The effects of auditory deprivation on the spatial distribution of cortical response thresholds to electrical stimulation of the adult cat cochlea were evaluated. Threshold distributions for single- and multiple-unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in three groups of animals: adult, acutely implanted animals ("acute group"); adult animals, 2 wk after deafening and implantation ("short-term group"); adult, neonatally deafened animals ("long-term group") implanted after 2-5 years of deafness. For all three groups, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The two low-threshold regions in the acute and the short-term group were arranged cochleotopically. This was reflected in a systematic shift of the cortical locations with minimum thresholds as a function of cochlear position of the radial and monopolar stimulation electrodes. By contrast, the long-term deafened animals maintained only weak or no signs of cochleotopicity. In some cases of this group, significant deviations from a simple tri-partition of the dorsoventral axis of AI was observed. Analysis of the spatial extent of the low-threshold regions revealed that the activated area in acute cases was significantly smaller than the long- and the short-term cases for both dorsal and ventral AI. There were no significant differences in the rostrocaudal extent of activation between long- and short-term deafening, although the total activated area in the short-term cases was larger than in long-term deafened animals. The width of the narrow high-threshold ridge that separated the dorsal and ventral low-threshold regions was the widest for the acute cases and the narrowest for the short-term deafened animals. The findings of relative large differences in cortical response distributions between the acute and short-term animals suggests that the effects observed in long-term deafened animals are not solely a consequence of loss of peripheral innervation density. The effects may reflect electrode-specific effects or reorganizational changes based on factors such as differences in excitatory and inhibitory balance.

1. Responses of neurons in primary auditory cortex (AI) of the barbiturate-anesthetized adult cat were studied using cochlear stimulation with electrical and acoustic stimuli. Neuronal responses to acoustic stimulation with brief biphasic clicks of the ear ipsilateral to the studied cortical hemisphere were compared with those evoked by electrical stimulation of the contralateral cochlea with brief biphasic electrical pulses delivered via a feline cochlear prosthesis. The contralateral ear was deafened immediately before implantation of the cochlear prosthesis. The feline cochlear prosthesis consisted of four bipolar electrode pairs and was placed in the scala tympani. Two bipolar electrode conditions were used for stimulation: one near radial pair with electrode spacing of 0.25-0.5 mm, and one longitudinal pair with electrode spacing of approximately 6 mm. 2. The firing rates obtained from single- and multiple-neuron recordings were measured as a function of stimulus repetition rate of electrical and acoustic pulses. From period histograms over a recording interval of 1,000 ms, the driven firing rate to repetition rates from 2 to 38 Hz was obtained and repetition rate transfer functions (RRTFs) were constructed. The RRTFs were characterized as low-pass or band-pass filters and several descriptors were obtained, such as the repetition rate producing the highest driven activity, high and low cutoff frequencies 6 dB below maximum firing rate, and maximum firing rate. 3. For a given neuron, the main characteristics of cortical RRTFs obtained with electrical and acoustic cochlear stimulation were quite similar. However, some small but statistically significant differences in the best repetition rate, cutoff frequencies, and maximum firing rate could be observed between the different stimulation modes. The proportion of band-pass RRTFs was larger for electrical stimulation (57%) than for acoustic stimulation (41%). The high cutoff frequencies for electrical stimulation were slightly but consistently higher than for acoustic RRTFs of the same neuron and the maximum firing rate for electrical stimulation was significantly higher than that evoked by ipsilateral acoustic stimulation. 4. The entrainment of cortical neurons to electrical and acoustic pulses was determined and entrainment profiles were constructed. For a given neuron, electrical entrainment profiles showed higher cutoff frequencies than with acoustic stimulation when judged with a fixed entrainment criterion of 0.25 spikes per event. The maximum entrainment seen for electrical stimulation was approximately 20% higher than seen for the same neuron with acoustic stimulation. 5. Correlation analysis of repetition coding and latency parameters revealed several relationships between these response aspects. Most prominent among them was a significant correlation between measures of the response latency and estimates of the ability to follow temporal repetitions for acoustic as well as electrical conditions. 6. Parametric and comparative evaluations of cortical res...

The Minimal Auditory Capabilities (MAC) Battery was administered individually to 75 hearing aid users with profound sensorineural hearing loss. The purposes of the study were (1) to determine the reliability of the individual tests, their range of difficulty, and their intercorrelations; (2) to undertake a standardization procedure based on data from this population; and (3) to assess the need for revisions. Difficulty ranged gradually from a mean score of 86% correct for the Spondee Same/Different test to a mean of 16% for the NU 6 monosyllabic word test. Alpha estimates of reliability were 0.89 or higher for all but the Everyday Sounds test (0.85), the Question/$tatement test (0.83), and the Spondee Same/Different test (0.81). Along with the reliability and range of difficulty results, intertest correlations provided no indication that any of the tests should be discarded. In the standardization of the MAC, the mean score correct on each test was set at 100 and the standard deviation at 10. Prepared tables for immediate conversion of a raw score to a standardized score for any test are included in an Appendix. Among revisions, also listed in the Appendix, the CID Everyday Sentences test was reduced to 20 items from 40, the SPIN High-Context Sentences were assigned key words, and the assessment of distinctive feature identification was discontinued. BACKGROUND