Interaction of Streaming and Attention in Human Auditory Cortex
March
Interaction of Streaming and Attention in Human Auditory Cortex
Alexander Gutschalk 0 1
Andr Rupp 0 1
Andrew R. Dykstra 0 1
0 Department of Neurology, Ruprecht-Karls-Universitat Heidelberg , Heidelberg , Germany
1 Academic Editor: Christian Friedrich Altmann, Kyoto University , JAPAN
Serially presented tones are sometimes segregated into two perceptually distinct streams. An ongoing debate is whether this basic streaming phenomenon reflects automatic processes or requires attention focused to the stimuli. Here, we examined the influence of focused attention on streaming-related activity in human auditory cortex using magnetoencephalography (MEG). Listeners were presented with a dichotic paradigm in which left-ear stimuli consisted of canonical streaming stimuli (ABA_ or ABAA) and right-ear stimuli consisted of a classical oddball paradigm. In phase one, listeners were instructed to attend the right-ear oddball sequence and detect rare deviants. In phase two, they were instructed to attend the left ear streaming stimulus and report whether they heard one or two streams. The frequency difference (F) of the sequences was set such that the smallest and largest F conditions generally induced one- and two-stream percepts, respectively. Two intermediate F conditions were chosen to elicit bistable percepts (i.e., either one or two streams). Attention enhanced the peak-to-peak amplitude of the P1-N1 complex, but only for ambiguous F conditions, consistent with the notion that automatic mechanisms for streaming tightly interact with attention and that the latter is of particular importance for ambiguous sound sequences.
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Competing Interests: The authors have declared
that no competing interests exist.
A major challenge for our central auditory system is to segregate simultaneous streams of
auditory information that we receive from two or more sound sources. Sequence parameters which
determine whether sequentially presented tones are grouped into the same stream have been
explored with the so-called streaming paradigm (aka stream segregation) [1]. This paradigm
typically uses sequential tone patterns of two or more different tones (e.g., A and B). A number
of physical parameters influence whether a sequence is perceived as one integrated stream of
alternating tones or as two segregated monotone sequences (e.g. inter-tone interval [2],
frequency separation, or F [3,4], and several others [5]).
An ongoing debate in the field is the extent to which focused attention influences the
streaming process. Bregman [1] suggested that primitive streaming cuessuch as Fcould
promote streaming at an early processing stage without requiring attention or other top-down
mechanisms. This hypothesis is supported by EEG studies which found that the occurrence of
the mismatch negativity (MMN), a component of the auditory evoked response elicited by a
change in an otherwise regular stimulus sequence [6], depends on the organization of auditory
streams [7,8]. Moreover, the transient waves P1m and N1m evoked by each tone of a sequence
increase in amplitude with frequency separation [9,10], probably as a consequence of selective
adaptation [11]. This F dependent modulation of the P1m and N1m covaries with listeners
rating of streaming perception. All of these evoked response componentsthe P1m, N1m, and
MMNcan be recorded while listeners are not engaged in the auditory stimulation and even
when attention is focused on another task.
However, Carlyon et al. [12] challenged Bregmans view and argued that streaming requires
focused attention towards the source to be segregated. These authors used dichotic stimuli and
instructed listeners to first listen to their right ear and perform an unrelated distractor task
while an ABA_ streaming sequence played in the left ear. Listeners were further instructed to
switch their attention to the left ear after the cessation of the distracting task and report their
streaming percept of the ABA_ triplets. The results showed that streaming was not stable at the
time of the attention switch, but rather required a new build-up period [13,14]. When
listeners rated streaming from the beginning of the sequence, in contrast, streaming was at a
constantly high level after identical time delays. Moreover, patients with neglect after
righthemisphere brain lesions, who have an attentional deficit for left-sided stimuli, showed lower
streaming rates for the left-sided sequences [12]. It is well known that focusing attention to one
of two sound streams enhances the N1 evoked by each tone of that stream, but typically the
stimuli in these experiments were chosen such that the two streams could be readily segregated
[15,16,17,18].
The influence of intentional listening on streaming has been known since at least the mid
70s: van Noorden [4] showed that the streaming threshold depends on listeners' attentional
set within a range of F and repetition rates. At the lower (upper) F border lies the fission
(temporal coh (...truncated)