Studying visual discrimination and stereopsis

In the 1960s your whole work was turned over to charting the course of the transmission of impulses in the visual system, through the geniculate nucleus towards the cerebral cortex?

We started off by plotting the projection of the visual field onto the lateral geniculate nucleus, finding where the different fibres go to in the nucleus. To do that, I realised, I would have to know much more about the eye itself and how it forms an image, and that was the real beginning of my work on the visual system. That's when we started to study the cat's eye in detail and I developed, with my colleagues, the schematic eye for the cat. A schematic eye is a mathematical model of an average eye. That had been done for the human eye by Gullstrand, way back before the First World War, but we were the first to prepare a schematic eye for any animal.

And that was absolutely essential in showing the relationship between visual input, optical stimulation, and what was coming through to the geniculate nucleus.

Well, you have to know what the optic nerve gives to the lateral geniculate, because the optic nerve joins the eye to the lateral geniculate nucleus. That was the beginning of the work. In the late 1960s I became interested in stereopsis, which is the ability to see in depth, to see that one object is further away than another object. We started single cell recording from the cerebral cortex - the visual parts at the back of the brain, the occipital lobe. Hubel and Wiesel had already done this as well. What was new was the realisation that the two eyes send impulses up to the brain that, by coming together on a single cell in the striate cortex, could form the basis for stereopsis. We started by studying the properties of the receptive fields. A receptive field is that little patch in the visual world – the outside world – that each cell keeps a watch on. Each cell is concerned with a little area in the visual world – that's its receptive field. The impulses from the two eyes go back to a single cell (the same cell) in the cerebral cortex, so that in effect that cell in the cerebral cortex looks out through both eyes at a little area we call a receptive field, and its special job is to report to the rest of the brain what is happening in that little area.

That little view of the world.

Yes. What the cells in the brain, in the cortex, do at that stage in the visual system is not to record seeing an actual object but rather to report to the rest of the brain the individual features of that object – geometrical properties such as lines and edges, corners and so on. A cell in the brain looks out through both eyes at the two receptive fields, one for each eye, and the cell's job is to report individual features of objects in those two little areas, which have to have exactly the same properties because they have to report the same features of the external object - they must be capable of recording a line at a particular angle, and edges and so on.

What we did in the 1960s was to study what happens when the two receptive fields come together. So, if cells in the cortex are going to report a particular feature in the external world, the two receptive fields have to be in register. They can't be separate because the cell would be reporting different features. What we did was to study how the responses of the cells in the cortex change as a result of the two receptive fields being in register.

Furthermore, in stereopsis or depth perception, a cell has to be able to report that, when the two receptive fields come into register, the feature of the object is closer to or further away from the fixation point, the point that the animal or human is actually looking at. It can do this with extraordinary precision, as a result of a property called receptive field disparity. When the two receptive fields are a bit out of register, the brain can tell the change in the visual angle that occurs. The human brain can do that to about 10 seconds of arc. In laboratory conditions humans can even do it to 2 or 3 seconds of arc. That's quite an incredible property. The human brain can tell when these two receptive fields are in register and when they're out of register even by 10 seconds of arc, and that 10 seconds of arc represents an image difference on the retina of the two eyes of about 1 micron, which is one thousandth of a millimetre and not much greater than the wavelength of light. Light has a wavelength of about half a micron. To do experiments to determine these things required very high precision work.

Focus questions

• How would the ability to see in three dimensions be an advantage to an organism?

• What important new observation about the brain and stereo vision did Bishop make as a result of recording the nerve impulses of a single cell in the brain?