Perceptual Coherence : Hearing and Seeing

72 Perceptual Coherence
overlap in the frequency range of different bands: One band may extend
from 1000 to 1050 Hz, while a second band may extend from 1005 to
1055 Hz, and so on. Only at the inferior colliculus in the auditory midbrain,
where the bandwidth does not change with intensity, do the correspondences
between the physiological and perceptual bandwidths emerge.
Any single cochlear frequency band is represented physically as a twodimensional
plane or lamina. The typical connection between cells is
within a single-frequency lamina. There is a topographical gradient of the
bandwidths at each isofrequency lamina: Neurons with broader bandwidths,
many beyond the measured critical band, are located more peripherally,
while neurons with narrower bandwidths are located more centrally.
Many cells in the inferior colliculus (as well as in lower nuclei) are excited
by the stimulation of one ear and inhibited by the stimulation of the
other ear. Pollak, Burger, and Klug (2003) suggested that due to excitatory
and inhibitory inputs from lower centers, trailing sounds can be inhibited
by the leading sound, yielding the precedence effect, or the law of the first
wavefront. Here, the locations of the successive echoes in a reverberant
room are suppressed and do not contribute to the perceived location of the
sound. All of the echoes, however, do combine to create the volume and
timbre of the sound.
At the Primary Auditory Cortex
The auditory pathway has a tortuous path passing through several brain nuclei
before reaching the auditory cortex. We might expect dramatic changes
in the receptive fields, given the many opportunities for neural convergence.
Yet if we assume that there will be homologies between the organization
of the visual and auditory systems, we would expect that organization
according to frequency would be the dominant factor (termed tonotopic) and
that there would be a primary auditory cortex (that may be subdivided) and
subsequent subregions. In these other regions of the auditory cortex, the
cells will have different properties, may not be tonotopic, and may mainly
have nonseparable frequency-time receptive fields (i.e., responsive to frequency
glides). However, in all regions, tonotopic frequency organization
would still be the overarching physiological concept. In what follows, I
first consider the overall properties of the organization of the auditory cortex.
Then I consider the characteristics of spectral-temporal receptive fields
and question whether these are auditory feature detectors that are tuned to
the unique requirements of a species.
Organization of the Primary Auditory Cortex The tonotopic organization
coming out of the cochlea is maintained along the entire auditory
pathway and results in a two-dimensional representation in the cortex. In