MIDDLE LATENCY RESPONSES TO ACOUSTICAL AND ELECTRICAL-STIMULATION OF THE COCHLEA IN CATS

The middle latency responses (MLR) to acoustical stimulation (A-MLR) a s well as to electrical stimulation (E-MLR) of the inner ear were reco rded in pentobarbital-anaesthetised cats. Monopolar and bipolar MLR re cordings were performed with electrodes located at different places on the primary auditory cortex (AI). The cochlea was electrically stimul ated (ES) through a single round-window electrode or through a multich annel intracochlear implant. The slope of amplitude-intensity function s of the A-MLR was steeper when the stimulus frequency of the acoustic al stimuli corresponded to the tonotopical recording place on the audi tory cortex. Other response properties (waveshape, thresholds and late ncies) were related to the recording site and stimulus frequency in on ly two-thirds of animals. Parameters of E-MLRs evoked by high-frequenc y (> 4 kHz) and low-intensity ES in hearing cats, which produced an el ectrophonic effect, were similar to parameters of acoustically evoked MLRs. In deafened cats, the properties of responses to extracochlear E S were different from those recorded to acoustical stimulation and the y were almost uniform in all cortical places, Variations in thresholds , in latencies and in the slope of the amplitude-intensity functions o f the E-MLRs recorded in individual tonotopical cortical places were o bserved when the auditory nerve was stimulated with different configur ations of electrodes through a multichannel intracochlear implant.