Across-species comparisons of psychophysical detection thresholds for electrical stimulation of the cochlea: I....

Several species have been, and continue to be, used as subjects in studies of electrical stimulation of the cochlea. Few attempts, however, have been made to determine if data obtained from different species are quantitatively or qualitatively similar. The present work compares psychophysical absolute detection threshold vs. frequency functions for sinusoidal stimuli obtained from humans, nonhuman primates, cats, and guinea pigs. Threshold data for monopolar and bipolar electrode configurations from both previously published and unpublished studies are compared. In general, within all four species, significant intersubject variation in detection threshold level was found, but slopes of threshold vs. frequency functions were relatively well conserved within a species, under the conditions studied. With one exception (cat bipolar stimulation), threshold functions reached a minimum at or near 100 Hz across species and electrode configurations. In all cases, thresholds were significantly lower for monopolar, as compared with bipolar, configurations. Statistically, there were no significant differences in absolute threshold level across species. Threshold levels increased with frequency above 100 Hz at a rate of 3.0-7.9 dB/octave, depending on both electrode configuration and species. Slopes were steeper for monopolar than for bipolar configurations. When slopes were averaged between 200 and 2000 Hz, no statistically significant differences in overall slopes were found, nor was there a significant interaction between electrode configuration and species. There were, however, consistent species differences within more restricted regions of the function. Human functions for both monopolar and bipolar stimulation were steeper than all animal functions in the range of 100-300 Hz. Within this range, the differences between slopes for human and nonhuman subjects were statistically significant. In addition, differences were noted in the frequency at which slope decreased, with slopes for nonhuman subjects showing the decrease at higher frequencies than did those for human subjects. These differences may be true species differences, or may reflect the influence of confounding variables associated with each experimental-subject model.