Very comprehensive and complete summary of the sixth task of the block GGZ2025, with many images and figures (original from the basic book). Summary contains material from the basic book (Kolb & Whishaw). Now available as bundle!
There was consensus that the visual cortex is hierarchically organized, with visual information
preceding from area V1 to V2 to V3. Each was though to process the information from the
preceding area. Today, this is considered as too simple, and has been replaced by the notion of
a distributed hierarchical process with multiple parallel and interconnecting pathways at each
level.
There are a few simple principles of the diagram of the visual
pathway;
- V1 (striate cortex) is the first processing level in the hierarchy,
receiving the largest input from the lateral geniculate nucleus of
the thalamus and projecting to all other occipital regions.
- V2, the second processing level, also projects to all other
occipital regions.
- After V2, three distinct parallel pathways emerge en route to
the parietal cortex, the multimodal superior temporal sulcus
(STS) and inferior temporal cortex for further processing (figure).
Areas V1 and V2 are functionally heterogeneous; both segregate processing for color, form
and motion. This in contrast to the areas that follow. In a sense, areas V1 and V2 appear to
serve as in-boxes into which different types of information are resembled before being sent on
to more specialized visual areas.
From V1 and V2 flow three parallel pathways that convey different attributes of vision.
Information from V1 goes to area V4, considered a color area (cells in V4 respond to both
form and color). Other information from V1 also goes to area V2 and then to area V5 (middle
temporal, or area MT), which is specialized to detect motion. Finally, input from areas V1
and V2 to V3 concerns dynamic form – the shape of objects in motion. Thus, vision
processing begins in the primary occipital cortex (V1), which has multiple functions, then
continues in more specialized zones.
Selective lesions up the hierarchy in V3, V4 and V5 produce different deficits;
❖ Lesions in area V4 cause people to see only in shades of gray. These patients not only
fail to perceive colors, but also fail to recall colors perceived before their injuries, or
even to image colors. So, loss of area V4 results in the loss of color cognition.
❖ Lesions in area V5 erase the ability to perceive objects in motions; objects vanish as
soon as they start moving.
❖ Lesions in V3 will affect form perception, but because area V4 also processes form, a
larger lesion of both V3 and V4 would be required to eliminate form perception.
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Areas V3, V4 and V5 all receive major input from area V1. People with lesions in V1 act as
though they are blind, but visual input can still get through to higher levels. People with V1
lesions seem unaware of visual input and can be shown to retain some aspects of vision only
by special testing. So, V1 must function for the brain to make sense of what the more
specialized visual areas are processing.
VISUAL FUNCTIONS BEYOND THE OCCIPITAL LOBE
Visual processing does not culminate in secondary areas such as V3, V4 and V5, but
continues within multiple visual regions in the parietal, temporal and frontal lobes. The table
summarizes the functions of regions in both ventral and dorsal streams.
Although it is tempting to regard
each ventral-stream region as
independent visual processor, all are
clearly responsive to all categories of
stimuli. The differences among the
regions are matter of degree, not the
mere presence of activity.
Vision is not unitary, but is composed
of many highly specific forms of
processing. These can be organized
into five general categories; vision
for action, action for vision, visual
recognition, visual space and visual
attention.
VISION FOR ACTION
This category is visual processing required to direct specific movements. When reaching for
a particular object, this is obviously guided by vision. In addition to guiding grasping, various
visual areas guide all kinds of specific movements, including the
eyes, head and whole body. Finally, vision for action must be
sensitive to the target’s movements. Catching a moving ball requires
specific information about its location, trajectory, speed and shape.
Vision for action is a function of the parietal visual areas in the
dorsal stream.
ACTION FOR VISION
In a more top-down process, the viewer actively searches for only part of the target objects
and attends to it selectively. When we look at a visual stimulus, we scan it with numerous eye
movements. These are not random, but tend to focus on important or distinct features.
When we scan a face, we make multiple eye movements directed towards the eye and mouth.
Curiously, we also direct more eye scans to the left visual field (right side of person’s face)
than to the right visual field.
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This scanning bias is important in the way that we process faces, because it is not found
when scanning other stimuli (see figure B). People with deficits in action for vision are likely
to have significant deficits in visual perception (figure C), although this is not studies
systematically.
When people are asked to rotate objects mentally, they usually make eye movements,
especially to the left. when people are acting in the dark they also make eye movements.
Curiously, if the eyes are closed, the movements stop. It appears easier to do many tasks in
the dark with closed eyes, because the visual system interferes with act by touch.
VISUAL RECOGNITION
We have the ability to recognize objects and respond to visual information. We can both
recognize faces and discriminate and interpret different expressions in those faces. Similarly,
we can recognize letters or symbols and assign meaning to them. We can recognize different
objects, but it is not reasonable to expect that we have different visual regions for each object.
There are some specialized areas in the temporal regions (like for faces and hands).
VISUAL SPACE
Visual information that comes from specific locations in space allows us to direct our
movements to objects in that space, and to assign meaning to those objects. Objects have
location both relative to an individual (egocentric space) and relative to one another
(allocentric space).
Egocentric visual space is central to controlling your actions toward objects. It seems likely
that visual space is coded in neural systems related to vision for action. In contrast, allocentric
properties of objects are necessary to construct a memory of spatial location.
❖ A key feature of allocentric spatial coding is its dependence on the identity of
particular features of the world. It is likely to be associated with the regions of visual
recognition.
In summary, different aspects of spatial processing probably occur in both the parietal and the
temporal visual regions, and respective functions are integrated in areas that interact and
exchange information.
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