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multi-tasking and cognitive capacity

Even when we do just one task, limits to cog capacity

All process take time (e.g. memory, retrieval, decision making)

Limits to input any one process (e.g. syntactic parsing) can handle

Representational/storage capacity limited (E.g. WM)

Capacity limits become even more obvious when resources must be shared between tasks, i.e.

Have to get more than one task done in certain time

At least some tasks time-critical (cant wait), so must either (try to) do them simultaneously/switch back and forth between them


examples of real-life multi-tasking

YouTube video

Cooking and ironing and baby-monitoring and phone-answering and door-bell

Other professions


limitations of multi-tasking and cog capacity

Theoretically (what is global computational architecture of mind/brain)?

Practically (efficiency, risk – ‘human error’ as source of accidents)


what do multi-tasking demands include?

When we try to do tasks simultaneously:

Comp for shared resources – dual-task interference

When try to switch between tasks, overhead inc:

Set-shifting/task-switching costs

Retrospective memory

Prospective memory

Monitoring for trigger conditions

Remembering meaning of ‘trigger’

Other demands on executive control

Planning, scheduling, prioritising, coordinating 2 task streams

Trouble-shooting, problem-solving when things go wrong/unexpected conditions arise

So multi-tasking not single competence and executive control processes critical


what happens if you drive and use a phone?

Use of handset while driving now illegal in UK

Use of hands-free phone not illegal per se, but can be prosecuted if use concurrent with accident

Think can drive competently while speaking on phone, esp. if using hands-free


Epidemiological studies show increased accidents (relative risk similar to driving at legal limit for alcohol)

Observational studies show delayed braking etc

Exp studies show impaired braking, detection of potential hazards, etc, esp. in young drivers


Straver, Drews and Crouch (2006) - study on relative risk of cell phone use and drinking

Driver in simulator, follows pacer car in slow lane of interstate for 15 minutes, tries to maintain distance, pacer car brakes occasionally

Baseline v alcohol v casual talk on hand-held/hands-free mobile

Mobile users: Slower reactions, more tail-end collisions, slower recovery

Alcohol: More aggressive driving (closer following, harder breaking)

No sig diffs between effects of talking on hand-held and hands-free


further driving simulator studies - just talking on hands-free phone

Reduced (50%) anticipatory glances to safety critical locations

Reduced (50%) later recognition memory of objects in driving env

Reduced (50%) amplitude of P300 to onset of brake light in car being followed

Increased p(unsafe lane change)


does talking to passenger have same effect as being on phone

Passengers sensitive to driver’s load

Passengers help spot hazards, turn-offs etc


measuring dual-task interference in lab

Just 2 tasks, designed for measurement and manip

Typically measures perf

On tasks A and B alone – should get 100% perf

On A and B combined

Does perf on each task deteriorate when other must also be performed?

Must measure perf on both tasks: P may be able to trade off perf of A and B


what are the sources of dual-task interference?

Slower and less accurate perf in dual-task conditions might be attributable to:

1.Comp for use of specialised domain-specific resources

Parts of body

Brain ‘modules’: Specific processes/representations

2. Comp for use of general-purpose processing capacity

‘Central processor?’

Pool of general-purpose processing resource

3. Limited cap of executive control mechanisms that set up and manage flow of info through system, and/ sub-optimal control strats


comp for domain-specific resources

2 continuous speech inputs cannot simultaneously be understood/repeated (but both can be monitored for target word meaning)

Perf spatial tracking task interferes with use of visual imagery to remember stuff – both use visuo-spatial WM

Where 2 tasks need to use same perceptual processes/same response mechanisms/same ‘central’ translation/coord processes, should expect dual-task interference (unless info rate low enough to switch use of that resource between tasks)

Doesn’t concurrent perf of any pair of tasks seems to result in some interference (relative to perf of either task by itself?)


comp for general-purpose processor?

Analogous to standard late 20th century digital computer, with CPU

Plausible account of brain architecture?

Others identified consciousness with ‘central processor’ – assumed to be required to pattern recognition, access to memory, decision making, action selection, awareness etc


comp for general-purpose resource pool

Kahneman (1975) proposed pool of general purpose resource/effort shared among concurrent tasks

Capacity of general purpose resource might vary:

Over people

Within people, over states of alertness: Level of ‘sustained attention’ (=available ‘resource/effort’)

Diminishes with boredom/fatigue

Increased with time of day (apart from post-lunch dip), presence of moderate stressors (e.g. noise, heat, but only up to a point), emotional arousal (up to a point), conscious effort


whose capacity is shared by any 2 tasks - central processor/resource pool

Sum of capacity demands doesn’t exceed available total

= No interference

Sum does exceed

= Interference

Increasing difficulty of one task should reduce the capacity available for other task

Hard to know (a priori - relating to or denoting reasoning or knowledge which proceeds from theoretical deduction rather than from observation or experience) how much capacity a given task should use

So, test theory by using pairs of tasks for which it seems obvious that each would require all/most of central capacity


Allport, Antonis and Reynolds (1972) - case of demanding tasks combined w/o interference

Uni of Reading Y3 music students (competent pianists) – tasks:

Sight-read Grade 2 (easier)/ grade 4 (harder) piano pieces

Shadow prose from Austen novel (easier)/text on Old Norse (harder)

Relatively little practice: 10 x 1 min shadowing (to reach criterion of 2 trials with no omissions), 2 x 1 min sight-reading, 7 x 1 min dual task

Exp: two sessions of 2 x 1 min dual task for each combo of easy and hard, and 1 min sight-reading/shadowing alone (order balanced)

Rate of shadowing and number of shadowing errors no diff w/ and w/o concurrent sight-reading

Concurrent shadowing also didn’t increase sight-reading errors

More shadowing errors for harder text, and more sight-reading errors for Grade 4 pieces (difficulty manips effective), but (by session 2):

Shadowing perf not influenced by difficulty of music piece

Sight-reading perf not influenced by difficulty of prose shadowed


further examples of demanding tasks w/o interference

Tasks combined w/o apparent inference:

Schaffer (1975): skilled visual copy-typing can be combined with shadowing of prose w/o interference

One task insensitive to difficulty of other

North (1977): continuous tracking and digit  key task

Identify digit with key press

Identify digit before present one

Classify successive pair of digits

No effect of difficulty of digit task on tracking delays


what is the claim of no central general-purpose processor/resource?

Pairs complex input-output translation tasks can be combined w/ little/no interference if they use non-overlapping ‘modules’ (diff input codes/modalities, action systems, central representations/networks)

Idea of general-purpose ‘central processor’ seems unnecessary

Wickens’ (1984) (incomplete) summary diagram of domain-specific cognitive models

But, even when tasks use completely diff modules, some interference may arise due to coord and control demands

i.e. as a consequence of load on (specialised) executive processes

e.g. Allport at al. error in one task briefly slows other


what shouldn't you do when driving?

Driving/navigation (visuo/spatial input --> hand/foot responses) and

Conversation (speech input meaning speech production)

Require use of diff ‘modules?’

Yes and no – use diff input and output modalities, but both require construction of ‘mental model’ (for driving: representation of route, goals, progress, meaning of road-signs, interp of observed events, prediction of potential hazards etc.)

Particularly obvious how construction of mental model for driving can be interfered with by conversation if ask driver to think about visuo-spatial array/imagine movements


the importance of practice

Tasks which cannot be combined w/o interference become easier to combine with practice

E.g. changing gear while driving

Spelke, Hirst and Neisser (1976): after 85h practice at

Reading storied (for comprehension) at same time as writing to dictation (6 weeks)

Reading storied concurrent with writing category of spoken words (11 weeks)

Some Ps showed little dual-task interference


Practicing one task automates it: reduces need for ‘executive’ control of constituent processes

Practicing combining tasks develops optimal control strats for combining particular task pair


what is Broadbent's objection to Allport-type exps?

With pairs continuous tasks like shadowing and sight-reading there is:

Some predictability in input (i.e. can anticipate)

Substantial lag between input and output (so must be temporary storage in WM of input and/output)

So, could be central processor switching between 2 tasks (time-sharing) – while processor services one task, input/output for other task could be stored in WM buffers

Test: processor-switching should be revealed if we use concurrent tasks w/ v. small lags between input and output, and next stimulus unpredictable

i.e. reaction time tasks


what is the psych refractory period/PRP?

2 choice reaction-time tasks, stimulus onsets separated by variable, v. short, interval (‘stimulus onset asynchrony’/SOA)

Schumacher et al. (2001)

Task 1 (auditory-vocal): low/medium/high tone

Task 2 (visual-manual): o--/-o-/--o --> index/middle/ring finger

A PRP effect, robust to practice, occurs even when stimuli and responses for 2 tasks in diff modalities

Pashler (1990) – capacity limitation must be in central translation processes

Perceptual processes --> central translation --> response execution


Pashler's theory

Pashler’s theory: response selection is bottleneck: can be perf for only one task at time

If second stimulus meanwhile arrives and identified, must wait until response selection mechanisms is free


other PRP explanations

RT task pairs where no PRP interference observed:

Repeat spoken letter name and move lever in direction of arrow (Greenwald and Shulman, 1973)

Repeat spoken word and visual same/diff (McLeod and Posner, 1984)

Saccadic eye movement and auditory/manual (Pashler et al., 1993)

All cases with very natural/practiced input-output mapping

When observed, PRP effect could arise not from structural bottleneck, but from cautious control strat Ps adopt to avoid producing response to second stimulus first (Meyer and Kieras, 1997, computational model)

If subject trained to adopt more liberal strat, PRP effect disappears (Schumacher et al., 2001)

Continues to be debated whether dual task costs in PRP paradigm due to structural bottleneck (response selection mechanism)/to ‘soft’ capacity limits attributable to control strats

Even if former, response-selection mechanism not ‘general-purpose processor’



Much interference between simultaneously perf tasks can be explained as due to comp for specialised resources

Although idea of general-purpose processing capacity for high-level cognition (=consciousness) has been popular, little ev for it

Cases of dual-task interference previously interpreted as due to comp for limited capacity of ‘general-purpose central-processor’/even generic response-selection mechanism, may be better explained in terms of

Comp for domain-specific resources


Capacity of (specialised) executive control mechanisms


Use of inefficient control strats