Response times to the first and second stimuli as a function of time between the onset of the stimuli (stimulus-onset asynchrony) and whether or not the order of the stimuli was known beforehand. Adapted from Pashler (1990).

are two stimuli (e g., two lights) and two responses (e.g., button presses), and the task is to respond to each stimulus as rapidly as possible. When the second stimulus is presented very shortly after the first one, there is generally a marked slowing of the response to the second stimulus: this is known as the psychological refractory period effect (see Welford, 1952).

It could be argued that the psychological refractory period occurs simply because people are not used to responding to two immediately successive stimuli. However, Pashler (1993) discussed one of his studies in which the effect was still observable after more than 10,000 practice trials.

Another objection to the notion that the delay in responding to the second stimulus reflects a bottleneck in processing is that the effect is due to similarity of stimuli and/or similarity of responses. According to the bottleneck theory, the psychological refractory period effect should be present even when the two stimuli and responses differ greatly. In contrast, the effect should disappear if similarity is crucial. Pashler (1990) used a tone requiring a vocal response and a visual letter requiring a button-push response. Some participants were told the order in which the stimuli would be presented, whereas the others were not. In spite of a lack of either stimulus or response similarity, there was a psychological refractory period effect, and it was greater when the order of stimuli was known than when it was not (see Figure 5.7). Thus, the findings provided strong support for the bottleneck position.

Pashler (1998, p. 177) ended his review with the following conclusion:

If there were no fundamental constraint preventing central stages of multiple tasks from being carried out simultaneously, one might expect that exceptions to PRP [psychological refractory period] interference would be encountered frequently But in fact,... only a handful of exceptions have been noted .These exceptions have generally been interpreted as indicating that certain specific neural pathways are capable of bypassing the central bottleneck.

Earlier we discussed studies (e.g., Hirst et al., 1980; Spelke et al., 1976) in which two complex tasks were performed remarkably well togetter. Such findings make it hard to argue for the existence of a bottleneck in processing. However, studies on the psychological refractory period have the advantage of very precise assessment of the time taken to respond to any given stimulus. The coarse-grained measures obtained in studies such as those of Spelke et al. (1976) and Hirst et al. (1980) may simply be too insensitive to permit detection of bottlenecks.

It has been assumed so far that there is a single bottleneck, but there may be multiple bottlenecks. Pashler (1998, p. 175) addressed this issue: "At present,...a single bottleneck seems sufficient to account for the response delays observed in 'standard' PRP designs involving pairs of choice RT [response time] tasks. In fact, results from these paradigms are difficult to square with the existence of multiple bottlenecks."

Pashler et al. (1994) studied split-brain patients, in whom the connections between the cortical hemispheres have been surgically cut. One stimulus-response task was presented to one hemisphere and the other was presented to the other hemisphere. If the bottleneck is located in the cortex, then it might be expected that these patients would not show the psychological refractory period effect. In fact, they had a normal effect, suggesting that sub-cortical structures underlie the effect.

The evidence from studies of the psychological refractory period indicates that there is a bottleneck, and that some processing is serial. However, the size of the psychological refractory period is typically not very large, and suggests that most processes do not operate in a serial way. As Pashler (1998, p. 184) pointed out, "The idea of obligatory serial central processing is quite consistent with a great deal of parallel processing."

Central capacity theories

A simple way of accounting for many dual-task findings is to assume there is some central capacity (e.g. central executive) which can be used flexibly across a wide range of activities. This central processor has strictly limited resources, and is sometimes known as attention or effort. The extent to which two tasks can be performed together depends on the demands that each task makes on those resources. If the combined demands of the two tasks do not exceed the total resources of the central capacity, then the two tasks will not interfere with each other. However, if the resources are insufficient, then performance disruption is inevitable.

One of the best known of the capacity theories was put forward by Kahneman (1973). He argued that attentional capacity is limited but the capacity can vary somewhat. More specifically, it is greater when task difficulty is high than when it is low, and it increases in conditions of high effort or motivation. Increased effort tends to produce physiological arousal, and this can be assessed in various ways (e.g., pupillary dilation).

There are various problems with Kahneman's (1973) theory. He did not define his key terms very clearly, referring to a "a nonspecific input, which may be variously labelled 'effort', 'capacity', or 'attention'." Another problem is that it is assumed that effort and attentional capacity are determined in part by task difficulty, but it is very hard to determine the difficulty of a task with any precision.

Bourke, Duncan, and Nimmo-Smith (1996) tested predictions of central capacity theory. They selected four tasks that were designed to be as different as possible:

1. Random generation: generating letters at random.

2. Prototype learning: working out the features of two patterns or prototypes exemplars.

3. Manual task: screwing a nut down to the bottom of a bolt and back up to the top bottom of a second bolt and back up, and so on.

4. Tone task: detecting the occurrence of a target tone.

The participants were given two of these tasks to perform together, with one task being identified as more important than the other. The basic argument was as follows: if there is a central or general capacity, then from seeing various , and then down to the the task making most demands on this capacity will interfere most with all three of the other tasks. In contrast, the task making fewest demands on this capacity will intefere least with all the other tasks.

What did Bourke et al. (1996) find? First, these very different tasks did interfere with each other. Second, the random generation task interfered the most overall with the performance of the other tasks, and the tone

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