Figure 7

Recording of neural responses in the inferior colliculus (ICC) of the guinea pig was used to document the regions of activation generated by intracochlear electrical stimulation. Prior to deafening the animal a series of pure tones was presented to the left ear and neural responses were recorded in the contralateral ICC using a multichannel recording probe. This procedure calibrated the depth of the recording probe, allowing the location of activity in the ICC to be directly related to the location of the site of stimulation in the cochlear spiral. The panels shown in A illustrate the location and strength of the neural response for pure tones of increasing frequency (5.7 kHz on the left to 26.9 kHz on the right). Increasing intensity is plotted along the abscissa for each panel and the response magnitude is represented by pixel color. The maximum response for low frequency tones is consistently located near the surface of the ICC. This focus shifts deeper as the acoustic signal increases in frequency. In all cases, the region of activation spreads across the ICC as loudness increases.

Panels B, C and D illustrate how the configuration of stimulating sites, in this case tripolar (B), bipolar (C) and monopolar (D) strongly affects the spread of excitation for stimulation with each channel of the electrode array. For each configuration, stimuli were presented with the cathodic phase (−) first on the contact indicated, i.e. the center contact in a tripolar configuration, the apical contact in a bipolar configuration and the intracochlear contact in a monopolar configuration. Under all conditions, stimulation of an apical location in the cochlea (a low frequency tone, electrode configuration 1,2,3 tripolar, 1,2 bipolar, or monopolar, resulted in neural activity focused near the surface of the ICC. Conversely, stimulation of a more basal electrode site or high frequency tone resulted in maximum activity deeper in the nucleus. Clearly, the neural responses to monopolar stimulation are more broadly distributed in the ICC than those for bipolar or tripolar stimuli.