Increasing diversity of neural responses to speech sounds across the central auditory pathway

Abstract

Neurons at higher stations of each sensory system are responsive to feature combinations not present in lower levels. As a result, the activity of these neurons becomes less redundant than lower levels. We recorded responses to speech sounds from inferior colliculus and primary auditory cortex neurons of rats, and tested the hypothesis that primary auditory cortex neurons are more sensitive to combinations of multiple acoustic parameters compared to inferior colliculus neurons. We independently eliminated periodicity information, spectral information and temporal information in each consonant and vowel sound using a noise vocoder. This technique made it possible to test several key hypotheses about speech sound processing. Our results demonstrate that inferior colliculus responses are spatially arranged and primarily determined by the spectral energy and the fundamental frequency of speech, whereas primary auditory cortex neurons generate widely distributed responses to multiple acoustic parameters, and are not strongly influenced by the fundamental frequency of speech. We found no evidence that inferior colliculus or primary auditory cortex was specialized for speech features such as voice onset time or formants. The greater diversity of responses in primary auditory cortex compared to inferior colliculus may help explain how the auditory system can identify a wide range of speech sounds across a wide range of conditions without relying on any single acoustic cue.

Speech training alters consonant and vowel responses in multiple auditory cortex fields

“…The response strength to speech sounds was quantified as the driven number of spikes evoked during 1) the first 40 ms of the neural response to the initial consonant and 2) the first 300 ms of the neural response to the vowel [1,3,8,20]. The onset latency to speech sounds was quantified as the latency of the first spike within the 40 ms window after sound onset.…”

“…For vowels, responses consisted of the 300 ms neural response to the vowel of the trained vowel pairs (‘dad’ vs. ‘deed’, ‘dad’ vs. ‘dood’, ‘dad’ vs. ‘dud’) using the mean rate over a 300 ms bin. Neural diversity was quantified by comparing the correlation coefficient (R) between the responses to speech sounds of randomly selected pairs of neurons with similar characteristic frequencies (within ¼ octave), as in previous studies [8,9]. …”

“…For example, deactivation of PAF impairs spatial sound localization ability, while deactivation of AAF impairs temporal sound identification ability [6,7]. Multiple auditory fields contain information about speech sound identity, although there are differences in the representation across fields [3,8,9]. At higher levels of the central auditory system, responses to speech sounds become more diverse and less determined by spectral power alone.…”

“…At higher levels of the central auditory system, responses to speech sounds become more diverse and less determined by spectral power alone. For example, speech responses in inferior colliculus are more influenced by spectral power compared to neurons in A1 [3,9,10]. PAF and VAF neurons with similar frequency tuning have speech responses that are more independent, which indicates that acoustic features other than spectral power are more important in these fields than in earlier fields [8].…”

Degraded speech sound processing in a rat model of fragile X syndrome

“…Speech sounds were identical to the sounds used in our previous studies and were spoken by a female native English speaker (Centanni et al, 2013b; Crystal T Engineer et al, 2014; Engineer et al, 2013, 2008; Perez et al, 2013; Ranasinghe et al, 2013). Five consonants were recorded in a ‘_æd’ context (‘dad’, ‘bad’, ‘gad’, ‘tad’, and ‘sad’), and five vowels were recorded in a ‘d_d’ context (/ae/ ‘dad’, /ε/ ‘dead’, /Λ/ ‘dud’, /i/ ‘deed’, and /u/ ‘dood’).…”