Perceptual Coherence : Hearing and Seeing

Transformation of Sensory Information Into Perceptual Information 85
fields. The authors pointed out that for natural stimuli such as the zebra
finch songs used here, there are strong spectral and temporal correlations.
For example, the overall amplitude envelopes of a set of frequency bands
(neglecting the fine variations within the envelope) are correlated over time
so that the onsets and offsets of one band can predict the corresponding
timing of another band. This suggests that the optimal stimulus representation
should remove such correlations in order to reduce the redundancies.
However, Theunissen et al. (2000) found that the correlations between
frequency bands did affect the spectral-temporal receptive fields because
simulated versions of songs that balanced the average frequency distribution
and rhythmic distribution did not yield firing patterns identical to those
of the actual sounds that contained the correlations. Two typical neurons
of this sort are shown in figure 2.32. Neuron (A) appears to be tuned to a
downward-moving frequency edge, while neuron (B) appears to be tuned
to a nonoverlapping combination of frequencies between 1500 and 2500 Hz,
followed by frequencies between 4000 and 6500 Hz.
Figure 2.32. The response of two auditory neurons to real conspecific songs
shows a distinct receptive field structure (C and D). The response, however, is very
weak to a sequence of tones that has the same frequency distribution as the real
conspecific songs, but a different sequence and rhythm (A and B). From “Spectral-
Temporal Receptive Fields of Nonlinear Auditory Neurons Obtained Using Natural
Sounds,” by F. E. Theunissen, K. Sen, and A. J. Doupe, 2000, Journal of Neuroscience,
20, 2315–2331.Copyright 2000 by the Society for Neuroscience. Reprinted
with permission.