Distribution of retinogeniculate cells in the tammar wallaby in relation to decussation at the optic chiasm.

Partial decussation of the optic nerve in mammals is related to the laterofrontal placement of the eyes. To investigate this relationship in the wallaby (Macropus eugenii), injections of wheat germ agglutinin-conjugated to horseradish peroxidase were made into one dorsal lateral geniculate nucleus to label retinal ganglion cell bodies in both retinas. Contralaterally, labelled ganglion cells were present across the nasotemporal axis, except for the far temporal retina where they were absent or very sparsely scattered compared with the density of labelled cells at similar nasal eccentricities in the same retinas. Ipsilaterally, labelling was confined to the temporal retina. Cell counts confirmed a visual streak and an area centralis in the contralateral projection. Diameters of labelled cells ranged from 9 microm to 30 microm with a hint of three categories of cells based on size. Only the large alpha-type cells were easily separated. Measurement of the acceptance angles of the eye in the anaesthetised animal showed about 15% of the horizontal visual field of each eye projects into a region of binocular overlap giving a binocular field of 50 degrees . The uniocular visual field extends from -25 degrees (nasally) to + 162 degrees (temporally) in azimuth, giving the wallaby a monocular visual field width of 187 degrees and a total visual field width of 324 degrees . In elevation, field ranges from 70 degrees inferior to +120 degrees superior, encompassing 190 degrees in the vertical plane. The wallaby shows partial decussation of optic nerve fibres projecting to the lateral geniculate nucleus that could allow stereopsis, plus an extensive panoramic field.

Lateral division of the lateral posterior region: connections with area 18 in cats.

To establish the topography of the lateral division of the lateral posterior region (LP1) projections to area 18, up to five different anatomical tracers were injected in separate rostrocaudal locations in area 18 of four adult cats, and patterns of retrogradely labeled LP1 cells were identified. LP1 inputs to area 18 arose from both caudal and rostral nuclei and were topological, organized in patterns that indicate that lower visual space is represented anteroventrally, and more central and upper visual space is represented caudodorsally. In the caudal LP1 nucleus, patches of labeled cells formed bands that ran parallel to the medial and lateral LP1 borders and encompassed medial portions of the nucleus. In rostral LP1, the patches of labeled cells formed clusters giving the connections with area 18 a more modular appearance, and were nearer the lateral LP1 border. Injections made nearest area centralis representations in area 18 labeled more neurons than injections in cortex representing more peripheral visual space. Also, neighboring injections in area 18 labeled overlapping patches of cells, but no double-labeled cells were observed. These findings are consistent with previous conclusions based on electrophysiological mapping studies, that two retinotopically organized nuclei constitute LP1.

Surface-view connectivity patterns of area 18 in cats.

To determine surface-view connectivity patterns of area 18, separate injections of up to six anatomical tracers were delivered to various rostrocaudal locations of area 18 in six normal cats. Subsequently, cortex was separated from subcortical structures, manually flattened, and cut parallel to the surface. Results reveal that ipsilateral cortical connections of area 18 with three regions of cortex are topological. In areas 17 and 19, separate patches of cells labeled with different tracers progressed in a rostrocaudal sequence corresponding to the order of the injections. A similar but less precise pattern of rostrocaudal labeling occurred in more lateral visual cortex, even though several presumptive visual areas were involved. Thus, anteromedial suprasylvian cortex projected to anterior area 18 while more posterolateral suprasylvian cortex projected to posterior area 18. There was no evidence of double-labeled cells projecting to separate regions in area 18. These results are more consistent with the concept of a single suprasylvian area projecting to area 18 cortex than several.

The role of ipsilateral and contralateral inputs from primary cortex in responses of area 21a neurons in cats.

Neuronal responses in cat visual area 21a were analyzed when the primary visual cortex (areas 17 and 18) was deactivated by cooling. Ipsilateral and contralateral cortices were deactivated separately. Results established that (1) cooling the ipsilateral primary cortex diminished the activity of all area 21a cells and, in 30%, blocked responsiveness altogether, and (2) cooling the contralateral primary cortex initially increased activity in area 21a cells but, with further cooling, reduced it to below the original level although only 9% of cells ceased responding. These findings were then compared to earlier results in which bilateral deactivation of the primary cortex greatly reduced and, in most cases, blocked the activity of area 21a cells (Michalski et al., 1993). Despite the response attenuation following cooling of the primary visual cortex (either ipsilateral or contralateral), neurons of area 21a retained their original orientation specificity and sharpness of tuning (measured as the half-width at half-height of the orientation tuning curve). Direction selectivity also tended to remain unchanged. We concluded that for area 21a cells (1) the ipsilateral primary cortex provides the main excitatory input; (2) the contralateral primary cortex supplies a large inhibitory input; and (3) the nature of orientation specificity, sharpness of orientation tuning, and direction selectivity are largely unaffected by removal of the ipsilateral hemisphere excitatory input or the contralateral hemisphere inhibitory input.

The effect of reversible cooling of cat's primary visual cortex on the responses of area 21a neurons.

1. Responses of sixty-four neurons in cortical area 21a were studied with areas 17 and 18 reversibly deactivated by cooling. From anatomical studies, most of area 21a input in the cat originates from these primary areas with no input from the A laminae of the lateral geniculate nucleus. 2. Both responses and spontaneous activity of all sixty-four area 21a neurons were markedly reduced when primary areas were cooled. In sixteen cells the responses were totally blocked. Temperatures of primary cortex required to produce total blockade varied between 25 and 4.5 degrees C. 3. The effect of cooling the primary visual cortex on the shape of orientation tuning curves was analysed in thirteen neurons from area 17 and in fifty-eight neurons from area 21a. In both areas, the width of the curve, when measured at its half-height, was preserved even when spike activity was reduced to below 10% of the original level. All neurons also retained their original directional preferences during cooling of the primary visual cortex. 4. The responsiveness of forty-seven neurons from area 21a was tested after rewarming of primary cortex. All but one neuron recovered their initial responsiveness within half an hour of restoring physiological temperature of primary visual cortex. 5. The results of the present study give an indication of the extent to which area 21a is sequentially related to the primary visual cortex in the processing of information.

Projections from the lateral division of the lateral posterior-pulvinar complex to area 21a and the striate cortex in the cat.

The axonal tracer HRP-WGA, injected into area 21a of the cat, was used in conjunction with AChE histochemistry to demonstrate that a reciprocal pathway links area 21a with the LPl (striate recipient zone) in the lateral posterior-pulvinar complex. Separate injections of the fluorescent tracers, Fast blue (FB) and Diamidino yellow (DY), were then placed simultaneously in the striate cortex and area 21a, respectively, to identify the relative position of cells projecting to both cortical areas and to pinpoint the location of efferent terminals in the LPl. Both FB and DY labelled cells and terminals are located together in the LPl but none of the neurons projecting to the striate cortex and area 21a was double labelled which would have indicated the presence of cells with axons projecting to both areas. Nonetheless, the intermingling of cells projecting to the striate cortex and area 21a, and the reciprocal character of both pathways, is consistent with a model in which the two cortical areas exchange signals via the LPl.

Response characteristics of the cells of cortical area 21a of the cat with special reference to orientation specificity.

1. Extracellular recording using tungsten-in-glass microelectrodes was conducted on 115 neurons in area 21a of fifteen anaesthetized cats. Quantitative analysis using computer-controlled display and collecting routines were used to investigate the excitatory and inhibitory regions of the receptive field and to see if interaction, within and between these regions, contributed to the response properties of the cells. 2. The responses of the cells in the sample appeared to arise from a single, homogeneous class. All cells had single discharge regions which responded with composite ON/OFF firing to a stationary flashing bar. The same region also responded to moving light and dark bars and edges. There was little evidence of inhibition as measured by the suppression of spontaneous or induced firing. Most cells had relatively small receptive fields (primary width: mean = 2.1 +/- 0.9 deg (S.D.); n = 108), all were binocular and were located within 15.0 deg of the visual axes. 3. All cells responded well to slowly moving stimuli but generally failed to respond to stimuli moving faster than 10.0 deg s-1. All responses were bi-directional and, although many showed evidence of length summation, there was no sign of linear summation. 4. Despite the absence of significant sideband inhibition many cells were acutely tuned for orientation (half-width at half-height: mean = 15.6 +/- 5.3 deg; n = 48). To investigate this property further, cells were analysed to assess the effect of changing the length of a moving bar stimulus on the acuteness of the orientation tuning curve. Short bars, of similar length to the width of the receptive field, had orientation tuning curves of equivalent sharpness to those obtained with longer bars. 5. The equivalence of orientation tuning for long and short bars stands in contrast to the results obtained for both simple (S) and complex (C) cells of the striate cortex where tuning for the longer bar is sharper than that for the shorter. The result from area 21a cells is consistent with the absence of sideband inhibition and can be related to an input from the striate cortex that passes through a threshold barrier. 6. The orientation tuning of cells of area 21a can be explained if it is assumed that they receive their major input from C or complex cells of the striate cortex in which firing must reach a threshold frequency to activate the recipient cell.

Area 21a in the cat and the detection of binocular orientation disparity.

Visual response properties were examined in 115 cells, recorded in area 21a of the cerebral cortex of anaesthetized and paralysed adult cats. Cells were binocular and had receptive fields consisting of a single uniform discharge region which fired with composite ON/OFF responses to stationary flashing stimuli. Most cells were sharply tuned for orientation, but this was unaffected by changes in stimulus length. This result is consistent with a model in which the cells of area 21a receive their input from C cells of the striate cortex. Evidence for this was obtained by studying the decline in the responsiveness in area 21a that accompanied the cooling of areas 17 and 18. There was little indication that the cells of area 21a were effective detectors of spatial disparity, but their sharp monocular orientation tuning and differences in the preferred orientation of ipsilateral and contralateral eyes hinted at a role in the detection of binocular orientation disparity. Our results, however, showed that the recorded binocular disparity curves could be accounted for by summing the two monocular contributions and there was no apparent novel binocular component.