Biomedical subjects
Publications and source records attributed to C Blakemore.
Random-dot stereograms when used as adaptation stimuli can influence the perceived depth of similar test stimuli. Adaptation for 1 minute is sufficient to evoke this three-dimensional aftereffect for several seconds. This aftereffect must occur after stereopsis because prior to stereopsis no relevant monocular cues exist in these adaptation and test stimuli.
1. Inspection of a high-contrast adapting grating produces two visual after-effects: (a) the contrast threshold is raised for test gratings of similar spatial frequency to that of the adapting pattern and (b) the apparent spatial frequency of test gratings shifts away from that of the adapting grating-higher frequencies seem higher and lower ones lower than they really are.2. Both after-effects are orientation-specific. A horizontal adapting grating influences neither the threshold nor the apparent spatial frequency of vertical test gratings.3. The magnitude of the two after-effects was measured with vertical test gratings as a function of (a) tilt of a high-contrast adapting grating and (b) contrast of a vertical adapting grating.4. At all frequencies of the test grating, the decline of both after-effects produced by an increase in tilt of approximately 6(3/4) degrees could be matched by a reduction in contrast by a factor of 2.5. We take this as evidence for a common neural origin for these two visual phenomena.
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1. Binocularly driven single units were recorded in the cat's striate cortex. For each neurone the two monocular receptive fields were stimulated simultaneously in order to assess the optimal positioning of the image in both eyes to give the best binocular response.2. The electrode was driven perpendicular to the surface of the brain to explore cortical columns, all the cells of which are known to have the same preferred target orientation.3. All orientation columns were found to fit into one of two classes according to their binocular organization.4. In a constant depth column the receptive fields of binocular neurones cover a small retinal area and they are laid out in almost identical arrays in the two eyes. Consequently, the horizontal disparity is practically the same for all the units. The depth column as a whole is viewing a thin sheet of visual space, a few degrees wide, floating at some distance from the cat. There may be about 0.6 degrees disparity difference between neighbouring depth columns.5. In a constant direction column the binocular units' fields are all super-imposed on the retina contralateral to the hemisphere containing the column. In the ipsilateral eye they are more scattered horizontally. Therefore the horizontal disparity varies enormously from cell to cell and the column as a whole is viewing a cylinder of visual space directed towards the contralateral eye. Neighbouring direction columns may vary by about 4 degrees in their oculocentric visual direction.6. This columnar arrangement is probably important for space perception in the cat. Activity in only one depth and one direction column would specify the orientation and the three-dimensional locus of an object in space.7. The two types of column may be involved in the control of disjunctive and conjugate eye movements.
1. Prolonged observation of a high-contrast grating pattern causes an apparent shift in the spatial frequency of gratings subsequently viewed with the same retinal region. Gratings of higher and lower frequency than the adapting pattern seem, respectively, higher and lower than in fact they are.2. There is no significant after-effect at the adapting frequency itself nor at frequencies more than two octaves away.3. For very low adapting frequencies, the after-effect remains centred at about 3.0 c/deg and declines in strength as the adapting frequency is successively lowered.4. The magnitude of the after-effect increases with the contrast of the adapting grating and the length of time spent in adaptation. It takes several hours to recover completely from 30 min adaptation.5. The phenomenon is orientation-specific: a horizontal adapting grating has no effect on vertical test gratings. There is partial interocular transfer of the after-effect.6. These findings provide further evidence that the visual system of man, like those of the cat and the monkey, contains neurones selectively sensitive to the orientation and dimensions of retinal images, and that these adaptable cells are actually involved in the encoding and perception of the size of simple patterns.
1. Depth discrimination, using disparity cues alone, was studied with a small fixation point and briefly exposed, vertical slit-shaped targets.2. The upper limit for reliable qualitative localization of a slit as nearer or further than the fixation point is 4-7 deg of absolute disparity in a convergent direction and 9-12 deg in a divergent direction. Even larger absolute disparities can be recognized in the peripheral visual field.3. Relative depth discrimination between two slit targets was measured as a function of their spatial position. The horopter (the locus of targets that appear to be fused binocularly) is the region of maximum stereoacuity and this does not necessarily coincide with the Vieth-Müller circle (the locus of zero geometric or absolute disparity). There is a gradual increase in stereo-threshold as the targets are moved out along the horopter, away from the fixation point into the peripheral visual field. The relative disparity threshold also rises, approximately exponentially, as the targets are moved in depth or absolute disparity away from the horopter.4. Relative depth discrimination is, then, operative over a very wide band of visual space around the horopter (about 3 deg of absolute disparity in the centre of the visual field and even more in the periphery).5. The findings are discussed in relation to the neurophysiology of binocular neurones of the cat cortex. The dimensions of visual space under observation by the binocular apparatus of cat and man are rather similar. The sharper decline of stereo-acuity with absolute disparity in the centre of the visual field may be related to the limits of bilateral representation of a central strip of retina in the human brain.
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If, after prolonged observation of a striped pattern, one views a grating of the same orientation with somewhat narrower bars, then the bars seem even thinner than in fact they are. Broader bars seem broader still. This finding implies a system of size-detecting channels in humnan vision. The phenomenon may underlie many of the classical figural aftereffects.
1. It was found that an occipital evoked potential can be elicited in the human by moving a grating pattern without changing the mean light flux entering the eye. Prolonged viewing of a high contrast grating reduces the amplitude of the potential evoked by a low contrast grating.2. This adaptation to a grating was studied psychophysically by determining the contrast threshold before and after adaptation. There is a temporary fivefold rise in contrast threshold after exposure to a high contrast grating of the same orientation and spatial frequency.3. By determining the rise of threshold over a range of spatial frequency for a number of adapting frequencies it was found that the threshold elevation is limited to a spectrum of frequencies with a bandwidth of just over an octave at half amplitude, centred on the adapting frequency.4. The amplitude of the effect and its bandwidth are very similar for adapting spatial frequencies between 3 c/deg. and 14 c/deg. At higher frequencies the bandwidth is slightly narrower. For lower adapting frequencies the peak of the effect stays at 3 c/deg.5. These and other findings suggest that the human visual system may possess neurones selectively sensitive to spatial frequency and size. The orientational selectivity and the interocular transfer of the adaptation effect implicate the visual cortex as the site of these neurones.6. This neural system may play an essential preliminary role in the recognition of complex images and generalization for magnification.
1. 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 1.5 degrees and the S.D. of the distribution is about 0.5 degrees .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 2.3 degrees , the S.D. of the distribution about 0.9 degrees . 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.
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