Brainstem timing: implications for cortical processing and literacy

Abstract

The search for a unique biological marker of language-based learning disabilities has so far yielded inconclusive findings. Previous studies have shown a plethora of auditory processing deficits in learning disabilities at both the perceptual and physiological levels. In this study, we investigated the association among brainstem timing, cortical processing of stimulus differences, and literacy skills. To that end, brainstem timing and cortical sensitivity to acoustic change [mismatch negativity (MMN)] were measured in a group of children with learning disabilities and normal-learning children. The learning-disabled (LD) group was further divided into two subgroups with normal and abnormal brainstem timing. MMNs, literacy, and cognitive abilities were compared among the three groups. LD individuals with abnormal brainstem timing were more likely to show reduced processing of acoustic change at the cortical level compared with both normal-learning individuals and LD individuals with normal brainstem timing. This group was also characterized by a more severe form of learning disability manifested by poorer reading, listening comprehension, and general cognitive ability. We conclude that abnormal brainstem timing in learning disabilities is related to higher incidence of reduced cortical sensitivity to acoustic change and to deficient literacy skills. These findings suggest that abnormal brainstem timing may serve as a reliable marker of a subgroup of individuals with learning disabilities. They also suggest that faulty mechanisms of neural timing at the brainstem may be the biological basis of malfunction in this group.

Figure 1.

A typical brainstem response of a NL child to the speech syllable /da/. Onset waves V and A are marked with arrows. Wave V is the positive peak at 7.76 ms, and wave A is the negative trough at 8.61 ms. The VA-complex slope (the transition between the peak and the trough) is defined as the microvolt (amplitude) change per millisecond between wave V and wave A.

Figure 2.

Distribution of brainstem-timing z-scores.

Figure 3.

A, Distribution of brainstem-timing parameters. Individual data of (left to right) wave V latency, wave A latency, and the VA complex slope. NL subjects are depicted with diamonds, LD subjects with circles. The dashed lines represent the 2 SD value in the NL group (excluding the outlier on the slope). LD individuals whose data fell above that value on any measureare defined as LD-. B, Grand average speech-ABR waveforms of the three groups. The NL group depicted with a thin line, the LD+ group with a dashed line, and the LD-group with a thick line. Waveforms of the LD+ and NL groups are (by definition) overlapping.

Figure 4.

MMN. Top row, Grand average responses to a rare /da/ presented amid a frequent /ga/ (solid line) versus /da/ presented alone (dashed line). No group differences were found in the responses to /da/ alone. Bottom row, The difference between the response to the rare /da/ and the response to /da/ presented alone. The black horizontal line indicates periods of significant MMN. Data shown are from Fz. The same pattern was seen at Cz.

Figure 5.

MMN distribution by group. Data are from Fz. MMN size: robust, MMN area >225 μV·ms and MMN duration >175 ms; medium (intermediate), MMN area >100 μV·ms and MMN duration >100 ms; small (small or missing), MMN duration <100 ms or area <100 μV·ms.

Figure 6.

Physiological abnormalities in individuals with LD. + indicates normal processing, - indicates abnormal processing at the brainstem (ABR) or cortical (MMN) levels. Clockwise from top center, LD subjects with normal physiology (ABR+, MMN+), LD subjects with normal brainstem but abnormal cortical physiology (ABR+, MMN-), abnormal brainstem and abnormal cortical processing (ABR-, MMN-), and abnormal brainstem but normal cortical processing (ABR-, MMN+).

Figure 7.

Distribution of literacy scores.