The Function of the Callosal Connections of the Visual Cortex

123

SUMMARY1. If classical partial decussation exactly segregates the projections of right and left hemi-retinae on to the two optic tracts, the images of an object in central vision, nearer or further than the fixation point, should project to separate hemispheres. This would prevent the encoding of retinal disparity by binocularly driven neurones of the visual cortex.2. It is proposed that there is a central vertical strip of retina in each eye which is represented in both hemispheres. The angular width of this strip should be exactly one half the actual range of horizontal disparities of binocular receptive fields near the central vertical meridian.3. By recording from single neurones in the area 17/18 region in both hemispheres of a cat, it was found that there is such a strip of bilateral projection. The centres of receptive fields for units from the two hemispheres overlap in the middle of the visual field by about 1.50 and the S.D. of the distribution is about 0.50.4. The horizontal disparities of the centres of binocular receptive fields were measured for samples of units representing different parts of the visual field. The range of horizontal disparity for fields near the area centralis is about 2.30, the S.D. of the distribution about 0 9°. The proposed relationship between bilateral projection and disparity coding is thus confirmed.5. The origin of the bilateral projection is a matter of speculation, but in the cat some of it is almost certainly due to imprecision in the nature of the nasotemporal division of optic nerve fibres at the optic chiasma. A case can be made, however, that the overlap is partly due to connexions through the corpus callosum between the two occipital lobes.6. Evidence for the importance of the callosal pathway in man is drawn from the effects on stereopsis of section of the chiasma and the callosum.

Section: Discussionmentioning

confidence: 99%

Binocular depth discrimination and the nasotemporal division

Blakemore¹

1969

123

“…The role of the callosal connexions at the level of the 17/18 border is currently associated with the necessity of binding together the two halves of the visual field represented separately in the two hemispheres in order to grant perceptual continuity across the vertical meridian and/or to subserve binocular functions such as depth perception (Choudhury, Whitteridge & Wilson, 1965;Hubel & Wiesel, 1967;Berlucchi & Rizzolatti, 1968;Blakemore, 1969Blakemore, , 1970Mitchell & Blakemore, 1970;Berlucchi, 1972; Bishop, 1973; Blakemore, Diao, Pu, Wang & Xiao, 1983;Payne, Pearson & Berman, 1984).…”

Section: Introductionmentioning

confidence: 99%

The transfer of visual information across the corpus callosum: spatial and temporal properties in the cat.

Berardi

Bisti

Maffei

1987

3. In split-chiasm cats, both v.e.p.s and single-unit recordings indicate that the contrast gain of the callosal pathway is considerably lower than the gain of the direct, geniculo-cortical system. Spatial and temporal acuities are lower for the callosal than for the direct system. 4. The same differences in contrast gain between the spjatial m.t.f. obtained for the callosal and the direct system have been found in alert split-chiasm cats.5. Our data suggest that the cross-talk between the hemispheres taking place across the corpus callosum is nearly abolished at low contrasts and high spatial and temporal frequencies.

“…5) and experiments with subjects possessing abnormalities of the visual pathways have established that foveal fine stereoscopy depends upon callosal projections between the two hemispheres (Blakemore, 1969;Mitchell & Blakemore, 1970). Further, the callosal fibres which connect corresponding points in the two hemispheres project, via area 18, to the boundary region between areas 17 and 18 of the monkey visual cortex, the region containing the representation of the vertical meridian in the visual field (Choudhury, Whitteridge & Wilson, 1965). That the suppression effect is restricted to stimuli located around the vertical meridian (Fig.…”

Section: Discussionmentioning

confidence: 99%

The response characteristics of an inhibitory binocular interaction in human vision.

Ruddock¹,

Waterfield²,

Wigley³

1979

SUMMARY 1. The contrast threshold level for visual detection of a linear grating consisting of parallel light and dark bars is increased by adaptation to a high contrast grating of spatial characteristics similar to those of the test grating (e.g. Blakemore & Campbell, 1969). This so-called contrast threshold elevation effect is significantly reduced if two adaptation gratings, with appropriate different spatial characteristics, are presented one to either eye (Ruddock & Wigley, 1976). We have studied the contrast threshold elevation effect obtained with a test and an adaptation grating presented to one eye and a second adaptation grating, referred to as the conditioning grating, presented to the other.2. Preliminary data are presented for three subjects with normal stereoscopic vision. In each case, the contrast threshold elevation effect for a pair of spatially identical test and adaptation gratings is significantly reduced by a conditioning grating of spatial frequency in the range 1-5 c/deg.3. Reduction in the contrast elevation effect is observed whether the conditioning and adaptation gratings are of the same or of different wave-lengths and the effect of the conditioning grating increases to a steady-state level over a period of some 30 see following onset of its presentation.4. Measurements were made with a 5 deg diameter circular test grating presented at retinal locations up to 8 deg in the horizontal and vertical meridians from a foveal fixation point. It was found that in the horizontal meridian, the amplitude of the suppression effect associated with the conditioning grating falls as the displacement angle of the test field from the fixation point increases. For displacements in the vertical meridian, however, the amplitude of the suppression effect is virtually independent of the retinal location of the test field.5. Under experimental conditions which yield suppression of the contrast threshold elevation effect for subjects with normal stereoscopic vision, no suppression was found for three subjects who possessed neither global nor fine stereoscopic vision. Results for a subject who possessed fine, but not global stereoscopy, did, however, show the suppression effect.