Perceptual Coherence : Hearing and Seeing

40 Perceptual Coherence
Figure 2.6. Representation of a simple on-off cell. The maximum excitation would
occur if the white bar of the grating falls completely on the on-center and the black
bars fall completely on the off-surrounds, as shown in (B). Simple on-off cells are
phase sensitive, as illustrated in (B). The excitation would be maximum in (B1), close
to baseline in (B2), maximally inhibited in (B3), and equal to baseline in (B4). If
the frequency of the grating decreases as in (A) or triples as in (C), the firing would be
close to baseline because the white and black bars fall in the on- and off-regions.
are integrated by the bipolar and horizontal cells (the orientation of the bars
does not matter because the receptive fields are circular). 2 The optimal grating
frequencies will be those in which the widths of the black-and-white bars
equal the widths of the center and surround. As shown figure 2.6, the maximum
increase in firing shown in (B1) will occur when the width of a white
bar (representing an increase in intensity) completely falls within the center
on-region and the flanking black bars (representing a decrease in intensity)
fall in the surround off-regions. By the same reasoning, the maximum
2. Typically, spatial gratings are described in terms of frequency: the number of blackand-white
cycles per degree of visual angle. This provides a convenient metric that is independent
of the distance from the viewer. For a constant frequency, the black-and-white bars must
increase in width as the grating moves away from the viewer.