Perceptual Coherence : Hearing and Seeing

Transformation of Sensory Information Into Perceptual Information 49
Figure 2.10. Feed-forward models can help explain the origin of oriented cortical
cells. To increase the spatial orientation resolution, the length of the on- and offregions
would be lengthened by combining a greater number of circular on-off cells
along the horizontal axis (B). To increase the frequency resolution, the number of
on- and off-regions would be incremented by combining cells along the vertical
axis (C). To vary the optimal spatial frequency, the number of circular cells that compose
each on- and off-region would be varied along the vertical axis (D). Adapted
from Foundations of Vision by B. Wandell, 1995, Sunderland, MA: Sinauer.
simply in terms of the summation of the excitation and inhibition from
many lateral geniculate cells.
The receptive fields of these cells are highly structured. They represent a
restricted retinal area, and they respond to a range of spatial frequencies
and orientations. We can imagine the receptive field to be a filter, or effectively
a multiplier. We take the illumination falling on the receptive field
and multiply it by the receptive field pattern to measure the effect of the illumination
on the spike rate of the cortical cell.
There are always processing limitations so that the transmitted information
must be constrained to some maximum value; improving the frequency
resolution must necessitate the reduction in orientation resolution as described
in chapter 1. A two-dimensional envelope with a Gaussian fall-off (in