Topography of auditory evoked cortical potentials in children with severe language impairment: the N1 component

Topographic maps of late auditory evoked potentials (AEPs) were obtained in a group of 20 children, aged 9-15 years, with severe language impairment (LI) and an age-matched control (C) group of 20 normal children. The study was focused on differences in the latency, amplitude and topography of the N1 component between the two groups and the potential diagnostic value of these variables. The stimulus was a pure tone at 500 Hz with a duration of 100 msec and a rise and fall time of 20 msec. The intensity was 75 dB HL. Six test sequences of 50 stimuli at an interval of 1.0 sec were presented to the left and right ear separately. The AEPs were recorded and analyzed with the Bio-Logic Brain Atlas III program. In the topographic maps a focus corresponding to N1 (FN1) was seen in 15 subjects after left-ear stimulation and in 17 subjects after right-ear stimulation in the LI group. In the C group FN1 was identified in all 20 subjects after left-ear stimulation and in 19 subjects after right-ear stimulation. The position of FN1 was in front of the interaural line and with a dominance on the side contralateral to the ear stimulated in both groups. Among the subjects with an FN1, 6 in the LI group and 4 in the C group had deviating topography. Non-focal maps were seen in 5 LI subjects and 1 C subject. The latencies of N1 were longer in the LI group and there was no decrease in latency with age. There were no differences in FN1 amplitudes between groups. The prolonged latencies in the LI subjects compared to the C subjects may be explained by a slower processing in central auditory pathways and the lack of decrease in latencies with age in the LI subjects might indicate that the disturbance persists and is not a pure delay of maturation. The diagnostic sensitivity of N1 latency, amplitude and topography, in selecting the LI subjects, was 40% with a specificity of 90%. Statistical mapping of a time epoch of 70-140 msec and corresponding to FN1 in the map showed regions of > or = 3 S.D. in 10 LI and 2 C subjects, which corresponds to a sensitivity of 50% and a specificity of 90%. The variability of results within the LI group may reflect different pathophysiological factors underlying the language impairment. In conclusion, topographic evaluation of auditory long-latency potentials may become a diagnostic tool in speech and language disorders.