One hypothesis is that preparation for an input requires some mental resources, and these resources are in short supply. Thus to perceive a messages one must commit the necessary resources. Once this is done there might not be enough resources left over for the perception of a second message. As a different way of stating this, we could argue that the mind has a limited capacity for handling inputs or, for that matter, for dealing with any tasks.
Perhaps the situations we have described (e g., listening to two simultaneous messages, or reading and listening at the same time) exceed this capacity. Of course, this does little more than rename the problem. Instead of talking about the observable limits on attentions we are now talking about hypothesized limits on some unspecified mental resources. What are these “resources?” Why is there a “limited capacity?” And, above all can we get some direct evidence for these (allegedly) limited resources? In a classic series of studies, Posner and Snyder (1975) gave subjects this simple task: A pair of letters was shown on a computer screen, and subjects had to decide, as swiftly as they could, whether the letters were the same or different. So a subject might see “A A” and answer “same,” or might see “A B” and answer “different.”
Before each pair, subjects saw a warning signal. In the neutral condition, the warning signal was a plus sign (“+”). This notified subjects that the stimuli were about to arrive but provided no other information. In a different condition, the warning signal was itself a letter. and actually matched the stimuli-to-come.
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