Implicit memory

Implicit memoryImplicit memory

• Schacter and many other researchers distinguish between implicit and explicit memory

• This lecture will begin by surveying some historical observations regarding implicit memory

• It will then focus on a particular type of implicit memory, habits or skills, that are mediated by the basal ganglia

• Particular attention will be given to lesion studies and studies of patients with Parkinson’s disease, which is associated with impaired basal ganglia functioning


• implicit memory– occurs when previous experiences facilitate

performance on a task that does not require conscious recollection of those experiences

• explicit memory– occurs when the task requires conscious

recollection of past experiences


• Historical survey– Maine de Biran proposed that after sufficient

repetition a habit can be executed automatically and unconsciously without awareness of the act itself or of the previous episodes in which the habit was learned

– Maine de Biran also developed a form of multiple memory system


• Historical survey– Carpenter (1874) noted the importance of

autobiographical recognition or awareness in normal memory

– NeurologyKorsakoff noted that amnesic patients were affected by

previously experienced events even though they were consciously unaware of these events

Claparede refusal of amnesic to shake hands after he had pricked her hand


• Historical survey– Neurology

Schneider 1912 showed that amnesics required less information across learning trials to identify fragmented pictures


• Historical survey– Psychiatry

Freud and Janet investigated patients suffering hysterical amnesia as a result of emotional trauma

these patients could not explicitly remember the traumatic event, but their memories of these events were expressed indirectly (implicitly)

Janet --hysterical amnesia consists of: 1. the inability of a subject to evoke memories consciously and voluntarily; and 2. Automatic, compelling, and untimely activation of these memories


• Modern research on implicit memory– effects of subliminally encoded stimuli

several studies have shown that stimuli that are not represented in subjective awareness (consciously) are nevertheless processed to high levels by the perceptual system


– e.g., Kunst-Wilson & Zajonc (1980)presented geometric shapes for 1 ms; claimed it was

too brief a period of time to permit its perception; subsequently tested recognition (forced choice) and preference (forced choice)

Results: recognition (RN) was at chance; Subjects preferred the previously presented geometric shape


– e.g., Bargh & Pietromonaco (1982)presented ‘hostile words’ and then later had

participants rate a target personresults showed that explicit recognition memory of

‘hostile words’ was at chance, although ratings of the target person were more negative than those who did not receive prior exposure

e.g., divided attention study of Eich (1984)


– e.g., divided attention study of Eich (1984)auditory divided attention taskunattended channel -- presented homophones (e.g.,

taxi fare)subsequently participants showed no explicit RN

memory in yes/no task, but tended to spell homophones in biased direction compared to baseline performance


• Learning and conditioning without awareness– participants learn rules or contingencies without

explicit memory for them– this phenomenon was studied in multitrial

learning experiments and in classical conditioning experiments


– Implicit learning studies of Rebersubjects were presented letter strings that were

organized according to rules of an artificial grammarReber reported that subjects could categorize these

strings correctly even though they were unable to consciously aware of the rules


– Repetition priming effectsfacilitation in processing of a stimulus as a function of

recent prior exposure to the same stimulusnote: repetition priming has been observed under a

wide variety of test conditions, none of which require explicit reference to a prior study episode

lexical decision (word/nonword) -- DV = latencyword identificationword stem or fragment completion (e.g., __ ss__ss__)


– Repetition priming effects– 1. used to study nature of lexical representation

e.g., studied the effects of auditory presentation on subsequent word identification and lexical decision tasks

results showed little or no priminge.g., morphologically similar words (e.g., seen)

facilitate priming of (sees), but visually similar words do not prime each other (e.g., seen) versus (seed)


– Repetition priming effects– 2. used to study relation of implicit to explicit memory– stimulated by studies of Warrington and Weiskrantz

on amnesics– this study showed that amnesics showed excellent

retention when they were asked to complete three-letter stems of recently presented words even though their yes/no recognition memory was impaired


– Repetition priming effects– 2. used to study relation of implicit to explicit memory

several studies have shown that variations in level or type of study processing have differential effects on priming versus remembering

e.g., Jacoby & Dallas (1981) showed that answering questions about the meaning of a target word improved yes/no recognition memory relative to answering questions about presence of a letter, but that word identification was unaffected


– Repetition priming effects– 2. used to study relation of implicit to explicit

memorymodality shiftJacoby & Dallas (1981) showed that changing from

auditory (at study) to visual (at test) severely attenuated priming effects as assessed by word identification, but had little effect on yes/no recognition performance


– Repetition priming effects– 2. used to study relation of implicit to explicit

memoryother factors that have been manipulatedstudy-test delay--variable effectsmanipulations that affect retroactive and

proactive interference (and hence explicit memory) have little effect on word-stem or word fragment completion


– Habit or skilled learning– Several lines of evidence suggest that the

caudate nucleus mediates a particular type of implicit memory, habit memory or memory for skills, that are incrementally acquired

– This type of memory can be dissociated from explicit memory

– Caudate nucleus is part of the basal ganglia

ExplicitMemory

ImplicitMemory

Facts(Semantic Memory)

Events(Episodic Memory)

Priming

SimpleClassical

Conditioning

Skills andHabits

NonassociativeLearning

EmotionalResponse

SkeletalMusculature

Squire and Knowlton (1994)[Squire (1987): Declarative vs. procedural memory]

Medial Temporal Lobe

Striatum

Neocortex

AmygdalaCerebellum

Reflex Pathways

Habits and skillsHabits and skills

• Characteristics of habits and skills– Habits are learned (acquired via experience dependent

plasticity)– Habits are performed repeatedly over time– Habits are performed almost automatically and

nonconsciously– Habits tend to involve an ordered, structured, action

sequence– Habits are elicited by a particular context or sequence

(either internal or external)– Habits can be habits of thought and habits of motor

expression


• Background – Anatomy– Basal ganglia (BG) plays an important role in

normal voluntary movement– Input to BG received from cerebral cortex and

output sent to brain stem and via the thalamus back to the prefrontal, premotor, and motor cortices

BG in relation to brain structuresBG in relation to brain structures

• Basal ganglia receives i/p from cerebral cortex

• o/p from BG goes from thalamus back to cerebral cortex

• o/p also goes to spinal cord


• Background – Basal ganglia– Consists of 4 major nuclei– Caudate nucleus – Putamen– Globus pallidus– Subthalamic nucleus– Striatum consists of caudate nucleus, putamen, and

ventral striatum

BGBG

• Coronal view of BG in relation to surrounding structures

• Medial view of BG: note left side is anterior; right side is posterior

Habits and skillsHabits and skills• Background (circuits in BG)

– There are several circuits connected to prefrontal regions of the cortex; these include

• Dorsolateral prefrontal circuit– Involved in executive function

• Lateral orbitofrontal circuit– Plays a major role in mediating empathic, socially

appropriate responses• Anterior cingulate circuit

– Appears to play an important role in motivated behavior


• Background (diseases associated with BG deficits) – Movement disorders associated with basal ganglia

• Parkinson’s disease (described by Parkinson 1817)– Hypokinetic disorder– Impaired initiation of movement (akinesia)– Reduced amplitude and velocity of movement (bradykinesia)– Involuntary tremulous movement– Lessened muscular power– Increased muscle tone (rigidity)– Shuffling gait


• Parkinson’s disease (described by Parkinson 1817)– Results from an insufficiency of a neurotransmitter (dopamine) in the

substantia nigra– It has been shown that px with PD have a deficiency in dopamine in the

striatum, most particularly in the putamen– in 1960s it was shown that injection of L-DOPA, the precursor of dopamine

led to a reversal of symptoms temporarily (for about 5 years)

• Huntington’s disease– Hyperkinetic disorder– Excessive movements– Heritable and results from a gene defect


• Role of BG in learning and memory– BG is involved in S-R association formation or

the formation of habits or skills, and this form of memory is dissociable from declarative memory

• Animal lesion study (see Eichenbaum, 2002 for further information)– This has been investigated in an 8-arm radial

maze with different task requirements (e.g., McDonald & White, 1993)


• Animal lesion study– Win-shift maze – maze is put in a room and the rat is permitted to

encode stimuli around the room to provide spatial cues– on every trial a food reward is placed at the end of the

maze; rat is released from centre of maze and is allowed to retrieve food

– optimal performance occurs if the rat enters each arm only once. This means the rat must have memory for the mazes that were previously entered


• Role of BG in learning and memory– Win-stay maze – – 8-arm radial maze used, but maze surrounded by a curtain,

and lamps were used to illuminate 4 randomly selected arms of the maze;

– food was baited in the illuminated mazes only; when an arm was entered for first time, the maze was rebaited; after second time the light was turned off and no more food was provided;

– task was to associate a particular stimulus (light) with food across all trials and it was expected that declarative memory was not required, but habit memory was required

–

radial arm mazesradial arm mazes

• Radial arm maze configurations to illustrate win-shift and win-stay procedures;

• + = rewarded arm


• Results– Win-shift– Rats with hippocampal damage made more errors than controls by

visiting previously visited maze arms– Rats with striatal damage were unimpaired compared to normal

animals (controls)– Win-stay– Normal control rats learned strategy over several sessions– Rats with hippocampal damage learned somewhat faster than

controls– Rats with striatal damage were badly impaired and were at about

chance levels of performance even after extended practice


• Conclusions– Selective damage to the hippocampus impairs

performance on a task requiring declarative memory

– Selective damage to the striatum interferes with habit learning

Sequence learningSequence learning

• Human research– Further support for the dissociation between habit

memory and declarative memory– Sequence learning


• Human research– Evidence suggests that amnesics are unimpaired in

manual sequence learning (Reber & Squire (1998). Journal of Cognitive Neuroscience 10, 248-265

– This perceptual-motor habit was investigated in a serial response time (SRT) task paradigm

– SRT task. Participants are shown a visual cue in 1 of 4 locations by pressing a key corresponding to the location of the cue

– Cues follow a repeating sequence of locations (n = 12; 124313214234)


• Amnesics and controls and controls were given SRT training. Other unimpaired control groups (Memory) memorized the sequence of responses (see Table for details). All groups had their implicit and explicit memory tested

• The Memory control groups memorized the sequence of responses by watching the computer screen and attempting to memorize the sequence of responses


• Results

• Amnesics and controls had decreasing SRT RTs to practiced SRT across trial blocks

• Next figure presents implicit memory performance by group

• Note amnesics and controls were the only groups to show implicit memory


• Top panel. Open bars present RT for sequence S1 (practiced by amn and con) open bars; shaded bar presents RT for N1 (unpracticed)

• Bottom panel—difference between S1 and N1 (measure of implicit memory


• This Figure presents for each group explicit memory performance, assessed by recognition memory performance of 5 sequences (1 target and 5 lures) on a scale from 0 to 100


• Results

• Note: amnesics showed little evidence of explicit memory, in comparison to other groups

• Memory controls (esp. young) showed explicit memory performance

Sequence LearningSequence Learning

• Filled squares – amnesic• Open square –combined

performance of young subjects

• Open triangle – controls• Open circle – Mem old

participants• Note: cross-over interaction


• Conclusions• Proposed that cross-over interaction shown in

Figure 5 reflects encapsulation of implicit and explicit memory in different memory systems

• Explicit memory mediated by hippocampal/diencephalic brain structures

• Implicit memory mediated by motor cortex, neostriatal, supplementary motor area


• Human research (PD)– Wilkinson 2009, Neuropsychologia, 47, 2564-

2573– Investigated implicit & explicit memory in SRT

task in PDs and controls– Hypothesized that PDs would be impaired in their

implicit memory for SRTs and would also be impaired in their explicit memory for explicitly presented SRTs


• Human research (PD)– Wilkinson 2009, Neuropsychologia, 47, 2564-2573– Exp. 1 – implicit SRT– Sequence was presented probabilistically to introduce

noise and decrease opportunities for explicit learning– E.g., one group of Ps were shown 3-1-4-3-2-4-2-1-3-4-1-

2, 85% of the time and 4-3-1-2-4-1-3-2-1-4-2-3, 15 % of the time

– 10 blocks of trials each block with 120 trials of SRT task


• Human research (PD)– Implicit sequence learning assessed by

comparing difference between probable and improbable sequence Reaction time


• PDs shown in back; controls shown in white

• ** designates blocks in which difference between probable and improbable sequence is significant (p < .05)

• Note: evidence of implicit learning for controls stronger than for PDs


• Human research (PD)– Explicit sequence learning – After final implicit SRT block presented, explicit sequence

learning was tested– Inclusion condition – participants were presented a

sequence of 5 cues from probable sequence and were instructed to recall the next correct response (Inclusion)

– Exclusion condition – participants were presented sequence of 5 cues from probable sequence and were instructed to respond with the next cue that was inconsistent with the training sequence


• Results show that for controls and PDs there was no significant difference between inclusion probable and baseline and between exclusion probable and baseline

• Hence no evidence of explicit memory for sequence


• Human research (PD)– Conclusion – Wilkinson data suggest that PDs are

impaired in probabilistic sequence learning relative to controls

– There is no evidence for explicit memory for either group– Findings suggest that BG plays a role in implicit sequence

learning; – Findings by Reber and Squire, which showed that

amnesics were not impaired in sequence learning, suggest that a form of habit memory was not mediated by MTL

Implicit memory- cognitive habitsImplicit memory- cognitive habits

• Human research: Cognitive habits– Traditionally the impairment in the basal ganglia was

known to result in motor deficits based on observations of patients with PD and Huntington’s disease

– Aside PD affects dopaminergic input to the striatum; Huntington’s disease affects cells in the striatum

– It now appears that BG is involved in acquisition of cognitive habits that involve the gradual formation of S-R associations


• Human research– Probabilistic classification learning– Knowlton and her colleagues have studied px and their

performance on the probabilistic classification task– weather prediction task– Participants are presented 1 to 3 of 4 cards and task was

to predict “sunshine” or “rain” based on cues presented; cards had complex geometric shapes

– Each card and card combination was probabilistically associated with 2 outcomes

– after making prediction participant was given the actual outcome (see next figure)

Probabilistic classification learningProbabilistic classification learning

• Panel A. Probability of sunshine for individual cards

• Panel B. conjoint probabilities associated with various card combinations and example of trial series


• Human research– Probabilistic classification learning– Note: task has been designed so that specific memory for

outcomes associated with card combinations is not as useful, first because there are several card combinations, and second, because any given card combination may have an outcome of ‘sunshine’ or ‘rain’ associated with it

– More useful is a general sense of the relationship between cues and outcomes across trials


• Human research• Probabilistic classification learning

– Results– Controls– Over 50 trials normal controls improved from pure guessing (50%) correct to

about 70 % correct (highest performance possible)– PD px did not show significant learning and performance was worse in px

with more severe Parkinsonian symptoms (Knowlton, 1994, Science, 273, 1399-1402)

– After training PD px and controls were given questions about the task and nature of the stimuli (declarative tasks); performance of the two groups did not differ

– Results from amnesics show an opposite dissociation – unimpaired performance on the weather prediction task and impaired performance on the declarative memory task (Knowlton, 1994, Learning and Memory 1, 106-120; Reber, 1996, Behavioral Neuroscience, 110, 859-869)


• Human research– Interactions between implicit and declarative

memory– When task conditions favor implicit learning

medial temporal activity decreases as striatal activity increases (Foerde, 2006; Foerde, 2007)


• Human research– Foerde et al. (2006) PNAS, 103, 11778-11783– This experiment investigated the hypothesis that the

declarative and habit memory systems compete with each other in tasks in which both systems are involved

– This raises the possibility that it might be possible to modulate the operation of these systems by introducing a secondary task

– Previous research has shown that performing a secondary task that requires working memory should decrease declarative memory and thereby bias (increase) the operation of the habit memory system


• Human research– Prediction then is that dividing attention should affect the

way in which a task is performed – Overview of procedure and design (see also next figure).

Participants were given weather prediction training (PCT task) for two different cities based on presentation of two sets of cues; half the cues for a given city were trained under single task conditions (ST) and half the cues for that city were trained under dual task conditions (DT)

– Secondary task was tone counting task in which participants counted the number of high tones


• Human research– After training participants were given a probe run in which

participants were shown cues that had been trained under ST or DT conditions and were required to predict sun or rain (done in fMRI); no tone task administered here

– After scanner, participants were given test of declarative memory which required flexible knowledge about task (e.g., participants shown all 4 single cues and asked to select cue most likely to be associated with sun; task repeated for two-cue combinations, and 3-cue combinations)

Foerde procedureFoerde procedure

• Tone counting• Train on weather

prediction (single, dual)• In DT condition report

number of high tones• Baseline button push

condition• Probe run – predict

weather


• Results– Behavioral results shown in next figure showed that PCT

performance during training was higher after run 2 than run 1 and marginally higher in ST than DT (See a)

– Probe task (see b) no difference between ST and DT in weather prediction performance in probe task

– Cue selection (test of declarative memory, see c) participants were significantly better in the ST than DT condition suggesting that a more flexible memory representation was obtained

• Fig a. PCT performance during training

• Fig b. PCT performance during probe task

• Fig c. declarative memory performance


• Results– fMRI results in next figure (see b) showed that probe

accuracy was correlated with right hippocampus activity in ST but not DT condition

– Figure d showed that probe accuracy was correlated with left putamen activity in DT but not in ST condition

– Recall hippocampus associated with declarative memory– Putamen is part of BG, which is implicated in habit

memory

fMRI resultsfMRI results


• Conclusions– Memory for PCT can be either MTL or striatal– Declarative memory depends on executive

functions, but habit learning does not appear to depend upon it (recall secondary task effects)

– Although PCT performance can be supported by MTL or striatal systems, the type of memory system mediating performance affects flexibility of memory (recall declarative memory test)


• Conclusions– In disease states involving dysfunction of the BG

medial temporal lobe may be present when normally striatum activity would dominate (Moody, 2004)

Implicit memory

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