The ancient symbol Ouroboros, a dragon that continually consumes itself, denotes self-reference.
Self-reference occurs in natural or formal languages when a sentence, idea or formula
refers to itself. The reference may be expressed either
directly—through some intermediate sentence or formula—or by means of
some encoding. In philosophy,
it also refers to the ability of a subject to speak of or refer to
itself, that is, to have the kind of thought expressed by the first
person nominative singular pronoun "I" in English.
In classical philosophy, paradoxes were created by self-referential concepts such as the omnipotence paradox of asking if it was possible for a being to exist so powerful that it could create a stone that it could not lift. The Epimenides paradox,
'All Cretans are liars' when uttered by an ancient Greek Cretan was one
of the first recorded versions. Contemporary philosophy sometimes
employs the same technique to demonstrate that a supposed concept is
meaningless or ill-defined.
In mathematics and computability theory, self-reference (also known as impredicativity) is the key concept in proving limitations of many systems. Gödel's theorem uses it to show that no formal consistent
system of mathematics can ever contain all possible mathematical
truths, because it cannot prove some truths about its own structure. The halting problem
equivalent, in computation theory, shows that there is always some task
that a computer cannot perform, namely reasoning about itself. These
proofs relate to a long tradition of mathematical paradoxes such as Russell's paradox and Berry's paradox, and ultimately to classical philosophical paradoxes.
In game theory,
undefined behaviors can occur where two players must model each other's
mental states and behaviors, leading to infinite regress.
In computer programming, self-reference occurs in reflection, where a program can read or modify its own instructions like any other data.
Numerous programming languages support reflection to some extent with
varying degrees of expressiveness. Additionally, self-reference is seen
in recursion (related to the mathematical recurrence relation) in functional programming, where a code structure refers back to itself during computation.
'Taming' self-reference from potentially paradoxical concepts into
well-behaved recursions has been one of the great successes of computer science, and is now used routinely in, for example, writing compilers using the 'meta-language' ML. Using a compiler to compile itself is known as bootstrapping. Self-modifying code is possible to write (programs which operate on themselves), both with assembler and with functional languages such as Lisp, but is generally discouraged in real-world programming. Computing hardware makes fundamental use of self-reference in flip-flops,
the basic units of digital memory, which convert potentially
paradoxical logical self-relations into memory by expanding their terms
over time. Thinking in terms of self-reference is a pervasive part of
programmer culture, with many programs and acronyms named
self-referentially as a form of humor, such as GNU ('GNU's not Unix') and PINE ('Pine is not Elm'). The GNU Hurd is named for a pair of mutually self-referential acronyms.
The biology of self-replication is self-referential, as embodied by DNA and RNA replication mechanisms. Models of self-replication are found in Conway's Game of Life and have inspired engineering systems such as the self-replicating 3D printer RepRap.
In art
A self-referencing work of graffiti apologizing for its own existence
Self-referencial graffiti. The painter drawn on a wall erases his own graffiti, and may be erased himself by the next facade cleaner.
Self-reference in art is closely related to the concepts of breaking the fourth wall and meta-reference, which often involve self-reference. The short stories of Jorge Luis Borges play with self-reference and related paradoxes in many ways. Samuel Beckett's Krapp's Last Tape
consists entirely of the protagonist listening to and making recordings
of himself, mostly about other recordings. During the 1990s and 2000s
filmic self-reference was a popular part of the rubber reality movement, notably in Charlie Kaufman's films Being John Malkovich and Adaptation, the latter pushing the concept arguably to its breaking point as it attempts to portray its own creation, in a dramatized version of the Droste effect.
Various creation myths invoke self-reference to solve the problem of what created the creator. For example, the Egyptian creation myth has a god swallowing his own semen to create himself. The Ouroboros is a mythical dragon which eats itself.
The Quran includes numerous instances of self-referentiality.
The surrealist painter René Magritte is famous for his self-referential works. His painting The Treachery of Images, includes the words "this is not a pipe", the truth of which depends entirely on whether the word ceci (in English, "this") refers to the pipe depicted—or to the painting or the word or sentence itself. M.C. Escher's art also contains many self-referential concepts such as hands drawing themselves.
In language
A word that describes itself is called an autological word (or autonym). This generally applies to adjectives, for example sesquipedalian (i.e. "sesquipedalian" is a sesquipedalian word), but can also apply to other parts of speech, such as TLA, as a three-letter abbreviation for "three-letter abbreviation".
A sentence which inventories its own letters and punctuation marks is called an autogram.
There is a special case of meta-sentence in which the content of
the sentence in the metalanguage and the content of the sentence in the
object language are the same. Such a sentence is referring to itself.
However some meta-sentences of this type can lead to paradoxes. "This is
a sentence." can be considered to be a self-referential meta-sentence
which is obviously true. However "This sentence is false" is a
meta-sentence which leads to a self-referential paradox.
Such sentences can lead to problems, for example, in law, where
statements bringing laws into existence can contradict one another or
themselves. Kurt Gödel claimed to have found such a paradox in the US constitution at his citizenship ceremony.
Self-reference occasionally occurs in the media when it is required to write about itself, for example the BBC reporting on job cuts at the BBC. Notable encyclopedias may be required to feature articles about themselves, such as Wikipedia's article on Wikipedia.
Fumblerules
are a list of rules of good grammar and writing, demonstrated through
sentences that violate those very rules, such as "Avoid cliches like the
plague" and "Don't use no double negatives". The term was coined in a
published list of such rules by William Safire.
In popular culture
Douglas Hofstadter's books, especially Metamagical Themas and Gödel, Escher, Bach,
play with many self-referential concepts and were highly influential in
bringing them into mainstream intellectual culture during the 1980s. Hofstadter's law, which specifies that "It always takes longer than you expect, even when you take into account Hofstadter's Law"
is an example of a self-referencing adage. Hofstadter also suggested
the concept of a 'Reviews of this book', a book containing only reviews
of itself, which has since been implemented using wikis and other technologies. Hofstadter's 'strange loop' metaphysics attempts to map consciousness onto self-reference, but is a minority position in philosophy of mind.
A self-refuting idea or self-defeating idea is an idea or statement whose falsehood is a logical consequence
of the act or situation of holding them to be true. Many ideas are
called self-refuting by their detractors, and such accusations are
therefore almost always controversial, with defenders stating that the
idea is being misunderstood or that the argument
is invalid. For these reasons, none of the ideas below are
unambiguously or incontrovertibly self-refuting. These ideas are often
used as axioms, which are definitions taken to be true (tautological assumptions), and cannot be used to test themselves, for doing so would lead to only two consequences: consistency (circular reasoning) or exception (self-contradiction).
It is important to know that the conclusion of an argument that is
self-refuting is not necessarily false, since it could be supported by
another, more valid, argument.
Variations
Directly self-denying statements
The Epimenides paradox
is a statement of the form "this statement is false". Such statements
troubled philosophers, especially when there was a serious attempt to
formalize the foundations of logic. Bertrand Russell developed his "Theory of Types" to formalize a set of rules that would prevent such statements (more formally Russell's paradox) being made in symbolic logic. This work has led to the modern formulation of axiomatic set theory. While Russell's formalization did not contain such paradoxes, Kurt Gödel showed that it must contain independent statements. Any logical system that is rich enough to contain elementary arithmetic contains at least one proposition whose interpretation is this proposition is unprovable (from within the logical system concerned), and hence no such system can be both complete and consistent.
Indirectly self-denying statements or "fallacy of the stolen concept"
Objectivists
define the fallacy of the stolen concept: the act of using a concept
while ignoring, contradicting or denying the validity of the concepts on
which it logically and genetically depends. An example of the stolen
concept fallacy is anarchist Pierre-Joseph Proudhon's statement, "All property is theft".
While discussing the hierarchical
nature of knowledge, Nathaniel Branden states, "Theft" is a concept that
logically and genetically depends on the antecedent concept of
"rightfully owned property"—and refers to the act of taking that
property without the owner's consent. If no property is rightfully
owned, that is, if nothing is property, there can be no such concept as
"theft." Thus, the statement "All property is theft" has an internal
contradiction: to use the concept "theft" while denying the validity of
the concept of "property," is to use "theft" as a concept to which one
has no logical right—that is, as a stolen concept.
Others have said the statement is fallacious only on a superficial
reading of Proudhon, devoid of context. Proudhon used the term
"property" with reference to claimed ownership in land, factories, etc.
He believed such claims were illegitimate, and thus a form of theft from
the commons.
Proudhon explicitly states that the phrase "property is theft" is
analogous to the phrase "slavery is murder". According to Proudhon, the
slave, though biologically alive, is clearly in a sense "murdered".
The "theft" in his terminology does not refer to ownership any more than
the "murder" refers directly to physiological death, but rather both
are meant as terms to represent a denial of specific rights.
Others point out that the difference between the two examples is that
"slavery is murder", unlike "property is theft", does not make a
statement that denies the validity of one of the concepts it utilizes.
Proudhon does not actually say all property is theft—he is
referring to a very specific kind of property rights. Proudhon favored
another kind, which he called possession, based on occupancy and use, a
sort of usufruct rights idea. In What is Property?
he therefore says with the apparent contradiction "property is theft"
to denote one sort he feels is this, "property is liberty", referring to
the kind he favored, and "property is impossible" to make it clear any
sort of property rights cannot be absolute. Separate concepts are
therefore laid out in a way that can be confusing, especially if one is
not familiar with them.
In logic
Self-refutation plays an important role in some inconsistency tolerant logics (e.g. paraconsistent logics and direct logic) that lack proof by contradiction.
For example, the negation of a proposition can be proved by showing
that the proposition implies its own negation. Likewise, it can be
inferred that a proposition cannot be proved by (1) showing that a proof
would imply the negation of the proposition or by (2) showing a proof
would imply that the negation of the proposition can be proved.
Examples
Brain in a vat
Brain in a vat is a thought experiment in philosophy which is premised upon the skeptical hypothesis
that one could actually be a brain in a vat receiving electrical input
identical to that which would be coming from the nervous system. Similar
premises are found in Descartes's evil demon and dream argument.
Philosopher Hilary Putnam argues that some versions of the thought experiment would be inconsistent due to semantic externalism.
For a brain in a vat that had only ever experienced the simulated
world, the statement "I'm not a brain in a vat" is true. The only
possible brains and vats it could be referring to are simulated, and it
is true that it is not a simulated brain in a simulated vat. By the same
argument, saying "I'm a brain in a vat" would be false.
Determinism
It has been argued, particularly by Christian apologists, that to call determinism a rational statement is doubly self-defeating.
To count as rational, a belief must be freely chosen, which according to the determinist is impossible
Any kind of debate seems to be posited on the idea that the parties involved are trying to change each other's minds.
The argument does not succeed against the compatibilistic
view, since in the latter there is no conflict between determinism and
free will. Moreover, the argument fails if one denies either of the
above or its implicit implications. That is, one could avoid the
argument by maintaining that free will is not required for rationality
or for trying to change one's mind. The latter is a sensible position
insofar as one could be determined to try to persuade someone of
something, and the listener could be determined to accept it. There is
no internal contradiction in that view.
One can also consider a deterministic computer algorithm which is
able to make a correct conclusion, such as a mathematical calculation
or fingerprint identification. However, on some notions of
"rationality", such programs are themselves not rational because they
simply follow a certain deterministic pre-programmed path and nothing
more. This does not apply if one takes on a position with regards to
rationality analogous to compatibilism, namely, one could simply view
rationality as the property of correctly executing the laws of logic, in
which case there simply is no contradiction with determinism. The
contradiction would arise if one defines "rationality" in a manner that
is incompatibilist. Some argue that machines cannot "think", and if
rationality is defined so that it requires human-like thought, this
might pose a problem. But the view that machines cannot "think" in
principle is rejected by most philosophers who accept a computational theory of mind.
Ethical egoism
It
has been argued that extreme ethical egoism is self-defeating. Faced
with a situation of limited resources, egoists would consume as much of
the resource as they could, making the overall situation worse for
everybody. Egoists may respond that if the situation becomes worse for
everybody, that would include the egoist, so it is not, in fact, in his
or her rational self-interest to take things to such extremes. However, the (unregulated) tragedy of the commons and the (one off) prisoner's dilemma are cases in which, on the one hand, it is rational for an individual to seek to take as much as possible even though that makes things worse for everybody, and on the other hand, those cases are not self-refuting since that behaviour remains rational even though
it is ultimately self-defeating, i.e. self-defeating does not imply
self-refuting. Egoists might respond that a tragedy of the commons,
however, assumes some degree of public land. That is, a commons
forbidding homesteading requires regulation. Thus, an argument against
the tragedy of the commons, in this belief system, is fundamentally an
argument for private property rights and the system that recognizes both
property rights and rational self-interest—capitalism.
More generally, egoists might say that an increasing respect for
individual rights uniquely allows for increasing wealth creation and
increasing usable resources despite a fixed amount of raw materials
(e.g. the West pre-1776 versus post-1776, East versus West Germany, Hong
Kong versus mainland China, North versus South Korea, etc.).
Eliminative materialism
The philosopher Mary Midgley states that the idea that nothing exists except matter
is also self-refuting because if it were true neither it, nor any other
idea, would exist, and similarly that an argument to that effect would
be self-refuting because it would deny its own existence. Several other philosophers also argue that eliminative materialism is self-refuting.
However, other forms of materialism may escape this kind of argument because, rather than eliminating the mental, they seek to identify it with, or reduce it to, the material. For instance, identity theorists such as J. J. C. Smart, Ullin Place and E. G. Boring state that ideas exist materially as patterns of neural structure and activity. Christian apologist J.P. Moreland states that such arguments are based on semantics.
Epimenides paradox
The first notable self-refuting idea is the Epimenides paradox,
a statement attributed to Epimenides, a Cretan philosopher, that "All
Cretans are liars". This cannot be true if uttered by a Cretan.
A more common example is the self-refuting statement "I am lying"
(because the first statement allows the possibility "some Cretans do
not speak the truth", the speaker being one of them). The second
statement has no third alternative—the speaker's statement is either
true or false.
Evolutionary naturalism
Alvin Plantinga argues in his evolutionary argument against naturalism
that the combination of naturalism and evolution is "in a certain
interesting way self-defeating" because if it were true there would be
insufficient grounds to believe that human cognitive faculties are
reliable.
Consequently, if human cognitive abilities are unreliable, then any
human construct, which by implication utilizes cognitive faculties, such
as evolutionary theory, would be undermined. In this particular case,
it is the confluence of evolutionary theory and naturalism that,
according to the argument, undermine the reason for believing themselves
to be true. Since Plantinga originally formulated the argument, a few
theistic philosophers and Christian apologists have agreed.
There has also been a considerable backlash of papers arguing that the
argument is flawed in a number of ways, one of the more recent ones
published in 2011 by Feng Ye (see also the references in the Evolutionary argument against naturalism article).
Foundationalism
The philosopher Anthony Kenny argues that the idea, "common to theists like Aquinas and Descartes and to an atheist like Russell"
that "Rational belief [is] either self-evident or based directly or
indirectly on what is evident" (which he termed "foundationalism"
following Plantinga)
is self-refuting on the basis that this idea is itself neither
self-evident nor based directly or indirectly on what is evident and
that the same applies to other formulations of such foundationalism. However, the self-evident impossibility of infinite regress can be offered as a justification for foundationalism. Following the identification of problems with "naive foundationalism", the term is now often used to focus on incorrigible beliefs (modern foundationalism), or basic beliefs (reformed foundationalism).
Philosophical skepticism
Philosophical skeptics state that "nothing can be known". This has caused some to ask if nothing can be known then can that statement itself be known, or is it self-refuting. One very old response to this problem is academic skepticism: an exception is made for the skeptic's own statement. This leads to further debate about consistency and special pleading. Another response is that one should suspend belief completely (i.e. pyrrhonian scepticism).
However it is not clear whether one can hold this belief consistently
since one needs to believe that one should suspend belief. It can be
argued that the pyrrhonian could suspend belief on every such principle
as one needing to believe something.
Relativism
It is often stated that relativism about truth must be applied to itself. The cruder form of the argument concludes that since the relativist is calling relativism an absolute truth, it leads to a contradiction.
Relativists often rejoin that in fact relativism is only relatively
true, leading to a subtler problem: the absolutist, the relativist's
opponent, is perfectly entitled, by the relativist's own standards, to reject relativism. That is, the relativist's arguments can have no normative force over someone who has different basic beliefs.
Solipsism
On the face of it, a statement of solipsism
is — at least performatively — self-defeating, because a statement
assumes another person to whom the statement is made. (That is to say,
an unexpressed private belief in solipsism is not self-refuting). This,
of course, assumes the solipsist would not communicate with a
hallucination, even if just for self-amusement.
One response is that the solipsist's interlocutor is in fact a
figment of their imagination, but since their interlocutor knows they
are not, they are not going to be convinced.
Verification and falsification principles
The
statements "statements are meaningless unless they can be empirically
verified" and "statements are meaningless unless they can be empirically
falsified" have both been called self-refuting on the basis that they
can neither be empirically verified nor falsified.
Similar arguments have been made for statements such as "no statements
are true unless they can be shown empirically to be true", which was a
problem for logical positivism.
My propositions are elucidatory in
this way: he who understands me finally recognizes them as senseless,
when he has climbed out through them, on them, over them. (He must so to
speak throw away the ladder, after he has climbed up on it.)
(6.54)
However, this idea can be solved in the sense that, even if the
argument itself is self-refuting, the effects of the argument elicit
understandings that go beyond the argument itself. Søren Kierkegaard describes it as such:
[The reader] can understand that
the understanding is a revocation--the understanding with him as the
sole reader is indeed the revocation of the book. He can understand that
to write a book and to revoke it is not the same as refraining from
writing it, that to write a book that does not demand to be important
for anyone is still not the same as letting it be unwritten.
Neural synchrony is the correlation of brain activity across two or more people over time. In social and affective neuroscience,
neural synchrony specifically refers to the degree of similarity
between the spatio-temporal neural fluctuations of multiple people. This
phenomenon represents the convergence and coupling of different
people's neurocognitive
systems, and it is thought to be the neural substrate for many forms of
interpersonal dynamics and shared experiences. Some research also
refers to neural synchrony as inter-brain synchrony, brain-to-brain
coupling, inter-subject correlation, between-brain connectivity, or
neural coupling. In the current literature, neural synchrony is notably
distinct from intra-brain synchrony—sometimes also called neural
synchrony—which denotes the coupling of activity across regions of a
single individual's brain.
Increasingly implemented by social and affective neuroscientists,
neural synchrony approaches represent an important theoretical and methodological
contribution to the field. Since its conception, studies of neural
synchrony have helped elucidate the mechanisms underlying social
phenomena, including communication, narrative processing, coordination,
and cooperation. By emphasizing the social dynamics of the brain, this
area of research has played a critical role in making neuroscience more
attuned to people's social proclivities—a perspective that is often lost
on individual-level approaches to understanding the brain.
Despite the growth of social cognition and cognitive neuroscience
prior to the early 2000s, research into the brain neglected
interpersonal processes, focusing mostly on the neural mechanisms of
individuals' behaviors.
Furthermore, neuroscience research that did probe social questions only
investigated how social processes affect neural dynamics in a single
brain.
Considering that researchers clearly recognized how interpersonal
interaction was fundamental to human cognition, the paucity of social
and multi-brain neuroscience research represented a tension in the
field. In response to the discrepancy between the complexity of social
interaction and the single-brain focus of cognitive neuroscience,
researchers called for a multi-person, interaction-oriented approach to
understanding the brain.
Early history
In 2002, the American neuroscientistP. Read Montague
articulated the need to examine the neural activity of multiple
individuals at one time. To this point, Montague and his colleagues
wrote, "Studying social interactions by scanning the brain of just one
person is analogous to studying synapses while observing either the
presynaptic neuron or the postsynaptic neuron, but never both
simultaneously." They performed the first brain scan of more than one person by using functional magnetic resonance imaging
(fMRI) to take simultaneous recordings of two people engaged in a
simple deception game. While this study marked the first example of
multi-brain neuroimaging, in 2005, King-Casas and others
combined neuroimaging with an economic exchange game to conduct the
first study that directly compared neural activity between pairs of
subjects.
Since then, multi-brain imaging studies have grown in popularity,
leading to the formation of preliminary neural synchrony frameworks.
Early conceptualizations of neural synchrony, mostly from the Hasson Lab at Princeton University,
were motivated by models of stimulus-to-brain coupling. In these
models, aspects of the physical environment emit mechanical, chemical,
and electromagnetic
signals, which the brain receives and translates into electrical
impulses that guide our actions and allow us to understand the world. Researchers presumed that the synchronization
of neural activity between two brains should leverage the same system
that binds one's neural activity to environmental stimuli. If the
stimulus is another person, then the perceptual system of one brain may
couple with the behaviors or emotions of the other person, causing
"vicarious activations" that manifest as synchronized neural responses across perceiver and agent. According to the theory, this process also occurs through more complex, synergistic interactions, especially when people communicate and convey meaning.
Further development
Over
the last two decades, neural synchrony has become an increasingly
common topic of study in social and affective neuroscience research,
spurring conceptual and methodological development. Along with an
emphasis on ecologically valid, naturalistic experimental designs, the
focus on multi-brain neuroscience studies has increased researchers'
ability to explore neural synchrony in social contexts. As a result,
conceptualizations of neural synchrony have been expanded to incorporate
a wider range of ideas, though it is often viewed as a neural correlate
for two or more people's shared experiences. Studies now involve a
variety of social processes, with applications spanning simple motor
synchronization to classroom learning.
Notable methodological advancements have come from the evolution of multi-brain imaging techniques beyond fMRI, especially magnetoencephalography/electroencephalography (MEG/EEG) and functional near-infrared spectroscopy (fNIRS)—methods which afford more socially interactive experimental designs. These technologies are also complemented by comprehensive data processing techniques that are useful in multi-brain analyses, such as Granger causality or Phase Locking Value (PLV).
As a progressively paradigmatic approach in social and affective
neuroscience, neural synchrony undergirds the field's search for the
brain basis of social interaction.
Methods
Hyperscanning
The
study of neural synchrony is predicated on advanced neuroimaging
methods, particularly hyperscanning. Coined in 2002 by Montague et al, hyperscanning refers to the method of simultaneously measuring the hemodynamic or neuroelectric responses of two or more brains as they engage with the same task or stimulus.
The ability to record time-locked activity from multiple brains makes
hyperscanning conducive to exploring the variation in activity across
brains. It also allows experimenters to examine various aspects of
neural recordings in naturalistic scenarios, from low-level stimulus
processing to high-level social cognition.
For these reasons, hyperscanning has helped foster a systematic
investigation of interpersonal dynamics at the level of the brain.
Though hyperscanning has become the most common imaging technique
for studying neural synchrony, researchers do not necessarily need to
scan brains simultaneously. Sometimes referred to as off-line
measurement, or "pseudo-hyperscanning";
this alternative approach follows the same basic premise as
hyperscanning, except that participants' brain activity is recorded one
at a time. Data from different scans of isolated participants are then
analyzed to compare functional similarities during identical tasks or
stimuli.
Imaging techniques
Hyperscanning
and off-line scanning methods can be achieved through common
noninvasive hemodynamic or neuroelectric brain imaging techniques. A
review of neural synchrony hyperscanning studies showed that the most
prevalent methods are EEG, fNIRS, and fMRI, which account for 47%, 35%,
and 17% of studies, respectively.
Each technique offers unique contributions to the understanding of
neural synchrony given their relative advantages and limitations.
EEG measures the brain's electrical activity through the scalp.
It is widely used to study neural synchrony because of its superior millisecond-range temporal resolution.
Though susceptible to head movements, EEG still allows for exploring
neural synchrony through naturalistic designs where people can interact
socially. The downside to EEG is its relatively poor spatial resolution, which makes it difficult to elucidate spatial qualities of brain activation in social contexts.
fNIRS uses near infrared waves to measure the blood-oxygen-level-dependent
(BOLD) response in the brain. It is an increasingly popular imaging
method for neural synchrony studies because of its portability and
motion tolerance, which makes it ideal for testing real-world social stimuli. fNIRS only measures the cortical regions of the brain,
and its temporal resolution is not as fine as EEG. However, the balance
between spatial and temporal properties, combined with subjects'
ability to move around and interact with relative freedom during
scanning, qualify fNIRS as a versatile option for exploring neural
synchrony.
fMRI uses magnetic resonance
to measure the brain's BOLD response. The major advantage of fMRI is
the precise spatial resolution. fMRI allows researchers to examine
in-depth neurocognitive processes that occur across brains. However,
fMRI has low temporal resolution, is highly sensitive to motion, and
requires that subjects lie flat in a loud MRI machine while interacting
with a screen. These factors pose limitations to the study of neural
synchrony, which often calls for naturalistic environments and tasks
that are representative of real-world social contexts.
Analysis
A
standard approach to investigating neural synchrony, especially with
data from naturalistic experimental designs, is inter-subject
correlation (ISC).
Often, ISC is the Pearson correlation, or robust regression, of
spatio-temporal patterns of neural activity in multiple subjects. In
ISC, an individual's brain responses are either correlated across the
average of the other subjects in a leave-one-out analysis, or all pairs
of subjects are correlated in a pairwise analysis.
This method leverages time-locked stimuli in order to understand how
brain activity across participants relates to different parts of the
task. Rather than focusing on the strength of activation in brain areas,
ISC explores the variability in neural activity across subjects, allowing researchers to probe the level of similarity or idiosyncrasy in people's brain responses.
Shared variance in neural activity is assumed to be indicative of
similar processing of identical stimuli or tasks. Similar to the general linear model,
it is important to compare ISC values to a null, which can be derived
from recordings of resting states or irrelevant stimuli. Because it
depends on extended designs that allow for activity recording over time,
ISC is especially conducive to social interaction studies, which makes
it a powerful approach for exploring neural synchrony in social
contexts. However, ISC depends on stimulus-driven responses, which poses
difficulties for researchers interested in resting-state activity.
Recently, inter-subject representational similarity analysis
(IS-RSA) has been put forward as a way to detect the individual
differences, or “idiosynchrony,” across people experiencing naturalistic
experimental stimuli. This analysis takes the neural synchrony of each
subject to the other subjects and relates it to known individual
behavioral measures, allowing researchers to compare multi-person-level
brain data with individual-level traits and behaviors.
Best practices
Neural synchrony is a relatively new area of study that affords a variety of approaches, and no prevailing paradigm
exists to collect, analyze, and interpret the data. Many decisions,
such as imaging techniques or analysis methods, depend on researchers’
goals. However, there are some generally agreed upon best practices when
designing these experiments. For example, sample sizes of about 30 are
necessary to acquire reliable and reproducible statistical ISC maps.
Furthermore, when studying shared responses, researchers typically
prefer a strong stimulus that is able to generate significant brain
responses, allowing researchers to detect greater levels of neural
synchrony across participants. The exception to this preference is when
researchers are more interested in the individual differences that drive
synchrony. In these cases, researchers should employ stimuli that are
strong enough to evoke neural synchrony, yet modest enough to maintain
sufficient neural variability that researchers can later relate to the
variability in behavioral measures.
One of the biggest considerations for conducting neural synchrony studies concerns the ecological
validity of the design. As an inherently social phenomenon, neural
synchrony calls for multidimensional stimuli that emulate the richness
of the social world.
Furthermore, by nature of how it is measured—through computing the
variance in multiple brains' responses to a task over time—neural
synchrony is particularly amenable to extended social stimuli.
Ecological designs are notably difficult in most neuroimaging studies,
yet they are especially important for capturing social processes, and
they also play to the strengths and affordances of neural synchrony
approaches.
Experimental evidence and implications
Communication
Examining
neural synchrony through multi-brain studies has offered insight into
the shared and idiosyncratic aspects of human communication. As a
potential neural mechanism for the effective transfer of information
across brains, neural synchrony has shown how brain activity temporally
and spatially couples when people communicate. Synchrony during
communication occurs in a number of brain frequencies and regions,
notably alpha and gamma bands, the temporal parietal junction, and inferior frontal areas.
In a seminal study, Stephens et al.
demonstrated this inter-brain link through an fMRI analysis of speakers
and listeners. Using the speaker's spatial and temporal neural
responses to model the listener's responses during natural verbal
communication, they found that brain activity synchronized in dyads in
both a delayed and anticipatory manner, but this synchrony failed to
occur when subjects did not communicate (e.g., speaking in a language
the listener does not understand). Greater synchrony across brains,
especially in the predictive anticipatory responses, indicated better
scores on comprehension measures. Building on this work, other research
has sought to pinpoint communicative factors associated with neural
synchrony. By manipulating conversation modality and instruction,
research has found that neural synchrony is strongest during
face-to-face conversations that incorporate turn-taking behavior and
multi-sensory verbal and nonverbal interaction.
Network structure dynamics also play a role in neural synchrony, such
that central figures, like conversation leaders, tend to show greater
neural synchrony than non-leaders with other discussion partners.
Neural synchrony is also found in nonverbal communication, such
as hand gestures and facial expressions. An early study found
synchronization across participants playing a game of charades. Using
fMRI to record brain activity as people gestured or watched the
gestures, researchers found synchronized temporal variation in brain
activity in mirror neuron and mentalizing systems.
Another study showed that communicative behaviors like shared gaze and
positive affect expression generated neural synchrony in romantic
partners, though not in strangers.
As a whole, neural synchrony studies surrounding verbal, multi-sensory,
and nonverbal communication demonstrate its potential as a tool for
exploring the underlying mechanisms of interpersonal communication.
Narrative processing
Another
focus of neural synchrony studies involves narrative processing. This
direction of research has some crossover with neural synchrony studies
of communication, but there remains sufficient interest in the
similarities and differences in how people specifically process
multimodal narrative information, such as watching movies, hearing
stories, or reading passages. Importantly, narrative processing studies
of neural synchrony observe hierarchical levels of processing that
unfold over time,
starting in areas responsible for low-level processing of auditory or
visual stimuli. As semantic information becomes more salient in the
narrative, synchronized processing moves to more integrative networks,
such as the inferior parietal lobe or temporal parietal junction.
Research shows that neural synchrony is indicative of the
similarity in people's narrative recall and understanding, even for
ambiguous narratives. One study demonstrated this phenomenon using Heider and Simmel's
classic paradigm, where simple shapes move around the screen in a way
that causes people to imbue the shapes with stories and social meaning.
Participants who interpreted the movement of shapes in similar ways
showed greater neural synchrony in cortical brain regions. This
connection between neural synchrony and similarity in comprehension
reliably occurs across other types of narratives, including listening to
stories and free viewing of visual content,
and it persists throughout different stages of the narrative, such as
consuming the story, recalling the story, and listening to another
person recall the story. Together, these findings highlight neural
synchrony as a reliable neural mechanism for the convergence of people's
hierarchical narrative processing, suggesting that synchrony plays a
critical role in how, if, and why we see meaning in the world similarly.
Coordination
The
pursuit of complex goals for individuals or groups depends on
successful coordination, and neural synchrony provides a window into the
underlying mechanisms of these processes as well. A review of
hyperscanning research shows that neural synchrony approaches have
explored coordination through a range of paradigms, including joint attention, movements, ideas, and tasks.
These findings also demonstrate synchronization across a variety of
brain areas associated with sharing actions and mentalizing, namely the
inferior and temporal parietal areas, as well as alpha band and other
frequencies. Furthermore, converging evidence suggests that inter-brain
models (i.e., neural synchrony) are more effective than intra-brain
models at predicting performance for tasks requiring social
coordination.
Understanding how coordination via joint attention relates to
neural synchrony, and how this relationship drives performance, is of
particular interest to researchers. Research shows that even simple
social interactions, like attention convergence, can induce synchrony.
For example, in a task where one participant must direct another
participant to a target location through eye gazing only, which requires
that both participants eventually coordinate eye movements, researchers
found significant neural synchrony in mentalizing regions of
interacting pairs. Other studies show strong neural synchrony during simple coordinated events like hand and finger movement imitation, humming, and even eye-blinking.
Coordination studies also find neural synchrony in more complex
social coordinations. A set of studies has demonstrated the prevalence
of neural synchrony in music production while people coordinate rhythms
and movements. Early studies showed that dyads of guitarists generate
greater low frequency band neural synchrony when playing together than
when playing solo.
Also, people who performed distinct roles in an intricate musical piece
showed synchrony between brains during periods of coordination.
Another series of studies examined pilots and copilots in a flight
simulator, finding that synchrony was strongest when the situation
demanded more social coordination, such as during stressful scenarios or
takeoff and landing.
These findings implicate neural synchrony as a reliable correlate of
social coordination, even when interactions call for coordination of
various forms and complexities.
Cooperation
As
measured through tasks that involve interactive decision-making and
games, results from the field suggest a close association between neural
synchrony and cooperation. Decision-making contexts and games that
demand greater levels of social, high-level, and goal-directed
engagement with other people are typically more conducive to neural
synchrony.
In this domain, researchers are particularly interested in how neural
synchrony levels vary depending on whether people collaborate, compete,
or play alone.
For example, one study that employed a computer video game found
high levels of neural synchrony - and better performance - across
subjects when they played on the same team, but this effect disappeared
when people played against each other or by themselves.
Similarly, researchers that administered a puzzle solving task found
neural synchrony for people when they are working as a team, yet
synchrony decreased for the same people when they worked separately or
watched others solve the puzzle. Another study using a classic prisoner's dilemma
game showed that participants experienced higher neural synchrony with
each other in the high-cooperation-context conditions than they did in
the low-cooperation-context conditions or when they interacted with the
computer.
Subjective measures of perceived cooperativeness mediated this effect.
Critically, the idea that neural synchrony is robust during cooperation,
that more interactive and demanding cooperative tasks recruit greater
neural synchrony, and that better cooperation often links to better
performance is corroborated throughout the neural synchrony literature.
Individual-level differences
Much
of the neural synchrony literature examines how stimuli drive responses
across multiple brains. Because these responses are often
task-dependent, it becomes hard to disentangle state-level factors from
individual-level factors (e.g., traits). However, creative experimental
designs, access to certain populations, and advances in analysis
methods, like IS-RSA, have offered some recent insight into how
individual-level differences affect neural synchrony.
Using an ambiguous social narrative, Finn et al.
report that individuals with high-trait paranoia showed stronger neural
synchrony with each other in socially-motivated cortical regions than
they did with low-trait paranoia subjects - a finding that also scales
when examining the semantic and syntactic similarities of their
narrative recall. Similarly, research shows that people's cognitive styles
affect their level of synchrony with each other. In response to viewing
a film, Bacha-Trams et al. demonstrated that holistic thinkers showed
greater neural synchrony with each other, and presumably understood the
film more similarly, than analytic thinkers did with each other. The two
groups also exhibited within-group synchrony in different brain
regions.
The idea that individual-level differences affect neural
synchrony extends to clinical areas as well. Some research indicates
that people who manage autism spectrum disorder exhibit distinct and diminished patterns of neural synchrony compared to people without autism spectrum disorder. Clinically driven discrepancies in neural synchrony have also been shown to increase along with symptom severity.
The brain-as-predictor approach
Neural
synchrony has major implications for the brain-as-predictor approach,
which encourages the use of neuroimaging data to predict robust,
ecologically valid behavioral outcomes. The brain-as-predictor approach
has been effective in predicting outcomes across a variety of domains,
including health and consumer choices. Given its social nature, neural
synchrony has the potential to build on brain-as-predictor models by
allowing for predictions about real-world social processes. Some
researchers have started to employ this approach.
In one study, members of a bounded social network watched a
battery of short audiovisual movies in an MRI scanner. Hypothesizing
that similarity in neural responses tracks with social closeness, the
researchers used the strength of neural synchrony measures across
participants to reliably predict real-world social network proximity and
friendship. Another example of how neural synchrony can be leveraged to
predict outcomes involves the use of neural reference groups, which can
predict behaviors like partisan stance on controversial topics at
above-chance levels. This approach requires identifying groups of people
that perceive and respond to the world in similar ways, measuring their
brain activity and dispositional attitudes related to any stimuli of
interest, and then using a synchrony-based classification method to
predict whether new individuals see the world similarly or differently
depending on their synchrony with the reference group. Together, these
findings illustrate the power and potential for neural synchrony to
contribute to brain-as-predictor models, ultimately framing neural
synchrony as a tool for understanding real-world outcomes above and
beyond behavioral measures alone.
Agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or irrigation
water. Agricultural wastewater treatment is required for continuous
confined animal operations like milk and egg production. It may be
performed in plants using mechanized treatment units similar to those
used for industrial wastewater. Where land is available for ponds, settling basins and facultative lagoons may have lower operational costs for seasonal use conditions from breeding or harvest cycles. Animal slurries are usually treated by containment in anaerobic lagoons before disposal by spray or trickle application to grassland. Constructed wetlands are sometimes used to facilitate treatment of animal wastes.
Nonpoint source pollution includes sediment runoff, nutrient
runoff and pesticides. Point source pollution includes animal wastes,
silage liquor, milking parlour (dairy farming) wastes, slaughtering
waste, vegetable washing water and firewater. Many farms generate nonpoint source pollution from surface runoff which is not controlled through a treatment plant.
direct application (e.g. aerial spraying or broadcasting over water bodies)
runoff during rain storms
aerial drift (from adjacent fields).
Some pesticides have also been detected in groundwater.
Farmers may use Integrated Pest Management (IPM) techniques (which can include biological pest control) to maintain control over pests, reduce reliance on chemical pesticides, and protect water quality.
There are few safe ways of disposing of pesticide surpluses other than through containment in well managed landfills or by incineration. In some parts of the world, spraying on land is a permitted method of disposal.
Animal wastes from cattle can be produced as solid or semisolid manure or as a liquid slurry. The production of slurry is especially common in housed dairy cattle.
Treatment
Whilst solid manure heaps outdoors can give rise to polluting
wastewaters from runoff, this type of waste is usually relatively easy
to treat by containment and/or covering of the heap.
Animal slurries require special handling and are usually treated
by containment in lagoons before disposal by spray or trickle
application to grassland. Constructed wetlands are sometimes used to facilitate treatment of animal wastes, as are anaerobic lagoons.
Excessive application or application to sodden land or insufficient
land area can result in direct runoff to watercourses, with the
potential for causing severe pollution. Application of slurries to land overlying aquifers can result in direct contamination or, more commonly, elevation of nitrogen levels as nitrite or nitrate.
The disposal of any wastewater containing animal waste upstream
of a drinking water intake can pose serious health problems to those
drinking the water because of the highly resistant spores present in
many animals that are capable of causing disabling disease in humans.
This risk exists even for very low-level seepage via shallow surface
drains or from rainfall run-off.
Some animal slurries are treated by mixing with straws and composted at high temperature to produce a bacteriologically sterile and friable manure for soil improvement.
Piggery waste is comparable to other animal wastes and is processed
as for general animal waste, except that many piggery wastes contain
elevated levels of copper
that can be toxic in the natural environment. The liquid fraction of
the waste is frequently separated off and re-used in the piggery to
avoid the prohibitively expensive costs of disposing of copper-rich
liquid. Ascarid worms and their eggs are also common in piggery waste and can infect humans if wastewater treatment is ineffective.
Silage liquor
Fresh or wilted grass or other green crops can be made into a semi-fermented product called silage
which can be stored and used as winter forage for cattle and sheep. The
production of silage often involves the use of an acid conditioner such
as sulfuric acid or formic acid. The process of silage making frequently produces a yellow-brown strongly smelling liquid which is very rich in simple sugars, alcohol,
short-chain organic acids and silage conditioner. This liquor is one of
the most polluting organic substances known. The volume of silage
liquor produced is generally in proportion to the moisture content of
the ensiled material.
Treatment
Silage liquor is best treated through prevention by wilting crops
well before silage making. Any silage liquor that is produced can be
used as part of the food for pigs. The most effective treatment is by
containment in a slurry lagoon and by subsequent spreading on land
following substantial dilution with slurry. Containment of silage liquor
on its own can cause structural problems in concrete pits because of
the acidic nature of silage liquor.
Milking parlour (dairy farming) wastes
Although
milk is an important food product, its presence in wastewaters is
highly polluting because of its organic strength, which can lead to very
rapid de-oxygenation of receiving waters. Milking parlour wastes also contain large volumes of wash-down water, some animal waste together with cleaning and disinfection chemicals.
Treatment
Milking parlour wastes are often treated in admixture with human sewage in a local sewage treatment
plant. This ensures that disinfectants and cleaning agents are
sufficiently diluted and amenable to treatment. Running milking
wastewaters into a farm slurry lagoon is a possible option although this
tends to consume lagoon capacity very quickly. Land spreading is also a
treatment option.
Wastewater
from slaughtering activities is similar to milking parlour waste (see
above) although considerably stronger in its organic composition and
therefore potentially much more polluting.
Treatment
As for milking parlour waste (see above).
Vegetable washing water
Washing of vegetables
produces large volumes of water contaminated by soil and vegetable
pieces. Low levels of pesticides used to treat the vegetables may also
be present together with moderate levels of disinfectants such as chlorine.
Treatment
Most vegetable washing waters are extensively recycled with the
solids removed by settlement and filtration. The recovered soil can be
returned to the land.
Firewater
Although
few farms plan for fires, fires are nevertheless more common on farms
than on many other industrial premises. Stores of pesticides,
herbicides, fuel oil for farm machinery and fertilizers can all help
promote fire and can all be present in environmentally lethal quantities
in firewater from fire fighting at farms.
Treatment
All farm environmental management plans should allow for containment
of substantial quantities of firewater and for its subsequent recovery
and disposal by specialist disposal companies.
The concentration and mixture of contaminants in firewater make them
unsuited to any treatment method available on the farm. Even land
spreading has produced severe taste and odour problems for downstream
water supply companies in the past.
