Learning of a Complex Arithmetic Skill in Amnesia - Science Direct

BRAIN

AND

COGNITION

8,

91-104 (1988)

Learning of a Complex Arithmetic Skill in Amnesia: Evidence for a Dissociation between Compilation and Production WILLIAM GRECC.

VA Medical

MICHAEL Neurology

Department,

Boston

MILBERG Center

P. ALEXANDER

University

School

of Medicine,

Braintree

Hospital

NEIL CHARNESS Psychology

Department,

of Waterloo

University

REGINA MCGLINCHEY-BERROTH GRECC,

VA Medical

Center,

West Roxbury,

Massachusetts

AND ANNA GRECC,

VA Medical

Center,

BARRETT West Roxbury,

Massachusetts

Two patients with severe amnesia following rupture of anterior communicating artery aneurysms were able to learn a complex algorithm for mentally squaring two-digit numbers. Although both patients learned the algorithm at a similar rate, one patient’s improvement was accounted for by savings in the steps of the algorithm. The other patient, however, showed little improvement in the steps while performance of the whole algorithm improved dramatically. Neither patient showed savings in the Hebb Digit Span procedure. The results suggested a dissociation between amnesics in their capacity to learn the constituent “proThis work was sponsored by grants to W. Milberg by the VA Merit Review 097443765001 and by NIA Grant ROlAG03354-03, by NSERC Grant A0790 to N. Charness, and by Grant NSO6209 to H. Goodglass at the Boston University School of Medicine. Thanks to John Gabrielli for his comments on this manuscript. Address correspondence and reprint requests to William Milberg, GRECC, 1400 VFW Parkway, VA Medical Center, West Roxbury, MA 02132. 91 0278-2626188 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

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ductions” or steps of a skill and their capacity to organize those productions into a single coherent act. 0 1988 Academic Press, Inc.

One theme that has emerged from analyses of skilled perceptual-motor behavior in humans is that the individual movements that make up a skill must be distinguished from the organization of those movements into a larger goal-directed act (see Gallistel, 1980). Anderson (1982) suggests that this distinction applies not only to perceptual-motor skills but to the more general domain of cognitive skill. He divides cognitive skill into two main stages. The first stage consists of specific goal-oriented steps called “productions.” The subject may already have these productions in his repertoire, but must adapt them to the goal at hand, adding them as needed until the goal can be achieved. Anderson argues that subjects must explicitly access each of these independent steps in their attempt to match them to a task. In this stage each production requires attention and working memory, making execution slow and inefficient. The second stage is the process of “compilation,” in which sequences of individual productions are collapsed into larger task-specific productions. These are accessed as a whole and hence require less working memory and attention than did the individually executed component steps. As the subject becomes more expert the steps of the skill become organized into larger units that can be executed with increasing speed and efficiency. Investigations of patients with severe memory deficits have provided evidence of some capacity to learn new skills. For example, H.M., severely amnesic following bilateral hippocampectomy, learned a number of perceptual-motor skill tasks at a normal or nearly normal rate (Milner, 1962; Milner, Corkin, & Teuber, 1968; Corkin, 1968). Skill learning has also been demonstrated in amnesic patients of different etiologies (Cermak, Lewis, Butters, & Goodglass, 1973). Normal or relatively preserved learning in amnesics has been demonstrated for cognitive skills in which sensorimotor factors have been minimized (Kinsbourne & Wood, 1975; Cohen, 1984; Glisky, Schacter, & Tulving, 1986). In most demonstrations of skill acquisition in amnesics, however, it has not been possible to examine separately learning based on improved performance of component steps from learning based on the “compilation” of those components into larger units of production. An exception was the recent report of an amnesic Korsakoff patient who was found capable of learning a seven-step algorithm for mentally squaring two-digit numbers at a rate comparable to normal controls his age (Chamess, Milberg, & Alexander, 1987). This learning occurred despite the patient’s inability to accurately describe the steps of the algorithm, coupled with his denial throughout the learning sessions that he had previously encountered the technique. The patient improved slightly on the individual steps, but significantly in overall performance. Learning

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93

the mental squaring skill was attributed more to changes in the “compilation” or organization of the steps, than to improvement in the steps themselves. In the present case study, further evidence that a cognitive skill may be “learned” either by facilitation of the individual “productions” or by better “compilation” was gathered from an analysis of two patients with amnesia following anterior communicating artery aneurysm rupture. The same mental squaring algorithm paradigm described by Charness, Milberg, and Alexander (1987) was used and two distinct patterns of performance were found. The cases provide further evidence that skill learning may be based either on improvements in the execution of individual steps, or on an improvement in the organization of those steps within the context of a whole goal-directed act. METHODS Subjects Case 1 J.D. had a subarachnoid hemorrhage from an aneursym at the junction of the left anterior cerebral artery and the anterior communicating artery (ACoA). The aneurysm was surgically clipped 3 days after onset of the illness. His postoperative course was marked by confusion. The patient is right-handed, a high-school graduate with college courses. He was 43 years old at the time of admission. He worked as a police officer but had been disabled for years with a chronic back injury. The elemental examination was normal. Mental status examination revealed normal language, praxis, drawing, topographical knowledge, and proverb interpretation. Serial sevens and digit span were normal but he was totally disoriented to place, time, nature of his illness, and course of his hospitalization. He could not recall anything of his illness or public or personal history for at least 1 year prior to his surgery. He frequently confabulated about personal and historical facts. He was cheerful and unconcerned. The elemental examination was normal. At 6 months after the onset, he acknowledged that he had a memory problem and was aware of his illness. He knew the names and roles of his outpatient therapists. He often said that he did not know about any current events, but, when prompted, he usually provided ample appropriate detail. Neuropsychological assessment. Partial assessment was carried out 6 weeks after onset. Results are summarized in Table 1. CT. Nonenhanced CT performed 6 weeks after onset demonstrated a patchy wedgeshaped hypodensity in the orbital cortex of the right frontal lobe, extending back to the suprasellar cistern, and a discrete area of hypodensity lying irregularly between the frontal horns of the lateral ventricles with slight extension into the anterior corpus callosum. These changes are compatible with infarction in the territory of a branch of the right anterior cerebral artery and in the distribution of the perforating branches of the ACoA.

Case 2 R.M. had a sudden onset of severe headache followed by a grand ma1 seizure. Taken to a local hospital, he was oriented but lethargic. Evaluation revealed a subarachnoid hemmorrhage and an aneurysm of the ACoA. The aneurysm was surgically clipped 2 days after onset. Postoperatively the patient was extremely lethargic and almost mute. Over the next 5 weeks, he became more akinetic. A ventriculoperitoneal shunt was placed. The patient was 40 years old at the time of admission. He is right-handed. He has an llth-

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TABLE NEUROPSYCHOLOGICAL

ASSESSMENT

1

RESULTS

FOR PATIENTS

J.D.

AND

R.M.

R.M. J.D. Wechsler memory scale Logical memory Paired associates Visual reproductions Full scale IQ Trials A B Famous faces 20s 30s 40s SOS 60s

70s California verbal A B A Immed

77 3/23 6.5/21 3/14 107 (WAIS-R) 27 set 90 set 4 5 3 6 5 5 5/6/10/g/13 5 0

2 month 74 2123 6121 2114 103 (WAIS)

9 month 86 4123 7121 6114

65 set 230 set 4 4 5 5 6 4 3/3/4/4/4 3 0

grade education and had been employed as a construction foreman. There is no history of prior neurological event. The patient consumed 6-10 beers nightly for years. On examination he was alert but slow. He had frequent episodes of sudden irritability and would refuse further assessment. Language, praxis, drawing, and proverb interpretation were normal. He was oriented only to place. He had no knowledge of his illness, and produced only very sparse recollection of any historical events in the past year. He confabulated dramatically, often utilizing likely occurrences from his previous work experiences. By 6 months after onset, he was generally aware of the nature of his illness and was approximately oriented and aware of his management plans, but he was unable to learn any new information on testing at the bedside and still confabulated. Neuropsychological assessment. Partial testing was completed at 2 months after onset. More complete testing was done 9 months after onset. The results are also summarized in Table 1. CT. A CT performed 3 months after onset demonstrated exactly the same two areas of hypodensity as patient J.D. in the right orbital frontal cortex and in the space between the frontal horns of the lateral ventricles. In addition, the ventricles were moderately enlarged and a ventriculoperitoneal shunt was in place in the right frontal horn. Neurosurgical evaluation had indicated that the shunt was functioning despite the large ventricles.

Apparatus All experimental tasks were administered via a Commondore 64 microcomputer attached to a Commodore 1702 color monitor. Voice response times were recorded with a handheld microphone attached to a Lafayette Instruments voice-operated relay (VOR) connected through the User Port. All other responses were entered on the keyboard by the examiner. Response timing was accurate to approximately ?34 msec due to limits in the scanning rate of the computer monitor.

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General Procedure For most of the tasks described below the patient was shown a flashing fixation point on the computer screen that lasted for 1 or 2 sec. This was followed by the number(s) to be operated upon, named, or recalled and the initiation of response timing. The subject’s verbal response triggered the VOR that removed the number from the screen. Response times were recorded and the experimenter entered the subject’s numerical answer into the computer via the keyboard. There was a 3- to 4-set interval between trials. The subject was instructed to respond as quickly as possible while maintaining accuracy. The experimenter(s) prompted and encouraged the patient during each task, and reminded him of the procedure when needed. The patients were tested either in their private hospital rooms or in a quiet testing room. Each session lasted approximately 90 min, beginning with an interview consisting (after the first session) of requests for the examiners names, the purpose of the current and previous sessions, and the number of previous sessions. All sessions were recorded on audio tape and transcribed. Patient J.D., who was tested soon after the onset of his amnesia, was extremely slow in performing all tasks. Patient J.D. received three initial test sessions and a 6-month follow-up session. The follow-up was included because of the wide discrepancy in the time postonset of amnesia between the initial testing of patient J.D. and patient R.M. The data from J.D.‘s 6-month follow-up were compared to the data from the fourth test session of R.M. Patient R.M., who was tested approximately 8 months postonset, received a total of six test sessions in 1 week. Three sessions equivalent to those administered to patient J.D. are used for analysis.

Task Procedures Hebb Digit Span (Drachman & Arbit, 1966) In this task, the patient’s forward digit span was established using a staircase procedure, starting with three digits and incrementing by one digit if the patient’s response was correct, and decrementing by one digit if the patient’s response was a trial incorrect. The procedure was halted when six reversals of direction were obtained. Span was thus defined as the mean of the string lengths at the last five reversals, rounded up to the nearest whole digit. Once this baseline span was determined, 30 trials of span +-digit strings were presented for recall. The string given on the 3rd trial was repeated on the 6th, 9th, 12th. etc., trial, unless recalled correctly, in which case it was replaced by a new string of equal length which in turn was repeated at the same interval. Strings were randomly generated from the digits 0 through 9, with repetitions of digits within strings permitted. Numbers were presented serially at the center of the screen at the rate of approximately 1 digit/set. The trial began with the presentation of an asterisk accompanied by a tone. Trials were initiated when the patient indicated he was ready. No feedback was provided.

Squaring

Components

The components of the squaring task (COMP) consisted of trials that tested the individual steps of the squaring procedure. In each trial a number was displayed on the screen preceded by a brief verbal description of the desired operation. The patient had to verbally provide an answer to each problem as described. The experimenter(s) elaborated on the instructions before each trial, and prompted the patients during the trials when necessary. The set of 148 total problems was randomly ordered. Trials in which a misreading of the digits occurred or where the trial was terminated incorrectly were repeated at the end. The seven types of problems comprising the “steps” of COMP were as follows:

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Step I. Find the nearest multiple of 10 (NMT) for a target number (e.g., given 27 generate 30). Step2. Find the other number (OTN) for a target that is as far from the target as is the nearest multiple of 10 (e.g., given 27, generate 24). Step 3. find a constant (C) that represents the absolute value of the difference between the target number and the values determined in Steps 1 and 2 (e.g., 27-30 or 27-24, C = 3). Because C is an automatic consequence of Step 2 it was not measured separately in COMP. Step 4. Find the product of the nearest multiple of 10 and the decade digit of the other number (Pl) (e.g., 30 x 20). Step 5. Find the product of the nearest multiple of 10 and the units digit of the other number (P2) (e.g., 30 x 4). Step 6. Find the sum of the latter two products (SUM) (e.g., 600 + 120) Step 7. Add the square of the constant C* to the sum of the two products in Step 6 (S + C2) (ei.g., 720 + 32). For the NMT and OTN operations, four problems involved a difference of 1, 2, 4, or 5, and two involved a difference of 3. There were 16 PI and 16 P2 problems and 32 which involved taking products which were external to the squaring procedure (EPl, EP2). There were also 16 SUM problems and 16 that involved sums external to the procedure (ESUM). Finally, there were 16 S + C* problems all from the squaring environment. Squaring Squaring (SQR) involved 66 problems between 1 and 99. The problems were divided into four classes. Classes 1, 2, and 3 consisted of the numbers l-12, decade numbers lo90, and two-digit numbers ending in 5 between 15 and 95, respectively. Class 4 consisted of numbers that could most efficiently be squared using the entire algorithm: 17, 18, 21, 42, 26, 32, 33, 37, 38, 41, 44, 46, 49, 52, 53, 57, 58, 61, 64, 66, 69, 72, 73, 77, 78, 81, 84, 86, 89, 92, 93. These numbers were further subdivided into two sets balanced as closely as possible for the size of the constant, the decades digit for NMT, and OTN and size of carries in the SUM Step. The first set designated the PRACTICE set was presented during each of the test sessions. On the final session of squaring the full set of problems from 1 to 99 were presented, with the second subset of the Class 4 problems designated as the transfer set. Classes 1 through 3 were treated as filler items. Only Class 4 problems which required the use of the full algorithm are presented for analysis in this paper. For each trial the number to be squared appeared on the screen following an instruction to square it and work through the problem by speaking out loud. The experimenter pressed a button to halt timing when the full answer was given, and then keyed in the response. The order of trials was randomized and incorrectly terminated trials were rescheduled at the end. Feedback was provided after the answer was entered. Several practice trials preceded the main sequence. The order of tasks for both subjects was as follows: Day 1: NAME, MULT, COMP, HEBB-SPAN; Days 2-6: MULT, SQR, HEBB-SPAN; Day 7: MULT, SQR, HEBB-SPAN, NAME, MULT, COMP, HEBB-SPAN. Simple multiplication (MULT) and digit naming (NAME) were originally included to examine changes in the most basic level skills required for the arithmetic algorithm. Both patients showed some improvement in MULT, and a slight decrement in NAME. The functions assessed by these tasks were felt to be measured by a number of the subtasks in the COMP task described below and are largely peripheral to the focus of the study. These data will therefore not be discussed further.

AMNESIA

AND SKILL PATIENT

r z

08

' k g

06

E b g

02

J.D.

6KNlliFULCW-U'

0.4

00

3 I.

2 PATIENT

0 L 2 E

0.6

tB

02

Ml.

04

I

2 TESTIN

FIG. 1. Proportion patient R.M.

97

q REPEATED DIGITS 8 WZlQPEATE6 DIOITS '*

I

T ‘D

COMPONENTS

3

SESSION

of digits recalled in correct serial position for patient J.D. and for

RESULTS Differences in performance between test sessions were analyzed with correlated t tests with a conservative significance level of .Ol to compensate for the increased likelihood of Type 1 error. Marginally significant differences are also reported. Hebb Digit Span J.D.

There was a slight change in the baseline digit span over the test sessions for patient J.D. His digit span was 6.0, 6.6, 8.6 in the first, second, and 6-month follow-up sessions, respectively. The proportion of digits recalled in the span + 1 condition did not change over the first two test sessions for either nonrepeated, t(19) < 1, or repeated digits, t(9) < 1 (see Fig. 1). There was a nonsignificant decrease in the proportion of digits recalled from the second session to the 6-month follow-up for the repeated digits, t(9) = 1.01, p < .lO. The decrease in the proportion of digits recalled in the nonrepeated conditions was significant, t(19) = 2.20, p < .05, from the second to follow-up sessions. This is perhaps not surprising given the increase in baseline span noted above. The proportion of digits recalled for repeated and nonrepeated strings

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ET AL.

y=185.285-27.78%

R'091

y=,,%W-21231x

R=OBJ

q

.

TESTING

JO PLAN01 FTl mul RT

SESSION

FIG. 2. Mean correct response time and regression analysis for squaring Class 4 items, for patients J.D. and R.M.

did not differ in session 1, t(28) < 1, session 2, r(28) < 1, or in the 6month follow-up, t(28) < 1. R.M. Like patient J.D. there was a slight change in the baseline digi span over the test sessions for patient R.M. His baseline digit span was 5.0, 5.6, 5.0 in the first, second, and third test sessions, respectively. As can be seen in the bottom half of Fig. 1, the proportion of digits recalled in the span + 1 condition did not change over the first two test sessions for either nonrepeated, t(19) < 1, or repeated digits, t(9) < 1. There was a slight increase in the proportion of digits recalled from the second session to the third session in both the nonrepeated, t(19) = 2.96, p < .Ol, and repeated, t(9) = 2.93, p < .05, span conditions. There was a small advantage for repeated strings in session 1, t(28) = 2.18, p < .05, but not in session 2, t(28) < 1, or in session 3, t(28) = 1.64, p > .lO. Mental Squaring Squaring (The Use of the Whole SQUARING Algorithm) Figure 2 shows the means and the results of a regression analysis. J.D. Patient J.D. showed no change in the speed of solving Class 4 problems from session 1 to session 2, t(6) < 1, but did improve significantly from session 2 to session 3 t(13) = 4.27, p < .Ol, and from session 3 to the follow-up session, t(13) = 5.37, p < .OOl (Fig. 2). The time to perform the problems in SQUARE was 146.23 set for the first session and 96.98 set for the last session, a difference of 49.25 sec. The time to perform the SQUARE problems was 73.21 set the 6-month follow-up, an additional decrease of 23.77 sec. R.M. Patient R.M. (Fig. 2) showed significant improvement from session 1 to session 2 for Class 4 problems, t(l8) = 4.99, p < .Ol. No further improvement in response time was seen from sessions 2 to 3, t(25) < 1, or from sessions 3 to 4, t(25) = 1.62. The time to perform the problems

AMNESIA

AND SKILL PATIENT

99

COMPONENTS

J.D. n

G

FIRST SfSSIo( m LAST SESICN I3 6tlCNTH

50

3-40

Famkw

2F

30

‘;I

20

= b %

IO 0 NFIT

OTN

P2

Pl

PATIENT s

5

gj

4

2 F

3

P

2

::

I

SW1

S+C2

R-H.

n FIRSTSfSSIcN a

LAST SfSSI’34

n OI

-

N-lT

OTN sauAaIN6

Pl

P2

sul

SC2

CDHPDNENTS

FIG. 3. Mean correct response time on squaring components for patient J.D. and for patient R.M.

in SQUARE was 115.6 set in the first session, and 41.6 set in the last session, a difference of 74.0 sec. Regression analysis showed a strong correlation and linear relationship between solution time and test sessions for J.D., R = .91, and R.M., R = .83. The slopes of the regression lines suggest the rate of improvement was similar for both patients. Components (The Steps of the SQUARING Measured)

Problems Zndividually

The trimmed mean correct scores for each component were compared across the first and last sessions of the initial test series for both patients. In addition performance changes from the last session of the initial test series to the 6-month follow-up were analyzed for patient J.D. These results are presented in Fig. 3. The results of the analysis of changes for each component are presented in Table 2. J.D. Patient J.D. showed significant improvements on components NMT and OTN, and a marginally significant improvement on S + C* (Fig. 3 and Table 2). J.D. also showed significant improvements from the last session to the 6-month follow-up on NMT and P2, and marginal improvements on Pl and S + C2.

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TABLE I-TEST

VALUES

2

FOR FIRST vs. LAST SESSION FOR TASK COMPONENTS

Patient: Component NMT OTN PI P2 SUM s + c2

J.D.

R.M.

t(l7) = 4.49*** t(l7) = 3.74** t(l5) = 1.35

r(l3) = 0.37 1(13) = 0.34

r(l5)

r(11) = 0.91

=

0.83

f(9)

=

1.41

r(l5) = 0.21

r(6)

=

2.03

r(l5)

t(3)

=

1.15

=

2.46*

Last session vs. 6-month follow-up patient J.D. Component NMT OTN Pl P2 SUM S+c2

r(l4) r(l5) t(l5) r(11) ~(12) t(12)

= = = = = =

3.83** 1.69 2.49* 5.10*** 1.27 2.42*

* p < .05. ** p < .Ol. *** p < .ool.

R.M. Patient R.M. did not improve significantly on any of the component tasks (Fig. 3 and Table 2). Comparison

of COMP to SQUARE

Although both subjects showed some change in the response time the components of the squaring task and the squaring task itself, relationship between total change in the time needed to complete steps of COMP and the change in the time needed to complete SQUARE problems was different for the two patients (Fig. 4).

JD

PATIENT

for the the the

"'

FIG. 4. Performance changes in response time from the first to last session for components and squaring, for patients J.D. and R.M.

AMNESIA

AND SKILL

COMPONENTS

101

i, PATIENT PATENT . . 6 “WTH

JO b-t? FOLLOW-UP FCR PATlEN .m

1

2 TESTING

3

4’

SESSION

FIG. 5. Mean number of instructions per trial for squaring Class 4 numbers, for patients J.D. and R.M.

J.D. The sum of the steps in COMP in the first session was 93.5 set, and 44.0 set in the last session, a difference of 49.54 sec. The sum of the steps in the follow-up session was 23.3 set, an additional decrease of 20.27 sec. Note that the change in the time to perform the SQUARE problems is almost identical to the change in the SUM of the steps at both comparison points! R.M. The sum of the steps in COMP in the first session was 17.6 set, and 13.5 set in the last session, a difference of 4.07 sec. Note that in the case of patient R.M. the change in the SQUARE problems was 70.0 set greater than the change in COMP! Analysis of Cues

Both patients required encouragement and prompting during the course of testing. After reading the transcripts of the test sessions it was noted that the need for prompting (henceforth called “cues”) could be a distinguishing feature of the patients’ ability to learn the algorithm. Cueing was used by the examiner whenever the patient paused for an inordinately long period of time between steps or specifically asked for help. Cues were divided into two general categories: Explicit cues were defined as cues that provided a specific description of the next appropriate step called for by the squaring algorithm; implicit cues were defined as the use of any encouragement to proceed to the next step, or general reference to the next step, without a specific description of how to go about that step. The experimenters used explicit cues primarily upon the patient’s request, or when the experimenters felt that the use of an implicit cue would not elicit an answer. Implicit cues were often but not exclusively used before explicit cues, particularly in the latter sessions. Data are expressed as cues per trial. J.D. The mean number of cues per trial (both explicit and implicit) needed by patient J.D. (Fig. 5) did not change from session 1 to session 2, t(6) = 2.20, p > .lO, but did decrease from session 2 to session 3,

102

MILBERG PATIENT

I

ET AL. J.D.

PATIENT

R.tl.

2

3

4

SESSION

FIG. 6. Percentage of total instructions accounted for by explicit and implicit cues, for patient J.D. and for patient R.M.

t(13) = 3.36, p < .Ol. A marginal decrease can also be seen from session 3 to the 6-month follow-up, t(13) = 2.53, p < .05. Over the four test sessions the number of cues per trial required by patient J.D. decreased by 54%. Examination of the proportion of explicit cues (explicit cues divided by total cues) shown in Fig. 6 reveals a significant decrease from the first session to the follow-up session, t(6) = 4.04, p < .OOl. The decrease in the proportion of explicit cues is obviously accompanied by a corresponding increase in the proportion of implicit cues per trial over the same sessions, f(6) = 3.67, p < .Ol. R.M. The mean number of cues per trial needed by patient R.M. (Fig. 5) decreased significantly from session 1 to session 2, t(17) = 4.42, p < .OOl, from session 2 to session 3, t(23) = 1.78, p < .05, and also from session 3 to session 4, t(23) = 3.05, p < .Ol. Over the four test sessions the number of cues per trial required by patient R.M. decreased by 87%. In contrast to patient J.D., the proportion of explicit cues required by patient R.M., as shown in fig. 6, increased from the first to the last session, t(17) = 4.18, p < .OOl, and the proportion of implicit cues decreased across the test sessions, t(17) = 8.55, p < .OOl. R.M. only required a mean of 1.2 cues per trial by the last test session. DISCUSSION

Both amnesic patients performance on a mental

showed a dramatic ability to improve their squaring task by using a complex arithmetic

AMNESIA

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103

algorithm. Furthermore it appeared that this learning was greater than would have been expected from Hebb Digit Span performance that, like the mental squaring paradigm, used frequent repetition and a savings measure of learning. It appeared that there is something special about the requirement to combine basic arithmetic operations in the service of a specialized algorithm that is not captured in tasks requiring the learning of relatively arbitrary sequences of new information, no matter how sensitively this learning is assessed. However, there was a difference in the relationship between each patient’s improvement on the mental squaring task and his improvement on the constituent steps of the squaring task. Patient J.D. improved equivalently on the steps of the algorithm and the whole mental squaring task. Patient R.M., however, improved minimally on the steps of the algorithm while improving significantly on the squaring algorithm itself. These results suggest that for patient J.D., improvement on the squaring task could primarily be accounted for by improvements in individual steps. For patient R.M. improvement on the squaring task could be accounted for only by assuming improved efficiency in combining the steps into the whole algorithm. It is possible that for patient J.D., any improvements in the use of the whole algorithm were overshadowed by improvements in the rate of general response speed summed over the individual steps. However, even with this caution, the relative improvement in mental squaring observed for patient R.M. exceeded the total improvement in mental squaring for patient J.D. The difference between these patients’ quality of performance suggests that mental skill learning procedures may be dissociable in amnesic patients. Some patients may improve the execution of the component steps of a skill, while others may improve the organization of those steps, even when neither has explicit recall of the tasks or their strategies for learning. An examination of the cueing data may provide some insight into the cause of the observed learning differences between these two patients. The post hoc analysis of the cueing data indicated that both patients needed cues (both explicit and implicit) to complete the squaring task. R.M. remembered the steps of the algorithm and he used them spontaneously, but often he would forget a specific step. In contrast J.D. needed to be prodded to continue calculations even though he was ultimately able to remember the step required. Improvement in the overall performance of the mental algorithm must be dependent on the capacity to spontaneously initiate its component steps. Without this capacity further refinement of the organization of the steps appears not to be possible. The generality of this phenomena must await similar “cueing” data reported and analyzed in other studies of skill acquisition in amnesics. Moscovitch, Winocur, and McLachlan (1986) recently demonstrated that amnesic patients could learn item-specific information and interitem associations when tested implicitly with a speed-reading task. The current

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results show that the learning of interitem associations extends to the complex chains of responses that comprise mental skills. This conclusion is consistent with the process of compilation described by Anderson (1982) and other theorists. It is possible that “compilation” is the critical preverved function that allows the learning of other skills in amnesic patients. REFERENCES Anderson, J. R. 1982. Acquisition of cognitive skill. Psychological Review, 89(4), 369406. Cermak, L. S., Lewis, R., Butters, N., & Goodglass, H. 1973. Role of verbal mediation in performance of motor tasks by Korsakoff patients. Perceptual and Motor Skills, 37, 259-262. Charness, N., Milberg, W. P., & Alexander, M. P. 1987. Teaching an amnesic a complex cognitive skill. Submitted to Brian and Cognition. Cohen, N. J. 1984. Preserved learning capacity in amnesia: Evidence for multiple memory systems. In L. Squire & N. Butters (Eds.), Neuropsychology of memory. New York: Guilford Press. Pp. 83-103. Corkin, S. 1968. Acquisition of motor skills after bilateral medial temporal lobe excision. Neuropsychologia,

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