THE VISUAL CORTEX

9.7.1 Anatomical Configuration of

the Visual Cortex (VC)

Cat visual cortex comprises three areas, named 17, 18, and

19, defined by cytoarchitectonic criteria (Otsuka & Hassler, 1962).

9.7.2 Physiological Orientation of

the Visual Cortex (VC)

The earlier mentioned concentric receptive field organiza­tion of retinal and geniculate levels gives way at the cor­tex (VC) to a different layout of excitatory and inhibitory areas. An exception is that antagonistic areas are arranged in elongated strips so that the most effective stimulus is a straight contour (or series of numerous contours) in a par­ticular orientation. With the exception of very few cortical cells with non-oriented receptive fields (Spinelli & Barrett, 1969), orientation specificity is a fundamental property of neurons in the cat’s visual cortex.

There is a generalized belief that excitatory inputs from LGN to cortex establish a coarse orientation bias which is greatly refined by inhibitory connections between columns to yield the sharp tuning characteristic of cat cortical cells. Some cells in cat visual cortex are responsive to other forms of visual stimulation. Cortical cells will produce bursts of impulses when spots of light traverse their receptive fields (Pettigrew, 1974) and complex, but not simple, cells can be activated by movement of textured stimuli composed of random noise, i.e., a stochastically generated array of cir­cular dots (Hammond & MacKay, 1977). Cortical cells will produce high amounts of impulses when spots of light tra­verse their receptive fields (Pettigrew, 1974) and complex (not simple) cells can be activated by movement of textured stimuli composed of random noise, that is, a stochastically generated array of circular dots (Hammond & MacKay, 1977).

9.7.3 The Cell Types in Visual Cortex

The cells in cat visual cortex can be categorized into dis­tinct groups, that is simple, complex, and hypercomplex, on the basis of other receptive field properties (Henry, 1977). Simple cells possess receptive fields that consist of adja­cent ON and OFF regions within which spatial summation occurs and can be mapped using stationary flashing targets (Hubel & Wiesel, 1961).

Simple cells in general possess little or no spontaneous activity (Pettigrew, Nikara, & Bishop, 1968a), frequently respond more vigorously to one direction of movement than multidirectional (Bishop, Coombs, & Henry, 1971), prefer slowly moving stimuli (Movshon, 1975; Orban, Kennedy, & Maes, 1978; Pettigrew et al., 1968a), respond only within a narrow range or orientations (Hammond & Andrews, 1978; Henry et al., 1974; Hubel & Wiesel, 1962; Watkins & Berkley, 1974), commonly display linear spatial summation when tested with sinusoidal gratings (Movshon, Thompson, & Tolhurst, 1978a), possess small receptive Fields (In com­parison to the other cell types) (Hubel & Wiesel, 1962), and are located predominantly in layer IV (Hubel & Wiesel, 1962).

On the other hand, complex cells respond very weakly to flashed stimuli anywhere within the receptive field and do not show spatial summation (Hubel & Wiesel, 1962). They have comparatively large receptive fields (Hubel & Wiesel, 1962), respond towards a broader range of orienta­tions (Rose & Blakemore, 1974; Watkins & Berkley, 1974; Wilson & Sherman, 1976), are highly nonlinear in their response to sinusoidal gratings (Movshon, Thompson, & Tolhurst, 1978b), have a higher level of spontaneous activ­ity (Lund, 1987; Pettigrew et al., 1968a), prefer relatively high speeds of movement (Movshon, 1975; Pettigrew et al., 1968a; Pollen & Ronner, 1975); and are concentrated in the cortical layers on either side of IV (Lund, 1987).

Shared features amongst the two types of cells include possession of larger receptive fields, less orientation selec­tivity, and higher preferred speeds with increasing retinal eccentricity.

The last category, hypercomplex cell; are sensitive to the length of an oriented stimulus (Hubel & Wiesel, 1965a), have comparatively low spontaneous activity (Rose & Blakemore, 1974), vary widely in their degree of orienta­tion tuning; and are encountered most often in areas 18 and 19, usually within layers II and III (Hubel & Wiesel, 1965a; Lund, 1987).

There are variable explanations on how the three cell types operate. One of these is that complex cells receive their input from simple cells, and hypercomplex from com­plex (Hubel & Wiesel, 1965a). A contrasting one states that the complex cells are working in parallel, not in series, with the simple cells. Simple and complex cells represent the cortical terminations of the X and Y projections, respec­tively, from the LGN (Hoffmann & Stone, 1971; Maffei & Fiorentini, 1973; Stone, 1972). Thus, fast-conducting axons (Y type) project to area 18, in which complex cells predom­inate, and slow-conducting axons to project to area 17, the cortical region in which simple cells are concentrated.

Neurons in areas 17 and 18 differ considerably in their spatial and temporal response properties, in a manner consistent with the belief that X cells project to area 17, while Y cells project predominantly to area 18 (Movshon, Thompson, & Tolhurst, 1978b). The area 17 neurons (whether simple or complex) preferred higher spatial fre­quencies and lower temporal frequencies relative to neu­rons in area 18. Thus, within area 17, simple and complex cells (Distinguished by their linearity of summation) did not differ in terms of the distribution of their preferred spa­tial frequencies, but simple cells did tend to be more nar­rowly tuned for spatial frequency (Movshon, Thompson, & Tolhurst, 1978b).

9.7.4 Binocularity in Visual Cortex Neurons

Here, we are discussing how the cortical neurons play a major role in making binocular visualization a possibility in cats. It is found that most cortical neurons possess two receptive fields, one believed to be associated with each eye.

For others, there exists various degrees of retinal dis­parity between the pair of monocular receptive fields, and these disparities are typically more pronounced in the hori­zontal direction (Barlow, Blakemore, & Pettigrew, 1967; Nikara, Bishop, & Pettigrew, 1968). Another form of bin­ocular modification in receptive fields of cortical neurons exists in area 17, that allows cyclorotational eye movements, and interocular orientation disparities as large as 15 degrees (Blakemore, Fiorentini, & Maffei, 1972; Nelson, Kato, & Bishop, 1977).

Another stunning difference is seen in area 18 of the cat, which has a class of binocular cells that have different pre­ferred directions of motion between the two eyes (Cynader & Regan, 1978; Pettigrew, 1973), and another class which has different preferred speeds of movement (Cynader & Regan, 1978).

9.7.5 Role of Auxiliary Visual Areas

The lateral supersylvian area is one of the major auxillary visual areas that are of interest. Cells in this area tend to show similar properties that are exhibited by the complex cells in areas 17 and 18 (Hubel & Wiesel, 1969). Some aston­ishing features that are unique to the cells in lateral super- sylvian area include lack of orientation selectivity, presence of extremely large receptive fields and paucity of fields rep­resenting the upper visual field (Spear & Baumann, 1975).

9.7.6 Visually Induced Potentials in The Cortex

This measures the combined activity from a large collec­tion of cortical neurons. This is achieved by determining the spatial frequency at which the evoked potential has fallen to zero (Berkley & Watkins & Berkley, 1974), or by estimating the contrast level at which the evoked potential is zero (Campbell, Maffei, & Piccolino, 1973).

9.7.7 Visual Defects

This section will summarise visual deficiencies in relation to age. There is a level of visual deprivation in animals (cats) during the first two to three months of life. This is mostly affecting visual areas, excluding the retina (Wiesel & Hubel, 1963a, 1963b). This defect can be both unilateral and bilateral on X and Y mechanisms (Garey & Blakemore, 1977a; Sherman, Hoffmann, & Stone, 1972; Sherman & Stone; 1973; Hirsch & Spinelli 1970). All instances are described below, with respect to different parts of the visual system.

9.7.8 Monocular and Binocular Deprivation

on the Lateral Geniculate Nucleus.

Monocular deprivation leads to a significant reduction in the proportion of Y cells sampled from the deprived laminae, while that of X cells remains relatively normal (Sherman et al., 1972; Sherman, Wilson, & Guillery, 1975). These physiological changes are accompanied by morpho­logical effects which are relatively specific to the Y system (Garey & Blakemore, 1977a, 1977b; Lin & Sherman, 1978; Sherman, Guillery, Kaas, & Sanderson, 1974). Central to the LGN; that is at the optic radiation, there is a normal complement of Y neurons following monocular deprivation (Eysel, Grusser, & Hoffman, 1979). This can be attributed to greater reduction in the size of somas of Y cells, com­pared to X cells (Eysel et al., 1979). On the other hand, X cells tend to be normal throughout all laminae, in both monocular and binocular instances.

In a nutshell, binocular deprivation produces a less severe, though still significant, reduction in Y-cell fre­quency throughout the lateral geniculate nucleus, mon­ocular crescent included, leaving X-cell frequency to be relatively normal (Sherman et al., 1972).

9.7.9 Deprivation at the Visual Cortex

Visual deprivation effects are so obvious at the level of visual cortex, even minor surgical interventions such as third eyelid flapping can show elicit some manifestations (Schechter & Murphy, 1976). Cells in cortical layer IV of eyes subjected to some levels of visual deprivation tend to shrink as compared to those innervating undisturbed eye (Shatz & Stryker, 1978). These effects of monocular depri­vation tend not to be uniformly distributed. Incidence of cortical cells responsive through the deprived eye increases in more peripheral regions of the visual field, with both simple and complex, display abnormal receptive field prop­erties (Wilson & Sherman, 1977).

Ultimately, physiological effects of monocular depriva­tion are found in both areas 17 and 18, except that in area 18, the contralateral pathway (That is, the hemisphere con­tralateral to the deprived eye) is much more resistant to the effects of monocular deprivation than is the ipsilateral pathway. This asymmetry between crossed and uncrossed projections is not observed in area 17 (Singer, 1978).

Other instances that can cause binocular visual defects include strabismus and alternating monocular occlusion (Blakemore, 1976; Hubel & Wiesel, 1965b).

On the other hand, binocular deprivation effects are relatively ess severe, except that there is a large increase in the number of visually unresponsive cells (Blakemore, Fiorentini, & Maffei, 1972; Imbert & Buisseret, 1975; Pettigrew, 1974). Among these are response of cells over a wider range of orientations (Hubel & Wiesel, 1965a), retinal disparities (Pettigrew, 1974), presence of fewer direction­ally selective cortical cells (Singer & Tretter, 1976) and an unusual number of cells with exceedingly large receptive fields which appear to lack inhibitory sidebands (Singer & Tretter, 1976).

9.7.10 Recovery from Monocular

Deprivation Effects

Are there possible practical ways to reverse of decrease the gross effects mentioned above?

Binocularity in some cells may be temporarily restored following prolonged monocular deprivation by intravenous administration of bicuculline, which is an antagonist of the putative inhibitory neurotransmitter gamma-aminobutyric acid (GABA) (Duffy et al., 1976). Permanent, though not practical way of reversal is the enucleation of the non­deprived eye, thus, inducing a compensatory hypertro­phy in the deprived eye (Kratz, Spear, & Smith, 1976). Another way is intraventricular injection (Kasamatsu & Pettigrew, 1976) or local perfusion (Pettigrew et al., 1968a) of 6-hydroxydopamine (6-HDA) prevented the usual loss of binocularity following monocular deprivation.