Time course of simultaneous masking in the starling's auditory forebrain

Simultaneous masking of pure tones was studied in the primary auditory fore brain of a songbird species, the European starling (Sturnus vulgaris). The responses of 32 multi-unit clusters in the input layer of the auditory neos triatum (field L2a) were recorded via radiotelemetry from freely moving bir ds. The probe was a 10-ms tone burst at the units' characteristic-frequency (CF) presented 20 dB above the threshold. The masker was an 80-ms tone bur st presented either at the units' CF (excitatory masker) or at a frequency located in inhibitory side-bands (inhibitory masker) of the units' tuning c urves. The probe was presented either 3 ms or 63 ms after masker onset. Pro bes presented at a 3-ms delay were influenced at significantly lower levels of an excitatory masker than probes presented at a 63-ms delay. The mean d ifference in masker level at the detection thresholds for both probe delays was 8 dB. No difference in masker level was observed for inhibitory-freque ncy maskers. The observed neural masking effects may be explained by at lea st four mechanisms: (1) swamping of the probe response by the response to t he masker, (2) a reduction of the probe response during neural adaptation o f the response to the masker, (3) a reduction of the probe response during side-band inhibition in the central nervous system, and (4) suppression ori ginating in the cochlea.