Behavioral evidence for the role of cortical θ oscillations in determining auditory channel capacity for speech

Abstract

Studies on the intelligibility of time-compressed speech have shown flawless performance for moderate compression factors, a sharp deterioration for compression factors above three, and an improved performance as a result of “repackaging”—a process of dividing the time-compressed waveform into fragments, called packets, and delivering the packets in a prescribed rate. This intricate pattern of performance reflects the reliability of the auditory system in processing speech streams with different information transfer rates; the knee-point of performance defines the auditory channel capacity. This study is concerned with the cortical computation principle that determines channel capacity. Oscillation-based models of speech perception hypothesize that the speech decoding process is guided by a cascade of oscillations with theta as “master,” capable of tracking the input rhythm, with the theta cycles aligned with the intervocalic speech fragments termed θ-syllables; intelligibility remains high as long as theta is in sync with the input, and it sharply deteriorates once theta is out of sync. In the study described here the hypothesized role of theta was examined by measuring the auditory channel capacity of time-compressed speech undergone repackaging. For all speech speeds tested (with compression factors of up to eight), packaging rate at capacity equals 9 packets/s—aligned with the upper limit of cortical theta, θmax (about 9 Hz)—and the packet duration equals the duration of one uncompressed θ-syllable divided by the compression factor. The alignment of both the packaging rate and the packet duration with properties of cortical theta suggests that the auditory channel capacity is determined by theta. Irrespective of speech speed, the maximum information transfer rate through the auditory channel is the information in one uncompressed θ-syllable long speech fragment per one θmax cycle. Equivalently, the auditory channel capacity is 9 θ-syllables/s.

The intelligibility of interrupted and temporally altered speech: Effects of context, age, and hearing loss

“…The nonlinearity in the effect of absolute interval duration on differences in performance between regularly gated and temporally expanded conditions is also consistent with the view that distinguishes between perceptual processing of interrupted speech at low and high interruption rates (Huggins, 1975;Samuel, 1991;Ghitza and Greenberg, 2009;Shafiro et al, 2011a;Ghitza, 2014;Shafiro et al, 2015). Recent studies by Ghitza (Ghitza and Greenberg, 2009;Ghitza, 2014) have suggested a neurologically plausible cortical mechanism that may be able to account for the nonlinear effects of time alterations.…”

“…Given the importance of slow varying envelope fluctuations for the perception of sentences (Fogerty et al, 2012), it is possible that changes in the sentence envelopes introduced by gating and further exacerbated by temporal alterations contributed to the decline in intelligibility in the ONH and OHI groups. It is also conceivable that lesser sensitivity to envelope changes might have obscured some of the acoustic landmarks used to synchronize speech input with cortical oscillators, thus negatively affecting the perceptual outcome (Ghitza, 2014).…”

“…Recent studies by Ghitza (Ghitza and Greenberg, 2009;Ghitza, 2014) have suggested a neurologically plausible cortical mechanism that may be able to account for the nonlinear effects of time alterations. From that perspective, the variable effects of interval duration in interrupted speech reflect synchronization patterns between input speech and the cortical theta rhythm, which approximately corresponds to syllable timing.…”

“…The general finding has been that for many interruption rates, high intelligibility can be maintained with as little as 25%-50% of the original signal remaining (Miller and Licklider, 1950;Nelson and Jin, 2004;Shafiro et al, 2011a;Saija et al, 2014;Shafiro et al, 2015). Some investigators have also adopted this experimental approach to study contributions of temporal constraints on the perceptual processing of speech (Garvey, 1953;Huggins, 1964;Heiman et al, 1986;Shafiro et al, 2011b;Saija et al, 2014;Ghitza, 2014). In a previous study, Shafiro et al (2011b) examined the intelligibility of sentences that were either gated at different rates (0.5-16 Hz) and a 50% duty cycle or similarly gated and then temporally compressed by concatenating the consecutive speech fragments retained after gating.…”

Entrained theta oscillations guide perception of subsequent speech: behavioural evidence from rate normalisation

“…Dupoux & Green, 1997). Ghitza (2014) demonstrated that the intelligibility of compressed speech deteriorates particularly sharply when syllable rates exceed the upper frequency of the theta range (>9 Hz; cf. Ghitza & Greenberg, 2009).…”