Across-channel processes in frequency modulation detection.

This study investigated how well listeners combine information about frequency changes imposed on different carrier frequencies. The pattern of frequency change over time was either identical or different across carriers; this is referred to as "coherence." Psychometric functions were measured for the detection of frequency modulation (FM) imposed on two sinusoidal carriers, with frequencies 1100 and 2000 Hz. The modulation of each carrier was equally detectable, as determined in preliminary experiments. A continuous pink noise background was used to mask the outputs of auditory filters tuned between the two carrier frequencies. In experiment 1, the carriers were gated synchronously with l-s steady state duration and 50-ms raised-cosine ramps. One cycle of 5-Hz sinusoidal FM was used, the carrier having unmodulated "fringes" on either side of this. The FM on the two carriers was symmetrically located about the temporal center of the stimulus. The relative timing of the onset of FM (lag) between the two carriers was systematically varied. When the FM overlapped partially or completely in time across carriers, detectability for coherent FM was often better than for incoherent FM, especially for lag = 0, and was also often better than predicted on the assumption that information about the FM on the two carriers was extracted independently and combined optimally. When the FM did not overlap in time across the carriers, the detectability of the combined FM was generally equal to or lower than the value predicted on this assumption. In experiment 2, the long steady-state fringes before and after the modulation were removed, and the modulation always started at the same time for the two carriers. The modulation rate was either 2.5, 5, or 10 Hz. Again, performance for coherent FM was generally better than for incoherent FM. The effect of FM coherence was greater at the lowest modulation rate but did not vary markedly with the number of modulation cycles. The detectability of coherent FM was well above the value predicted on the assumption that information from the two carrier frequencies was processed independently and combined optimally. These results indicate the auditory system has higher sensitivity to FM when the FM is coherent across carriers. Possible models to account for the results are discussed.