Directional Sensitivity of Neurons in the Primary Auditory (AI) Cortex: Effects of Sound-Source Intensity Level

Abstract

Transient sounds were delivered from different directions in virtual acoustic space while recording from single neurons in primary auditory cortex (AI) of cats under general anesthesia. The intensity level of the sound source was varied parametrically to determine the operating characteristics of the spatial receptive field. The spatial receptive field was constructed from the onset latency of the response to a sound at each sampled direction. Spatial gradients of response latency composing a receptive field are due partially to a systematic co-dependence on sound-source direction and intensity level. Typically, at any given intensity level, the distribution of response latency within the receptive field was unimodal with a range of approximately 3–4 ms, although for some cells and some levels, the spread could be as much as 20 or as little as 2 ms. Response latency, averaged across directions, differed among neurons for the same intensity level, and also differed among intensity levels for the same neuron. Generally, increases in intensity level resulted in decreases in the mean and variance, which follows an inverse Gaussian distribution. Receptive field models, based on response latency, are developed using multiple parameters (azimuth, elevation, intensity), validated with Monte Carlo simulation, and their spatial filtering described using spherical harmonic analysis. Observations from an ensemble of modeled receptive fields are obtained by linking the inverse Gaussian density to the probabilistic inverse problem of estimating sound-source direction and intensity. Upper bounds on acuity is derived from the ensemble using Fisher information, and the predicted patterns of estimation errors are related to psychophysical performance.

A Rate Code for Sound Azimuth in Monkey Auditory Cortex: Implications for Human Neuroimaging Studies

“…Previous studies in auditory cortex of cats have shown that response latency varies with sound location, and have suggested that temporal information might be used as part of the code for sound location (Middlebrooks et al, 1994Brugge et al, 1996Brugge et al, , 1998Brugge et al, , 2001Eggermont, 1998;Eggermont and Mossop, 1998;Xu et al, 1998;Brugge and Reale, 2000;Furukawa et al, 2000;Jenison et al, 2001;Furukawa and Middlebrooks, 2002;Reale et al, 2003;Stecker et al, 2005b). We confirmed that first spike latency varies with sound location in primate auditory cortex as well.…”

“…One such possibility has been that spike timing information might encode sound location (Middlebrooks et al, 1994). Evidence that first spike latency and/or the temporal pattern of the response varies with sound location has been reported in cats and ferrets (Middlebrooks et al, 1994Brugge et al, 1996Brugge et al, , 1998Brugge et al, , 2001Eggermont, 1998;Eggermont and Mossop, 1998;Xu et al, 1998;Brugge and Reale, 2000;Furukawa et al, 2000;Jenison et al, 2001;Furukawa and Middlebrooks, 2002;Reale et al, 2003;Mrsic-Flogel et al, 2005;Stecker et al, 2005b). This has also been shown to be true for awake cats .…”

Encoding Stimulus Information by Spike Numbers and Mean Response Time in Primary Auditory Cortex

“…Instead, it is sufficient to have a mechanism that simply counts the number of spikes in the response, and marks their mean time. The feasibility of a decoding scheme based on spike latency has been discussed by others (Brugge et al, 2001;Furukawa and Middlebrooks, 2002;Jenison and Reale, 2003;Reale et al, 2003).…”

Populations of auditory cortical neurons can accurately encode acoustic space across stimulus intensity

“…Unilateral lesions of the auditory cortex have been shown to produce deficits in contralesional space in a variety of species (1)(2)(3)(4), indicating a critical role for auditory cortex. However, despite considerable effort to determine how the cerebral cortex processes acoustic space, our understanding remains rudimentary (e.g., [5][6][7][8][9][10][11][12][13][14][15][16][17]. From these studies and others, we know that (i) acoustic space is not topographically organized in the mammalian cerebral cortex and (ii) single neuron spatial receptive fields are very broad and, in themselves, unlikely to account for localization ability.…”