Brain-stem, middle latency and late cortical evoked potentials in children with speech and language disorders

The topography of the brain-stem (ABR), middle latency (MLR) and cortical (ACR) evoked responses was investigated in children with normal speech and language development and those with either a language or motor speech disorder. The aim was to determine whether it is possible to discriminate between the groups of children in terms of the evoked potential characteristics. There were significant inter-group differences, particularly relating to the amplitude of the different responses. The ABR in both the language and motor speech groups exhibited smaller amplitudes for waves I, III and V than the control group, with no change in latency. Two explanations were suggested; firstly abnormal functioning of the peripheral hearing mechanism even though the hearing thresholds were normal which could be a secondary effect due to deprivation of normal speech and language development; and secondly far-field recording effects due to differences in the electrical conductivity of tissue and the distance separating the generator site and recording electrodes. The MLR in the motor speech group was significantly larger at the mastoid and temporal electrode sites than either the control or language groups. This was considered to be an enhanced myogenic response like the other exaggerated brain-stem reflexes seen in congenital suprabulbar paresis. Significantly larger amplitudes of the ACR were also recorded from the motor speech group at the Cz electrode site. This was thought to be due to underactivity of some normal cortical inhibitory system and not a direct result of increased MLR amplitude. The ACR in the language disordered children exhibited an abnormal left temporal hemispheric dominance and a more inverted or 'dissimilar' wave form at the T3 electrode site on the correlation analysis. These findings suggest impaired functioning of the left temporal cortex in our children who have failed to develop language normally. We feel that this has more significance for the language abnormality than the low amplitude ABRs which were observed in both the language and motor speech disordered children.