Response of Neurons in the Cochlear Nuclei to Variations in Noise Bandwidth and to Tone-Noise Combinations

Abstract

Experiments were performed in which bands of noise were widened (arithmetically) around the best frequencies of single units in the cochlear nuclear complex of the cat. Two types of effect that were noted earlier by Greenwood and Maruyama [J. Neurophysiol. 28, 863–892 (1965) and unpublished observations] were observed more precisely: (1) summation, in which increasing bandwidth from narrow widths (constant spectrum level) produced increases in firing and (2) “suppression,” in which increasing bandwidth beyond the range in which summation occurred produced systematic reductions in firing. In monotonic units, in which summation was more readily observed, increasing bandwidth was approximately equivalent to increasing the spectrum level of a narrow band of constant width; the subsequent suppression at larger bandwidths ranged from slight to very marked. The bandwidths at which summation ceased and suppression began decreased somewhat at higher spectrum levels and were similar in width on a log scale for units of differing best frequency but, for several reasons, these “turnover” bandwidths are not readily interpretable. Tone-noise-combination stimuli were also used to study some units, and it was possible to repeat and extend earlier observations by Greenwood and Maruyama to the effect that—depending on intensity—a band of noise centered at one frequency may eliminate, i.e., mask, a unit's response to a tone at a different frequency either (a) by inhibiting that response or (b) activating the unit itself so that the tone has no additional effect. These results are similar to recent observations of primary unit behavior.