Neump\~rhologro, V-01. 29, No. 7, pp. 629-640, 1991 Printed in Great Bntain. 0028&3932/91 S3.00+0.00 Pergamon Press plc RECALL OF THE END-POSITION OF EXAMINER-DEFINED ARM MOVEMENTS BY PATIENTS WITH FRONTAL- OR TEMPORAL-LOBE LESIONS GABRIEL LEONARD* and BRENDA MILNER Department of Neurology and Neurosurgery, McGill University and the Montreal Neurological Institute, 3801 University Street, Montreal, Quebec, Canada H3A 2B4 (Received 27 September 1990; accepted 2 February 1991) Abstract-Sixty-three patients with unilateral temporal- or frontal-lobe excisions and 14 normal control subjects were tested on a kinesthetic task in which they had to recall the end-position of arm movements determined by the examiner. Patients with left frontal-lobe or small right frontal-lobe excisions performed normally, whereas patients with temporal-lobe excisions that included extensive removal of the hippocampus performed poorly, but only after a delay during which an interpolated task was carried out. Subjects with large right frontal-lobe removals were impaired in delayed recall, and the presence of an interpolated task did not exacerbate the impairment, which was equal for the two arms. The results point to an important role played by the right frontal lobe in the maintenance of kinesthetic-location cues over time, but not in their initial encoding. INTRODUCTION LEONARD and MILNER [12] have demonstrated that patients with large excisions from the right frontal lobe have difficulty in reproducing accurately the extent of examiner-defined arm movements, the movements being made without the aid of vision. The deficit was manifest not only on the arm contralateral to the lesion but also on the ipsilateral one. Patients with left frontal-lobe excisions, or with temporal-lobe removals from either hemisphere, demonstrated no such difficulty. The impairment in the right frontal-lobe group was not dependent on recall-condition, being apparent irrespective of delay, thus strengthening the notion that the deficit was primarily one of encoding. It was concluded that the right frontal lobe is critically involved in the monitoring of information related to movement. The possibility that the impairment of patients with large right frontal-lobe exisions on the arm-movement task might be specific to the reproduction of distance traversed remained an open question. Therefore, the present study examined how well patients with frontal- or temporal-lobe lesions could encode and subsequently recall the end-position of simple arm movements (again with vision excluded), when the distance traversed offered no useful cue, the starting position being changed for each movement. *Address for correspondence: Gabriel Leonard, Department of Neuropsychology, Montreal Neurological Institute, 3801 University Street, Montreal, P.Q., Canada H3A 2B4. 629
Transcript
Neump\~rhologro, V-01. 29, No. 7, pp. 629-640, 1991 Printed in Great Bntain.
0028&3932/91 S3.00+0.00 Pergamon Press plc
RECALL OF THE END-POSITION OF EXAMINER-DEFINED ARM MOVEMENTS BY PATIENTS WITH FRONTAL- OR
TEMPORAL-LOBE LESIONS
GABRIEL LEONARD* and BRENDA MILNER
Department of Neurology and Neurosurgery, McGill University and the Montreal Neurological Institute, 3801 University Street, Montreal, Quebec, Canada H3A 2B4
(Received 27 September 1990; accepted 2 February 1991)
Abstract-Sixty-three patients with unilateral temporal- or frontal-lobe excisions and 14 normal control subjects were tested on a kinesthetic task in which they had to recall the end-position of arm movements determined by the examiner. Patients with left frontal-lobe or small right frontal-lobe excisions performed normally, whereas patients with temporal-lobe excisions that included extensive removal of the hippocampus performed poorly, but only after a delay during which an interpolated task was carried out. Subjects with large right frontal-lobe removals were impaired in delayed recall, and the presence of an interpolated task did not exacerbate the impairment, which was equal for the two arms. The results point to an important role played by the right frontal lobe in the maintenance of kinesthetic-location cues over time, but not in their initial encoding.
INTRODUCTION
LEONARD and MILNER [12] have demonstrated that patients with large excisions from the right frontal lobe have difficulty in reproducing accurately the extent of examiner-defined arm movements, the movements being made without the aid of vision. The deficit was manifest not only on the arm contralateral to the lesion but also on the ipsilateral one. Patients with left frontal-lobe excisions, or with temporal-lobe removals from either hemisphere, demonstrated no such difficulty. The impairment in the right frontal-lobe group was not dependent on recall-condition, being apparent irrespective of delay, thus strengthening the notion that the deficit was primarily one of encoding. It was concluded that the right frontal lobe is critically involved in the monitoring of information related to movement.
The possibility that the impairment of patients with large right frontal-lobe exisions on the arm-movement task might be specific to the reproduction of distance traversed remained an open question. Therefore, the present study examined how well patients with frontal- or temporal-lobe lesions could encode and subsequently recall the end-position of simple arm movements (again with vision excluded), when the distance traversed offered no useful cue, the starting position being changed for each movement.
*Address for correspondence: Gabriel Leonard, Department of Neuropsychology, Montreal Neurological Institute, 3801 University Street, Montreal, P.Q., Canada H3A 2B4.
629
630 G. LEONARD and B. MILNER
A second goal of the investigation was to examine the possible contribution of verbal and visuo-spatial strategies to the encoding, storage and recall of location information derived from kinesthesis. If verbal labels are of critical importance, then it would be predicted that patients who have had left temporal-lobe removals, and who typically display a selective impairment in verbal memory [14], would perform poorly after a delay. If, on the other hand, forming a visual representation of the end-position of an arm movement can facilitate its maintenance in memory, one might expect to find an impairment after extensive right hippocampal removals, given the known importance of the right hippocampus for the visual recall of location [S, 20, 221.
Subjects
METHOD
Each patient who participated in the study had undergone a unilateral cortical excision at the Montreal Neurological Hospital. The operations were carried out for the relief of focal cerebral seizures. In most instances the epileptogenic lesion dated from birth or early life, and was static in nature, but the group also included 15 cases of indolent tumour. Patients who had electrographic abnormalities arising independently from both hemispheres, or who had evidence of fast-growing tumours or diffuse cerebral damage were excluded, as were those patients known to have right-sided, or bilateral, speech representation (as demonstrated by preoperative intracarotid Amytal tests [3]). Only patient between the ages of 16 and 60 with Full-Scale Wechsler IQ ratings above 80 took part in the experiment.
The hand preference of each subject had been assessed by means of a modified version of the questionnaire developed by CROVITZ and ZENER [6]. On this scale, a score of 29 or less signifies a strong preference for the right hand [15]; all subjects accepted for these studies had a score below this cut-off point.
In order to ensure that subjects had no motor or sensory deficits that could have interfered with their test performance, the strength and the somatosensory status of the hands were determined beforehand. Grip-strength was measured by having the subjects pull on a wooden handle connected to a Dillon Tensile-Force Gauge graduated in pounds [23]. Three readings were taken for each hand in a balanced order, beginning with the hand ipsilateral to the lesion. For men, scores below 75 lb for the right hand, and 74 lb for the left hand resulted in exclusion from the study. The corresponding scores for women were 50 lb for the right hand and 49 lb for the left hand. These cut-off scores, which were two standard deviations below the group means, had been established for a normal control group well matched to the patienis in the present investigation with respect to age, education and occupational status [ll]. The sensory status of the hands was also determined by quantitative testing [4]; the measures used included two- point discrimination on the palm and sense of passive movement of the fingers. All patients obtained normal scores for both hands on these tests and on a test of tactual object recognition [4].
Sixty-three patients and 14 normal control subjects were assigned to groups, as shown in Table 1. Also given in Table 1 are the sex distribution, mean age and mean education for all the groups and the mean Wechsler Full-Scale IQ and the time of testing for the patient groups. Separate one-way analyses of variance failed to reveal any significant differences between the groups with respect to age (F= 1.51, d.f. =7,69, P=O.17) or education (F= 1.95, d.f. = 7, 69, P=O.67). The patient groups did not differ significantly from one another in terms of IQ (F=0.66, d.f. = 6, 56, P= 0.67).
Temporal-lobe groups. The patients within the temporal-lobe groups were subdivided according to the extent of removal from the hippocampal region. This method of classification resulted in four groups: two left temporal, one (LTh) with small and one (LTH) with large removals from the hippocampal region; and two right temporal-lobe groups, one (RTh) with small and one (RTH) with large hippocampal excisions. The hippocampal removals were classified as small or large on the basis of the surgeon’s drawing and report at the time of operation. The removal was considered to be small if no more than the pes of the hippocampus was excised; the hippocampal removals were classified as large if the excision encroached further upon the body of the hippocampus or the parahippocampal gyrus. The removals in all cases included the anterior temporal neocortex and the amygdala. Two patient with left temporal-lobe lesions and two with right-sided lesions were operated upon for removal of low-grade astrocytomas.
Frontal-lobe groups. For the patients with frontal-lobe resection, the extent as well as the side of the removal was considered important, because, in our previous study of kinesthetic memory for distance traversed [12], only patients with large frontal-lobe lesions had had a deficit. Since the left hemisphere was dominant for speech in all cases, the removals from the left frontal lobe tended to be more conservative than those from the right, and only two patients had large removals; therefore, the subjects who had left frontal-lobe lesions were assigned to one group (LF) for all analyses, The patients with right frontal-lobe excisions had roughly equal numbers of subjects with small (RFS) and with large (RFL) resections of frontal cortex; hence these patients were maintained in two separate groups. The classification of the lesions into small and large was carried out by an independent judge.
KINESTHETIC LOCATION
Table 1. Main subject groups
631
Group Sex Age Education Wechsler IQ Time of testing
M F Mean Range Mean Range Mean Range Postoperative Follow-up
The extent of the removals in these nine patients is illustrated in Figure 1. The removals varied from small frontopolar (An. Be,), or small orbitofrontal (Ho. Sp.), to large dorsolateral excisions that included the medial cortex (MO. Au., No. Sa., Dw. Mi.). Two patients (An. Co., Wi. Wh.) had limited removals of the dorsomedial cortex and two (An. Be., Ke. Ke.) had dorsolateral removals, which also included the frontal pole; the other four patients had removals of the lateral convexity as well as the medial aspect of the frontal lobe. The LF group included four cases of low-grade astrocytoma (An. Co., Dw. Mi., Ho. Sp. and Wi. Wh), two cases of arteriovenous malformation (An. Be., Ke. Ke.), and one case of brain-scarring consequent to an old infarct (No. Sa.).
Right frontal-lobe group with small remooals
Figure 2 shows the cortical excisions for the nine patients in this group. Three patients had excisions ofthe inferior lateral cortex, and the remaining six had lesions that partially encroached on the dorsolateral and medial surfaces. Six patients had low-grade gliomas (Ro. Bo., Br. Fe., Br. Fo., Cl. Ji., Ro. Ma., El. Sa.).
Rightji-ontal-lobe cyoup with large removals
Illustrated in Fig. 3 are the cortical removals for the six patients in the RFL group. All these patients had extensive removals from the lateral convexity as well as from the medial cortex, except for the patient Bo. Cr., whose removal spared the orbital cortex. One patient (Ru. Ma.) had a low-grade astrocytoma.
Normal control subjects
A normal control group (NC) of I6 subjects was selected to match the patient groups as far as possible with respect to sex, age and level of education (see Table I).
Apptrr~rtus trrtd prowdurr
The apparatus used was a slightly modified version of the Manual Lever D, described by SULZER [24]. Figure 4 shows the examiner’s view of the apparatus, which consisted of a movable lever (0.9 cm in diameter) mounted on a flat board. The radius of the lever was 30 cm. as measured from the centre of the shaft to the top of the grip. The total possible range of movement was 140” (73.3 cm ofarc), and the scale, which was attached to the examiner’s side of the instrument. was marked in I steps. A metal plate, mounted on the same side as the scale, had holes drilled at each degree of arc, thus enabling the examiner to insert a metal rod to stop the lever at any predetermined position. The machine was kept covered, so that subjects on entering the room were unable to see it.
At the beginning of the testing session. after the subject had been seated comfortably in the adjustable chair with the chin resting on the chin-rest (behind the opaque screen), the subject was permitted to explore the range of movement of the lever. Subjects were then told that they would be asked to make arm movements to a particular location. which would be defined by the experimenter. In order to ensure that the subjects understood the task requirements. six practice trials were given. three with each hand, preceded by the following instructions:
632 G. LEONARD and B. MILNER
An. Be.
c3
Ho. "/";-7\
HIJ.
No. Sa.
@J
Fig. 1. Diagrams based on the surgeon’s drawings at the time of operation, showing the estimated extent of the cortical excisions for subjects in the left frontal-lobe group. For all figures illustrating frontal-lobe resections, the medial view (above) and the inferior view (below) will be included
whenever available, together with the lateral view.
“I am now going to place your hand on the lever. You are then to move the lever until it comes to a stop; remember where this stop is, and replace your hand on the table.”
Following a short delay, during which time the lever was moved by the experimenter to a different starting-position, the subject’s hand was again placed on the lever and the subject was instructed to return the lever to its previous stopping position (i.e. the criterion end-point). Upon completion of these practice trials, the examiner said:
“From now on, you will be asked to recall the stopping-point either immediately after you have moved to it, or following a delay during which you may or may not have had to count backwards by threes from a number that I will read to you, (Here an example was given and the subject had the opportunity to practice counting backwards by threes.) The starting-position of the movement will always be different, so that trying to remember the distance moved would not be a useful strategy. You will be asked to make 36 movements with each hand, and I would like you to try to be as accurate as you can on every trial. The test is designed to examine how accurately you can recall the location of the various stopping-points.”
Da. Co. m KINESTHETIC LOCATION
Ro. Bo.
633
Br. Fe. ,
GI. Ji.
Fig. 2. Diagrams showing the estimated extent of cortical excision for the patients in the right frontal-lobe group with small removals.
Each subject received 72 trials, 36 with each hand. For reproduction of the various locations, four recall-conditions were employed: less than 5 set (termed “immediate”), 30 set unfilled interval, I5 set with an interpolated counting task, and 30 set also with interpolated counting. The test employed I8 different locations, repeated four times. Locations, for the right hand, began at 128” (see Fig. 4), and were evenly distributed across the scale, except that the vertical position (90”) was omitted; the mirror image (32”) was used for the left hand. Trials involving the same recall-condition were not allowed to follow one another more than once within a block. To randomize the order of presentation, two blocks of trials were constructed for each hand. Blocks were presented either in the order ABBA (beginning with the hand ipsilateral to the lesion) or BAAB (beginning with the hand contralateral to the lesion). These orders were alternated within groups for successive subjects. The experiment lasted approximately 85 min and was performed in one session.
RESULTS
The data were analysed in terms of three dependent variables: (1) absolute error (the difference between the criterion movement and the subject’s actual response-score, without respect to sign), (2) variable error (a measure of within-subject variance that takes into account the error scores for each subject about the subject’s own mean), and (3) constant error (the average algebraic error). A separate analysis of variance was performed on each of
634 G. LEONARD and B. MILNER
00. Gr. i?rs Jo. Ed.
B
Da. Ya.
(29 Ra. Mi.
Ma. Si.
Fig. 3. Diagrams showing the estimated extent of cortical excision for the patients in the right frontal-lobe group with large removals.
these dependent measures. Hence, an 8 x 4 x 2 design (group x recall-condition x hand) was used, with repeated measures over the last two factors. Only absolute and constant error results are reported here, because the variable and absolute error results were very similar.
Absolute error
Main effect ofhand. There was no indication of a hand by group interaction in this analysis (F=0.64, d.f. = 7,69, P>O.72), and therefore the results reported below are collapsed across hands. There was, however, a main effect of hand (F= 6.41, d.f. = 1,69, P < 0.02), the right- hand performance being more accurate than the left (left, mean = 8.34”; right, mean = 7.74”).
Group x recall-condition interaction. The absolute-error analysis yielded a two-way interaction between group and recall-condition (F= 2.06, d.f. = 21, 207, P < 0.01). Post hoc examination of this interaction, with respect to between-group differences, revealed no significant findings for immediate recall (see Fig. 5). Following 30 set with no interference, the RFL group displayed a significant loss of information, relative not only to the control subjects but also to each of the other groups (Q > 2.87 vs all groups).
Figure 6 shows that, after 15 set during which a distracting task was performed, the RFL group was again impaired with respect to the NC and also to the LF groups (Q=5.85, P<O.Ol; and Q=4.27, PcO.05, respectively). Following a 30 set delay with distraction the patients with RFL excisions were impaired relative to the NC (Q = 6.74, PC 0.01 ), the LTh (Q=4.84, P<O.O5), the RFS (Q=4.51, P<O.O5)and the LFgroups (Q=4.39, P<O.O5).The patients in the LTH and RTH groups were also impaired in their recall relative to the NC
KINESTHETIC LOCATION 635
Fig. 4. Modified version of the Manual Lever D [24]. Above: examiner’s view. Below: subject’s position with respect to the apparatus. The arrow indicates the direction of movement for the right
hand.
group at the 30 set recall-condition with distraction (Q=5.90, P<O.Ol; Q=4.04, P~0.05, respectively). The RTh group also appeared to have difficulty with this recall-condition, but the difference between their mean score and that of the NC group just failed to reach significance (Q = 3.71 where a value of 3.86 is required).
Main Q&v qf recall-condition. There was a significant main effect of recall-condition (F= 89.35, d.f. = 3,207, P<O.OOl). Further exploration of this result showed performance to be most accurate in immediate recall (see Fig. 8). Recall after 30 set, although worse than
636 G. LEONARD and B. MILNER
T
::
:::: :::: ..‘. ::::
‘...
Group LTh LTH LF RTh RTH RFS RFL NC
13 6 9 9 11 9 6 14
Fig. 5. Absolute error: group x recall-condition interaction, without counting. The RFL group is impaired relative to the NC group and is significantly worse than all of the other patient groups in the
30 set recall-condition.
LTh LTH LF RTh RTH RFS RFL NC
N 13 6 9 9 11 9 6 14
Fig. 6. Absolute error: group x recall-condition interaction, with counting. The RFL group is impaired relative to the normal control group under both recall conditions. In addition, the LTH and
RTH groups are worse than the NC group following 30 set with counting.
immediate recall (Q = 9.76, P < 0.01) was significantly better than that after 1 S or 30 set with interpolated activity (Q = 3.84, P-C 0.01, Q = 12.67, PtO.O1, respectively). In addition, 15 set with counting was associated with more accurate recall than 30 set with the same distraction (Q = 8.82, P < 0.01 ), the latter being the most difficult condition for all groups.
Main effect of group. The analysis yielded a main effect of group (F=3.32, d.f.=7, 69, P< 0.01); post hoc comparisons showed that the performance of the NC, RFS, LTh and LF
KINESTHETIC LOCATION 637
Recall Condbm
Fig. 7. Absolute error: main effect of recall-condition collapsed across group. Increased error is associated with time elapsed and with the performance of an interpolated task.
WIthout Countmg With Countmg
30s 1%
Recall Condition
Fig. 8. Constant error: main effect of recall-condition. There is a general tendency to undershoot following a 30 set untilled interval, whereas overshooting is a feature of recall-conditions that include
an interpolated task.
groups was superior to that of the RFL group (Q=6.51, P~0.01; Q=4.75, PcO.05,
Q = 4.60, P-c 0.05; and Q = 4.32, P < 0.05, respectively).
Constant error
Main @xt qf rrcall-condition. The significant main effect of recall-condition (F= 10.64, d.f. = 3,207, P<O.OOl ) is illustrated in Fig. 8. Following the 30 set recall interval, subjects had a tendency to undershoot; whereas following the same time interval, during which an interpolated task was performed, a slight tendency to overshoot was observed. The only post hoc comparison not to reach significance was between the immediate recall condition and the 30 set recall condition with the interpolated task (Q=O.67, P>O.O5).
638 G. LEONARD and B. MILNER
Main efSect of hand. There was a main effect of hand evident in the constant-error analysis, consequent to a small but consistent difference between the hands (F= 11.49, d.f. = 1, 69, PcO.01); the mean score for the left hand was 1.07”, representing a slight tendency to overshoot, whereas the overall mean for the right hand was -0.08”, signifying no bias for that hand.
DISCUSSION
The patients with small hippocampal removals from either the left or right temporal lobe performed the present location-recall task normally under all recall conditions; in contrast, both the left and right temporal-lobe groups with large hippocampal lesions were impaired under the 30 set recall-condition with the interpolated task. The latter findings suggest that subjects may have depended on both verbal and visual strategies (in addition to kinesthetic information) to encode, maintain and recall their limb positions. Indeed, when the subjects were asked, at the end of the test, to report the strategy they had employed to remember the end-position of the movements, the majority of subjects in each group reported using both verbal labels (e.g. time on a clock) and visual images (e.g. angles on a protractor), in addition to feelings in the arm and hand.
The deficit displayed by the patients in the LTH group suggests that the use of verbal labels was of some importance. The appearance of the deficit was contingent on the inclusion of the hippocampus in the removal, patients in the LTh group being normal in their performance; this fact underlines the role that the hippocampal region plays in maintaining information over time, especially in the face of interference [S]. The fact that a verbal interpolated task can interfere with recall accuracy is not surprising in the light of SHEA’S [21] findings with normal subjects. Shea demonstrated that, when supplied with relevant verbal labels (e.g. “40”“, or “3:30 pm”) to assist recall of location, subjects perform better than in the absence of such cues. Shea concluded that the application of a verbal label constitutes a useful strategy that can lead to increased accuracy at recall. Similarly, the present results appear to be consistent with the demonstration of POSNER and K~NICK [lS] and of LAABS [lo], that the recall of kinesthetic location deteriorates in the presence of verbal interpolated activity.
The fact that patients with extensive removal from the right hippocampal region were also impaired in recalling kinesthetically derived location-information, following 30 set of counting, parallels CORSI’S finding for the recall of visual location [S], and is concordant with the results of RAINS for a tactually guided spatial-location task [20], in which he required the subject (who was blindfolded) to locate the head of a map pin protruding from a cork surface delineated by a raised wooden ring. Rains’s findings supported those of CORSI [S], in that only patients with extensive removals from the right hippocampal region were impaired on his tactual location task. Unlike the present finding, the deficit in Rains’s study was limited to the hand contralateral to the removal.
An important contribution of the hippocampus to memory for visual location has also been demonstrated in non-human primates by PARKINSON and MISHKIN [17]. These investigators trained monkeys, on the basis of a single acquisition trial, to remember the location of a particular object; after bilateral hippocampal lesions, these animals were unable to relearn the task. The deficit, which appears to be comparable to that seen in human subjects with right hippocampal excisions [22], was explained by Parkinson and Mishkin as a failure to associate object with place. ANGELI et al. [1] went on to demonstrate that monkeys with such lesions also had difficulty in remembering location as such. The difficulty
KINESTHETIC LOCATION 639
of the hippocampectomized animals in Parkinson and Mishkin’s study and in that of Angeli et al. was not accentuated by the inclusion of the amygdala in the removal. Hence these experimenters were able to conclude that the hippocampus was crucial for normal performance on their location tasks.
The results for the frontal-lobe groups in the present study extend our earlier findings for distance traversed [12], in that the patients with large right frontal-lobe removals were again impaired, but this time in their ability to recall the end-position of their arm movements after a short delay. It is noteworthy that the patients who had undergone small removals from the right frontal lobe performed as well as normal control subjects. Nevertheless, the size of the lesions alone is not sufficient to explain the observed deficit, because those patients who had left frontal-lobe excisions comparable in size to the large right frontal-lobe lesions (My. Au., Hu. Ma., Dw. Mi., No. Sa., see Fig. 1) were normal in their ability to perform the task. Thus, the laterality of the lesion also appears to be a significant factor.
The overall analyses revealed no hand differences, hence ruling out the possibility that the deficits were consequent to awkwardness or slight reduction in strength in the hand contralateral to the removal [ll]. The finding of bilateral deficits following unilateral frontal-lobe lesions is consistent with the results of other movement-related studies [S, 9, 13, 251.
The results of the present study diverge from those for distance in the important respect that for location the deficit after right frontal lobectomy was only evident after a delay, when an impairment was found irrespective of whether or not a distracting task was performed during the intratrial interval. Thus, the findings suggest that the right frontal lobe is critically involved in maintaining cues relevant to body position in memory over short periods of time, but not for the initial encoding of the information. This interpretation is therefore in terms of a short-term memory deficit and is reminiscent of the results for delayed paired-comparison tasks reported by PRISKO [ 191 for patients with frontal-lobe lesions, where the introduction of a delay was critical in bringing about a deficit. The findings for human subjects parallel the delayed-response deficits observed in monkeys with principalis lesions [2, 161. Certainly the large frontal-lobe lesions of the patients in the present study would include the homologue in man of the principalis region [7], but none of the small removals would overlap this region.
Acknowledgements-This study is based on a thesis submitted by Gabriel Leonard in September 1987 to McGill University-in partial fulfillment of the requirements for the Phil degree. The research was supported by Grant MT 2624 and a career investigatorship from the Medical Research Council of Canada to Brenda Milner. We thank Drs T. Rasmussen, W. Feindel, A. Olivier and J. G. Villemure of the Montreal Neurological Hospitial for the opportunity to study their patients, and for providing detailed descriptions of the surgical removals.
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