Vision
The paradox of vision is that retinal information is sent to specialized
areas that process selected features of the visual world, often using
information that is not always referred to the conscious picture, and at the
same time these specialized units interact to produce an integrated picture of
what is going on out there. As long as your keep your eyes open, that picture is
continuous and remarkably stable, despite that fact that everything is
changing. While our visual world is information rich, very little visual
experience is stored in the brain, even as short-term working memory. Pictures
are not stored, but the meaning of visual events is remembered. The short
term memory for objects just seen varies from 1.5 to about 5 objects.
In his classic text, A Vision of the Brain, Semir Zeki stated: “The brain
strives to acquire knowledge about the external world through the sense of
vision… But this is no easy matter since the visual world is in a continual
state of change. Thus the brain can only acquire knowledge about the invariant
properties of objects and surfaces if it is able to discard the continually
changing information reaching it …a visual precept does not reside in any given
visual area, even if the area is critical for recognizing certain features, but
rather it is the result of ongoing activity in several interconnected visual
areas”
A different view that contradicts Zeki is that the brain is more interested
in moving events and tends to ignore static events. Our inborn interest is in
detecting the changing behavior of other humans and we survive better if we
notice even subtle changes in the environment. The best idea is to recognize
that the brain is skilled in detecting novel events and at the same time can
produce a remarkably stable picture as a monitor image in consciousness.
Light reflected from a scene is focused on the retina where receptors
activate nerve cells that send data to the occipital cortex. The primary
receptive cortex is V1 also known as the striate cortex. If you destroy the
striate cortex, vision is deleted from consciousness. Other clusters of neurons
respond to features such as spatial position, wavelength (color), motion, and
form. Color information is processed in area V4 in the fusiform gyrus of
the temporal lobe. The right fusiform gyrus contributes to the conscious
recollection of visual attributes
of familiar objects.
The eye is the only visible part of the brain. The retina inside can be
easily examined with an ophthalmoscope. The macula is a small but visible area
of the retina that originates all the information for color and detailed
imaging. The derivation of visual meaning begins with strategies of scanning the
environment and orienting to events that promise rewards or threaten punishment.
Images enter the eye and enter a series of processing steps in the thalamus
(lateral geniculate body) and then to the occipital cortex. Visual information
is combined with all other information in other areas of the cerebral cortex.
The visual cortex is subdivided into 6 regions, V1 to V6. All the fibers from
the retina via LGB go to V1 first and then are distributed in a recursive manner
to the other five regions. Information about object form, color and movement are
processed separately in parallel channels.
Retinal Diagram from Helga Kolb. Simple Anatomy of the Retina.
Webvision. The Organization of the Retina and Visual System
http://webvision.med.utah.edu/book/part-i-foundations/simple-anatomy-of-the-retina/
Specialized vision processors are linked to other sensory processors and
interact. Thus, your understanding of what you see is influenced by what you
hear and visa versa. This is an ancient animal strategy, combine the input from
many sensory sources and you will survive better. Not only do cognition
processors interact but all are influenced at the same time by context
processors and all can be shifted abruptly by interrupts such as emotions.
Tovee described two streams of visual information that project to prefrontal
cortical areas. A ventral system projects to the cortex of the inferior
convexity and a dorsal system projects to the dorsolateral prefrontal
region.
He stated:” As one moves along both of these visual streams from the retina,
through the primary visual cortex to the visual association areas, the response
characteristics of neurons change. Neurons higher up the pathways have larger
receptive fields and respond to more complex stimuli. In the highest association
areas, the activity of the neurons plays an important role in working memory.
For example, in the primate inferior temporal cortex there are neurons
responsive to complex biological stimuli such as faces. Some neurons in the
monkey IT cortex maintain a high firing rate during the delay between stimuli
(which can be as long as 10 to 15 seconds), as though they are actively
maintaining a memory of the sample stimulus for comparison with the test
stimulus. However, if a new stimulus is presented during the delay between the
sample and test, the maintained neural activity is abolished. This neural
activity seems to represent a form of visual rehearsal which can be easily
disrupted, but which may still be an aid to short-term memory formation.”
Vision is a composite of specialized parallel processing sequences.
Some of these processing sequences eventually emerge as a conscious picture. At
each stage in the visual path, information is extracted and decisions are made
before and while the picture is conscious.
It is not obvious that the conscious picture is required by cognitive
processors to derive meaning from what is seen. Vul and MacLeod used quickly
alternating images to demonstrate that: “cortical mechanisms track color much
faster than perception, responding well to color alternations that are too rapid
to be perceptible. The more restricted frequency response of the conscious
perception of color suggests that extra integrative steps give conscious color
perception a time course substantially slower than that of early cortical
mechanisms.”
There are many examples of humans who are blind after their primary visual
cortex is damaged but still respond to visual information. Damage to other brain
areas may leave the picture intact in consciousness, but face or object
recognition fails so that a face can be seen clearly but cannot be identified;
objects exist but lose their meaning. As a general proposition, the meaning of
the contents of a picture is known before the picture is perceived. The wrong
impression is that the meaning is derived from consciousness and somehow depends
on the perceiver’s conscious participation.
Tsao et al suggested that monkeys and humans are similar, processing
different object categories in specialized areas of the cerebral cortex.
Functional magnetic resonance imaging (fMRI) studies, for example, have shown
that humans have a ventral temporal area tuned to recognizing faces, and that
macaques have face-responsive neurons scattered throughout temporal cortex,
similar in relative size and number to face patches in humans.
The progress in task related imaging is remarkable. Sometimes incidental
observations are more informative than the measurements planned by the
researchers. In one study for example, the authors reflect on the highly
recursive nature of cortical processing. They stated that visual input entering
visual cortex is not conveyed to other brain regions in a serial fashion, but
rather is processed in multiple parallel feedback loops. Neural activity from a
visual stimulus can persist over several hundred milliseconds, lasting longer
than the stimulus. Iconic memory relates to ongoing cortical processing in
higher visual areas, such as lateral occipital cortex. The time course and decay
of iconic memory reflect temporal properties of recursive computations in visual
areas, where percept-related activity has been shown to persist after the
stimulus ends.
Ganel et al made a distinction between visual processing that guides movement
and visual processing that leads to a conscious picture. They stated: “The
visual perception of object shape depends on 'holistic' processing in which a
given dimension cannot be perceptually isolated from the other dimensions of the
object. The visual control of action (such as grasping an object) is
mediated by cortical areas that are largely independent of those mediating
conscious perception, processing only the most action-relevant dimension of an
object, ignoring non-relevant object features.”
