Attention in Streaming

Elyse Sussman

“Attending to the sound initiates the buildup of streaming” - the first statement under the heading of Attention in the Position Statement - presupposes that streaming never occurs without the active process of selecting out a subset of sounds from all those available. A position I would disagree with. Recent data from my lab support the stance that attention is not required to initiate the buildup process. These data, I am quick to point out, do not preclude a role for attention in the buildup process or in the streaming process. It is not an either/or proposition. This leads to the next point…

Is attention needed to maintain the organization that is selected out? There is evidence that if attention is focused on a subset of sounds switching attention away from the sounds can reset the stream segregation process - switching back to the sounds starts the buildup process again. The logical conclusion from this is that attention is needed to maintain the stream segregation process or it is lost. However, consider a different scenario, in which the sounds are not focused on; attention is focused on a task involving the visual modality. Neurophysiological evidence evoked by the unattended sounds in this situation indicates that the sounds are organized to two streams when the subject focuses on the visual task. Now ask the same question. Is attention needed to maintain the streaming organization of the sounds that are outside the focus of attention? Neurophysiological evidence suggests it isn’t.

However, it has been suggested that the reason that the two stream organization is represented outside the focus of attention (as evidenced by MMN, e.g.) is because covert attention at the beginning of the sound stream initiates the segregation of the to-be-ignored sounds. This diverted attention to the start of the sound sequence could initiate the buildup so that the sounds would stream. Attention would then not be needed to maintain the streaming organization while subjects perform the visual task -- i.e., attention is not needed to maintain the two stream organization only to initiate the buildup process (and hence MMN is elicited).

These two situations seem to be at odds with each other. If attention is needed to select out a subset of the streams and this selection process is what maintains the organization then attention to initiate the streaming process should not be enough to maintain the two stream organization if attention is then not used to continue selection of the sounds. That is, if attention is focused elsewhere, evidence for two streams organization (i.e., MMN) should not be elicited.

One possible explanation is that attention is needed to maintain a streaming organization only when the input is ambiguous -- according to stimulus-driven factors such as the 3 conditions described as being perceptually distinguishable, sharing a consistent feature, or being temporally distinct. If the stimuli are unambiguous then attention-switching may have less effect on the organization.