Individual differences in auditory processing in specific language impairment: a follow-up study using event-related potentials and behavioural thresholds

Abstract

It has frequently been claimed that children with specific language impairment (SLI) have impaired auditory perception, but there is much controversy about the role of such deficits in causing their language problems, and it has been difficult to establish solid, replicable findings in this area. Discrepancies in this field may arise because (a) a focus on mean results obscures the heterogeneity in the population and (b) insufficient attention has been paid to maturational aspects of auditory processing. We conducted a study of 16 young people with specific language impairment (SLI) and 16 control participants, 24 of whom had had auditory event-related potentials (ERPs) and frequency discrimination thresholds assessed 18 months previously. When originally assessed, around one third of the listeners with SLI had poor behavioural frequency discrimination thresholds, and these tended to be the younger participants. However, most of the SLI group had age-inappropriate late components of the auditory ERP, regardless of their frequency discrimination. At follow-up, the behavioural thresholds of those with poor frequency discrimination improved, though some remained outside the control range. At follow-up, ERPs for many of the individuals in the SLI group were still not age-appropriate. In several cases, waveforms of individuals in the SLI group resembled those of younger typically-developing children, though in other cases the waveform was unlike that of control cases at any age. Electrophysiological methods may reveal underlying immaturity or other abnormality of auditory processing even when behavioural thresholds look normal. This study emphasises the variability seen in SLI, and the importance of studying individual cases rather than focusing on group means.

Fig. 1

The auditory ERP of the oldest control participant tested at Time 2, electrode Fz.

Fig. 2

Scatterplot showing relationship between FD thresholds of participants in the SLI group and control group at Time 1 and Time 2. The threshold corresponds to the frequency at which the higher tone had to be set to be distinguished from a 600 Hz standard tone. The four participants with thresholds over 700 Hz at time 1 constituted the original poor-FD group. Points falling on the dotted line correspond to cases obtaining the same score at Time 1 and Time 2. Those falling below the line show improvement from Time 1 to Time 2.

Fig. 3

Individual auditory ERPs (average of electrodes Fz, FCz, F3, F4, FC3 and FC4) for younger participants with SLI (left column) and controls (right column) with dotted grey line indicating age-appropriate grand average comparison waveform for controls (see text). Significance of ICCs (one-tailed) denoted by * = .05, ** = .025 and *** = .01. Shading denotes cases from original SLI-poor FD group.

Fig. 4

Individual auditory ERPs (average of electrodes Fz, FCz, F3, F4, FC3 and FC4) for older participants with SLI (left column) and controls (right column) with dotted grey line indicating age-appropriate grand average comparison waveform for controls (see text). Significance of ICCs (one-tailed) denoted by * = .05, ** = .025 and *** = .01. Shading denotes cases from original SLI-poor FD group. ! denote case where ICC with younger controls is higher.