Altered Low-Gamma Sampling in Auditory Cortex Accounts for the Three Main Facets of Dyslexia

Abstract

It has recently been conjectured that dyslexia arises from abnormal auditory sampling. What sampling rate is altered and how it affects reading remains unclear. We hypothesized that by impairing phonemic parsing abnormal low-gamma sampling could yield phonemic representations of unusual format and disrupt phonological processing and verbal memory. Using magnetoencephalography and behavioral tests, we show in dyslexic subjects a reduced left-hemisphere bias for phonemic processing, reflected in less entrainment to ≈30 Hz acoustic modulations in left auditory cortex. This deficit correlates with measures of phonological processing and rapid naming. We further observed enhanced cortical entrainment at rates beyond 40 Hz in dyslexics and show that this particularity is associated with a verbal memory deficit. These data suggest that a single auditory anomaly, i.e., phonemic oversampling in left auditory cortex, accounts for three main facets of the linguistic deficit in dyslexia.

From temporal processing to developmental language disorders: mind the gap

“…For example, a proposed link between auditory processing and literacy development has implicated neural oscillation entrainment to multiple timescales [26,89,90,112]. Although still vague, this approach connects with recent ideas about the implementation of speech perception in the brain [113,114].…”

“…However, other recent neural oscillation data are inconsistent with temporal sampling at the syllable rate and call for a reinterpretation of data previously thought consistent with rapid processing [89,90]. Simplified schematic diagram of hypothetical relations among theoretical and empirical constituents of the temporal processing approaches to developmental language disorders.…”

Speech rhythm and temporal structure: Converging perspectives?

“…The AMPH approach to speech rhythm would predict that atypical entrainment at slower rates would have consequences throughout the phonological system. Indeed, a study of adult French-speaking dyslexics (Lehongre et al 2011) has suggested atypical oscillatory entrainment to faster gamma rate (~30 Hz) amplitude modulations (these authors measured entrainment to a complex white noise stimulus linearly increasing in modulation rate between 10−80 Hz ). Using the auditory steady-state response (ASSR) as a measure of oscillatory entrainment (a power increase at the rate of stimulation), the French study found that the ASSR at 30 Hz (low gamma rate) in auditory cortex was not left-dominant in dyslexic adults, in contrast to control participants.…”

Hemispheric Asymmetry of Auditory Steady-State Responses to Monaural and Diotic Stimulation

“…Studies have, however, shown that the dominant ASSR generator is determined by the modulation rate. That is, 80 Hz ASSRs are thought to be predominantly generated by brainstem sources, whereas low-frequency ASSRs (such as 4 and 20 Hz) are dominated by cortical sources (e.g., Herdman et al 2002;Lehongre et al 2011;Millman et al 2010). This idea is supported by studies in animals (e.g., Frisina et al 1990;Joris et al 2004) and in humans (e.g., Giraud et al 2000) showing that cortical neurons are more specialized than brainstem neurons to follow slow modulations.…”