Efficient across-frequency integration: evidence from psychometric functions

Across-frequency integration of complex signals was investigated by measuring psychometric functions [log (d') versus signal level in dB SPL] for detection of brief and long signals presented in broadband noise. The signals were tones at 630, 1600, and 4000 Hz, and a nine-tone complex with components spaced at one-third-octave frequencies between 630 and 4000 Hz. The phase relationship of the components in the complex was varied such that adjacent components were in phase (at 0 degrees), 90, or 180 degrees out of phase. Signal durations (defined in terms of the number of cycles between the half-amplitude points of the Gaussian envelopes) of 4.7 and 150 cycles were tested. Results for six normal-hearing listeners showed that the slopes of the psychometric functions were steeper for the brief than for the long signals, and steeper for the tone complexes than for the tones, particularly for the brief signals. This suggests that the transformation from signal intensity to decision variable may be different for brief complex signals than for tonal signals and long complex signals. Thresholds obtained from the psychometric functions were in excellent agreement with those obtained with an adaptive procedure that employed three interleaved tracks. For the long signals, the threshold improvement for the tone complexes relative to a single tone was well described by a 5* log (n) integration rule. However, the threshold improvement for brief signals obeyed a more efficient integration rule of 7 to 8* log (n). A portion of this effect could be accounted for by the phase relationship of the tone complexes; thresholds for brief signals were lowest when the components were in phase at the envelope peak of the signal. This finding indicates that temporal synchrony across auditory channels may enhance detection of brief multi-tone complexes.