Earlier we discussed research (e.g., Spelke et al., 1976) suggesting two complex tasks can be performed very well together with minimal disruption.
However, the participants had much flexibility in terms of when and how they processed the tasks. Thus, it is possible there were interference effects that went unnoticed because of lack of experimental control and/or insensitivity of measurement.
We turn now to perhaps the most sensitive type of experiment for detecting dual-task interference. There are two stimuli (e.g., two lights) and two responses (e.g., button presses), one associated with each stimulus. Participants respond to each stimulus as rapidly as possible. When the two stimuli are presented at the same time (dual-task condition), performance is typically worse on both tasks than when each task is presented on its own (single-task condition).
When the second stimulus is presented very shortly after the first one, there is typically a marked slowing of the response to the second stimulus. This is the psychological refractory period (PRP) effect. It is a robust effect – Ruthruff et al. (2009) found there was still a large PRP effect even when participants were given strong incentives to eliminate it. Note that when the time interval between the two stimuli is increased, there is much less slowing
of response to the second stimulus.
KEY TERM
Psychological refractory period (PRP) effect
The slowing of the response to the second of two stimuli when presented close together in time.
The PRP effect has direct relevance to the real world. Hibberd et al. (2013) studied the effects of a simple in-vehicle task on braking performance when the vehicle in front braked and slowed down. They obtained a classic PDP effect – braking time was slowed down most when the in-vehicle task was presented very shortly before the vehicle in front braked.
How can we explain the PRP effect? Several theorists assume task performance involves three successive stages: (1) perceptual, (2) central (e.g., deciding which response to make) and (3) motor (preparing and executing a response). According to the central bottleneck theory (originally pro posed by Welford, 1952), a bottleneck “pre vents more than one central decision process from operating at any given moment” (Pashler et al., 2008, p. 313).
The notion of a central bottleneck remains the most influential explanation for the PRP effect. It can also explain many dual-task costs when individuals perform two tasks at the same time. However, other explanations of the PRP effect are possible. First, it may occur because participants in most studies receive insufficient practice to eliminate it. Second, the PRP effect may occur because people decide their performance will be better if they engage in serial rather than parallel processing.
Figure 5.21
Examples of stimuli used to study rule-based (top) and information-integration (bottom) category learning. The boundaries between the A and B categories are indicated by solid lines. A simple verbal rule (e.g., respond “A” if the bars are narrow and “B” if they are wide) is sufficient on the rule-based task. In contrast, there is no simple verbal description of the categories with the information-integration task.
From Ashby and Crossley (2012). With permission of Wiley.
Findings
Hesselmann et al. (2011) used event-related potentials (ERPs; see Glossary) to clarify the processes involved in the PRP effect. They focused on the P300 component of the ERP, which probably reflects central decision processes. The amount of slowing of responses on the second task was closely matched by the amount of slowing in the onset of P300. However, there was no slowing of earlier ERP components reflecting perceptual processing. Thus, the PRP effect occurred because of a slowing of central decision processes rather than of perceptual processes, precisely as predicted by the central bottleneck theory.
Evidence apparently problematical for the notion of a bottleneck was reported by Schumacher et al. (2001). They used two tasks: (1) say “one”, “two”
or “three” to low-, medium- and high-pitched tones, respectively; and (2) press response keys corresponding to the position of a disc on a computer screen.
These two tasks were performed together for over 2,000 trials, at the end of which some participants performed them as well together as singly. In general, those performing each task on its own especially well showed the smallest PRP effect.
Strobach et al. (2013) carried out a study that was very similar to that of Schumacher et al. (2001), with participants receiving over 5,000 trials involving single-task or dual-task conditions. However, they did not find evidence that dual-task costs were eliminated after extensive practice. As is shown in Figure 5.22, dual-task costs for the auditory task reduced from 185 to 60 ms with practice, and those for the visual task reduced from 83 to 20 ms.
Why did this difference in findings between Strobach et al. (2013) and Schumacher et al. (2001) occur? In both studies, participants were rewarded for fast responding on single-task and dual-task trials. However, the way the reward system was set up in the Schumacher et al. study may have led participants to exert more effort in dual-task than single-task trials. This potential bias was absent from the Strobach et al. study. This difference in reward structure may explain why there was much more evidence of dual-task costs in the Strobach et al. study.
Figure 5.22
Reaction times on correct trials only over eight experimental sessions under dual-task (auditory and visual tasks) and single-task (auditory or visual task) conditions.
From Strobach et al. (2013). Reprinted with permission of Springer.
How does dual-task practice benefit performance? This issue was addressed by Strobach et al. (2013). The main effect of practice was to speed up the central response-selection stage in both tasks. There was also an effect of practice on the perceptual stage of the auditory task. In contrast, practice had no effect at all on the motor or response stage on either task.
We have seen the central response-selection stage is of crucial importance in explaining interference effects between two tasks performed at the same time. There is much evidence suggesting the left posterior lateral prefrontal cortex plays an important role in response selection (Filmer et al., 2013).
However, most research has obtained only correlational evidence to support that claim. Filmer et al. applied transcranial direct current cathodal stimulation to decrease excitability of that brain region. Dual-task costs (slower responding on dual-task trials compared to single-task trials) were significantly reduced by cathodal stimulation.
The precise reasons why cathodal stimulation had this effect is unclear. However, the study is important in showing that the left posterior lateral prefrontal cortex is probably causally involved in response selection.
Finally, we return to the PRP effect. As mentioned earlier, it is possible the effect might disappear if participants received prolonged practice. In fact, practice typically reduces but rarely eliminates the effect. For example, Pashler (1993) found the PRP effect was still observable after over 10,000 practice trials.
Is it possible that participants typically engage in serial processing in PRP tasks because they choose to rather than because they must do so. Miller et al. (2009) argued that serial processing in PRP tasks generally leads to superior performance than parallel processing. However, their theoretical analysis indicated parallel processing would have the edge when the stimuli associated with the two tasks are mostly presented close together in time. As predicted,
there was a shift from predominantly serial processing towards parallel processing when that was the case.
Miller et al. (2009) used very simple tasks, and it is likely that parallel processing is more likely to be found with simple than with complex tasks. In a study discussed earlier, Han and Marois (2013) used two tasks, one of which was relatively difficult. The participants engaged in serial processing even when the use of parallel processing was encouraged by financial rewards.
Interactive exercise:
Definitions of attention
Summary and conclusions
The findings from most studies of the psychological refractory period effect and dual-task interference are consistent with the cognitive bottleneck theory.
This is so for behavioural and neuroscience research. Practice typically produces a substantial reduction in the PRP effect and dual-task interference.
However, these effects are rarely eliminated, which suggests central decision processes typically occur serially. However, there is some evidence for parallel processing when the two tasks are both easy. The predominance of serial processing over parallel processing may occur in part because it is generally associated with superior levels of performance (Miller et al., 2009).
CHAPTER SUMMARY
• Focused auditory attention. The task of attending to one voice among several (the cocktail party problem) is a challenging one for automatic speech recognition systems. Human listeners use several top-down and bottom-up processes to select one voice. There is more limited processing of unattended than attended messages although unattended messages often receive some semantic processing. The restricted processing of unattended messages may reflect a bottleneck that can be found at various stages of processing.
• Focused visual attention. Visual attention can resemble a spotlight or zoom lens. However, it is very flexible and can also resemble multiple spotlights. Visual attention is typically object-based in everyday life. However, it can also be space-based or feature-based depending on the precise task requirements, recent experience and individual differences. According to Lavie’s load theory, we are more susceptible to distraction when our current task involves low perceptual load and/or high cognitive load. There is much support for this theory, but the effects of perceptual and cognitive load are often not independent as predicted.
There is a stimulus-driven ventral attention network and a goal-directed dorsal attention network involving different (but partially overlapping) brain networks. There is some lack of clarity concerning the precise brain regions associated with each network and little is currently known about how the two attentional systems interact.
• Disorders of visual attention. Neglect occurs when damage to the ventral attention network in the right hemisphere impairs the functioning of the undamaged dorsal attention network. This impaired functioning of the dorsal attention network is due to reduced activation and alertness within the left hemisphere. Extinction is due to biased competition for attention between the two hemispheres combined with reduced attentional capacity. Prism-adaptation and alertness training have both proved effective in treating the symptoms of neglect.
• Visual search. One problem with airport security checks is that there are numerous possible target objects. Another problem is the rarity of targets, which produces excessive caution in reporting targets. According to feature integration theory, object features are processed in parallel and are then combined by focused attention in visual search. In fact, there is more parallel processing than assumed by this theory. Much parallel processing occurs because much information is typically extracted from the peripheral visual field. In everyday life, general scene knowledge is used to focus visual search on areas of the scene most likely to contain the target object.
• Cross-modal effects. In the real world, we often coordinate information from two or more sense modalities. The ventriloquism effect shows vision dominates sound because an object’s location is typically indicated more precisely by vision. Temporal ventriloquism shows temporal judgements can be dominated by auditory stimuli because the auditory modality is typically more precise than the visual modality at discriminating temporal relations. Auditory or vibrotactile warning signals informative about the direction of danger and/or time to collision speed up drivers’ braking times in an emergency speed up braking reaction times.
• Divided attention: dual-task performance. Driving performance is impaired substantially by a secondary task (e.g., mobile-phone use). This is often due to inattentional blindness or reduced attention to peripheral objects.
Multiple-resource theory and threaded cognition theory both assume that dual-task performance depends on several processing resources each of which has limited capacity. This permits two tasks to be performed together successfully provided they use different processing resources. This general approach has proved successful but de-emphasises high-level executive processes (e.g., monitoring and coordinating two tasks at the same time).
Some neuroimaging studies have found underadditivity under dual-task conditions (less activation than for the two tasks performed separately). This may indicate that people have limited general processing resources. Other neuroimaging studies have found that dual-task conditions can introduce new processing demands of task coordination associated with activation within the dorsolateral prefrontal cortex and cerebellum.
• Automatic processing. Shiffrin and Schneider distinguished between slow, flexible controlled processes and fast, automatic ones. Automatic processes are generally goal-unrelated, unconscious, efficient and fast. This could occur because automatic processes require only the direct retrieval of relevant information from long-term memory. The typical existence of a psychological refractory period (PRP) effect can be
explained by a processing bottleneck at the stage of response selection. However, massive practice may sometimes eliminate this bottleneck.
Alternatively, the PRP effect may occur because participants choose to engage in serial processing rather than because they cannot use parallel processing.
Further reading
• Bartolomeo, P., de Schotten, M.T. & Chica, A.B. (2012). Brain networks of visuospatial attention and their disruption in visual neglect. Frontiers in Human Neuroscience, 6, Article 110. The authors provide a thorough account of theory and research on visual neglect.
• Chan, L.K.H. & Hayward, W.G. (2013). Visual search. Wiley Interdisciplinary Review – Cognitive Science, 4: 415–29. Louis Chan and William Hayward discuss the main theoretical approaches to visual search.
• Corbetta, M. & Shulman, G.L. (2011). Spatial neglect and attention networks. Annual Review of Neuroscience, 34: 569–99. The complexities of the brain mechanisms underlying visual neglect are discussed thoroughly in this review article.
• Kastner, S. & Nobre, A.C. (eds) (2014). The Oxford handbook of attention. Oxford: Oxford University Press. This edited book contains numerous chapters by leading authorities on the issues discussed in this chapter.
• McDermott, J.H. (2009). The cocktail party problem. Current Biology, 19: R1024–7. Josh McDermott provides an informative account of some of the main factors involved in focused auditory attention.
• Moors, A. (2013). Automaticity. In D. Reisberg (ed.), The Oxford handbook of cognitive psychology. Oxford: Oxford University Press. Issues relating to automaticity are discussed comprehensively by Agnes Moors.
• Wolfe, J.M., Võ, M.L.-H., Evans, K.K. & Greene, M.R. (2011). Visual search in scenes involves selective and nonselective pathways. Trends in Cognitive Sciences, 15: 77–84. Current theoretical models of visual search are discussed and evaluated by Jeremy Wolfe.
• Wu, W. (2014). Attention. Hove: Psychology Press. Wayne Wu considers attention from psychological, neuroscience and philosophical perspectives.