Assignment: Perceiving Upright Faces

Task switching Stephen Monsell

School of Psychology University of Exeter, Exeter, EX4 4QG, UK

Everyday life requires frequent shifts between cognitive tasks. Research reviewed in this article probes the con- trol processes that reconfigure mental resources for a change of task by requiring subjects to switch fre- quently among a small set of simple tasks. Subjects’ responses are substantially slower and, usually, more error-prone immediately after a task switch. This ‘switch cost’ is reduced, but not eliminated, by an opportunity for preparation. It seems to result from both transient and long-term carry-over of ‘task-set’ activation and inhibition as well as time consumed by task-set reconfiguration processes. Neuroimaging studies of task switching have revealed extra activation in numerous brain regions when subjects prepare to change tasks and when they perform a changed task, but we cannot yet separate ‘controlling’ from ‘con- trolled’ regions.

A professor sits at a computer, attempting to write a paper. The phone rings, he answers. It’s an administrator, demanding a completed ‘module review form’. The pro- fessor sighs, thinks for a moment, scans the desk for the form, locates it, picks it up and walks down the hall to the administrator’s office, exchanging greetings with a col- league on the way. Each cognitive task in this quotidian sequence – sentence-composing, phone-answering, con- versation, episodic retrieval, visual search, reaching and grasping, navigation, social exchange – requires an appropriate configuration of mental resources, a pro- cedural ‘schema’ [1] or ‘task-set’ [2]. The task performed at each point is triggered partly by external stimuli (the phone’s ring and the located form). But each stimulus affords alternative tasks: the form could also be thrown in the bin or made into a paper plane. We exercise intentional ‘executive’ control to select and implement the task-set, or the combination of task-sets, that are appropriate to our dominant goals [3], resisting temptations to satisfy other goals.

Goals and tasks can be described at multiple grains or levels of abstraction [4]: the same action can be described as both ‘putting a piece of toast in one’s mouth’ and ‘maintaining an adequate supply of nutrients’. I focus here on the relatively microscopic level, at which a ‘task’ consists of producing an appropriate action (e.g. conveying to mouth) in response to a stimulus (e.g. toast in a particular context). One question is: how are appropriate task-sets selected and implemented? Another is: to what extent can we enable a changed task-set in advance of the relevant stimulus – as suggested by the term ‘set’?

Introspection indicates that we can, for example, set ourselves appropriately to name a pictured object aloud without knowing what object we are about to see. When an object then appears, it is identified, its name is retrieved and speech emerges without a further ‘act of intention’: the sequence of processes unfolds as a ‘prepared reflex’ [5,6].

Many task-sets, which were initially acquired through instruction or trial and error, are stored in our memories. The more we practice a task, or the more recently we have practised it, the easier it becomes to re-enable that task- set. At the same time, in the absence of any particular intention, stimuli we happen to encounter evoke ten- dencies to perform tasks that are habitually associated with them: we unintentionally read the text on cereal packages or retrieve the names of people we pass in the street. More inconveniently, stimuli evoke the tendency to perform tasks habitually associated with them despite a contrary intention. The standard laboratory example of this is the Stroop effect [7]: we have difficulty suppressing the reading of a colour name when required to name the conflicting colour in which it is printed (e.g. ‘RED’ printed in blue). Brain damage can exacerbate the problem, as in ‘utilization behaviour’, which is a tendency of some patients with frontal-lobe damage to perform the actions afforded by everyday instruments, such as matches, scissors and handles, even when these actions are contextually inappropriate [8].

Hence the cognitive task we perform at each moment, and the efficacy with which we perform it, results from a complex interplay of deliberate intentions that are governed by goals (‘endogenous’ control) and the avail- ability, frequency and recency of the alternative tasks afforded by the stimulus and its context (‘exogenous’ influences). Effective cognition requires a delicate, ‘just- enough’ calibration of endogenous control [9] that is sufficient to protect an ongoing task from disruption (e.g. not looking up at every movement in the visual field), but does not compromise the flexibility that allows the rapid execution of other tasks when appropriate (e.g. when the moving object is a sabre-toothed tiger).

To investigate processes that reconfigure task-set, we need to induce experimental subjects to switch between tasks and examine the behavioural and brain correlates of changing task. Task-switching experiments are not new (Box 1), but the past decade has seen a surge of interest, stimulated by the development of some novel techniques for inducing task switches and getting subjects to prepare for them (Box 2), and some surprising phenomena revealed thereby, as well as by the broader growth of interest in control of cognition (e.g. [10]).Corresponding author: Stephen Monsell ([email protected]).

Review TRENDS in Cognitive Sciences Vol.7 No.3 March 2003134

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Task switching: basic phenomena In a task-switching experiment, subjects are first pre- trained on two or more simple tasks afforded by a set of stimuli (Figs 1 and 2 provide examples). Each task requires attention to, and classification of, a different element or attribute of the stimulus, or retrieval from memory or computation of a different property of the stimulus. Then, a stimulus is presented on each of a series of trials and the subject performs one of the tasks. There are several methods for telling the subject which task to perform (Box 2), but in all cases the task sometimes changes from one trial to the next, and sometimes does not. Thus, we can examine performance or brain activation on and following trials when the task changes for evidence of extra processing demands that are associated with the need to reconfigure task-set. We can also examine the effects of localized brain damage, transient magnetic stimulation (TMS) or pharmacological interventions on behavioural indices of switching efficiency. Four phenom- ena of primary interest (of which the first three are illustrated in Figs 1 and 2) are described below.

Switch cost (task-repetition benefit) Generally, responses take longer to initiate on a ‘switch trial’ than on a ‘non-switch’ or task-repetition trial, often by a substantial amount (e.g. 200 ms relative to a baseline of 500 ms).Also, theerrorrateisoftenhigherafterataskswitch.

Preparation effect If advance knowledge is given of the upcoming task and time allowed to prepare for it, the average switch cost is usually reduced.

Residual cost Preparation generally does not eliminate the switch cost. In the examples shown, the reduction in switch cost seems to have reached a substantial asymptote, the ‘residual

cost’, after ,600 ms of preparation. Substantial residual costs have been reported even when 5 s or more is allowed for preparation (e.g. [11,12]).

Mixing cost Although performance recovers rapidly after a switch (Fig. 1), responses remain slower than when just one task must be performed throughout the block: there is a long- term as well as a transient cost of task switching.

These phenomena have been demonstrated with a wide range of different tasks and they are modulated by numerous other variables. What explains them?

Sources of the switch cost Time taken by control operations To change tasks, some process or processes of ‘task-set reconfiguration’ (TSR) – a sort of mental ‘gear changing’ – must happen before appropriate task-specific processes can proceed. TSR can include shifting attention between stimulus attributes or elements, or between conceptual criteria, retrieving goal states (what to do) and condition– action rules (how to do it) into procedural working memory (or deleting them), enabling a different response set and adjusting response criteria. TSR may well involve inhi- bition of elements of the prior task-set as well as activation of the required task-set.

An account of the switch cost that appeals intuitively is that it reflects the time consumed by TSR. The preparation effect then suggests that, if sufficient time is allowed, TSR can, to some extent, be accomplished under endogenous control, before the stimulus onset. The residual cost is more perplexing. Rogers and Monsell [13] suggest that

Box 1. Early research on task-set and task switching

The intentional and contextual control of ‘set’ (‘Einstellung’) was discussed in 19th and early 20th century German experimental psychology. In 1895, von Kries used as examples the way the clef sign changes the action performed to play a note on the musical stave, and the way the current state of a game changes how one sets oneself to respond to an opponent’s behaviour [58]. Exner and the Wurzburg school described the ‘prepared reflex’, and, in 1910, Ach described experiments on overlearned responses competing with the acqui- sition of a novel stimulus–response mapping, see [6]. Until recently, in the English-language literature, ideas about control of task-set have been stimulated mainly by the observation of impairments of control, both in everyday action and as a result of neurological damage, see [2], despite some experimentation on normal executive function in cognitive laboratories [5].

The invention of the task-switching paradigm is credited to Jersild [59] who had students time themselves working through a list of items, either repeating one task or alternating between two. Some task pairs (adding 3 to vs. subtracting 3 from numbers) resulted in dramatic alternation costs; others (adding 3 to a number vs. writing the antonym of an adjective) did not. Jersild’s paradigm was revived, and his results replicated using discrete reaction-time measurements, by Biederman and Spector [60]. Despite this work and some pioneering task-cueing studies (e.g. [61–63]) it was only in the mid 1990s that the present surge of research on task switching developed.