| This study examined the responses of the primary auditory cortex (area A1) to acoustical and electrical stimulation of the cochlea. Experiments were conducted in ketamine-anesthetized guinea pigs. Acoustical stimuli consisted of tones and narrowband noises presented to normal hearing animals. Electrical stimuli were single biphasic pulses presented through a 6-channel cochlear implant (Cochlear Ltd.) to acutely deafened animals. The extent of cochlear activation with manipulated by varying acoustical bandwidth and electrode configuration. The spatio-temporal pattern of neural spike activity was measured simultaneously across 16 cortical locations spanning ∼2–3 octaves of the cortical tonotopic axis. This pattern, averaged over 10 to 40 presentations of any particular stimulus, was defined as the “cortical image” of that stimulus. The cortical image of a restricted cochlear stimulus, i.e., acoustical tones or electrical pulses in tripolar or bipolar electrode configurations, showed a restricted focus of activity along the cochleotopic gradient. As the cochlear place of stimulation was changed from apical (low frequency) to basal (high frequency), the focus shifted from rostral to caudal. When cochlear activation was broadened, by increasing the stimulus level or changing the electrode configuration, the cortical image broadened. Interactions among cochlear-implant channels were studied with two-channel electrical stimuli that varied in spatial and temporal spacing. The response to a stimulus on one channel was facilitated by stimulation of a second channel. Channel interaction occurred, in some conditions when a single sub-threshold pulse preceded a second pulse in time by 1 ins or more. For simultaneous stimulation, interaction for monopolar stimulation was greater than that for bipolar, which was greater than that for tripolar. For non-simultaneous stimulation interaction was abolished by 160 μs for MP stimulation but persisted for bipolar stimulation. The results of this study demonstrated that, compared to the monopolar configuration, focused electrode configurations such as tripolar and bipolar signal the place of cochlear stimuli more precisely and minimize channel interaction during simultaneous two-channel stimulation. The monopolar configuration, however, appears to produce a shorter acting stimulus that might minimize channel interaction when channels are stimulated non-simultaneously. |
