Perceptual Coherence : Hearing and Seeing

The Transition Between Noise and Structure 191
visual system. The lack of such delay lines has led Shamma (2001) to suggest
that there is direct spatial comparison of the movement of the sound
wave along the basilar membranes of the near and far ears because, that can
directly represent the interaural delay.
Second, for vision, there is the problem of tracking the movement of objects
in space. Starting with Reichardt (1961), models for the perception of
movement are based on delay lines feeding into coincident detectors. Imagine
an edge that moves spatially. The outputs from every receptor would be
a set of delay lines (including one with zero delay), and those would be
paired one-to-one with delay lines from other receptors at different spatial
positions. Each such pair of delay lines would converge on one coincidence
detector. The speed and direction of movement would be calculated by
the cross-correlations of the spike patterns between pairs of delay lines by
the coincidence detectors (this is covered more completely in chapter 5).
I have conceptualized the conversion of auditory or visual excitation
into separate, but overlapping sensory channels. Each channel is most sensitive
to a particular auditory or spatial frequency, and the excitation of
each channel is a function of the stimulus energy at that frequency. As described
in chapter 2, the excitation can be conceptualized in terms of the
firing rate of each channel or in terms of the timing between the spikes of
the neurons. There need not be only one mechanism. 13
To the extent that the neurons are phase-locked to the excitation, then
each repetition should generate the identical distribution of intervals between
the spikes. There are several possible mechanisms that we could postulate
that maintain the neural spike sequence so that matches can be found
among the repeating units. Cariani (1999) hypothesized that there are sets
of recurrent delay lines that detect the repetition. Any signal creates a set of
memory traces that encode the spike timings. Each trace is delayed by a
different time interval. The new incoming signal is compared to every delayed
version of the current memory trace and for each delay, a coincidence
detector passes on those spikes that occur in the same time bins from the
circulating trace and the present signal. There would be many coincident
spikes for the delay lines that match the repetition rate of the signal, but
only a small number of coincident spikes for the delay lines that do not.
13. Given that organisms need different kinds of information about the external world, we
would expect that there would be several ways in which neural excitation could and would
code that information. Victor (2000) presented an information theory method for estimating
whether using the coincidence in the timing of the spikes improves the information transmission
above that obtained by simply counting the number of spikes. He estimated that the gain
is roughly 20-30%. Using a similar method, Victor (2000) argued that when several neurons
signal the identical stimulus property (i.e., neurons that have the same best frequency), transmission
is better if the spike trains of each such neuron are kept separate, rather than summing
all together. The gain here is smaller, about 10%. This is the same conclusion as Reich et al.
(2001).