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HLTB21H3 Study Guide - Angioma, Cortical Blindness, Superior Temporal Sulcus


Department
Health Studies
Course Code
HLTB21H3
Professor
Caroline Barakat

Page:
of 19
Chapter 13: The Occipital Lobes
The Autonomy of the Occipital Lobes
The occipital lobes form the posterior pole of the cerebral hemispheres, lying
beneath the occipital bone at the back of the skull. On the medial surface of each
hemisphere, the occipital lobe is distinguished from the parietal lobe by the
parietal- occipital sulcus.
Within the visual cortex, however, are three clear landmarks. The most prominent
is the calcarine sulcus, which contains much of the primary cortex. The calcarine
sulcus divides the upper and lower halves of the visual world. On the ventral
surface of each hemisphere are two gyri (lingual and fusiform). The lingual gyrus
includes part of visual cortical regions V2 and VP, whereas V4 is in the fusiform
gyrus.
Subdivisions of the Occipital Cortex
The discovery that area V1 is functionally
heterogeneous— that a given cortical area may
have more than one distinct function— was
unexpected. Area V2 also is heterogeneous
when stained with cytochrome oxidase, but,
instead of blobs, stripes are revealed. Because of
the distinct stripes, the visual cortex is
sometimes called the striate cortex.
The distribution of color function across much of the occipital cortex and beyond
(that is, areas V1, V2, V4, V8) is important because, until recently, the perception
of form or movement was believed to be color- blind. It has now be-come clear
that color vision is integral to the analysis of position, depth, motion, and the
structure of objects.
Connections of the Visual Cortex
V1 (the striate cortex) is the primary vision area: it receives the largest input
from the lateral geniculate nucleus of the thalamus, and it projects to all other
occipital regions. V1 is the first processing level in the hierarchy.
V2 also projects to all other occipital regions. V2 is the second level.
After V2, three distinct, parallel pathways emerge en route to the parietal cortex,
superior temporal sulcus (STS), and inferior temporal cortex, for further
processing.
The parietal pathway, or dorsal stream, has a role
in the visual guidance of movement, and the
inferior temporal pathway, or ventral stream, is
concerned with object perception (including color).
The middle pathway along the superior temporal
sulcus (the STS stream) is probably important in
visuospatial functions and in the perception of
certain types of movements.
A Theory of Occipital Lobe Function
In a sense, areas V1 and V2 appear to serve as mailboxes into which different
types of information are assembled before being sent on to the more specialized
visual areas.
It is not surprising to discover that selective lesions up the hierarchy in areas V3,
V4, and V5 produce specific deficits in visual processing. People who suffer
damage to area V4 are able to see only in shades of gray. Curiously, patients not
only fail to perceive colors but also fail to recall colors perceived before their
injuries or even to imagine colors. In a real sense, the loss of area V4 results in the
loss of color cognition, or the ability to think about color.
Similarly, a lesion in area V5 produces an inability to perceive objects in motion.
Objects at rest are perceived but, when the objects begin to move, they vanish. In
principle, a lesion in area V3 will affect form perception but because area V4 also
processes form, a rather large lesion of both V3 and V4 would be required to
eliminate form perception.
People with V1 lesions seem not to be aware of visual input and can be shown to
retain some aspects of vision only by special testing. Thus, when asked what they
see, patients with V1 damage often reply that they see nothing. Nonetheless, they
can act on visual information, indicating that they do indeed “ see.”
Visual Functions Beyond the Occipital Lobe
One conclusion that we can make is that vision is not unitary but is com-posed of
many quite specific forms of processing. These different forms 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.
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 object and attends selectively to it. When we look at a visual stimulus, we
do not simply stare at it; rather, we scan the stimulus with numerous eye
movements. These movements are not random but tend to focus on important or
distinct features of the stimulus.
Visual Recognition
We enjoy the ability both to recognize objects and to respond to visual
information. For example, we can both recognize specific faces and dis-criminate
and interpret different expressions in those faces. Similarly, we can recognize
letters or symbols and assign meaning to them.
Visual Space
Visual information comes from specific locations in space. This information
allows us to direct our movements to objects in space and to assign meaning to
objects. But spatial location is not a unitary characteristic. Objects have location
both relative to an individual (egocentric space) and relative to one another
(allocentric space).
Egocentric visual space is central to the control of your actions to-ward objects. It
therefore seems likely that visual space is coded in neural systems related to vision
for action. In contrast, the allocentric properties of objects are necessary for you to
construct a memory of spatial location.
Visual Attention
We cannot possibly process all the visual information available. This page has
shape, color, texture, location, and so on, but the only really important
characteristic is that it has words and images. Thus, when we read the page, we
select specific aspects of visual input and attend to them selectively.
Independent mechanisms of attention are probably required both for the guidance
of movements (in the parietal lobe) and for object recognition ( in the temporal
lobe).
Vision evolved first for motion, not for recognition. Simple organisms can detect