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The Visual Pathway from Retina to Cortex (continued)

Figure 15.5 The course of the optic radiations from the lateral geniculate nucleus of the thalamus to the striate cortex of the occipital lobe is illustrated in a lateral view of the left side of the brain.

The LGN neurons (4° visual afferents) send their axons in the internal capsule to the occipital lobe where they terminate in the striate cortex (Figure 15.5).

• The LGN axons fan out as the optic radiations of the internal capsule and travel through the temporal, parietal and occipital lobes.
  
• The LGN axons in the sublenticular segment of the optic radiations pass below the lenticular nuclei, loop around the inferior horn of the lateral ventricle within the temporal lobe and swing posteriorly to form Meyer’s loop.
  
  • Once around the inferior horn, they travel up to the inferior bank of the striate cortex, where they terminate.
    
• The LGN axons in the retrolenticular segment of the internal capsule pass superiorly through the parietal lobe to end in the superior bank of the striate cortex.

The striate cortex (Figure 15.6) is considered to be the primary visual cortex or V1, as

• most LGN axons terminate in V1
  
• all V1 neurons respond to visual stimuli exclusively
  
• ablating V1 results in blindness
  
• electrical stimulation of V1 elicits visual sensations.
  

The striate cortex is involved in the initial cortical processing of all visual information necessary for visual perception and its damage results in loss of vision in the contralesional hemifield.

Figure 15.6 The topographic map of the left halves of the visual fields in the medial aspect of the right striate cortex. Note that the neurons representing the visual field center extend around the occipital pole into the lateral surface of the occipital lobe. This results in a disproportionate representation of the central field when compared to the cortical area representing the peripheral visual field.

The color (kLGN), shape (pLGN) and movement (mLGN) information from the thalamus are sent to different neurons within V1 for further processing in V1 and then sent onto different areas of the extrastriate visual cortex.

Figure 15.7 The responses of a "shape-form" type primary visual cortex neuron is recorded while a light bar is flashed on and off the screen. For each of the frames, the light bar has a different orientation. The neuron displays a preference (i.e., produces a maximal response) for a light bar centered and parallel to the long axis of the receptive field.

V1 blob cells: Some V1 cells resemble kLGN neurons. They are

• monocular (i.e., respond to stimulation of one eye only).
  
• color sensitive.
  
• characterized by small, concentric receptive fields.
  
• found in clusters (.e., blob cells).
  
• a special target of the kLGN axon terminals.
  

The P-stream information processed by the V1 blob cells is used in color perception, color discrimination and the learning and memory of the color of objects. The blob cells are the "color" processing cells of V1.

V1 interblob cells: Most V1 interblob cells are

• binocular (i.e., respond to stimulation of either eye).
  
• not color sensitive.
  
• characterized by elongated (rectangular-shaped) receptive fields that may or may not have a center-surround type organization.
  
• found around the clusters of color-sensitive V1 blob cells.
  
• exhibit ocular dominance (i.e., respond best to stimulation of a preferred eye).
  
• exhibit orientation specificity (i.e., respond best when the stimulus is oriented in a particular plane).
  

Location specific V1 interblob cells: One subset of V1 interblob cells responds best when the stimulus is in a specific location of the receptive field (i.e., they also exhibit location specificity).

The P-stream information processed by the V1 interblob cells that exhibit orientation and location specificity but are not motion sensitive is used in object perception, discrimination, learning and memory or in spatial orientation. These interblob cells are the "shape/form" processing cells and the "location" processing cells of V1.

Movement sensitive V1 interblob cells: A second subset of interblob cells respond best to moving stimuli (i.e., exhibit movement sensitivity, Figure 15.8) without a preference for the direction of movement.

Figure 15.8 The responses of a "motion sensitive" primary visual cortex neuron recorded in response to movement of a light bar across the neuron's receptive field from left to right.

Figure 15.9 The responses of a "motion sensitive" primary visual cortex neuron recorded in response to movement of a light bar across the neuron's receptive field. The neuron responds vigorously to movement in one direction (i.e., from left to right as in Figure 15.8) and poorly to movement in the opposite direction (i.e., from right to left). Consequently, this neuron exhibits directional sensitivity.

Direction specific V1 interblob cells: A third subset displays a preference for movement in a particular direction (i.e., some also exhibit directional sensitivity, Figure 15.9).

The M-stream of information processed by the motion sensitive V1 interblob cells is used to detect object movement and direction/velocity of movement and to guide eye movements. These motion-sensitive interblob cells are the "motion detecting” cells of V1.

Extrastriate Visual Cortex. The extrastriate cortex includes all of the occipital lobe areas surrounding the primary visual cortex (Figure 15.4, Areas 18 & 19). The extrastriate cortex in non-human primates has been subdivided into as many as three functional areas, V2, V3, and V4. The primary visual cortex, V1, sends input to extrastriate cortex and to visual association cortex. The information from the “color”, “shape/form”, "location" and “motion” detecting V1, neurons are sent to different areas of the extrastriate cortex (Figure 15.10).

Damage to extrastriate cortex does not result in a “simple loss of vision”; rather it results in higher order visual perceptual deficits including the failure to recognize objects, colors and/or movement of objects.

Figure 15.10 The flow of visual information from the primary visual cortex to other cortical areas depends on the type of information being processed. Information used to locate objects and detect their motion is sent to more superior cortex (a.k.a. the dorsal stream). Information necessary to detect, identify and use color and shape information is sent to inferior cortical areas (a.k.a., the ventral stream).

Visual Association Cortex. The visual association cortex extends anteriorly from the extrastriate cortex to encompass adjacent areas of the posterior parietal lobe and much of the posterior temporal lobe (Figure 15.4, Areas 7, 20, 37 & 39). In most cases, these areas receive visual input via the extrastriate cortex, which sends color, shape/form, location and motion information to different areas of the visual association cortex (Figure 15.10).

The Dorsal Stream: The neurons in the parietal association cortex and superior and middle temporal visual association cortex (Areas 7 and 39 and the superior part of Area 37 in Figure 15.4) have binocular receptive fields and process P-channel information about object location and M-channel information about object movement.

These dorsally located visual association neurons are responsible for producing our sense of

• spatial orientation
  
• binocular fusion/depth perception
  
• the location, the movement and the movement direction and velocity of objects in space.
  

The dorsal stream processes information about the “where” of the visual stimulus (Figure 15.10).

Damage the dorsal visual association cortex results in deficits in spatial orientation, motion detection and in guidance of visual tracking eye movements.

The Ventral Stream: The neurons in the inferior temporal visual association cortex (Area 20 and the inferior part of Areas 37 & 39 in Figure 15.4) process P-channel information about object color and form.

These ventrally located visual association neurons are responsible for processing information necessary for our abilities to

• recognize objects and colors
  
• read text and
  
• learn and remember visual objects (e.g., words and their meanings)
  

This ventral stream processes information about the “what” of the visual stimulus (Figure 15.10).

Damage to the inferior visual association cortex produces deficits in complex visual perception tasks, attention and learning/memory.