Middle-latency response: basic properties and prepositions for clinical application

Abstract

The principal characteristics of the middle-latency response (MLR) were systematically studied in the consecutive experimental sessions. Both healthy subjects and patients were investigated and clicks and tone-peeps of various frequencies were applied. The main results of the long-term experience could be summarized as follows: The MLR covers the frequencies from 12 to 100 Hz. The main spectral constituents of the early MLR compontents distribute over 40 Hz, and those of the late components over 20 Hz. The MLR reaches the maximal amplitudes on the vertex. In central and periaural areas the late MLR components possess the same and the early ones the opposite polarity. With the mastoid reference the superimposition of the sonomotor potentials upon the MLR takes place. With the ear-lobe reference the MLR mostly lacks muscle contaminations. Detection thresholds of the MLRs approximately those of the psychoacoustic sensation. The identification of individual MLR components do not differ significantly from each other. An increase in the stimulus level is followed by the MLR peak latency (PL) shortening and the amplitude increase. The amplitude curves of the early and middle MLR components are steeper and that of the late components is flatter. At all stimulus levels the amplitude values of the middle MLR components exceed those of the early and late ones. Over the course of the long-term acoustic stimulation the amplitudes of the MLR remain stable. Neither PLs nor amplitudes of the MLR depend systematically on the polarity of the acoustic stimuli. Under monaural stimulation the greater MLRs are registered from contra- vs ipsilateral scalp areas. The amplitude asymmetry progresses to the later MLR components. Bilateral MLRs do not differ from each other by the PLs. The MLR follows high rates of acoustic stimulation and possesses a rapid recovery of responsiveness. At a conditioning interval of 50 ms the MLR restoration is completed. Prominent binaural convergence of an occlusive type is characteristic for the MLR. The amount of convergence does not depend on the stimulus frequency and the polarity. It intensifies with an increase in the stimulus level. At high stimulus levels the MLRs to the low-frequency tone-peeps contain both the frequency specific and non-specific contributions. At threshold levels the frequency specificity of the MLRs looks perfect. No great effects of preceding shocks upon the MLR to the following clicks are evidenced in intermodal experiments. However, an initial part of the MLR is altered in most test records, outlining a strip of the probable interaction. At threshold sensation levels the MLR identification scores are higher, the amplitudes are greater, and the PLs are shorter in females than in males. The PL differences increase to the later MLR components. As a result, the interpeak intervals are shorter in females than in males. The MLR parameters in the patients with temporal epilepsy do not differ from those in healthy individuals. The MLR has a subcortial, polylevel, and monomodal rather than a cortical, monolevel, and polymodal genesis.