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.
Two major classes of cells in V1 are the simple and
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.
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,
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,
We propose that the activity of the S1 simple cells
in V1 plays a central role in the illusiory effect.
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).