Abstract:
This study examined whether the detection of frequency modulation (FM) on two carriers depends on the coherence of the FM across carriers. Psychometric functions were measured for detecting sinusoidal FM of carriers with frequencies 1100 and 2000 Hz. The modulators for the two carriers were either in phase (coherent) or in anti-phase (incoherent). The modulation rate was either 2.5, 5, or 10 Hz. One or more cycles of modulation were used. The modulation of each carrier was equally detectable, as determined in a preliminary experiment. A continuous pink noise background was used to mask the outputs of auditory filters tuned between the two carrier frequencies. Detectability was better for coherent FM than for incoherent FM. The effect of FM coherence was greatest at the lowest modulation rate, possibly indicating that phase locking plays a role [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 97, 2468--2478 (1995)]. The detectability of the 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 imply the existence of one or more mechanisms sensitive to FM coherence. [Work supported by the MRC(UK), the British Council, and an ORS award.]