The probe-signal method and auditory-filter shape: results from normal- and hearing-impaired subjects

In the probe-signal method, subjects are required to detect a signal in noise that is presented on the majority of trials at an "expected" (target) frequency but on a minority of trials at an "unexpected" probe frequency. Detection of the probes worsens with increasing separation between the target and probe frequencies. This result has often been interpreted as indicating that subjects monitor the output of a single auditory filter centered at the target frequency. To test this idea, a two-stage experiment was conducted. In the first stage, auditory-filter shapes were estimated using the notched-noise method at center frequencies of 1000, 1259, 1585, and 2000 Hz. These were the frequencies that were used for the targets in the second stage of the experiment. In the second stage, low-pass filtered white noise was presented continuously. On each trial, a cue tone was presented at one of the four possible target frequencies. The specific frequency was selected randomly on each trial. This was followed by two observation intervals during one of which a further sinusoidal tone was presented. This tone was either a target (the same as the cue frequency) (on 60% of trials), or had one of four possible probe frequencies corresponding to that target. The four probe frequencies were chosen to correspond to specific points on the estimated response curve of the auditory filter centered at the target frequency. The percentage of correct detections of a given probe was compared with that obtained in a separate condition where the frequency of the tone was fixed throughout, the cue frequency always equaled the target frequency, and the target was attenuated by an amount corresponding to the attenuation of the auditory filter at the probe frequency. Two subjects with normal hearing and two subjects with unilateral cochlear hearing loss were used. Comparison of the results for the normal and impaired ears suggests that the detectability of the probes is governed more by the selectivity of the auditory filters than by the ratios of the expected and probe frequencies. However, detection of the probes was generally better than would occur if subjects monitored the output of a single auditory filter centered at the target frequency.