Academic journal article Perception and Psychophysics

Some Characteristics of Auditory Spatial Attention Revealed Using Rhythmic Masking Release

Academic journal article Perception and Psychophysics

Some Characteristics of Auditory Spatial Attention Revealed Using Rhythmic Masking Release

Article excerpt

The tuning of auditory spatial attention with respect to interaural level and time difference cues (ILDs and ITDs) was explored using a rhythmic masking release (RMR) procedure. Listeners heard tone sequences defining one of two simple target rhythms, interleaved with arhythmic masking tones, presented over headphones. There were two conditions, which differed only in the ILD of the tones defining the target rhythm: For one condition, ILD was 0 dB and the perceived lateral position was central, and for the other, ILD was 4 dB and the perceived lateral position was to the right; target tone ITD was always zero. For the masking tones, ILD was fixed at 0 dB and ITDs were varied, giving rise to a range of lateral positions determined by ITD. The listeners' task was to attend to and identify the target rhythm. The data showed that target rhythm identification accuracy was low, indicating that maskers were effective, when target and masker shared spatial position, but not when they shared only ITD. A clear implication is that at least within the constraints of the RMR paradigm, overall spatial position, and not ITD, is the substrate for auditory spatial attention.

Over a century ago, William James (1890) suggested that focusing attention might be accomplished by muscular changes in the appropriate sense organ. Such early selection at the receptor would lift from the central nervous system much of the burden of extracting target information from competing stimuli. This process is a familiar aspect of overt orienting with vision, in which eye movements control which objects in a visual scene fall on the fovea. In certain animals with prominent and highly directional pinnae, head and pinna movements may perform a similar function for spatial focusing of audition. In humans, this effect is limited, and since the cochlea lacks an equivalent to the fovea, it is less obvious how attentional selection of auditory spatial location might be implemented early in the peripheral auditory system.

Information about the azimuthal position of a sound source is encoded by the auditory system in two primary ways: ( 1 ) for any source displaced from the sagittal plane, the sound reaches the closer ear earlier than it does the farther one, giving rise to an interaural time difference (ITD) cue. (2) Particularly for high frequencies, the acoustic shadow cast by the head results in the sound being more intense at the closer ear, yielding an interaural level difference (ILD).1 This article is concerned with the tuning of auditory spatial attention with respect to these cues and with whether directing auditory attention to some spatial position defined only by an ILD is equivalent to directing attention to the same position defined by an ITD. This question is closely related to whether auditory spatial attention operates relatively early or late in the auditory pathway, as discussed below.

A potential substrate for early attentional selection by spatial location was described in a visionary paper by Jeffress (1948). He proposed a neural mechanism in the auditory pathway for the detection of ITDs, whereby an array of central units respond to coincidences in the signals arriving from the two ears via fibers of different lengths, which function as delay lines, so that each "coincidence detector" is tuned to a particular ITD. Jeffress's proposal, which has since found physiological support (Goldberg & Brown, 1969; Yin & Chan, 1990), raises the possibility that listeners could selectively monitor those coincidence detectors associated with a particular target azimuth and thus provides a potential mechanism for spatial attention involving low-level selection by ITD. Elaborations of such a scheme have been implemented in computational approaches to auditory scene analysis (e.g., Bodden, 1996).

Figure 1 a is a schematic representation of the auditory system in which ITD and ILD cues are processed separately before the two are combined to yield an integrated spatial percept. …

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