Perceptual Coherence : Hearing and Seeing

80 Perceptual Coherence
Figure 2.28. The spectral-temporal receptive fields of four representative
auditory cells. Nearly all neurons had multiple excitation and inhibitory regions
and seem tuned to auditory characteristics that define objects. Adapted from “Optimizing
Sound Features for Cortical Neurons,” by R. C. DeCharms, D. T. Blake, and
M. M. Merzenich, 1998, Science, 280, 1439–1444.
surface, finding stimulus edges (onsets) in time, finding stimulus movements,
and finding feature conjunctions.”
Shamma (2001) and Shamma and Klein (2000) measured the receptive
fields in A1 using sounds made up of many harmonics across a five-octave frequency
range. The amplitudes of the harmonics are modulated across the logarithmic
frequency axis by a sine wave that drifts either up or down in
frequency. The sine wave creates an amplitude envelope across the frequency
range that gradually shifts in frequency, as shown in figure 2.29. By varying
the frequency, rate of drift, and amplitude (depth of modulation) of the sine
wave, it is possible to measure the spectral-temporal response field from the
resulting firing pattern. Examples of rippled spectra are shown in figure 2.29. 5
Researchers (Depireaux, Simon, Klein, & Shamma, 2001; Kowalski,
Depireaux, & Shamma, 1996) investigated whether the spectral and temporal
components of the neural response were separable and independent. In
5. The ripple stimuli are modeled after drifting sinusoidal gratings, discussed in chapter 5,
used to investigate second-order motion.