working memory
Post on 09-Jan-2016
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Working Memory
Active short-term memory – Maintenance of task-relevant informationonline. Keeps relevant information available.
Like RAM in a computer
Close correspondence between working memory and consciousness
Delayed response tasks are used to WM in animals
Spatial delayed response
Object delayed response
also alternation tasks
In all cases, a response is based on some previous event that is relevant on the current trial, but not necessarily the next.
Sample
What Delay
Test Array
Where Delay
What-Then-Where Task
4 objects and 4 locations were used
Choice
Fixation (2000 msec)
Sample (500 msec)
Delay (1000 msec)
Test (500 msec)
S1
S2
S3
C1
C2
C3
Delayed paired associate task
Important features of WM tasks
A cue is followed by a delay and then two or morechoices
Delay is relatively short (about a second to about a minute).
Small number of cues that are used repeatedly. A singlecue is only relevant for one trial at a time and animalsmust remember
Delay tasks require that:
1. Animals discriminate among cues
2. Maintain memory across a delay
3. Evaluate choice phase and make correctdecision
Brain areas involved in WM?
Today’s take-home message:
Working memory is not simply short-term memory.It is an active process in which subjects must select andmaintain information. It is under volitional control.
Two fundamental observations of behavior duringa typical WM task (delayed match-to-sample)
1. Performance increases with increases in sample
Correct choice
Sample Delay Test
Time
2. Performance decreases with increases in the delay
(Much of this work has been conducted in pigeons)
Also:Match rule retained inreference memory.
The copy/trace model was meant to highlight what WMis NOT.
Other observations of DMS performance:
1. Changing the illumination of the testing box during retention intervalinterferes w/ performance. (doesn't matter whether light->dark dark->light) This depends on degree of "surprise". It will habituate. Expected events have no effect on performance.
2. Surprising samples are more memorable.
Suggests that animals actively process info about the SM during delay.When surprised, attentional resources drawn to event, less left for actively maintaining the sample.
Another example: Directed Forgetting
Next: Neurophysiologicalevidence for WM as anactive process
Sample
What Delay
Test Array
Where Delay
What-Then-Where Task
4 objects and 4 locations were used
Choice
'What" Delay Activity in PF Cortex: Single Cell
Time from sample onset (msec)
0 500 1000 1500 2000 2500
Spikes per second
20
30
40
50
60
70
80
90
100 'What' Delay 'Where' Delay
Sample “A”
Sample “B”
a40_1_01
'What' and 'Where' Delay Activity: Single Cell
'What' Delay
Time from sample onset (msec)
0 200 400 600 800 1000 1200 1400
Sp
ike
s p
er s
eco
nd
20
40
60
80
100
Sample A
Sample B
a43_1_32
'Where' Delay
1400 1600 1800 2000 2200 2400 2600 2800
Down
Up
A monkey test of attention
Sample
Array Trials
Delay Test stimuli
Nonmatch Match
Remember the identity and location of only the relevant object
Cue Trials
750 msec 1500 msec Bar release
Neural correlate of awareness in the prefrontal cortex
Time from sample onset (msec)
0 1000 2000
Spikes per second
0
50
100
Time from sample onset
0 1000 2000
Sample Test
“A” alone “B” alone
Neural correlate of awareness in the prefrontal cortex
Time from sample onset (msec)
0 1000 2000
Spikes per second
0
50
100
Time from sample onset
0 1000 2000
Sample Test
Attend “A” “A” alone “B” alone
Neural correlate of awareness in the prefrontal cortex
Time from sample onset (msec)
0 1000 2000
Spikes per second
0
50
100
Time from sample onset
0 1000 2000
Sample Test
Attend “A” “A” alone Attend “B” “B” alone
WM is not a passive copy of a sensory stimulusinstead an active process So, in contrast to the copy/trace model, WM is:1. Active - Ss actively select, encode and maintain, what is heldin WM.2. Not a simple match rule. Simple maintenance of record of samplethen application of a matching rule. Rather something more complexis going on. One thing that may be happening is that instead ofa matching rule, Ss develop an expectation, they anticipate whatis going to happen and base their behavior on this expectation
What is info is maintained in the delayof WM tasks?
Retrospective versus Prospective processing
Correct choice
Sample Delay Test
Time
There is a considerable amount of evidence that animals tend to relyon a prospective code for performance in delay tasks.
In symbolic DMS, increasing the delay between sample and choice stimuli causes an increase confusions between similar choice stimuli, not similar samples.
Fixation (2000 msec)
Sample (500 msec)
Delay (1000 msec)
Test (500 msec)
S1
S2
S3
C1
C2
C3
Fixation (2000 msec)
Sample (500 msec)
Delay (1000 msec)
Test (500 msec)
S1
S2
S3
C1
C2
C3
Delayed paired associate task
Reaction Time [msec]
250 300 350 400 450
Fre
quen
cy o
f occ
uren
ce [%
]
0
5
10
15
20
S1S2
S3
C1
C2C3
S1 à C1C1 à C1
S2 à C2C2 à C2
S3 à C3C3 à C3
Time [msec]
0 500 1000
Pro
spec
tive
Inde
x
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
***
* P<0.01
Conclusions
Working memory is an active process. It is under volitionalcontrol and can be contrasted with passive or automaticforms of memory.
Working memory is prospective. It is geared toward the future, notmerely a record of past events.
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