Perceiving and counting sounds.

Observers counted sequences of 20-msec tones that were presented to the same spatial location or alternated between spatial locations. Counting accuracy increased with increases in the silent interval between tones and increased at a faster rate when the tones were presented to the same location. In the second study, subjects monitored the same sequences of tones for a probe tone that was either higher or lower than the other tones. Probe recognition improved with increases in the silent interval between tones, and there was no significant decrement in monitoring tones alternated between spatial locations. In the next two studies, observers processed a sequence of tones presented at the same spatial location, but the tones could alternate in frequency by slightly more than an octave. Counting accuracy was poorer when the tones alternated in frequency. When subjects monitored these sequences of tones for the duration of a probe tone, recognition leveled off at a lower asymptote for sequences of tones alternating in frequency. The deficit in counting sounds alternating between spatial locations or frequencies appears to be due to a process that has difficulty integrating successive tones that are perceived at different spatial locations or pitch levels.