SUMMARY1. 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.

No abstract

In 1970 the Boulder Committee described the basic principles of the development of the CNS, derived from observations on the human embryonic cerebrum. Since then, numerous studies have significantly advanced our knowledge of the timing, sequence and complexity of developmental events, and revealed important inter-species differences. We review current data on the development of the human cerebral cortex and update the classical model of how the structure that makes us human is formed.

S'UMMARY 1. Binocularly driven units were investigated in the cat's primary visual cortex.2. It was found that a stimulus located correctly in the visual fields of both eyes was more effective in driving the units than a monocular stimulus, and much more effective than a binocular stimulus which was correctly positioned in only one eye: the response to the correctly located image in one eye is vetoed if the image is incorrectly located in the other eye.3. The vertical and horizontal disparities of the paired retinal images that yielded the maximum response were measured in 87 units from seven cats: the range of horizontal disparities was 6.60, of vertical disparities 2.20.4. With fixed convergence, different units will be optimally excited by objects lying at different distances. This may be the basic mechanism underlying depth discrimination in the cat. INTROD'UCTIONThe image formed by a single eye is a two-dimensional projection of the three-dimensional world which entirely lacks representation of those distances in the three-dimensional world that are possibly of greatest survival value to an animal, namely the distances of the external objects from the eye. It is true that the third dimension of apparent depth can be added to the visual image of a single eye by using a number of indirect cues, such as the angular subtense of an object of known size, motion parallax, accommodative effort, and the obscuration of distant objects by nearer ones. However, these cues can only be utilized in special circumstances, and most of them require rather complex image-processing. In animals where the visual fields of the two eyes overlap the situation becomes much more favourable, for between them the different projections * Harkness Fellow, 1965-67. t Present address:

The mammalian cerebral cortex is innervated by a large number of corticocortical connections. The number of connections makes it difficult to understand the organization of the cortical network. Nonetheless, conclusions about the organization of cortical systems drawn from examining connectional data have often been made in a speculative and informal manner, unsupported by any analytic treatment. Recently, progress has been made toward more systematic ways of extracting organizing principles from data on the network of connections between cortical areas of the monkey. In this article, we extend these approaches to the cortical systems of the cat. We collated information from the neuroanatomical literature about the corticocortical connections of the cat. This collation incorporated 1139 reported corticocortical connections between 65 cortical areas. We have previously used an optimization technique (Scannell and Young, 1993) to analyze this database in order to represent the connectional organization of cortical systems in the cat. Here, we report the connectional database and analyze it in a number of further ways. First, we employed rules from Felleman and Van Essen (1991) to investigate hierarchical relations among the areas. Second, we compared quantitatively the results of the optimization method with the results of the hierarchical method. Third, we examined quantitatively whether simple connection rules, which may reflect the development and evolution of the cortex, can account for the experimentally identified corticocortical connections in the database. The results showed, first, that hierarchical rules, when applied to the cat visual system, define a largely consistent hierarchy. Second, in both auditory and visual systems, the ordering of areas by hierarchical analysis and by optimization analysis was statistically significantly related. Hence, independent analyzes concur broadly in their ordering of areas in the cortical hierarchies. Third, the majority of corticocortical connections, and much of the pattern of connectivity, were accounted for by a simple “nearest-neighbor-or-next-door-but-one” connection rule, which may suggest one of the mechanisms by which the development of cortical connectivity is controlled.

No abstract

No abstract

No abstract