Evoked response 'forward masking' functions in chinchillas

Evoked potentials were obtained from the inferior colliculus of awake chinchillas using a 'forward masking' stimulus paradigm. The response was elicited using a brief probe tone presented 10 dB above threshold. Forward masking functions were then measured by determining the intensity of the masker needed to produce a 50% reduction in evoked response amplitude over a range of time intervals between masker offset and probe onset. The time course of forward masking was evaluated by fitting an exponential function to the results. The average time constant values of forward masking functions ranged from 50 to 90 ms. Although the constants at 0.5 and 8.0 kHz were somewhat longer than those at the midfrequencies, the differences were not statistically significant. The evoked response forward masking functions were quite similar to those obtained psychophysically; this suggests that it may be an efficient method of estimating the time course of forward masking in difficult to test subjects.