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Summary Task 6: Attention disorders and agnosia (GGZ2025)
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Task 6: Attention disorders and agnosiaPART 1___________________________________________________________
Kolb, B. & Whishaw, I.Q. (2015). Fundamentals of Human Neuropsychology
(7th Edition) H13
13.1: Occipital lobe anatomy (p.353)
Connections of the visual cortex
By the late 1960s, men thought that the visual cortex is hierarchically
organized, with visual information proceeding from area V1 to V2 to V3. Each
was thought to elaborate on processing in the preceding area. However this
hierarchical view is too simple and has been replaced by the notion
of a distributed hierarchical process with multiple parallel and
interconnecting pathways at each level. A few simple principles:
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
o Multimodal superior temporal sulcus (STS) and inferior
temporal cortex for further processing. Information to and
from the dorsal and ventral streams converges in the STS
stream, which flows from area V1 into the superior temporal
sulcus.
Flows from area V1 Into the superior temporal
sulcus.
o Dorsal stream (parietal pathway): participates in the visual guidance of
movement.
It flows from area V1 to the posterior parietal visual areas.
o Ventral stream (including both STS and inferior temporal pathway) is
concerned with object perception (including colour and faces) and
perceiving certain types of movements.
It flows from area V1 to the temporal visual areas.
13.2: A theory of Occipital- lobe function
Areas V1 and V2 appear to serve as in- boxes into which different types of information
are assembled before being sent to more specialized visual areas. From areas V1 and V2
flow three parallel pathways that convey different attributes of vision.
Information derived from the blob areas of V1 goes to area V4, considered a colour area.
Cells in area V4 are not solely responsive to colour; some cells respond to both form and
colour.
Other information from area V1 also goes to area V2 and then to area V5 (also known as
middle temporal, or area MT), which is specialized to detect motion.
Finally, an input from areas V1 and V2 to area V3 concerns dynamic form—the shape of
objects in motion.
Vision processing begins in the primary occipital cortex (V1), then continues in more
specialized cortical zones.
1
,Selective lesions in visual areas produce specific deficits:
People who suffer damage to area V4 are able to see only in shades of gray. They also fail to
recall colours perceived before their injuries, or even imagine colors. the loss of area V4
results in the loss of colour cognition— the ability to think about colour.
A lesion in area V5 erases the ability to perceive objects in motion. Objects at rest are
perceived, but when the objects begin to move, they vanish.
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.
Areas V3, V4, and V5 all receive major input from area V1. People with V1 lesions seem
unaware of visual input and can be shown to retain some aspects of vision only by special
testing . 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.”
o Visual input can still get through to higher levels, partly through small projections of
the lateral geniculate nucleus to area V2 and partly through projections from the
colliculus to the thalamus (the pulvinar) to the cortex.
Visual functions beyond the occipital lobe
Visual processing in humans 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. Table 13.1
summarizes the putative (vermoedelijke) functions of
regions in both the ventral and dorsal streams.
Ventral stream; several regions appear to be tuned
selectively to identify body parts such as hands
(EBA and FBA), faces (FFA), or moving bodies
(STSp). Another region, PPA, has a totally different
function; analyzing information about the
appearance and layout of scenes.
o These ventral stream regions are not
independent visual processors, they all are
clearly responsive to all categories of
stimuli. The differencesamong the regions
are a matter of degree, not the mere
presence, of activity.
o Goes to V4, V3, V2
Dorsal stream: regions specialized for moving the
eyes (LIP) or for object-directed grasping (AIP, PRR). Not all neurons in these regions control
movements directly. Some appear to be “purely visual” and are presumed to take part in
converting visual information into the necessary coordinates for action.
o Goes to V5, V2, V3A
One conclusion is that vision is not unitary but is composed of many highly specific forms of
processing. These forms can be organized into five general categories: vision for action, action for
vision, visual recognition, visual space, and visual attention.
Vision for action – visual processing required to direct specific movements. When reaching
for a particular object, such as a cup, the fingers form a specific pattern that enables a person
to grasp the cup. This movement is obviously guided by vision, because people do not need
to shape their hands consciously as they reach.
o Various visual areas guide all kinds of specific movements, including those of the
eyes, head, and whole body.
2
, o Vision for action is a function of the parietal visual areas in the dorsal stream.
Action of vision- the viewer actively searches for only part of the target object and attends to
it selectively. A top- down process. 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.
o When we scan a face, we make multiple eye movements directed toward the eyes
and mouth. Curiously, we also direct more eye scans to the left visual field than to
the right visual field. This scanning bias may be important in the way that we process
faces because it is not found when scanning other stimuli.
o Top- down: looking for certain features of the cup, you know what you are looking
for.
Visual recognition - We have the ability both to recognize objects and to respond to visual
information. We can both recognize specific faces and discriminate and interpret different
expressions in those faces. We can recognize different foods, tools, or body parts, but it is
not reasonable to expect that we have different visual regions for each category or object.
o We have some specialized areas in the temporal regions, for biologically significant
information, such as faces and hands, as well as regions for objects and places.
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
(spatial location). Objects have location both relative to an individual (egocentric space) and
relative to one another (allocentric space).
o Egocentric visual space is central to controlling your actions toward objects. It
therefore seems likely that visual space is coded in neural systems related to vision
for action. The allocentric properties of objects are necessary for you 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. Thus, it is likely to be
associated with the regions of visual recognition.
o Different aspects of spatial processing occur in both the parietal and temporal visual
regions, and respective functions are integrated in areas that interact and exchange
information.
Visual attention – We don’t process all available visual information, when you read a page for
example, you select specific aspects of visual input and attend to them selectively. neurons in
the cortex have various attentional mechanisms: neurons may respond selectively to stimuli
in particular places or at particular times or if a particular movement is to be executed.
o Independent mechanisms of attention are probably required both for guiding
movements (in the parietal lobe) and for recognizing objects (in the temporal lobe).
Visual pathways beyond the occipital lobe
Vision evolved first for motion, not for recognition. Simple organisms can detect light and move to or
from it. For example, the single-cell organism Euglena ;sunlight helps manufacture food in his
aquatic environment, it is an advantage for Euglena to move toward the light. For Euglena, vision
acts to guide movement—the most primitive form of vision for action.
As primitive animals interact with their environment, they are adapted to learn more about it.
Distinct visual systems thus evolved to recognize objects in the environment.
System of knowing what an object is includes the visual information flow from area V1 to the
temporal lobe in the ventral stream.
System controlling visually guided movements includes the flow of information from area V1
to the parietal lobe in the dorsal stream.
Figure 13.5 suggests a fairly simple flow-through of information along the dorsal and ventral
streams, but interaction with subcortical regions occurs at each step along the ventral
stream.
3
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