Signal detection in amplitude-modulated maskers. II. Processing in the songbird's auditory forebrain

In the natural environment, acoustic signals have to be detected in ubiquit ous background noise. Temporal fluctuations of background noise can be expl oited by the auditory system to enhance signal detection, especially if spe ctral masking components are coherently amplitude modulated across several auditory channels (a phenomenon called 'comodulation masking release'). In this study of neuronal mechanisms of masking release in the primary auditor y forebrain (field L) of awake European starlings (Sturnus vulgaris), we de termined and compared neural detection thresholds for 20-ms probe tones pre sented in a background of sinusoidally amplitude modulated (10-Hz) noise ma skers. Responses of a total of 34 multiunit clusters were recorded via radi otelemetry with chronically implanted microelectrodes from unrestrained bir ds. For maskers consisting of a single noise band centred around the record ing site's characteristic frequency, a substantial reduction in detection t hreshold (21 dB on average) was found when probe tones were presented durin g envelope dips rather than during envelope peaks. Such effects could also explain results obtained for masking protocols where the on-frequency noise band was presented together with excitatory or inhibitory flanking bands t hat were either coherently modulated (in-phase) or incoherently modulated ( phase-shifted). Generally, masking release for probe tones in maskers with flanking bands extending beyond the frequency range of a cell cluster's exc itatory tuning curve was not substantially improved. Only some of the neuro physiological results are in agreement with behavioural data from the same species if only the average population response is considered. A subsample of individual neurons, however, could account for behavioural thresholds.