Unstable representation of sound: a biological marker of dyslexia

Abstract

Learning to read proceeds smoothly for most children, yet others struggle to translate verbal language into its written form. Poor readers often have a host of auditory, linguistic, and attention deficits, including abnormal neural representation of speech and inconsistent performance on psychoacoustic tasks. We hypothesize that this constellation of deficits associated with reading disorders arises from the human auditory system failing to respond to sound in a consistent manner, and that this inconsistency impinges upon the ability to relate phonology and orthography during reading. In support of this hypothesis, we show that poor readers have significantly more variable auditory brainstem responses to speech than do good readers, independent of resting neurophysiological noise levels. Thus, neural variability may be an underlying biological contributor to well established behavioral and neural deficits found in poor readers.

Figure 1.

Auditory brainstem responses of poor readers are more variable than those of good readers. Responses from a representative good reader (left) and poor reader (right) are plotted to illustrate the increased variability in the poor readers relative to good readers. A, B, The response analyses regions are marked by dashed lines. Poor readers have more variable responses (lower r values) than good readers when calculating consistency between the first half of the recording and the second half (A) or between the even events and the odd events (B). Groups did not differ in the overall magnitude or prestimulus magnitude of the response.

Figure 2.

Neural variability in poor readers primarily reflects trial-by-trial inconsistency, not neural fatigue. Two possible mechanisms of response variability are illustrated. A, One mechanism of response variability is fatigue over time. This would be reflected by high variability when comparing responses from the first (black) and second half (gray) of the recording. When response consistency is calculated by comparing the first and second halves of the recording, poor readers had more variable brainstem responses relative to good readers in both the formant transition (dark gray; p < 0.005) and vowel (black; p < 0.05) portions of the response, and were marginally different from the average readers for the response to the formant transition (dark gray; p = 0.052). B, Another mechanism of response variability is trial-by-trial variability, reflected by less consistent responses when comparing odd-numbered events (black) to even-numbered events (gray). The dashed line is included to make the potential temporal jitter among responses more apparent. In the subset of data in which assessment of this mechanism was possible (n = 64), poor readers again had more variable responses than good readers in both the formant transition (dark gray) and vowel (black) portions (p < 0.005 and p < 0.05, respectively), and were marginally worse than average readers in the formant transition (dark gray; p = 0.095). Because our results are identical for the two analysis techniques, they suggest that trial-by-trial variability throughout the recording is the dominant cause of weaker response consistency in poor readers.