Signal detection in amplitude-modulated maskers. I. Behavioural auditory thresholds in a songbird

Vertebrates have evolved mechanisms to exploit amplitude modulations in bac kground noise for improving signal detection. However, the mechanisms under lying this masking release are not yet well understood. Here we present evi dence for masking release observed in European starlings (Sturnus vulgaris, Aves) that were trained in a Go/NoGo paradigm to report the detection of a short tone (20 ms) in 100% sinusoidally amplitude-modulated noise maskers (400 ms duration). Maskers centred at the tone frequency were composed of o ne, three, or five spectrally adjacent noise bands each of auditory filter bandwidth. Envelopes of the masking noise bands were either in-phase (i.e. coherent) or successively phase shifted by 90 degrees (i.e. incoherent). A release from masking of up to 28 dB was observed for detection of signals p resented in dips of the envelope of coherent maskers compared with those pr esented in peaks of coherent maskers and in incoherent maskers. For maskers limited to one auditory filter (i.e. limited to the analysis channel tuned to the test signal) this masking release was about 10 dB less than that ob served for maskers allowing a comparison across three or five auditory filt ers. This indicates that both within-channel cues and across-channel cues a re important for signal detection. These behavioural data provide the refer ence for the study of responses of auditory forebrain neurons in the same s pecies reported in a companion paper [Nieder & Klump (2001) Eur. J. Neurosc i., 13, 1033-1044].