Clinical applications of the middle latency response.

The protocol developed in our laboratory for the assessment of hearing, involving the middle latency response (MLR), is described. Applications are illustrated in cases: (1) where the MLR was used to identify residual low frequency hearing and (2) where the MLR provided the only available threshold data because brainstem damage compromised the use of the ABR for hearing assessment. An understanding of the MLR generating system can provide insight into the mechanisms responsible for the inconsistency of the MLR in children. Animal models and human data indicate that maturation of the generating network may involve an early developing, variable system influenced by the mesencephalic reticular formation, and a later developing, stable system dominated by the thalamo-cortical pathway. If the thalamo-cortical system is not yet mature, one might expect the response to vary depending upon the subject's level of alertness. The observed MLR may then be dominated by other, sleep-dependent systems involving the reticular formation.