The Neural Control of Vision
O. Illusions

We now propose a new theory called the S1 simple-cell theory:

This theory is based on a number of basic facts as well as a few assumptions. Most of the basic facts have been described in previous sections.
Here is the essential list of facts:

  1. As derived from the preceding displays (Slide Show A15-2 to A15-15), it appears the the Hermann grid illusion cannot be attributed to events occuring at the level of the retina or the lateral geniculate nucleus. Figure 3b suggests that orientation selective neurons play a role. We propose therefore that the effect arises in the cortex, most likely in area V1.

  2. Two major classes of cells in V1 are the simple and the complex.

  3. In majority of V1 cells is orientation selective. For any spatial location there is an ample representation of various orientations. The receptive fields are larger than in the retina. The size of receptive fields at any given eccentricity varies over quite a range with complex cells having larger receptive fields than simple cells.

  4. Several sub-classes of simple cells have been described. The S1 type simple cells receive selective excitatory input from either ON or OFF LGN cells (Schiller (1982) Nature, 297, 580-583). These cells are called the ON S1 and the OFF S1 simple cells. In a sample of 245 simple cells we had studied 23% were S1 type. Half were ON and half OFF (Schiller et al. (1976) J Neurophysiol, 39, 1288-1319).

  5. The responses of these cells increases when the length of an optimally oriented bar in the receptive field is increased over a moderate range with vaired degrees of summation among the cells (Schiller et al. (1976) J Neurophysiol, 39, 1288-1319).

  6. We propose that the activity of the S1 simple cells in V1 plays a central role in the illusiory effect.

  7. The activation of ON cells in the retina, LGN and V1 by elicits the perception of lightness, whereas the activation of OFF cells in the retina, LGN and V1 by elicits the perception of darkness (see slide show 1. VII. The Function of the ON and OFF Channels in Vision).