Attention To Tasks A Performing Multiple Tasks Simultaneously

In addition to coordinating the processing of sensory stimuli, attention must participate in the coordination of tasks. There are many situations in which humans must perform multiple tasks concurrently, as such having a conversation while driving. Performance is impaired when multiple tasks are performed concurrently, even when the tasks are highly practiced, as with driving and speaking. Attentional processes may be involved in selecting an individual task for current behavior, with a cost in performance occurring when attention is either divided or switched between multiple tasks.

The biased competition model we used to describe spatial attention can also be applied to performing multiple tasks such as the Stroop task, in which different tasks must be performed in different blocks of trials. In the Stroop task, observers view words that name colors (e.g., "red" or "blue") presented in different colors of ink. The words can be written in a compatible ink color ("red" written in red ink) or in incompatible colors ("red" written in blue ink). Observers perform one of two tasks—either reading the word or naming the ink color. Because word reading is more practiced than ink color naming, observers can read words with little effect of the ink color; observers can just as quickly read color words printed in a compatible ink color as color words printed in an incompatible ink color. In contrast, color naming is highly influenced by the word; observers are slower to name ink colors used to print an incompatible word ("red" printed in blue ink) than ink colors used to print a compatible word ("red" printed in red ink). In the Stroop task, the top-down task demands (i.e., the task observers are asked to perform) and the bottom-up stimulus features (the word and ink color) both guide behavior. The color naming task is difficult because the bottom-up inputs are stronger for words than for ink colors. Thus, there is a stronger bias for word reading than for color naming, allowing the word reading task to compete more effectively for the control of behavior.

One difficulty in using the Stroop task to study attention to tasks is that one of the tasks, word reading, is much easier than the other. An attentional phenomenon that demonstrates competition between two equally difficult tasks is the ''attentional blink.'' In the attentional blink task, observers view a stream of approximately 20 stimuli presented one at a time at a rate of about 10 stimuli per second; observers are asked to detect two targets from this stream. For example, the first target (T1) may be a number that observers must classify as even or odd, and the second target (T2) could be a letter that observers must classify as a consonant or vowel. Observers make both responses at the end of the stimulus stream. Observers often fail to identify T2 if it appears shortly after T1; if T2 appears somewhat later, observers more accurately report its identity. The temporary impairment in identifying T2 is referred to as the attentional blink because it is similar to the consequence of a T1-triggered eyeblink (i.e., a brief period during which subsequent targets cannot be detected).

The attentional blink does not appear to be caused by sensory-level interference between the two targets. Instead, the failure in reporting T2 is more central, resulting from a failure to store T2 in a durable form that can be reported at the end of the stimulus stream. In terms of a biased competition account, top-down task constraints related to the T1 task may bias this item to be coded into working memory. After T1 begins to be encoded into working memory, the task bias can begin to switch to the T2 task, but this reconfiguration takes time. Thus, if T2 appears soon after T1, it may not be efficiently encoded into working memory and may be overwritten by the next item in the input sequence. At the end of the stimulus sequence, reporting T2 is difficult because of this shallow encoding into working memory—T2 is identified but not reported, suggesting that selection may operate relatively late in the visual processing stream (i.e., after stimulus identification).

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