https://en.wikipedia.org/wiki/Visual_modularity
In cognitive neuroscience, visual modularity is an organizational concept concerning how vision works. The way in which the primate visual system operates is currently under intense scientific scrutiny. One dominant thesis is that different properties of the visual world (color, motion, form and so forth) require different computational solutions which are implemented in anatomically/functionally distinct regions that operate independently – that is, in a modular fashion.
In cognitive neuroscience, visual modularity is an organizational concept concerning how vision works. The way in which the primate visual system operates is currently under intense scientific scrutiny. One dominant thesis is that different properties of the visual world (color, motion, form and so forth) require different computational solutions which are implemented in anatomically/functionally distinct regions that operate independently – that is, in a modular fashion.
Motion processing
Akinetopsia is an intriguing condition brought about by damage to the Extrastriate cortex MT+ that renders humans and monkeys unable to perceive motion, seeing the world in a series of static "frames" instead
and indicates that there might be a "motion centre" in the brain. Of
course, such data can only indicate that this area is at least necessary
to motion perception, not that it is sufficient; however, other
evidence has shown the importance of this area to primate motion
perception. Specifically, physiological, neuroimaging, perceptual,
electrical- and transcranial magnetic stimulation
evidence (Table 1) all come together on the area V5/hMT+. Converging
evidence of this type is supportive of a module for motion processing.
However, this view is likely to be incomplete: other areas are involved
with motion perception, including V1, V2 and V3a and areas surrounding V5/hMT+ (Table 2). A recent fMRI study put the number of motion areas at twenty-one.
Clearly, this constitutes a stream of diverse anatomical areas. The
extent to which this is ‘pure’ is in question: with Akinetopsia come
severe difficulties in obtaining structure from motion. V5/hMT+ has since been implicated in this function as well as determining depth.
Thus the current evidence suggests that motion processing occurs in a
modular stream, although with a role in form and depth perception at
higher levels.
| Methodology | Finding |
|---|---|
| Physiology (single cell recording) | Cells directionally and speed selective in MT/V5 |
| Neuroimaging | Greater activation for motion information than static information in V5/MT |
| Electrical-stimulation & perceptual | Following electrical stimulation of V5/MT cells perceptual decisions are biased towards the stimulated neuron’s direction preference |
| Magnetic-stimulation | Motion perception is also briefly impaired in humans by a strong magnetic pulse over the corresponding scalp region to hMT+ |
| Psychophysics | Perceptual asynchrony among motion, color and orientation. |
| Methodology | Finding |
|---|---|
| Physiology (single cell recording) | Complex motion involving contraction/expansion and rotation found to activate neurons in medial superior temporal area (MST) |
| Neuroimaging | Biological motion activated superior temporal sulcus |
| Neuroimaging | Tool use activated middle temporal gyrus and inferior temporal sulcus |
| Neuropsychology | Damage to visual area V5 results in akinetopsia |
Color processing
Similar converging evidence suggests modularity for color. Beginning with Gowers’ finding that damage to the fusiform/lingual gyri in occipitotemporal cortex correlates with a loss in color perception (achromatopsia), the notion of a "color centre" in the primate brain has had growing support. Again, such clinical evidence only implies that this region is critical to color perception, and nothing more. Other evidence, however, including neuroimaging and physiology converges on V4 as necessary to color perception. A recent meta-analysis has also shown a specific lesion common to achromats corresponding to V4. From another direction altogether it has been found that when synaesthetes experience color by a non-visual stimulus, V4 is active.
On the basis of this evidence it would seem that color processing is
modular. However, as with motion processing it is likely that this
conclusion is inaccurate. Other evidence shown in Table 3 implies
different areas’ involvement with color. It may thus be more instructive
to consider a multistage color processing stream from the retina
through to cortical areas including at least V1, V2, V4, PITd and TEO. Consonant with motion perception, there appears to be a constellation of areas drawn upon for color perception.
In addition, V4 may have a special, but not exclusive, role. For
example, single cell recording has shown that only V4 cells respond to
the color of a stimuli rather than its waveband, whereas other areas
involved with color do not.
| Other areas involved with color/Other functions of V4 |
|---|
| Wavelength sensitive cells in V1 and V2 |
| anterior parts of the inferior temporal cortex |
| posterior parts of the superior temporal sulcus (PITd) |
| Area in or near TEO |
| Shape detection |
| Link between vision, attention and cognition |
Form processing
Another clinical case that would a priori suggest a module for modularity in visual processing is visual agnosia. The well studied patient DF is unable to recognize or discriminate objects
owing to damage in areas of the lateral occipital cortex although she
can see scenes without problem – she can literally see the forest but
not the trees. Neuroimaging
of intact individuals reveals strong occipito-temporal activation
during object presentation and greater activation still for object
recognition.
Of course, such activation could be due to other processes, such as
visual attention. However, other evidence that shows a tight coupling of
perceptual and physiological changes
suggests activation in this area does underpin object recognition.
Within these regions are more specialized areas for face or fine grained
analysis, place perception and human body perception. Perhaps some of the strongest evidence for the modular nature of these processing systems is the double dissociation between object- and face (prosop-) agnosia. However, as with color and motion, early areas are implicated too, lending support to the
idea of a multistage stream terminating in the inferotemporal cortex
rather than an isolated module.
Functional modularity
One of the first uses of the term "module" or "modularity" occurs in the influential book "Modularity of Mind" by philosopher Jerry Fodor.
A detailed application of this idea to the case of vision was published
by Pylyshyn (1999), who argued that there is a significant part of
vision that is not responsive to beliefs and is "cognitively
impenetrable".
Much of the confusion concerning modularity exists in
neuroscience because there is evidence for specific areas (e.g. V4 or
V5/hMT+) and the concomitant behavioral deficits following brain insult
(thus taken as evidence for modularity). In addition, evidence shows
other areas are involved and that these areas subserve processing of
multiple properties (e.g. V1)
(thus taken as evidence against modularity). That these streams have
the same implementation in early visual areas, like V1, is not
inconsistent with a modular viewpoint: to adopt the canonical analogy in
cognition, it is possible for different software to run on the same
hardware. A consideration of psychophysics
and neuropsychological data would suggest support for this. For
example, psychophysics has shown that percepts for different properties
are realized asynchronously. In addition, although achromats experience other cognitive defects they do not have motion deficits when their lesion is restricted to V4, or total loss of form perception. Relatedly, Zihl and colleagues' akinetopsia
patient shows no deficit to color or object perception (although
deriving depth and structure from motion is problematic, see above) and
object agnostics do not have damaged motion or color perception, making
the three disorders triply dissociable.
Taken together this evidence suggests that even though distinct
properties may employ the same early visual areas they are functionally
independent. Furthermore, that the intensity of subjective perceptual
experience (e.g. color) correlates with activity in these specific areas
(e.g. V4), the recent evidence that synaesthetes
show V4 activation during the perceptual experience of color, as well
as the fact that damage to these areas results in concomitant behavioral
deficits (the processing may be occurring but perceivers do not have
access to the information) are all evidence for visual modularity.
