The British scientist Colin Cherry was working in an electronics research laboratory at the Massachusetts Institute of Technology, but became involved in psychological research. What fascinated Cherry was the "cocktail party" problem: how are we able to follow just one conversation when several people are all talking at once? Cherry (1953) found that this ability involves using physical differences (e.g., sex of speaker; voice intensity; speaker location) to maintain attention to a chosen auditory message. When Cherry presented two messages in the same voice to both ears at once (thereby eliminating these physical differences), listeners found it very hard to separate out the two messages on the basis of meaning alone.
Cherry also carried out studies in which one auditory message had to be shadowed (i.e., repeated back out loud) while a second auditory message was played to the other ear. Very little information seemed to be extracted from the second or non-attended message. Listeners seldom noticed when that message was spoken in a foreign language or in reversed speech. In contrast, physical changes (e.g., a pure tone) were nearly always detected. The conclusion that unattended auditory information receives practically no processing was supported by other evidence. For example, there was very little memory for unattended words even when they were presented 35 times each (Moray, 1959).
Broadbent (1958) felt the findings from the shadowing task were important. He was also impressed by data from a memory task in which three pairs of digits were presented dichotically, i.e., three digits were heard one after the other by one ear, at the same time as three different digits were presented to the other ear. Most participants chose to recall the digits ear by ear rather than pair by pair. Thus, if 496 were presented to one ear and 852 to the other ear, recall would be 496852 rather than 489562. Broadbent (1958) accounted for the various findings as follows (see Figure 5.2):
• Two stimuli or messages presented at the same time gain access in parallel (at the same time) to a sensory buffer.
• One of the inputs is then allowed through a filter on the basis of its physical characteristics, with the other input remaining in the buffer for later processing.
• This filter prevents overloading of the limited-capacity mechanism beyond the filter; this mechanism processes the input thoroughly (e.g., in terms of its meaning).
This theory handles Cherry's basic findings, with unattended messages being rejected by the filter and thus receiving minimal processing. It also accounts for performance on Broadbent's dichotic task, because the filter selects one input on the basis of the most prominent physical characteristic distinguishing the two inputs
(i.e., the ear of arrival). However, it is assumed incorrectly that the unattended message is always rejected at an early stage of processing. The original shadowing experiments used participants with very little experience of shadowing messages, so nearly all their available processing resources had to be allocated to shadowing. Underwood (1974) asked participants to detect digits presented on either the shadowed or the non-shadowed message. Naive participants detected only 8% of the digits on the non-shadowed message, but an experienced researcher in the area (Neville Moray) detected 67% of them.
In most of the early work on the shadowing task, the two messages were rather similar (i.e., they were both auditorily presented verbal messages). Allport, Antonis, and Reynolds (1972) found the degree of similarity between the two messages had a major impact on memory for the non-shadowed message. When shadowing of auditorily presented passages was combined with auditory presentation of words, memory for the words was very poor. However, when shadowing was combined with picture presentation, memory for the pictures was very good (90% correct). If two inputs are dissimilar, they can both be processed more fully than was allowed for on Broadbent's filter theory.
In the early studies, it was concluded that there was no processing of the meaning of unattended messages because the participants had no conscious awareness of their meaning. However, meaning may be processed without awareness. Von Wright, Anderson, and Stenman (1975) presented two lists of words auditorily, with instructions to shadow one list and ignore the other. When a word that had previously been associated with electric shock was presented on the non-attended list, there was sometimes a physiological reaction (galvanic skin response). The same effect was produced by presenting a word very similar in sound or meaning to the shocked word. Thus, information on the unattended message was sometimes processed for sound and meaning, even though the participants were not consciously aware that a word related to the previously shocked word had been presented.
Broadbent's (1958) proposed an inflexible system of selective attention that cannot account for the great variability in the amount of analysis of the non-shadowed message. The same inflexibility of the filter theory is shown in its assumption that the filter selects information on the basis of physical features. This assumption is supported by the tendency of participants to recall dichotically presented digits ear by ear. However, Gray and Wedderburn (1960) made use of a version of the dichotic task in which "Who 6 there" might be presented to one ear as "4 goes 1" was presented to the other ear. The preferred order of report was determined by meaning (e.g., "who goes there" followed by "4 6 1"). The fact that selection can be based on the meaning of presented information is inconsistent with filter theory.
Treisman (1960) found with the shadowing task that the participants sometimes said a word that had been presented on the unattended channel. This is known as "breakthrough", and typically occurs when the word on the unattended channel is highly probable in the context of the message on the attended channel. Even in those circumstances, however, Treisman (1960) only observed breakthrough on 6% of trials.
Findings such as those of Treisman (1960) led Treisman (1964) to propose a theory in which the filter reduces or attenuates the analysis of unattended information (see Figure 5.2). Whereas Broadbent had suggested that there was a bottle-neck early in processing, Treisman claimed that the location of the bottleneck was more flexible. She proposed that stimulus analysis proceeds systematically through a hierarchy starting with analyses based on physical cues, syllabic pattern, and specific words, and moving on
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