Journal of Neurophysiology, Vol 41, Issue 6 1535-1545, Copyright © 1978 by APS
Functional characteristics of cochlear nucleus in behaving cat examined by acoustic masking of electrical stimuli
C. J. Frederickson and G. M. Gerken
1. Cats were trained, using an operant procedure, to detect and respond to
electrical stimulation delivered in the vicinity of the cochlear nucleus.
The electrical stimuli were presented both in silence and in synchrony with
repeated noise bursts to determine whether detection thresholds for the
electrical stimuli were elevated by the acoustic masking noise. 2. For
stimulation sites centered within auditory structures (cochlear nucleus or
acoustic nerve root), the acoustic maskers caused a consistent elevation of
the electrical detection thresholds. For stimulation sites that were in or
bordered on nonacoustic neural structures (e.g., vestibular), the acoustic
maskers caused little or no elevation of electrical detection thresholds.
3. The magnitude of the acoustic masking effect was monotonically related
to the intensity of the acoustic masker across the range of intensities
tested. 4. The magnitude of the masking effect was strongly dependent on
the relative timing of the stimulus pulse and the masker noise burst.
Maximum masking occurred when the pulse just followed the onset of the
neural activity in cochlear nucleus evoked by the masker burst. Less
masking occurred when the electrical pulse occurred at the middle or end of
the masker burst, and still less when the pulse occurred just prior to the
onset (backward masking) or just after the offset (forward masking) of the
masker burst. 5. The magnitude of the masking effect also depended on the
frequency of the acoustic masker. For tone bursts, masking was maximal for
each electrode at a particular frequency and declined monotonically for
masker frequencies above or below the optimal frequency. 6. It is concluded
that the masking of an electrical stimulus by an acoustic stimulus depends
on a direct interaction between the neural responses evoked by the two
stimuli, and that similar central, neural interactions may contribute to
acoustic masking of acoustic stimuli. It is also concluded that the
technique of masking an electrical stimulus by an acoustical stimulus is a
precise and useful tool for the study of sensory-neural organization in
intact behaving animals.
