[Frontiers in Bioscience, 3, d494-501, May 6, 1998] |
SYNAPTIC MECHANISMS IN AUDITORY CORTEX FUNCTION
Dept. of Psychobiology, University of California, Irvine, 2205 Biological Science II, Irvine, CA 92697-4550
Received 4/1/98 Accepted 4/20/98
5. RESPONSES OF AUDITORY CORTEX TO ACOUSTIC STIMULI
To appreciate cellular mechanisms in AC, it is helpful to review briefly the kinds of stimuli to which AC neurons respond. Well-established features of primary AC include the orderly representation of stimulus frequency, or tonotopic arrangement (and the resulting formation of isofrequency bands), and the sensitivity of neurons to binaural stimuli (1, 2, 3, 4, 38). These features reflect the ability of AC neurons to code for physical properties of acoustic stimuli including their frequency, intensity, and location in space. Neurons in primary AC respond with lowest threshold to a single frequency, the characteristic frequency (CF), and respond with higher thresholds to other, nearby frequencies. At CF, neurons respond to steadily increasing stimulus intensities either with firing rates that increase to a point and then plateau (producing monotonic intensity functions) or rates that increase to a point and then decrease (nonmonotonic functions). Sound waves from acoustic sources in the environment take different paths to each ear, and produce interaural time and intensity differences (ITDs and IIDs) at the two ears. These differences are used by neurons in several auditory regions to code for stimulus location. In AC, neurons optimally code ITDs and IIDs that reflect stimuli within 45 deg of the position directly in front of the animal (1). The involvement of AC in processing information about frequency and location has been implied by lesion studies. For example, lesioning an entire isofrequency band impairs the ability of animals to determine the location of a sound of that frequency (6). Conversely, lesions that encompassed all of AC except for a narrow isofrequency band leaves animals unable to discriminate the location of sounds except for the frequency of the spared representation.
Auditory event-related potentials (ERPs) are extracellular field potentials elicited in response to acoustic events. Because they can involve noninvasive recording techniques, ERPs are used extensively in studies of animal behavior and human psychophysics (7). Some ERP components are modified by psychological processes such as attention, whereas other components may reflect pre-attentive automatic feature analysis (e.g., the mismatch negativity, MMN). Only the earliest ERP components (£ 20 ms latency) overlap with the single unit responses of auditory physiology studies, and very little is understood regarding the mechanisms underlying longer-latency components (with latencies up to several hundred ms). Long-latency potentials may involve diffuse neuromodulatory systems (32, 39) or slow ionotropic mechanisms (40, 41).