A comparison of Broadbent's theory (top); Treisman's theory (middle); and Deutsch and Deutsch's theory (bottom).

to analyses based on individual words, grammatical structure, and meaning. If there is insufficient processing capacity to permit full stimulus analysis, then tests towards the top of the hierarchy are omitted.

Another important aspect of the theory proposed by Treisman (1964) was that the thresholds of all stimuli (e.g., words) consistent with current expectations are lowered. As a result, partially processed stimuli on the unattended channel sometimes exceed the threshold of conscious awareness. This aspect of the theory helps to account for the phenomenon of breakthrough.

Treisman's theory accounted for the extensive processing of unattended sources of information that had proved embarrassing for Broadbent. However, the same facts were also explained by Deutsch and Deutsch (1963). They argued that all stimuli are fully analysed, with the most important or relevant stimulus determining the response (see Figure 5.2). This theory places the bottleneck in processing much nearer the response end of the processing system than did Treisman's attenuation theory. As a result, the theory proposed by Deutsch and Deutsch (1963) is often called late-selection theory, whereas the theories of Broadbent (1958) and Treisman (1964) are termed early-selection theories.

Treisman and Geffen (1967) had participants shadow one of two auditory messages, and tap when they detected a target word in either message. According to Treisman's theory, there should be attenuated analysis of the non-shadowed message, and so fewer targets should be detected on that message. According to Deutsch and Deutsch, there is complete perceptual analysis of all stimuli, and so there should be no difference in detection rates between the two messages. In fact, detection rates were much higher on the shadowed than the non-shadowed message (87% vs. 8%, respectively).

Effects of attention condition (divided vs. focused) and of type of non-target on target detection. Data from Johnston and Wilson (1980).

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