Auditory attention is divisible: segregated tone streams can be tracked simultaneously.

Can auditory attention be split? We addressed this question using rapid sequences of tones alternating in frequency between 2 remote registers. In these rapid sequences, consecutive tones could not be perceptually linked; the tones were instead inevitably segregated into 2 concurrent melodic streams. Listeners had to determine if the 2 melodies interleaved in a sequence were exact transpositions of each other or not. This task could be performed successfully. More crucially, performance was better when each component tone of 1 melody was immediately transposed in the other melody than when component i of 1 melody was a transposition of component i-1 of the other melody. Nevertheless, because the melodies were segregated, listeners were unable to determine which was the leading melody when 2 interleaved melodies were immediate transpositions of each other. Our results are inconsistent with the hypothesis that listeners compared concurrent melodic streams using a memory-based serial-processing strategy. It instead appears that listeners were able to track such streams in parallel. Therefore, attention can be split between concurrent sensory streams even when the physical entities making up these streams do not overlap in time.

A late-emerging auditory deficit in autism.

OBJECTIVE: Individuals with autism spectrum disorders (ASD) show enhanced perceptual and memory abilities in the domain of pitch, but also perceptual deficits in other auditory domains. The present study investigated their skills with respect to "echoic memory," a form of short-term sensory memory intimately tied to auditory perception, using a developmental perspective. METHOD: We tested 23 high-functioning participants with ASD and 26 typically developing (TD) participants, distributed in two age groups (children vs. young adults; mean ages: ∼11 and ∼21 years). By means of an adaptive psychophysical procedure, we measured the longest period for which periodic (i.e., repeated) noise could be reliably discriminated from nonperiodic (i.e., plain random) noise. On each experimental trial, a single noise sample was presented to the participant, who had to classify this sound as periodic or nonperiodic. RESULTS: The TD adults performed, on average, much better than the other three groups, who performed similarly overall. As a function of practice, the measured thresholds improved for the TD participants, but did not change for the ASD participants. Thresholds were not correlated to performance in a test assessing verbal memory. The variance of the participants' response biases was larger among the ASD participants than among the TD participants. CONCLUSION: The results mainly suggest that echoic memory takes a long time to fully develop in TD humans, and that this development stops prematurely in persons with ASD.

Enhancing a tone by shifting its frequency or intensity.

When a test sound consisting of pure tones with equal intensities is preceded by a precursor sound identical to the test sound except for a reduction in the intensity of one tone, an auditory "enhancement" phenomenon occurs: In the test sound, the tone which was previously softer stands out perceptually. Here, enhancement was investigated using inharmonic sounds made up of five pure tones well resolved in the auditory periphery. It was found that enhancement can be elicited not only by increases in intensity but also by shifts in frequency. In both cases, when the precursor and test sounds are separated by a 500-ms delay, inserting a burst of pink noise during the delay has little effect on enhancement. Presenting the precursor and test sounds to opposite ears rather than to the same ear significantly reduces the enhancement resulting from increases in intensity, but not the enhancement resulting from shifts in frequency. This difference suggests that the mechanisms of enhancement are not identical for the two types of change. For frequency shifts, enhancement may be partly based on the existence of automatic "frequency-shift detectors" [Demany and Ramos, J. Acoust. Soc. Am. 117, 833-841 (2005)].