Cortical pathways

In the cortex, the general flow of information runs vertically – that is, to cells in other layers above and below the activated cells. The cortex contains columns of cells, [column a volume of cells stretching the entire depth of the cerebral cortex, which all have some physiological property in common (e.g. the preferred orientation of the bar or edge stimulus to which they respond, in the case of a column in the primary visual cortex)] which respond to similar properties of the stimulus and lie alongside other columns that respond to different aspects or features of the world. The earlier work of Hubel and Wiesel (1968) emphasized this vertical organization. They discovered that, unlike the retina and LGN, where neurons respond best to spots of light, many cortical neurons respond best to straight lines or edges. Some cells respond best to vertical lines (figure 8.11), others to diagonals, others to horizontals, and so on for all orientations around the clock. There is a very fine-grained, high-resolution representation of image-edge orientation at this stage of sensory processing. Moreover, each cell is sensitive only to lines in a relatively small area of the retinal image – the cell’s receptive field. The cells are also selective for the spacing between parallel lines (the spatial frequency), and in many cases also for the direction of stimulus movement, the colour of the stimulus and its distance. These cortical cells form the basis for the tilt after-effect and the other after-effects described above. The activity in these cells probably also underlies our perception of orientation, motion, etc. Even if we knew nothing about the neural organization of the visual system, we could suggest the existence of mechanisms with some of the properties of these cortical neurons, which we would infer from the properties of visual after-effects. However, the further evidence that has been obtained by researchers regarding these brain mechanisms gives us greater confidence in their actual reality, and shows how psycho ogy and neurophysiology can interact to form a satisfyingly interlocking pattern of evidence.

[Horace Barlow (1921– ) is a physiologist whose insights into the possible relationships between perception and neural activity have guided much thinking in the field. Barlow is especially well known for his discussion of whether we possess ‘grandmother cells’. These are single neurons whose activity would reflect the presence of an elderly female relative. More generally, do we possess cells within our brain that respond selectively to very specific familiar visual experiences in our environment, such as the sight of our car, our house or our grandmother]