other way around, when making a downward saccade, the eyes converged to enable alignment with crossed disparity in that part of the field. The phenomenon can be interpreted as an adaptation of rapid binocular eye movements to the statistics of the 3D environment, in order to minimize the need for corrective vergence movements at the end of saccades.

Pathophysiologic saccades

Saccadic oscillations not fitting the normal function are a deviation from a healthy or normal condition. Nystagmus is characterized by the combination of 'slow phases', which usually take the eye off the point of regard, interspersed with saccade-like "quick phases" that serve to bring the eye back on target. Pathological slow phases may be due to either an imbalance in the vestibular system or damage to the brainstem "neural integrator" that normally holds the eyes in place. On the other hand, opsoclonus or ocular flutter are composed purely of fast-phase saccadic eye movements. Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.

Eye movement measurements are also used to investigate psychiatric disorders. For example, ADHD is characterized by an increase of antisaccade errors and an increase in delays for visually guided saccade. Various pathological conditions also alter microsaccades and other fixational eye movements.

Paroxysmal eye–head movements, termed aberrant gaze saccades, are an early symptom of GLUT1 deficiency syndrome in infancy.

Saccade adaptation

When the brain is led to believe that the saccades it is generating are too large or too small (by an experimental manipulation in which a saccade-target steps backward or forward contingent on the eye movement made to acquire it), saccade amplitude gradually decreases (or increases), an adaptation (also termed gain adaptation) widely seen as a simple form of motor learning, possibly driven by an effort to correct visual error. This effect was first observed in humans with ocular muscle palsy. In these cases, it was noticed that the patients would make hypometric (small) saccades with the affected eye, and that they were able to correct these errors over time. This led to the realization that visual or retinal error (the difference between the post-saccadic point of regard and the target position) played a role in the homeostatic regulation of saccade amplitude. Since then, much scientific research has been devoted to various experiments employing saccade adaptation.

Reading

Saccadic eye movement allows the mind to read quickly, but it comes with its disadvantages. It can cause the mind to skip over words because it does not see them as important to the sentence, and the mind completely leaves it from the sentence or it replaces it with the wrong word. This can be seen in "Paris in the the Spring". This is a common psychological test, where the mind will often skip the second "the", especially when there is a line break in between the two.

When speaking, the mind plans what will be said before it is said. Sometimes the mind is not able to plan in advance and the speech is rushed out. This is why there are errors like mispronunciation, stuttering, and unplanned pauses. The same thing happens when reading. The mind does not always know what will come next. This is another reason that the second "the" can be missed.

Vision

Saccadic masking

Main article: Saccadic masking

It is a common but false belief that during the saccade, no information is passed through the optic nerve to the brain. Whereas low spatial frequencies (the 'fuzzier' parts) are attenuated, higher spatial frequencies (an image's fine details) that would otherwise be blurred by the eye movement remain unaffected. This phenomenon, known as saccadic masking or saccadic suppression, is known to begin prior to saccadic eye movements in every primate species studied, implying neurological reasons for the effect rather than simply the image's motion blur. This phenomenon leads to the so-called stopped-clock illusion, or chronostasis.

A person may observe the saccadic masking effect by standing in front of a mirror and looking from one eye to the next (and vice versa). The subject will not experience any movement of the eyes or any evidence that the optic nerve has momentarily ceased transmitting. Due to saccadic masking, the eye/brain system not only hides the eye movements from the individual but also hides the evidence that anything has been hidden. Of course, a second observer watching the experiment will see the subject's eyes moving back and forth. The function's main purpose is to prevent an otherwise significant smearing of the image. (You can experience your eye saccade movements by using your cellphone's front-facing camera as a mirror, hold the cellphone screen a couple of inches away from your face as you saccade from one eye to the other—the cellphone's signal processing delay allows you to see the end of the saccade movement.)

Spatial updating

When a visual stimulus is seen before a saccade, subjects are still able to make another saccade back to that image, even if it is no longer visible. This shows that the brain is somehow able to take into account the intervening eye movement. It is thought that the brain does this by temporarily recording a copy of the command for the eye movement, and comparing this to the remembered image of the target. This is called spatial updating. Neurophysiologists, having recorded from cortical areas for saccades during spatial updating, have found that memory-related signals get remapped during each saccade.

Trans-saccadic perception

It is also thought that perceptual memory is updated during saccades so that information gathered across fixations can be compared and synthesized. However, the entire visual image is not updated during each saccade. Some scientists believe that this is the same as visual working memory, but as in spatial updating the eye movement has to be accounted for. The process of retaining information across a saccade is called trans-saccadic memory, and the process of integrating information from more than one fixation is called trans-saccadic integration.

Comparative physiology

Saccades are a widespread phenomenon across animals with image-forming visual systems. They have been observed in animals across three phyla, including animals that do not have a fovea (most vertebrates do) and animals that cannot move their eyes independently of their head (such as insects). Therefore, while saccades serve in humans and other primates to increase the effective visual resolution of a scene, there must be additional reasons for the behavior. The most frequently suggested of these reasons is to avoid blurring of the image, which would occur if the response time of a photoreceptor cell is longer than the time a given portion of the image is stimulating that photoreceptor as the image drifts across the eye.

In birds, saccadic eye movements serve a further function. The avian retina is highly developed. It is thicker than the mammalian retina, has a higher metabolic activity, and has less vasculature obstruction, for greater visual acuity. Because of this, the retinal cells must obtain nutrients via diffusion through the choroid and from the vitreous humor. The pecten is a specialised structure in the avian retina. It is a highly vascular structure that projects into the vitreous humor. Experiments show that, during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the pecten oculi acts as an agitator, propelling perfusate (natural lubricants) toward the retina. Thus, in birds, saccadic eye movements appear to be important in retinal nutrition and cellular respiration.

Two-streams hypothesis

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Two-streams_hypothesis

The two-streams hypothesis is a model of the neural processing of vision as well as hearing. The hypothesis, given its initial characterisation in a paper by David Milner and Melvyn A. Goodale in 1992, argues that humans possess two distinct visual systems. Recently there seems to be evidence of two distinct auditory systems as well. As visual information exits the occipital lobe, and as sound leaves the phonological network, it follows two main pathways, or "streams". The ventral stream (also known as the "what pathway") leads to the temporal lobe, which is involved with object and visual identification and recognition. The dorsal stream (or, "how pathway") leads to the parietal lobe, which is involved with processing the object's spatial location relative to the viewer and with speech repetition.

History

The dorsal stream (green) and ventral stream (purple) are shown. They originate from a common source in the visual cortex

Several researchers had proposed similar ideas previously. The authors themselves credit the inspiration of work on blindsight by Weiskrantz, and previous neuroscientific vision research. Schneider first proposed the existence of two visual systems for localisation and identification in 1969. Ingle described two independent visual systems in frogs in 1973. Ettlinger reviewed the existing neuropsychological evidence of a distinction in 1990. Moreover, Trevarthen had offered an account of two separate mechanisms of vision in monkeys back in 1968.

In 1982, Ungerleider and Mishkin distinguished the dorsal and ventral streams, as processing spatial and visual features respectively, from their lesion studies of monkeys – proposing the original where vs what distinction. Though this framework was superseded by that of Milner & Goodale, it remains influential.

One hugely influential source of information that has informed the model has been experimental work exploring the extant abilities of visual agnosic patient D.F. The first, and most influential report, came from Goodale and colleagues in 1991 and work is still being published on her two decades later. This has been the focus of some criticism of the model due to the perceived over-reliance on findings from a single case.

Two visual systems

Goodale and Milner amassed an array of anatomical, neuropsychological, electrophysiological, and behavioural evidence for their model. According to their data, the ventral 'perceptual' stream computes a detailed map of the world from visual input, which can then be used for cognitive operations, and the dorsal 'action' stream transforms incoming visual information to the requisite egocentric (head-centered) coordinate system for skilled motor planning. The model also posits that visual perception encodes spatial properties of objects, such as size and location, relative to other objects in the visual field; in other words, it utilizes relative metrics and scene-based frames of reference. Visual action planning and coordination, on the other hand, uses absolute metrics determined via egocentric frames of reference, computing the actual properties of objects relative to the observer. Thus, grasping movements directed towards objects embedded in size-contrast-ambiguous scenes have been shown to escape the effects of these illusions, as different frames of references and metrics are involved in the perception of the illusion versus the execution of the grasping act.

Norman proposed a similar dual-process model of vision, and described eight main differences between the two systems consistent with other two-system models.

Factor	Ventral system (what)	Dorsal system (how)
Function	Recognition/identification	Visually guided behaviour
Sensitivity	High spatial frequencies - details	High temporal frequencies - motion
Memory	Long-term stored representations	Only very short-term storage
Speed	Relatively slow	Relatively fast
Consciousness	Typically high	Typically low
Frame of reference	Allocentric or object-centered	Egocentric or viewer-centered
Visual input	Mainly foveal or parafoveal	Across retina
Monocular vision	Generally reasonably small effects	Often large effects e.g. motion parallax

Dorsal stream

The dorsal stream is proposed to be involved in the guidance of actions and recognizing where objects are in space. The dorsal stream projects from the primary visual cortex to the posterior parietal cortex. It was initially termed the "where" pathway since it was thought that the dorsal stream processes information regarding the spatial properties of an object. However, later research conducted on a famous neuropsychological patient, Patient D.F., revealed that the dorsal stream is responsible for processing the visual information needed to construct the representations of objects one wishes to manipulate. Those findings led the nickname of the dorsal stream to be updated to the "how" pathway. The dorsal stream is interconnected with the parallel ventral stream (the "what" stream) which runs downward from V1 into the temporal lobe.

General features

The dorsal stream is involved in spatial awareness and guidance of actions (e.g., reaching). In this it has two distinct functional characteristics—it contains a detailed map of the visual field, and is also good at detecting and analyzing movements.

The dorsal stream commences with purely visual functions in the occipital lobe before gradually transferring to spatial awareness at its termination in the parietal lobe.

The posterior parietal cortex is essential for "the perception and interpretation of spatial relationships, accurate body image, and the learning of tasks involving coordination of the body in space".

It contains individually functioning lobules. The lateral intraparietal sulcus (LIP) contains neurons that produce enhanced activation when attention is moved onto the stimulus or the animal saccades towards a visual stimulus, and the ventral intraparietal sulcus (VIP) where visual and somatosensory information are integrated.

Effects of damage or lesions

Damage to the posterior parietal cortex causes a number of spatial disorders including:

• Simultanagnosia: where the patient can only describe single objects without the ability to perceive it as a component of a set of details or objects in a context (as in a scenario, e.g. the forest for the trees).
• Optic ataxia: where the patient can't use visuospatial information to guide arm movements.
• Hemispatial neglect: where the patient is unaware of the contralesional half of space (that is, they are unaware of things in their left field of view and focus only on objects in the right field of view; or appear unaware of things in one field of view when they perceive them in the other). For example, a person with this disorder may draw a clock, and then label all twelve of the numbers on one side of the face and consider the drawing complete.
• Akinetopsia: inability to perceive motion.
• Apraxia: inability to produce discretionary or volitional movement in the absence of muscular disorders.

Ventral stream

The ventral stream is associated with object recognition and form representation. Also described as the "what" stream, it has strong connections to the medial temporal lobe (which is associated with long-term memories), the limbic system (which controls emotions), and the dorsal stream (which deals with object locations and motion).

The ventral stream gets its main input from the parvocellular (as opposed to magnocellular) layer of the lateral geniculate nucleus of the thalamus. These neurons project to V1 sublayers 4Cβ, 4A, 3B and 2/3a successively. From there, the ventral pathway goes through V2 and V4 to areas of the inferior temporal lobe: PIT (posterior inferotemporal), CIT (central inferotemporal), and AIT (anterior inferotemporal). Each visual area contains a full representation of visual space. That is, it contains neurons whose receptive fields together represent the entire visual field. Visual information enters the ventral stream through the primary visual cortex and travels through the rest of the areas in sequence.

Moving along the stream from V1 to AIT, receptive fields increase their size, latency, and the complexity of their tuning. For example, recent studies have shown that the V4 area is responsible for color perception in humans, and the V8 (VO1) area is responsible for shape perception, while the VO2 area, which is located between these regions and the parahippocampal cortex, integrates information about the color and shape of stimuli into a holistic image.

All the areas in the ventral stream are influenced by extraretinal factors in addition to the nature of the stimulus in their receptive field. These factors include attention, working memory, and stimulus salience. Thus the ventral stream does not merely provide a description of the elements in the visual world—it also plays a crucial role in judging the significance of these elements.

Damage to the ventral stream can cause inability to recognize faces or interpret facial expression.

Two auditory systems

Ventral stream

comprehension of the phrase 'my cat' in the extended version of Hickok and Poeppel's dual pathway model

Along with the visual ventral pathway being important for visual processing, there is also a ventral auditory pathway emerging from the primary auditory cortex. In this pathway, phonemes are processed posteriorly to syllables and environmental sounds. The information then joins the visual ventral stream at the middle temporal gyrus and temporal pole. Here the auditory objects are converted into audio-visual concepts.

Dorsal stream

The function of the auditory dorsal pathway is to map the auditory sensory representations onto articulatory motor representations. Hickok & Poeppel claim that the auditory dorsal pathway is necessary because, "learning to speak is essentially a motor learning task. The primary input to this is sensory, speech in particular. So, there must be a neural mechanism that both codes and maintains instances of speech sounds, and can use these sensory traces to guide the tuning of speech gestures so that the sounds are accurately reproduced."

repetition of the phrase 'what is your name?' in the extended version of Hickok and Poeppel's dual pathway model

In contrast to the ventral stream's auditory processing, information enters from the primary auditory cortex into the posterior superior temporal gyrus and posterior superior temporal sulcus. From there the information moves to the beginning of the dorsal pathway, which is located at the boundary of the temporal and parietal lobes near the Sylvian fissure. The first step of the dorsal pathway begins in the sensorimotor interface, located in the left Sylvian parietal temporal (Spt) (within the Sylvian fissure at the parietal-temporal boundary). The spt is important for perceiving and reproducing sounds. This is evident because its ability to acquire new vocabulary, be disrupted by lesions and auditory feedback on speech production, articulatory decline in late-onset deafness and the non-phonological residue of Wernicke's aphasia; deficient self-monitoring. It is also important for the basic neuronal mechanisms for phonological short-term memory. Without the Spt, language acquisition is impaired. The information then moves onto the articulatory network, which is divided into two separate parts. The articulatory network 1, which processes motor syllable programs, is located in the left posterior inferior temporal gyrus and Brodmann's area 44 (pIFG-BA44). The articulatory network 2 is for motor phoneme programs and is located in the left M1-vBA6.

Conduction aphasia affects a subject's ability to reproduce speech (typically by repetition), though it has no influence on the subject's ability to comprehend spoken language. This shows that conduction aphasia must reflect not an impairment of the auditory ventral pathway but instead of the auditory dorsal pathway. Buchsbaum et al found that conduction aphasia can be the result of damage, particularly lesions, to the Spt (Sylvian parietal temporal). This is shown by the Spt's involvement in acquiring new vocabulary, for while experiments have shown that most conduction aphasiacs can repeat high-frequency, simple words, their ability to repeat low-frequency, complex words is impaired. The Spt is responsible for connecting the motor and auditory systems by making auditory code accessible to the motor cortex. It appears that the motor cortex recreates high-frequency, simple words (like cup) in order to more quickly and efficiently access them, while low-frequency, complex words (like Sylvian parietal temporal) require more active, online regulation by the Spt. This explains why conduction aphasiacs have particular difficulty with low-frequency words which requires a more hands-on process for speech production. "Functionally, conduction aphasia has been characterized as a deficit in the ability to encode phonological information for production," namely because of a disruption in the motor-auditory interface. Conduction aphasia has been more specifically related to damage of the arcuate fasciculus, which is vital for both speech and language comprehension, as the arcuate fasiculus makes up the connection between Broca and Wernicke's areas.

Criticisms

Goodale & Milner's innovation was to shift the perspective from an emphasis on input distinctions, such as object location versus properties, to an emphasis on the functional relevance of vision to behaviour, for perception or for action. Contemporary perspectives however, informed by empirical work over the past two decades, offer a more complex account than a simple separation of function into two-streams. Recent experimental work for instance has challenged these findings, and has suggested that the apparent dissociation between the effects of illusions on perception and action is due to differences in attention, task demands, and other confounds. There are other empirical findings, however, that cannot be so easily dismissed which provide strong support for the idea that skilled actions such as grasping are not affected by pictorial illusions.

Moreover, recent neuropsychological research has questioned the validity of the dissociation of the two streams that has provided the cornerstone of evidence for the model. The dissociation between visual agnosia and optic ataxia has been challenged by several researchers as not as strong as originally portrayed; Hesse and colleagues demonstrated dorsal stream impairments in patient DF; Himmelbach and colleagues reassessed DF's abilities and applied more rigorous statistical analysis demonstrating that the dissociation wasn't as strong as first thought.

A 2009 review of the accumulated evidence for the model concluded that whilst the spirit of the model has been vindicated the independence of the two streams has been overemphasised. Goodale & Milner themselves have proposed the analogy of tele-assistance, one of the most efficient schemes devised for the remote control of robots working in hostile environments. In this account, the dorsal stream is viewed as a semi-autonomous function that operates under guidance of executive functions which themselves are informed by ventral stream processing.

Thus the emerging perspective within neuropsychology and neurophysiology is that, whilst a two-systems framework was a necessary advance to stimulate study of the highly complex and differentiated functions of the two neural pathways; the reality is more likely to involve considerable interaction between vision-for-action and vision-for-perception. Robert McIntosh and Thomas Schenk summarize this position as follows:

We should view the model not as a formal hypothesis, but as a set of heuristics to guide experiment and theory. The differing informational requirements of visual recognition and action guidance still offer a compelling explanation for the broad relative specializations of dorsal and ventral streams. However, to progress the field, we may need to abandon the idea that these streams work largely independently of one other, and to address the dynamic details of how the many visual brain areas arrange themselves from task to task into novel functional networks.

Entheogen

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Entheogen

 

12th century frescoes depicting alleged entheogenic mushrooms in Christian art

Entheogens are psychoactive substances that induce alterations in perception, mood, consciousness, cognition, or behavior for the purposes of engendering spiritual development or otherwise in sacred contexts. Anthropological study has established that entheogens are used for religious, magical, shamanic, or spiritual purposes in many parts of the world. Entheogens have traditionally been used to supplement many diverse practices geared towards achieving transcendence, including divination, meditation, yoga, sensory deprivation, asceticism, prayer, trance, rituals, chanting, imitation of sounds, hymns like peyote songs, drumming, and ecstatic dance. The psychedelic experience is often compared to non-ordinary forms of consciousness such as those experienced in meditation, near-death experiences, and mystical experiences. Ego dissolution is often described as a key feature of the psychedelic experience.

Nomenclature

The neologism entheogen was coined in 1979 by a group of ethnobotanists and scholars of mythology (Carl A. P. Ruck, Jeremy Bigwood, Danny Staples, Richard Evans Schultes, Jonathan Ott and R. Gordon Wasson). The term is derived from two words of Ancient Greek, ἔνθεος (éntheos) and γενέσθαι (genésthai). The adjective entheos translates to English as "full of the god, inspired, possessed," and is the root of the English word "enthusiasm." The Greeks used it as praise for poets and other artists. Genesthai means "to come into being". Thus, an entheogen is a drug that causes one to become inspired or to experience feelings of inspiration, often in a religious or "spiritual" manner.

Ruck et al. argued that the term hallucinogen was inappropriate owing to its etymological relationship to words relating to delirium and insanity. The term psychedelic was also seen as problematic, owing to the similarity in sound to words about psychosis and also because it had become irreversibly associated with various connotations of the 1960s pop culture. In modern usage, entheogen may be used synonymously with these terms, or it may be chosen to contrast with recreational use of the same drugs. The meanings of the term entheogen was formally defined by Ruck et al.:

In a strict sense, only those vision-producing drugs that can be shown to have figured in shamanic or religious rites would be designated entheogens, but in a looser sense, the term could also be applied to other drugs, both natural and artificial, that induce alterations of consciousness similar to those documented for ritual ingestion of traditional entheogens.

— Ruck et al., 1979, Journal of Psychedelic Drugs

In 2004, David E. Nichols wrote the following about the nomenclature used for serotonergic hallucinogens:

Many different names have been proposed over the years for this drug class. The famous German toxicologist Louis Lewin used the name phantastica earlier in this century, and as we shall see later, such a descriptor is not so farfetched. The most popular names—hallucinogen, psychotomimetic, and psychedelic ("mind manifesting")—have often been used interchangeably. Hallucinogen is now, however, the most common designation in the scientific literature, although it is an inaccurate descriptor of the actual effects of these drugs. In the lay press, the term psychedelic is still the most popular and has held sway for nearly four decades. Most recently, there has been a movement in nonscientific circles to recognize the ability of these substances to provoke mystical experiences and evoke feelings of spiritual significance. Thus, the term entheogen, derived from the Greek word entheos, which means "god within," was introduced by Ruck et al. and has seen increasing use. This term suggests that these substances reveal or allow a connection to the "divine within." Although it seems unlikely that this name will ever be accepted in formal scientific circles, its use has dramatically increased in popular media and internet sites. Indeed, in much of the counterculture that uses these substances, entheogen has replaced psychedelic as the name of choice, and we may expect to see this trend continue.

History

Main article: History of entheogenic drugs

See also: Entheogenic drugs and the archaeological record

Entheogens have been used by indigenous peoples for thousands of years.

R. Gordon Wasson and Giorgio Samorini have proposed several examples of the cultural use of entheogens that are found in the archaeological record. Hemp seeds discovered by archaeologists at Pazyryk suggest early ceremonial practices by the Scythians occurred during the 5th to 2nd century BCE, confirming previous historical reports by Herodotus.

Most of the well-known modern examples of entheogens, such as Ayahuasca, peyote, psilocybin mushrooms, and morning glories are from the native cultures of the Americas. However, it has also been suggested that entheogens played an important role in ancient Indo-European culture, for example by inclusion in the ritual preparations of the Soma, the "pressed juice" that is the subject of Book 9 of the Rigveda. Soma was ritually prepared and drunk by priests and initiates and elicited a paean in the Rigveda that embodies the nature of an entheogen:

Splendid by Law! declaring Law, truth speaking, truthful in thy works, Enouncing faith, King Soma!... O [Soma] Pavāmana (mind clarifying), place me in that deathless, undecaying world wherein the light of heaven is set, and everlasting lustre shines.... Make me immortal in that realm where happiness and transports, where joy and felicities combine...

The kykeon that preceded initiation into the Eleusinian Mysteries is another entheogen, which was investigated (before the word was coined) by Carl Kerényi, in Eleusis: Archetypal Image of Mother and Daughter. Other entheogens in the Ancient Near East and the Aegean include the opium poppy, datura, and the unidentified "lotus" (likely the sacred blue lily) eaten by the Lotus-Eaters in the Odyssey and Narcissus.

According to Ruck, Eyan, and Staples, the familiar shamanic entheogen that the Indo-Europeans brought knowledge of was Amanita muscaria. It could not be cultivated; thus it had to be found, which suited it to a nomadic lifestyle. When they reached the world of the Caucasus and the Aegean, the Indo-Europeans encountered wine, the entheogen of Dionysus, who brought it with him from his birthplace in the mythical Nysa, when he returned to claim his Olympian birthright. The Indo-European proto-Greeks "recognized it as the entheogen of Zeus, and their own traditions of shamanism, the Amanita and the 'pressed juice' of Soma – but better, since no longer unpredictable and wild, the way it was found among the Hyperboreans: as befit their own assimilation of agrarian modes of life, the entheogen was now cultivable." Robert Graves, in his foreword to The Greek Myths, hypothesises that the ambrosia of various pre-Hellenic tribes was Amanita muscaria (which, based on the morphological similarity of the words amanita, amrita and ambrosia, is entirely plausible) and perhaps psilocybin mushrooms of the genus Panaeolus. Amanita muscaria was regarded as divine food, according to Ruck and Staples, not something to be indulged in, sampled lightly, or profaned. It was seen as the food of the gods, their ambrosia, and as mediating between the two realms. It is said that Tantalus's crime was inviting commoners to share his ambrosia.

Uses and purpose

Entheogens have been used in various ways, e.g., as part of established religious rituals or as aids for personal spiritual development ("plant teachers"). There are also instances where people have been given entheogens without their knowledge or consent (e.g., tourists given ayahuasca).

In religion

See also: Religion and drugs

Mazatec people performing a Salvia ritual dance in Huautla de Jiménez

Shamans all over the world and in different cultures have traditionally used drugs, especially psychedelics, for their religious experiences. In these communities the absorption of drugs leads to dreams (visions) through sensory distortion. The psychedelic experience is often compared to non-ordinary forms of consciousness such as those experienced in meditation, and mystical experiences. Ego dissolution is often described as a key feature of the psychedelic experience.

Entheogens used in the contemporary world include biota like peyote (Native American Church), extracts like ayahuasca (Santo Daime, União do Vegetal), and synthetic drugs like 2C-B (Sangoma). Entheogens also play an important role in contemporary religious movements such as the Rastafari movement.

Hinduism

Main article: Entheogenic use of cannabis

Bhang is an edible preparation of cannabis native to the Indian subcontinent. It has been used in food and drink as early as 1000 BCE by Hindus in ancient India. The earliest known reports regarding the sacred status of cannabis in the Indian subcontinent come from the Atharva Veda estimated to have been written sometime around 2000–1400 BCE, which mentions cannabis as one of the "five sacred plants... which release us from anxiety" and that a guardian angel resides in its leaves. The Vedas also refer to it as a "source of happiness," "joy-giver" and "liberator," and in the Raja Valabba, the gods send hemp to the human race.

Buddhism

Flowering San Pedro, an entheogenic cactus that has been used for over 3,000 years. Today the vast majority of extracted mescaline is from columnar cacti, not vulnerable peyote.

It has been suggested that the Amanita muscaria mushroom was used by the Tantric Buddhist mahasiddha tradition of the 8th to 12th century.

In the West, some modern Buddhist teachers have written on the usefulness of psychedelics. The Buddhist magazine Tricycle devoted their entire fall 1996 edition to this issue. Some teachers such as Jack Kornfield have suggested the possibility that psychedelics could complement Buddhist practice, bring healing and help people understand their connection with everything which could lead to compassion. Kornfield warns however that addiction can still be a hindrance. Other teachers such as Michelle McDonald-Smith expressed views which saw entheogens as not conducive to Buddhist practice ("I don't see them developing anything").

The fifth of the Pancasila, the ethical code in the Theravada and Mahayana Buddhist traditions, states that adherents must: "abstain from fermented and distilled beverages that cause heedlessness". The Pali Canon, the scripture of Theravada Buddhism, depicts refraining from alcohol as essential to moral conduct because intoxication causes a loss of mindfulness. Although the Fifth Precept only names a specific wine and cider, this has traditionally been interpreted to mean all alcoholic beverages.

Judaism

Main article: Cannabis and Judaism

The primary advocate of the religious use of cannabis in early Judaism was Polish anthropologist Sula Benet, who claimed that the plant kaneh bosem קְנֵה-בֹשֶׂם mentioned five times in the Hebrew Bible, and used in the holy anointing oil of the Book of Exodus, was cannabis. According to theories that hold that cannabis was present in Ancient Israelite society, a variant of hashish is held to have been present. In 2020, it was announced that cannabis residue had been found on the Israelite sanctuary altar at Tel Arad dating to the 8th century BCE of the Kingdom of Judah, suggesting that cannabis was a part of some Israelite rituals at the time.

While Benet's conclusion regarding the psychoactive use of cannabis is not universally accepted among Jewish scholars, there is general agreement that cannabis is used in talmudic sources to refer to hemp fibers, not hashish, as hemp was a vital commodity before linen replaced it. Lexicons of Hebrew and dictionaries of plants of the Bible such as by Michael Zohary (1985), Hans Arne Jensen (2004) and James A. Duke (2010) and others identify the plant in question as either Acorus calamus or Cymbopogon citratus, not cannabis.

Christianity

See also: Sacramental wine and real presence of Christ in the Eucharist

Many Christian denominations disapprove of the use of most illicit drugs. Nevertheless, scholars such as David Hillman suggest that a variety of drug use, recreational and otherwise, is to be found in the early history of the Church.

The historical picture portrayed by the Entheos journal is of fairly widespread use of visionary plants in early Christianity and the surrounding culture, with a gradual reduction of use of entheogens in Christianity. R. Gordon Wasson's book Soma prints a letter from art historian Erwin Panofsky asserting that art scholars are aware of many "mushroom trees" in Christian art.

The question of the extent of visionary plant use throughout the history of Christian practice has barely been considered yet by academic or independent scholars. The question of whether visionary plants were used in pre-Theodosian Christianity is distinct from evidence that indicates the extent to which visionary plants were utilized or forgotten in later Christianity, including heretical or quasi-Christian groups, and the question of other groups such as elites or laity within orthodox Catholic practice.

Peyotism

The shrine at Tel Arad, where the earliest use of cannabis in the Near East is thought to have occurred during the Kingdom of Judah

The Native American Church (NAC) is also known as Peyotism and Peyote Religion. Peyotism is a Native American religion characterized by mixed traditional as well as Protestant beliefs and by sacramental use of the entheogen peyote.

The Peyote Way Church of God believe that "Peyote is a holy sacrament, when taken according to our sacramental procedure and combined with a holistic lifestyle".

Santo Daime

Santo Daime is a syncretic religion founded in the 1930s in the Brazilian Amazonian state of Acre by Raimundo Irineu Serra, known as Mestre Irineu. Santo Daime incorporates elements of several religious or spiritual traditions including Folk Catholicism, Kardecist Spiritism, African animism and indigenous South American shamanism, including vegetalismo.

Ceremonies – trabalhos (Brazilian Portuguese for "works") – are typically several hours long and are undertaken sitting in silent "concentration", or sung collectively, dancing according to simple steps in geometrical formation. Ayahuasca, referred to as Daime within the practice, which contains several psychoactive compounds, is drunk as part of the ceremony. The drinking of Daime can induce a strong emetic effect which is embraced as both emotional and physical purging.

União do Vegetal

União do Vegetal (UDV) is a religious society founded on July 22, 1961, by José Gabriel da Costa, known as Mestre Gabriel. The translation of União do Vegetal is Union of the Plants referring to the sacrament of the UDV, Hoasca tea (also known as ayahuasca). This beverage is made by boiling two plants, Mariri (Banisteriopsis caapi) and Chacrona (Psychotria viridis), both of which are native to the Amazon rainforest.

In its sessions, UDV members drink Hoasca Tea for the effect of mental concentration. In Brazil, the use of Hoasca in religious rituals was regulated by the Brazilian Federal Government's National Drug Policy Council on January 25, 2010. The policy established legal norms for the religious institutions that responsibly use this tea. The Supreme Court of the United States unanimously affirmed the UDV's right to use Hoasca tea in its religious sessions in the United States, in a decision published on February 21, 2006.

A Native American peyote drummer (c. 1927)

By region

See also: Regional forms of shamanism

Africa

The best-known entheogen-using culture of Africa is the Bwitists, who used a preparation of the root bark of Tabernanthe iboga. Although the ancient Egyptians may have been using the sacred blue lily plant in some of their religious rituals or just symbolically, it has been suggested that Egyptian religion once revolved around the ritualistic ingestion of the far more psychoactive Psilocybe cubensis mushroom, and that the Egyptian White Crown, Triple Crown, and Atef Crown were evidently designed to represent pin-stages of this mushroom. There is also evidence for the use of psilocybin mushrooms in Ivory Coast. Numerous other plants used in shamanic ritual in Africa, such as Silene capensis sacred to the Xhosa, are yet to be investigated by western science. A recent revitalization has occurred in the study of southern African psychoactives and entheogens (Mitchell and Hudson 2004; Sobiecki 2002, 2008, 2012).

Among the amaXhosa, the artificial drug 2C-B is used as entheogen by traditional healers or amagqirha over their traditional plants; they refer to the chemical as Ubulawu Nomathotholo, which roughly translates to "Medicine of the Singing Ancestors".

Americas

See also: Aztec use of entheogens and Entheogenics and the Maya

Entheogens have played a pivotal role in the spiritual practices of most American cultures for millennia. The first American entheogen to be subject to scientific analysis was the peyote cactus (Lophophora williamsii). One of the founders of modern ethno-botany, Richard Evans Schultes of Harvard University documented the ritual use of peyote cactus among the Kiowa, who live in what became Oklahoma. While it was used traditionally by many cultures of what is now Mexico, in the 19th century its use spread throughout North America, replacing the toxic mescal bean (Calia secundiflora). Other well-known entheogens used by Mexican cultures include the alcoholic Aztec sacrament, pulque, ritual tobacco (known as 'picietl' to the Aztecs, and 'sikar' to the Maya (from where the word 'cigar' derives)), psilocybin mushrooms, morning glories (Ipomoea tricolor and Turbina corymbosa), and Salvia divinorum.

Datura wrightii is sacred to some Native Americans and has been used in ceremonies and rites of passage by Chumash, Tongva, and others. Among the Chumash, when a boy was 8 years old, his mother would give him a preparation of momoy to drink. This supposed spiritual challenge should help the boy develop the spiritual wellbeing that is required to become a man. Not all of the boys undergoing this ritual survived. Momoy was also used to enhance spiritual wellbeing among adults. For instance, during a frightening situation, such as when seeing a coyote walk like a man, a leaf of momoy was sucked to help keep the soul in the body.

The mescal bean Sophora secundiflora was used by the shamanic hunter-gatherer cultures of the Great Plains region. Other plants with ritual significance in North American shamanism are the hallucinogenic seeds of the Texas buckeye and jimsonweed (Datura stramonium). Paleoethnobotanical evidence for these plants from archaeological sites shows they were used in ancient times thousands of years ago.

Asia

Salvia divinorum (Herba de Maria)

The indigenous peoples of Siberia (from whom the term shaman was borrowed) have used Amanita muscaria as an entheogen.

In Hinduism, Datura stramonium and cannabis have been used in religious ceremonies, although the religious use of datura is not very common, as the primary alkaloids are strong deliriants, which causes serious intoxication with unpredictable effects.

Also, the ancient drink Soma, mentioned often in the Vedas, appears to be consistent with the effects of an entheogen. In his 1967 book, Wasson argues that Soma was Amanita muscaria. The active ingredient of Soma is presumed by some to be ephedrine, an alkaloid with stimulant properties derived from the soma plant, identified as Ephedra pachyclada. However, there are also arguments to suggest that Soma could have also been Syrian rue, cannabis, Atropa belladonna, or some combination of any of the above plants.

Europe

Fermented honey, known in Northern Europe as mead, was an early entheogen in Aegean civilization, predating the introduction of wine, which was the more familiar entheogen of the reborn Dionysus and the maenads. Its religious uses in the Aegean world are intertwined with the mythology of the bee.

Dacians were known to use cannabis in their religious and important life ceremonies, proven by discoveries of large clay pots with burnt cannabis seeds in ancient tombs and religious shrines. Also, local oral folklore and myths tell of ancient priests that dreamed with gods and walked in the smoke. Their names, as transmitted by Herodotus, were "kap-no-batai" which in Dacian was supposed to mean "the ones that walk in the clouds".

The growth of Roman Christianity also saw the end of the two-thousand-year-old tradition of the Eleusinian Mysteries, the initiation ceremony for the cult of Demeter and Persephone involving the use of a drug known as kykeon. The term 'ambrosia' is used in Greek mythology in a way that is remarkably similar to the Soma of the Hindus as well.

A theory that naturally-occurring gases like ethylene used by inhalation may have played a role in divinatory ceremonies at Delphi in Classical Greece received popular press attention in the early 2000s, yet has not been conclusively proven.

Mushroom consumption is part of the culture of Europeans in general, with particular importance to Slavic and Baltic peoples. Some academics argue that the use of psilocybin- and/or muscimol-containing mushrooms was an integral part of the ancient culture of the Rus' people.

Middle East

It has been suggested that the ritual use of small amounts of Syrian rue^{[by whom?]} is an artifact of its ancient use in higher doses as an entheogen (possibly in conjunction with DMT-containing acacia).

John Marco Allegro argued that early Jewish and Christian cultic practice was based on the use of Amanita muscaria, which was later forgotten by its adherents, but this view has been widely disputed.

Oceania

In general, indigenous Australians are thought not to have used entheogens, although there is a strong barrier of secrecy surrounding Aboriginal shamanism, which has likely limited what has been told to outsiders. A plant that the Australian Aboriginals used to ingest is called Pitcheri, which is said to have a similar effect to that of coca. Pitcheri was made from the bark of the shrub Duboisia myoporoides. This plant is now grown commercially and is processed to manufacture an eye medication.

There are no known uses of entheogens by the Māori of New Zealand aside from a variant species of kava, although some modern scholars have claimed that there may be evidence of psilocybin mushroom use. Natives of Papua New Guinea are known to use several species of entheogenic mushrooms (Psilocybe spp, Boletus manicus).

Kava or kava kava (Piper Methysticum) has been cultivated for at least 3,000 years by a number of Pacific island-dwelling peoples. Historically, most Polynesian, many Melanesian, and some Micronesian cultures have ingested the psychoactive pulverized root, typically taking it mixed with water. In these traditions, taking kava is believed to facilitate contact with the spirits of the dead, especially relatives and ancestors.

Research

Notable early testing of the entheogenic experience includes the Marsh Chapel Experiment, conducted by physician and theology doctoral candidate Walter Pahnke under the supervision of psychologist Timothy Leary and the Harvard Psilocybin Project. In this double-blind experiment, volunteer graduate school divinity students from the Boston area almost all claimed to have had profound religious experiences subsequent to the ingestion of pure psilocybin.

Beginning in 2006, experiments have been conducted at Johns Hopkins University, showing that under controlled conditions psilocybin causes mystical experiences in most participants and that they rank the personal and spiritual meaningfulness of the experiences very highly.

Except in Mexico, research with psychedelics is limited due to ongoing widespread drug prohibition. The amount of peer-reviewed research on psychedelics has accordingly been limited due to the difficulty of getting approval from institutional review boards. Furthermore, scientific studies on entheogens present some significant challenges to investigators, including philosophical questions relating to ontology, epistemology and objectivity.

Legal status

Mandala-like round window above the altar at Boston University's Marsh Chapel, site of Marsh Chapel Experiment

Main article: Convention on Psychotropic Substances

Some countries have legislation that allows for traditional entheogen use.

Australia

Main article: Dimethyltryptamine § Australia

Between 2011 and 2012, the Australian Federal Government was considering changes to the Australian Criminal Code that would classify any plants containing any amount of DMT as "controlled plants". DMT itself was already controlled under current laws. The proposed changes included other similar blanket bans for other substances, such as a ban on any and all plants containing mescaline or ephedrine. The proposal was not pursued after political embarrassment on realisation that this would make the official Floral Emblem of Australia, Acacia pycnantha (golden wattle), illegal. The Therapeutic Goods Administration and federal authority had considered a motion to ban the same, but this was withdrawn in May 2012 (as DMT may still hold potential entheogenic value to native or religious peoples).

United States

In 1963 in Sherbert v. Verner the Supreme Court established the Sherbert Test, which consists of four criteria that are used to determine if an individual's right to religious free exercise has been violated by the government. The test is as follows:

For the individual, the court must determine

• whether the person has a claim involving a sincere religious belief, and
• whether the government action is a substantial burden on the person's ability to act on that belief.

If these two elements are established, then the government must prove

• that it is acting in furtherance of a "compelling state interest", and
• that it has pursued that interest in the manner least restrictive, or least burdensome, to religion.

This test was eventually all-but-eliminated in Employment Division v. Smith 494 U.S. 872 (1990) which held that a "neutral law of general applicability" was not subject to the test. Congress resurrected it for the purposes of federal law in the federal Religious Freedom Restoration Act (RFRA) of 1993.

In City of Boerne v. Flores, 521 U.S. 507 (1997) RFRA was held to trespass on state sovereignty, and application of the RFRA was essentially limited to federal law enforcement. In Gonzales v. O Centro Espírita Beneficente União do Vegetal, 546 U.S. 418 (2006), a case involving only federal law, RFRA was held to permit a church's use of a DMT-containing tea for religious ceremonies.

Some states have enacted State Religious Freedom Restoration Acts intended to mirror the federal RFRA's protections.

Peyote is listed by the United States DEA as a Schedule I controlled substance. However, practitioners of the Peyote Way Church of God, a Native American religion, perceive the regulations regarding the use of peyote as discriminating, leading to religious discrimination issues regarding about the U.S. policy towards drugs. As the result of Peyote Way Church of God, Inc. v. Thornburgh the American Indian Religious Freedom Act of 1978 was passed. This federal statute allow the "Traditional Indian religious use of the peyote sacrament", exempting only use by Native American persons.

In literature

Many works of literature have described entheogen use; some of those are:

• The drug melange (spice) in Frank Herbert's Dune universe acts as both an entheogen (in large enough quantities) and an addictive geriatric medicine. Control of the supply of melange was crucial to the Empire, as it was necessary for, among other things, faster-than-light (folding space) navigation.
• Consumption of the imaginary mushroom anochi [enoki] as the entheogen underlying the creation of Christianity is the premise of Philip K. Dick's last novel, The Transmigration of Timothy Archer, a theme that seems to be inspired by John Allegro's book.
• Aldous Huxley's final novel, Island (1962), depicted a fictional psychoactive mushroom – termed "moksha medicine" – used by the people of Pala in rites of passage, such as the transition to adulthood and at the end of life.
• Bruce Sterling's Holy Fire novel refers to the religion in the future as a result of entheogens, used freely by the population.
• In Stephen King's The Dark Tower: The Gunslinger, Book 1 of The Dark Tower series, the main character receives guidance after taking mescaline.
• The Alastair Reynolds novel Absolution Gap features a moon under the control of a religious government that uses neurological viruses to induce religious faith.
• A critical examination of the ethical and societal implications and relevance of "entheogenic" experiences can be found in Daniel Waterman and Casey William Hardison's book Entheogens, Society & Law: Towards a Politics of Consciousness, Autonomy and Responsibility (Melrose, Oxford 2013). This book includes a controversial analysis of the term entheogen arguing that Wasson et al. were mystifying the effects of the plants and traditions to which it refers.