1 CS 630: Cognitive Systems. Dario Salvucci, Drexel University.
Lecture 6:
Multitasking
CS 630: Cognitive Systems. Dario Salvucci, Drexel University. 2
Our Multitasking World
talking &
driving
cooking &
reading a book writing paper &
reading email
watching game &
talking to friends listening &
note-taking
The Challenges of Multitasking
■ Technological challenges
– user interfaces, hardware, networking...
■ Scientific challenges – how do we multitask?
– when is multitasking easy or difficult?
– how does it affect task performance?
■ Societal challenges
– when is multitasking useful?
– when is multitasking inappropriate?
– when is multitasking dangerous?
The Multitasking Continuum
talking &
driving cooking &
reading a book writing paper &
reading email watching game &
talking to friends listening &
note-taking
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The Multitasking Continuum
Concurrent Multitasking
(e.g., PRP, driver distraction)
Sequential Multitasking
(e.g., task interruptions)
seconds minutes hours
Time between Task Switches talking &
driving
cooking &
reading a book writing paper &
reading email watching game &
talking to friends listening &
note-taking
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Threaded Cognition
■ Goal: Unifying theory of multitasking – ... across the multitasking continuum – ... across laboratory and real-world domains – ... across different levels of abstraction
■ Approach: Computational cognitive modeling (obviously J)
– Threaded cognition
– in the ACT-R cognitive architecture
Threaded Cognition
■ Your brain is a “Thought Kitchen”
– with resources and processes
• central resource: the cook
• other resources: oven, stove, mixer, etc.
Threaded Cognition
■ Concurrent multitasking is a basic skill—best represented by a simple general mechanism
■ Threaded cognition...
– allows concurrent execution of multiple
“streams of thought” = threads
– takes models A, B... predicts behavior of A+B
■ Theoretical components
– (1) Resources that perform relevant processing
• derived from the ACT-R architecture
– (2) Processing principles that define task
allocation
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Processing Principles (1) Threaded Processing
– Cognition maintains a set of active goals that produce “threads” of processing
• many domains are nicely represented as threads – some are more obvious – e.g., driving + dialing – some are less obvious – e.g., list-memory tasks
– In ACT-R terms, this means maintaining multiple goals at a time
• In the past, ACT-R had only one goal at a time
• then it had a “goal stack” (inspired by tasks with a robust subgoal structure, like Tower of Hanoi)
• now, several active goals
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Processing Principles (2) Resource Exclusivity
– Resources execute processing requests serially, exclusively for one request/task at a time
• resources can be massively parallel themselves, within the resource
– e.g., visual processing
• but resources can serve only one goal at a time
– (caveat: what about resources like motor? — are the hands independent? fingers? hands from feet? etc.)
Processing Principles (3) Resource Usage
– Threads acquire and release resources in a greedy, polite manner.
• greedy: used as soon as available
• polite: threads free resources ASAP
(4) Conflict Resolution
– When threads contend for the procedural resource, the thread with the highest urgency proceeds.
• highest urgency = least recently used
• simple mechanism for balancing thread processing
Threaded Models
■ Now let’s look at some models that use
threaded cognition…
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Threads-1
(set-‐task "cs630.ThreadsTask”
(add-‐dm (type-‐number isa type-‐number) (answer-‐phone isa answer-‐phone)
(goal-‐focus type-‐number) (goal-‐focus answer-‐phone)
(p type-‐number*Bind-‐Birst =goal>
isa type-‐number current-‐x nil ?visual-‐location>
state free buffer empty ?visual>
state free buffer empty
==>
+visual-‐location>
isa visual-‐location screen-‐x lowest )
(p type-‐number*Bind-‐next =goal>
isa type-‐number current-‐x =x ?visual-‐location>
state free buffer empty ?visual>
state free buffer empty
==>
+visual-‐location>
isa visual-‐location screen-‐x lowest > screen-‐x =x )
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Threads-1
(p type-‐number*encode =goal>
isa type-‐number =visual-‐location>
isa visual-‐location screen-‐x =x ?visual>
state free buffer empty
==>
+visual>
isa move-‐attention screen-‐pos =visual-‐location =goal>
current-‐x =x )
(p type-‐number*type =goal>
isa type-‐number =visual>
isa text value =digit ?manual>
state free
==>
+manual>
isa press-‐key key =digit )
(p type-‐number*done =goal>
isa type-‐number current-‐x =x ?visual-‐location>
state error
==>
-‐goal>
)
Threads-1
(p answer-‐phone*encode =goal>
isa answer-‐phone =aural-‐location>
isa audio-‐event ?aural>
buffer empty state free
==>
+aural>
isa ring event =aural-‐location )
(p answer-‐phone*done =goal>
isa answer-‐phone =aural>
isa ring ?vocal>
state free
==>
+vocal>
isa speak string "Hello!"
-‐goal>
)
Threads-1
0.000 vision unrequested [vision~62]!
0.000 procedural start!
0.050 procedural ** TYPE-NUMBER*ENCODE ** [type-number]!
0.050 vision move-attention!
0.135 vision encoding-complete [text~66]!
0.185 procedural ** TYPE-NUMBER*TYPE ** [type-number]!
0.185 motor press-key "1"!
0.235 procedural ** TYPE-NUMBER*FIND-NEXT ** [type-number]!
0.235 vision find-location [vision~72]!
0.285 procedural ** TYPE-NUMBER*ENCODE ** [type-number]!
0.285 vision move-attention!
0.300 audio audio-event [audio-event~63]!
0.300 audio unrequested [audio-event~63]!
0.350 procedural ** ANSWER-PHONE*ENCODE-SOUND ** [answer-phone]!
0.350 audio attend-sound!
0.370 vision encoding-complete [text~75]!
0.435 motor preparation-complete!
0.485 motor initiation-complete!
0.585 motor output key 1!
0.735 motor finish-movement!
0.785 procedural ** TYPE-NUMBER*TYPE ** [type-number]!
0.785 motor press-key "2"!
0.835 procedural ** TYPE-NUMBER*FIND-NEXT ** [type-number]!
0.835 vision find-location [vision~84]!
0.850 audio audio-encoding-complete [ring~78]!
0.885 procedural ** TYPE-NUMBER*ENCODE ** [type-number]!
0.885 vision move-attention!
0.935 motor preparation-complete!
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Threads-1
0.935 procedural ** ANSWER-PHONE*DONE ** [answer-phone]!
0.935 declarative store chunk [answer-phone]!
0.935 speech speak "Hello!"!
0.970 vision encoding-complete [text~87]!
0.985 motor initiation-complete!
1.085 motor output key 2!
1.085 speech preparation-complete!
1.135 speech initiation-complete!
1.135 speech output-speech "Hello!"!
1.235 motor finish-movement!
1.285 procedural ** TYPE-NUMBER*TYPE **!
1.285 motor press-key "3"!
1.335 procedural ** TYPE-NUMBER*FIND-NEXT **!
1.335 vision find-location [vision~95]!
1.385 procedural ** TYPE-NUMBER*ENCODE **!
1.385 vision move-attention!
1.435 speech finish-movement!
1.435 motor preparation-complete!
1.470 vision encoding-complete [text~98]!
1.485 motor initiation-complete!
1.585 motor output key 3!
1.735 motor finish-movement!
1.785 procedural ** TYPE-NUMBER*TYPE **!
...!
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Threads-2
(set-‐task "cs630.ThreadsTask")
(add-‐dm (start isa goal) )
(goal-‐focus start)
(p start-‐multitasking =goal>
isa goal
==>
+goal>
isa type-‐number +goal>
isa answer-‐phone )
...
< same as before>
Threads-2
0.000 vision unrequested [vision~62]!
0.000 procedural start!
0.050 procedural ** START-MULTITASKING **!
0.050 declarative store chunk [start] (start isa goal)!
0.050 declarative store chunk [type-number~65]!
0.100 procedural ** TYPE-NUMBER*ENCODE ** [type-number~65]!
0.100 vision move-attention!
0.185 vision encoding-complete [text~70]!
0.235 procedural ** TYPE-NUMBER*TYPE ** [type-number~65]!
0.235 motor press-key "1"!
0.285 procedural ** TYPE-NUMBER*FIND-NEXT ** [type-number~65]!
0.285 vision find-location [vision~76]!
0.300 audio audio-event [audio-event~63]!
0.300 audio unrequested [audio-event~63]!
0.335 procedural ** TYPE-NUMBER*ENCODE ** [type-number~65]!
0.335 vision move-attention!
0.385 procedural ** ANSWER-PHONE*ENCODE-SOUND ** [answer-phone~67]!
0.385 audio attend-sound!
...!
0.835 procedural ** TYPE-NUMBER*TYPE ** [type-number~65]!
0.835 motor press-key "2"!
0.885 audio audio-encoding-complete [ring~82]!
0.885 procedural ** TYPE-NUMBER*FIND-NEXT ** [type-number~65]!
0.885 vision find-location [vision~88]!
0.935 procedural ** ANSWER-PHONE*DONE ** [answer-phone~67]!
0.935 declarative store chunk [answer-phone~67]!
0.935 speech speak "Hello!"!
...!
Threads-3
(set-‐task "cs630.ThreadsTask")
(add-‐dm (type-‐number isa type-‐number) )
(goal-‐focus type-‐number)
...
(p handle-‐sound*encode =goal>
=aural-‐location>
isa audio-‐event ?aural>
buffer empty state free
==>
+aural>
isa ring event =aural-‐location +goal>
isa handle-‐sound +goal> =goal )
(p handle-‐sound*phone =goal>
isa handle-‐sound =aural>
isa ring ?vocal>
state free
==>
+vocal>
isa speak string "Hello!"
+goal>
isa converse )
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Threads-3
0.000 vision unrequested [vision~62]!
0.000 procedural start!
0.050 procedural ** TYPE-NUMBER*ENCODE **!
0.050 vision move-attention!
0.135 vision encoding-complete [text~66]!
0.185 procedural ** TYPE-NUMBER*TYPE **!
0.185 motor press-key "1"!
0.235 procedural ** TYPE-NUMBER*FIND-NEXT **!
0.235 vision find-location [vision~72]!
0.285 procedural ** TYPE-NUMBER*ENCODE **!
0.285 vision move-attention!
0.300 audio audio-event [audio-event~63]!
0.300 audio unrequested [audio-event~63]!
0.350 procedural ** HANDLE-SOUND*ENCODE **!
0.350 declarative store chunk [type-number]!
0.350 audio attend-sound!
0.370 vision encoding-complete [text~75]!
0.435 motor preparation-complete!
0.485 motor initiation-complete!
0.585 motor output key 1!
0.735 motor finish-movement!
0.785 procedural ** TYPE-NUMBER*TYPE ** [type-number~80]!
0.785 motor press-key "2"!
0.835 procedural ** TYPE-NUMBER*FIND-NEXT ** [type-number~80]!
0.835 vision find-location [vision~87]!
0.850 audio audio-encoding-complete [ring~81]!
0.885 procedural ** TYPE-NUMBER*ENCODE ** [type-number~80]!
0.885 vision move-attention!
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Threads-3
0.935 motor preparation-complete!
0.935 procedural ** HANDLE-SOUND*PHONE ** [handle-sound~79]!
0.935 declarative store chunk [handle-sound~79]!
0.935 speech speak "Hello!"!
0.970 vision encoding-complete [text~90]!
0.985 motor initiation-complete!
1.085 motor output key 2!
1.085 speech preparation-complete!
1.135 speech initiation-complete!
1.135 speech output-speech "Hello!"!
1.235 motor finish-movement!
1.285 procedural ** TYPE-NUMBER*TYPE ** [type-number~80]!
1.285 motor press-key "3"!
1.335 procedural ** TYPE-NUMBER*FIND-NEXT ** [type-number~80]!
1.335 vision find-location [vision~100]!
1.385 procedural ** TYPE-NUMBER*ENCODE ** [type-number~80]!
1.385 vision move-attention!
1.435 speech finish-movement!
1.435 motor preparation-complete!
1.470 vision encoding-complete [text~103]!
1.485 motor initiation-complete!
1.585 motor output key 3!
1.735 motor finish-movement!
1.785 procedural ** TYPE-NUMBER*TYPE ** [type-number~80]!
1.785 motor press-key "4"!
!
< “converse” goal is still active and proceeds here... >!
To the Laboratory...
■ Let’s look at threaded cognition in two laboratory tasks:
– tracking & choice – dual-choice tasks
Tracking & Choice
■ Manual tracking appears in many forms
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Tracking
■ In many experiments, it’s much more controlled
– either: try to keep a pointer on a target
– or: try to keep a cursor within a target range
– while the movement is generated using a pseudo-random forcing function
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Tracking & Choice
■ Experiment: Martin-Emerson & Wickens (1992)
– tracking: keep the cursor in the target area
• hard vs. easy tracking, depending on forcing function
– choice: see an arrow pointing {left, right}, press key to respond
• arrow separated from target area by offset that varies between 0° and 35° of visual angle
target area cursor
choice stimulus
offset
Tracking & Choice
■ Experiment: Martin-Emerson & Wickens (1992)
Track.java
public void start () {
processDisplay();
Utilities.shuffle (offsetIndices);
offsetIndex = 0;
offsetCount = 0;
lastArrowTime = 5.0;
addPeriodicUpdate (.020);
}
void updateTarget (double time) {
double pi2 = 2 * Math.PI;
if (easy)
{
tx = (2.86 * Math.sin (0.00 + (pi2 * (time / 16.670)))) + (1.15 * Math.sin (1.57 + (pi2 * (time / 6.250)))) + (0.57 * Math.sin (3.93 + (pi2 * (time / 9.091))));
ty = (2.29 * Math.sin (0.79 + (pi2 * (time / 8.000))))
+ (1.72 * Math.sin (4.72 + (pi2 * (time / 11.110)))) + (1.72 * Math.sin (2.36 + (pi2 * (time / 50.000))));
}
else
...
<< move target to (tx,ty) >>
}
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Track
(set-‐task "cs630.Tracking")
(sgp :v nil :emma t )
(start-‐hand-‐at-‐mouse)
(add-‐dm (track-‐goal isa track) (choice-‐goal isa choice) )
(goal-‐focus track-‐goal) (goal-‐focus choice-‐goal)
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Track
;; Tracking Task
(p Bind-‐target =goal>
isa track ?visual-‐location>
state free -‐ buffer requested ?visual>
state free buffer empty
==>
+visual-‐location>
isa visual-‐location kind cross )
(p move-‐to-‐target =goal>
isa track =visual-‐location>
kind cross ?visual>
state free buffer empty ?manual>
state free
==>
+visual>
isa move-‐attention screen-‐pos =visual-‐location +manual>
isa move-‐cursor loc =visual-‐location )
(p repeat-‐track =goal>
isa track =visual>
isa cross ?manual>
state free
==>
)
Track
;; Choice Task (p Bind-‐arrow =goal>
isa choice ?visual-‐location>
state free -‐ buffer requested ?visual>
state free buffer empty
==>
+visual-‐location>
isa visual-‐location kind text :attended nil )
(p arrow-‐not-‐found =goal>
isa choice ?visual-‐location>
state error
==>
-‐visual-‐location>
)
(p encode-‐arrow =goal>
=visual-‐location>
kind text ?visual>
state free buffer empty
==>
+visual>
isa move-‐attention screen-‐pos =visual-‐location )
(p respond-‐left =goal>
isa choice =visual>
isa text value "<"
?manual>
state free
==>
+manual>
isa punch hand left Binger pinkie )
(p respond-‐right ...
)
Track
1849.957 procedural ** FIND-ARROW ** [choice-goal]!
1849.957 vision error!
1850.007 procedural ** ARROW-NOT-FOUND ** [choice-goal]!
1850.020 vision unrequested [vision~27824]!
1850.057 procedural ** FIND-TARGET ** [track-goal]!
1850.057 vision find-location [vision~27827]!
1850.107 procedural ** MOVE-TO-TARGET ** [track-goal]!
1850.107 vision move-attention!
1850.107 motor move-cursor vision~27827!
1850.107 motor preparation-complete!
1850.148 vision encoding-complete [cross~27832]!
1850.157 motor initiation-complete!
1850.242 eye preparation-complete [cross~27832]!
1850.315 eye execution-complete [cross~27832]!
1850.529 motor move cursor (255 10)!
1850.579 motor finish-movement!
1850.629 procedural ** REPEAT-TRACK ** [track-goal]!
1850.679 procedural ** FIND-ARROW ** [choice-goal]!
1850.679 vision find-location [vision~27836]!
1850.729 procedural ** ENCODE-ARROW ** [track-goal]!
1850.729 vision move-attention!
1850.864 eye preparation-complete [text~27839]!
1850.995 eye execution-complete [text~27839]!
1851.023 vision encoding-complete [text~27839]!
1851.073 procedural ** RESPOND-LEFT ** [choice-goal]!
1851.073 motor punch left pinkie!
1851.123 procedural ** FIND-TARGET ** [track-goal]!
...!
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Track
■ Process timeline
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Tracking & Choice
■ Results: Choice response time
Tracking & Choice
■ Results: Tracking error
Dual-Choice Tasks
■ A choice task means – get a (simple) stimulus – produce a (simple) response
■ Dual-choice tasks ask a person to do 2 choice tasks at almost the same time
■ Several factors are often varied in experiments using this paradigm
– perceptual modality: visual / aural – motor modality: manual / vocal
– cognitive difficulty of stimulus
àresponse
mapping
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Dual-Choice Tasks
■ Example (with consistent S à R mappings) – visual-manual task
– aural-vocal task
O – – – – O – – – – O –
low tone
one mid tone
two high tone
three
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Dual-Choice Tasks
■ In dual-choice tasks, there’s usually a delay between one stimulus and the other
– called the “SOA” = “stimulus onset asynchrony”
– e.g., if we start the aural-vocal task at time = 0, we might present the visual-manual task at time = {.000 .050 .150 .250 .500 1.000}
■ Compared to the single-task case, how long will the visual-manual task take when...
– SOA is big?
(stimuli far apart)– SOA = 0?
(concurrent stimuli)– somewhere in between?
The PRP Effect
■ Psychological Refractory Period (PRP) effect – “refractory” from analogy with cells returning
to normal after excitation (not a great analogy, but it stuck)
The PRP Effect
■ What causes the PRP effect?
■ For a long time, it was assumed to be a solid indicator of a “cognitive bottleneck”
■ A box-diagram depiction:
– this is the “response-selection” bottleneck – it’s also a bit misleading...
Is there really an inherent cognitive bottleneck?
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Experiments
■ Schumacher et al. (2001), Experiment 1 – people can achieve perfect time-sharing!
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Experiments
■ Schumacher et al. (2001), Experiment 1 – people can achieve perfect time-sharing!
– table with final results after learning only...
Single-Task Dual-Task
Aural-Vocal 446 ms 456 ms
Visual-Manual 281 ms 283 ms
Experiments
■ Schumacher et al. (2001), Experiment 2 – PRP effect comes from instructions /
constraints: do Task 1, then do Task 2
Perfect Time-Sharing
■ Perfect time-sharing follows readily from threaded cognition
– in this case, everything works out perfectly; no
interference, no dual-task/PRP effect!
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Dual-Choice Tasks
■ An inconsistent S à R mapping requires an extra step to retrieve the mapping
O – – – – – – O
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Dual-Choice Tasks
■ An inconsistent S à R mapping requires an extra step to retrieve the mapping
– with possible interference, as here (A)
Dual-Choice Tasks
■ Increased perceptual difficulty makes perception for the tasks run into each other
O O O O O O O O
Dual-Choice Tasks
■ Increased perceptual difficulty makes perception for the tasks run into each other
– with possible interference, as here (B)
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Problem State
■ Problem state = temporary information required during task execution
– roughly speaking, a task’s “mental context”
– in ACT-R: stored in the imaginal buffer
■ Example: Solving 3+4
– encode “3”, then “+”, then “4”
– all this is now held in the problem state / imaginal buffer
– in this case, used to pass along information for retrieval
– can also be used to remember new information
• i.e., associate 3, +, 4, and then 7 with one another
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Problem State
■ Example: Writing a paper
– what do you need to keep in mind as you’re writing a...
• sentence?
• paragraph?
• section?
– where do you maintain the least amount of information?
■ Example: Tracking, or Driving – no problem state needed!
Memory & Interruptions
■ Memory clearly plays an important role in interruptions
■ What are the (at least two) important features of human memory?
– information strengthens with use – information decays over time
Memory for Goals
■ Memory for goals theory (Altmann &
Trafton, 2002) + ACT-R memory theory (Anderson et al., 2004)
– to suspend a task, people encode (rehearse) the current goal until it’s readily available in memory
• in ACT-R, each retrieval boosts a chunk’s activation, making it easier to recall
• e.g., rehearse “I’m ordering a platypus” a few times
– to resume the task, people simply recall the goal
• in ACT-R, associated cues can facilitate recall
• e.g., seeing computer, or browser on platypus web page
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Memory for Goals
■ Memory for goals as threads...
■ Encoding
■ Retrieval
Primary task Secondary task Primary task
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Memory for Goals
■ Memory for goals as threads...
■ Encoding
■ Retrieval
Rehearsal Retrieval
Primary task
Secondary task
Primary task
Memory for Goals
■ How many retrievals/rehearsals is a “good”
number for a typical interruption?
Interruption Study
■ Monk, Trafton, & Boehm-Davis (2008) – explored effects of interruption duration &
demand
on primary-task resumption – primary task: programming a VCR
– interruption duration: 3, 8, or 13 seconds – interrupting task
• no-task: just wait
• track: manual tracking task
• n-back: compare current and previous letters (<,>)
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Interruption Study
■ Model
– model for primary task
• full model not needed…
• specified declarative chunk to represent the goal
• estimated time parameter for performing first action
– models for interrupting tasks
• no-task: trivially waits
• tracking: does the tracking
• n-back: simplified from previous work
(Juvina & Taatgen, 2007)– uses declarative resource to retrieve last item!
– model for interruption process described earlier – but when exactly should encoding occur?
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Interruption Study
■ Encoding strategies
– S1: Encode during the entire interruption – S2: Encode for n seconds, concurrently with
secondary task
– S3: Encode until retrieval takes no more than n seconds, concurrently with the secondary task – S4: Encode for n seconds prior to the
interruption, ending at the onset of the interruption
– S5: Encode for a few (3) retrievals prior to the interruption, ending at the onset of the interruption
Interruption Study
Monk et al. (2008)
Interruption Study
Monk et al. (2008) Model – S1
(rehearse entire interruption)
no effect of duration
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Interruption Study
Monk et al. (2008) Model – S5
(rehearse few times before interrupt)
duration effect too large;
no n-back interaction
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Interruption Study
Monk et al. (2008) Model – S4
(rehearse for n sec before interrupt)
no n-back interaction
Interruption Study
Monk et al. (2008) Model – S3
(rehearse until retrieval < n sec)
strange n-back interaction (interference forces too much encoding!)
Interruption Study
Monk et al. (2008) Model – S2
(rehearse for n sec after interrupt)
Yes!
n-back interaction due to declarative interference
R2=.94
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Interruption Study
■ ACT-R model for S2 – interleaved with tracking
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