Perceptual Coherence : Hearing and Seeing

192 Perceptual Coherence
This process creates a new set of delayed traces that in turn are compared
to the next signal. Over time, the relative signal strength of the delay line
at the period of the signal would increase due to a high number of coincidence
spikes, and the strength of the other delay lines would decrease.
These sorts of models seem to capture our implicit understanding of
how we make sense of repeating patterns. To provide an equivalent sort of
visual stimulus, we could construct a random dot kinematogram similar to
the ones shown in figure 1.1. The observer would see a streaming pattern of
dots and would have to judge each time the pattern recycled. A similar twodimensional
recurrent delay line model would work.
Moreover, it seems that models based on the timings among the spikes
can also account for the change in perceived periodicity and pitch for alternating
random noise segments. As described above, a sequence such as
ABABAB is perceived as repeating AB units. But, as the number of repeats
is increased from
AABBAABB to AAAAAAAABBBBBBBBAAAAAAAABBBBBBBB
the perception shifts to repeating A and B units. The perceived pitch doubles
and the timbre of the noise shifts back and forth from As to Bs. The sequence
is not perceived as repeating AAAAAAAABBBBBBBB units,
even though this 16-segment unit is a perfect repeat. We can speculate that
there are limits to the ability to find periodicities at long intervals, so that
perceptual strength would decrease for the repeating units based on 16
noise segments. If the noise segment were 2 ms (a frequency of 500 Hz),
the duration of 8 As followed by 8 Bs would be 32 ms (31 Hz), close to the
lower limit for tonal perception.
All of these models depend on the existence of neurons capable of detecting
the coincidence of incoming excitations. In the auditory pathway,
octopus cells fit the requirements for a coincidence detector (Oertel, Bal,
Gardner, Smith, & Joris, 2000). The octopus cells are found in the mammalian
cochlear nucleus and cross the bundle of auditory fibers. Several
features of the octopus cells make them superbly suited to detecting spike
coincidences. First, each octopus cell spans about one third of the frequency
array of the auditory fibers and therefore can integrate the firings
across a wide range of frequencies. In mice, a single octopus cell could be
expected to receive inputs from auditory nerve cells that span two to three
octaves. Anywhere from 60 to 240 fibers would converge on a single cell,
and roughly one tenth to one third of the fibers seem to be necessary to trigger
the octopus cell. Second, the summation of synaptic potentials from
many fibers within 1 ms is required to cause a firing. Third, octopus cells
can fire rapidly with exceptionally well-timed spikes to periodic broadband
sounds like clicks. Fourth, octopus cells can phase-lock at very high rates,
between 800 and 1,000 spikes per second and respond to tones above