Ripple Analysis in Ferret Primary Auditory Cortex. I. Response Characteristics of Single Units to Sinusoidally Rippled Spectra

Abstract

We compared the response properties of single units to tones and sinusoidally, rippled spectral stimuli in the primary auditory cortex (AI) of the, barbiturate-anesthetized ferret. Using two- tone stimuli, we determined the response area of each cell and parameterized it in terms of best frequency (BF), the bandwidth of the excitatory responses at 20dB above threshold (BW20), and an asymmetry index measuring the balance of inhibition and excitation around the BF.Using frequency-modulated (FM) tones, we also determined a directional sensitivity index for the cell. Using broadband stimuli (1-20 kHz) with sinusoidally modulated spectral envelopes (ripples), we measured the response magnitude of each cell as a function of ripple frequency (W) and ripple phase (F), and then reconstructed the magnitude and phase of a ripple transfer function. Most cells (approximately 90 %) were tuned to a specific ripple frequency, denoted as a characteristic ripple frequency (Wo) . Most cells also exhibited a linear ripple phase as a function of W. The intercept of the phase function defined as the characteristic ripple phase (Fo), and is interpreted as the best ripple phase to drive the cell; the slope of the phase function reflects the location of the response area of the cell along the tonotopic axis. By inverse Fourier transforming the transfer function, we obtain the response field (RF) of the cell, an analogue of the response area measured with tonal stimuli. Like the response area, the RF was parametrized by the following measures: BFRF, which is the location of the maximum of the RF along the tonotopic axis, Wo , which is roughly inversely proportional to the width of the RF, and Fo which reflects the asymmetry of the RF. In the ferret Wo , ranges from 0.2 to 3 cycles/octave, with the average of the distribution around 1.0. Fo , ranges over the full cycle in a Gaussian-like distribution around 0o. For a subgroup of cells the sinusoidal modulations of the spectrum were presented both on linear and logarithmic amplitude scale. The responses were not notably different. The effect of the variations of amplitude of the sinusoidal modulation was studied. The largest effect was observed for the magnitude transfer function, which increased with amplitude and then saturated. The parameters Wo and Fo did not vary significantly with ripple amplitude. Typically, cells respond best to intermediate sound levels of the ripple stimulus, i.e., the magnitude transfer function shows a nonmonotonic dependence on overall stimulus level. The phase function and Wo do not depend much on level. The effects of a few nonlinearities on the responses are examined briefly. Effects of nonlinearities as threshold and saturation of the neural firing rates are examined. It is found that (non)monoticity of the rate level function of a cell could be distinguished from its ripple response characteristics. The RF of a cell closely corresponds to the response area measured with tone stimuli. Regression analysis shows that: (A) BFRF is, very similar to the tonal BF; (B) Wo is inversely correlated to the excitatory bandwidth; (C) Fo is correlated to the asymmetry of the response area.Responses to rippled spectra in AI resemble closely the response properties to sinusoidal gratings in the primary visual cortex (VI). This provides a unified framework within which to interpret the functional organization of both corticies. Basic differences between the two systems, however, are also evident as the lack in AI of a substantial simple/complex distinction in the responses.It is hypothesized that AI effectively analyzes an arbitrary input spectrum into a weighted sum of ripple components of different ripple frequencies and phases. This analysis is performed locally around each BF by a two-dimensional bank of filters tuned to different Wo and Fo values. Psychophysical support and implications of this hypothesis are also discussed in relation to the perception of timbre and other auditory tasks.