Perceptual Coherence : Hearing and Seeing

Transformation of Sensory Information Into Perceptual Information 69
the 100 Hz fundamental. Correlogram representations, and timing models in
general, have been rather successful in matching results from human pitch
judgment experiments.
Given the frequency limitations to phase-locking, it is best to hypothesize
that both rate and timing are used by the auditory system: Timing coding
occurs up to the frequency determined by the phase-locking limit, and
rate coding occurs at frequencies beyond that. As you go up the auditory
system, the phase-locking limit decreases so that by the primary auditory
cortex, that limit may be only 40 to 50 Hz (T. Lu, Liang, & Wang, 2001). T.
Lu and Wang (2004) argued that rate and timing coding may encode different
properties of the auditory input or encode the same property by means
of different populations of neurons. In that work, roughly 40% of the cortical
neurons synchronized to the onsets of rapidly occurring clicks (down to
a 30 ms interval between clicks), while the remaining 60% of the cells did
not have any timing information. T. Lu and Wang (2004) argued that the
latter cells are a transformed representation of the stimulus that could encode
other properties.
At the Auditory Brainstem and Midbrain
One strong similarity between the spatial receptive fields in vision and the
frequency receptive fields in audition is that both occur at different resolutions.
Spatial receptive fields and critical bands vary in bandwidth and are
densely distributed along the frequency dimension. There is local coding in
both. A second similarity is the spatial overlap of receptive fields in vision
and the frequency overlap of critical bands in audition.
This firing pattern is carried by the auditory nerve to the cochlear nucleus.
Here, parallel processing channels are created that abstract certain
acoustic features. One such enhanced feature is the degree of amplitude
Figure 2.23. Derivation of a population peristimulus histogram and population
all-order interval histogram. The population peristimulus histogram sums the number
of spikes across fibers with different characteristic frequencies at time points after
stimulus onset. The derivation of an all-order peristimulus histogram for a single
fiber is shown schematically in (A), (B), and (C). The stimulus waveform is shown
in (A), the spikes are shown in (B), and all of the intervals between spikes are
shown in (C). An example of a population all-order interval histogram is shown below.
The histogram covers a 50 ms interval beginning 240 ms after stimulus onset.
At the top of the left column is the stimulus waveform, below that the spikes for
fibers with different characteristic frequencies, and at the bottom the total number
of spikes at each time point. At the top of the right column is the autocorrelation for
the stimulus waveform, below that is the intervals between spikes for fibers with
different characteristic frequencies, and at the bottom is the frequencies of the intervals
between spikes. Figure courtesy of Dr. Peter Cariani.