Sound temporal envelope and time-patterns of activity in the human auditory pathway : an fMRI study

Abstract

The temporal envelope of sound strongly influences the intelligibility of speech, pattern analysis, and the grouping of sequential stimuli. This thesis examined the coding of sound temporal envelope in the time-patterns of population neural activity of the human auditory pathway. Traditional microelectrode recordings capture the fine time-pattern of neural spiking in individual neurons, but do not necessarily provide a good assay of temporal coding in neural populations. In contrast, functional magnetic resonance imaging (fMRI), the technique chosen for the present study, provides an indicator of population activity over a time-scale of seconds, with the added advantage that it can be used routinely in human listeners. In a first study, it was established that the time-pattern of cortical activity is heavily influenced by sound repetition rate, whereas the time-pattern of subcortical activity is not. In the inferior colliculus, activity to prolonged noise burst trains (30 s) increased with increasing rate (2/s - 35/s), but was always sustained throughout the train. In contrast, the most striking sound rate dependence of auditory cortex was seen in the time-pattern of activity. Low rates elicited sustained activity, whereas high rates elicited "phasic" activity, characterized by strong adaptation early in the train and a robust response to train offset. These results for auditory cortex suggested that certain sound temporal envelope characteristics are encoded over multiple seconds in the time-patterns of cortical population activity. A second study tested this idea more fully by using a wider variety of sounds (e.g., speech, music, clicks, tones) and by systematically varying different sound features.

(cont.) Important for this test was the development of a new set of basis functions for use in a general linear model that enabled the detection and quantification of the full range of cortical activity patterns. This study established that the time-pattern of cortical activity is strongly dependent on sound temporal envelope, but not sound level or bandwidth. Namely, as either rate or sound-time fraction increases, the time-pattern shifts from sustained to phasic. Thus, shifts in the time-pattern of cortical activity from sustained to phasic signal subsecond differences in sound temporal envelope. These shifts may be fundamental to the perception of successive acoustic transients as either distinct or grouped acoustic events.