Concurrent motion detection based on dynamic changes in interaural delay.

The ability to detect a dynamic change in the interaural delay of a pure tone in the presence of a distracter tone of a different frequency was investigated in four conditions: (1) a control condition in which no distracter tone was present, (2) the distracter tone was stationary (fixed interaural delay), (3) the distracter had an interaural delay that changed in the same direction as that of the target tone, i.e., concurrent auditory motion in the same direction, and (4) the distracter had an interaural delay that changed in a direction opposite to that of the target tone, i.e., concurrent auditory motion in opposite directions. In a cued single-interval two-alternative forced-choice design, the observer had to determine if the target tone had a constant or dynamic interaural delay. The target was a 500-Hz tone and the distracter was a tone with a frequency of 300, 510, 550, 600, 800, or 1000 Hz. Detection was also examined for a range of stimulus durations, rates of change in interaural delay (i.e., velocity), and extent of change in interaural time difference (i.e., 'distance'). Results showed that the best performance (highest d') was associated with the no-distracter condition, followed by the stationary-distracter, opposite-direction, and same-direction conditions, respectively. Detection improved with increasing frequency difference between distracter and target tones, but was nonetheless lower than that associated with the no-distracter condition, even when the distracter frequency was several critical bands removed from the target frequency.