Within- and across-channel processing in auditory masking: a physiological study in the songbird forebrain

Synchronous envelope fluctuations in different frequency ranges of an acoustic background enhance the detection of signals in background noise. This effect, termed comodulation masking release (CMR), is attributed to both processing within one frequency channel of the auditory system and comparisons across separate frequency channels. Here we present data on CMR from a study in field L2 of the auditory forebrain of the European starling (Sturnus vulgaris) using two 25-Hz-wide bands of masking noise that provide the opportunity to distinguish between within-channel and across-channel effects. Acoustically evoked responses were recorded from unrestrained birds via radio telemetry. The signal was a 800 msec pure tone presented at the most sensitive frequency of the units in a previously determined frequency-tuning curve (FTC). One band of masking noise was centered on the signal frequency while the flanking band of noise was presented either within the limits of the excitatory FTC (i.e., within the same frequency channel as the on-frequency masker) or in the suppression area of the FTC (i.e., in a separate channel). For flanking bands inside the excitatory FTC, signal detection thresholds based on the rate code were lower in noise maskers with identical envelope fluctuations (comodulated) than in maskers with uncorrelated envelopes resulting in a neural CMR of approximately 4-7 dB. For flanking bands inside the suppression areas, the neural CMR was reduced. Although the average neural CMR was below the behaviorally determined CMR, a subsample of between 11 and 26% of the recording sites resembled the behavioral performance.