We have recently discovered a paradoxical aftereffect associated with
inhibition in the gerbil auditory midbrain. Single neurons in the infe
rior colliculus (IC) were assessed for sensitivity to a virtual motion
stimulus produced by modulating the interaural level difference (ILD)
, a major cue for sound localization. The class of neuron studied was
predominantly excited by contralateral stimulation and inhibited by ip
silateral stimulation. Sound pressure level was modulated trapezoidall
y at the ipsilateral ''inhibitory'' ear, whereas the contralateral ''e
xcitatory'' level remained constant. When the inhibitory stimulus was
decreased within a range of sound levels that maintained suppression u
nder static conditions, an unexpected discharge was often elicited, ap
parently because of an aftereffect of synaptic inhibition. In contrast
, when the inhibitory stimulus was increased within a range of sound l
evels that produced only modest suppression under static conditions, n
euronal discharge was often profoundly suppressed. In many cases the '
'conditioned enhancement'' or ''conditioned suppression'' persisted fo
r several seconds after the modulation of ILD, and such conditioned re
sponses were influenced by the modulation depth and rate. To test the
effect of inhibition in the IC directly, glycine and GABA were pulsed
from a glass recording pipette during a constant monaural excitatory s
timulus. The acoustically elicited discharge rate was potentiated mark
edly if preceded immediately by the brief (0.5-10 sec) application of
inhibitory transmitter. Collectively, these results revealed unusually
long-lasting effects of inhibition that may establish a new range of
acoustic cues to which the neuron responds best. This may have broad i
mplications for processing ensuing auditory stimuli.