Masking of Auditory Responses in the Medulla Oblongata of Goldfish

Abstract

ABSTRACT The nervous activity of single auditory neurones in goldfish brain have been measured. Four types of acoustic stimuli were used, (1) pure tones, (2) noise of one-third octave band width, (3) noise of one-octave band width with centre frequency equal to the pure tone, and (4) white noise. Except for white noise, these stimuli produced the same response to equal sound pressures. The white noise response was less, presumably because the frequency range covered by a single neurone is far narrower than the range of white noise. The conclusion has been reached that for low-frequency acoustic signals, the acoustic power over a frequency band of one to two octaves is integrated by the nervous system. The masking effect of background noise on the acoustic threshold of single units to pure tones is strongest when the noise band has the same centre frequency as the test tone. In this case the tone threshold increases linearly with the background noise level. When the noise band was centred at a different frequency from the tone, the masking effect decreased at a rate of 20–22 dB/octave for the first one-third octave for a tone frequency of 250 Hz. For a tone of 500 Hz the masking effect of lower frequencies was stronger and was reduced by only some 9 dB/octave for the first one-third octave.