Brnin
Rrsearch
B~rlleiin.
Vol.
25, pp. 293-29X.
c Pergamon
Press
plc,
1990. Printed
II361 -9230190
in the U.S.4
$3 (K) +
.OO
Changes in Retention of a Visual Discrimination Task Following Unilateral and Bilateral Transections of Temporo-Entorhinal Connections in Rats TROND
MYHRER’
AND EVY GRIN1
IVERSEN
Norwe,qian Defence Research Establishment, Division for Environmental N-2007 Kjeller, Norway-
Received
MYHRER,
3 1 January
Toxicoloq
1990
T. AND E. G. IVERSEN.
Changes in retenrion qf a ~.isud di.wriminafion connections in rats. BRAIN RES BULL 25(2) 293-298,
rusk fo//owin,y
unilurernl
trncl hilarerd
1990.-In a previous study it wac shown that the lateral entorhinal cortex (LEC) in the left hemisphere contains higher concentration of glutamate than in the right hemiapherc. The purpose of the present study was to examine potential relationship betwjeen this neurochemical asymmetry and behavior. Rats with either left, right. or bilateral transections of the fiber connections between the temporal cortex and LEC were tested for postoperative retention of a visual discrimination task. Because histological verification of lesions was crucial in this study. analvae\ of neurochemistry had to be omitted. The results showed that both left and bilateral lesions resulted in impaired retention. but the &lateral group was even more impaired than the left group. The rats with lesions in the right hemisphere, however. used fewer trials to reach the learning criterion than the control group, but they did not make fewer errors than the controls. The results are discussed in term\ of lateralization of mnemonic processes. tranwcrions
Temporal
of temporo-entorhinal
region
Lesions
Memory
Hemispheric
asymmetry
DAMAGE to the hippocampal formation has been regarded as critical in bitemporal amnesia in humans (22). However, results from subsequent animal research have led to doubt about whether the hippocampal formation or related structures are critically involved in bitemporal amnesia (24). Bilateral hippocampal lesions in the rat have not been shown to produce a general memory deficit. but a decrement only in a special kind of short-term memory (working memory). A growing body of evidence implies that bilateral hippocampal dysfunction impairs working memory more than reference memory, and that unilateral dysfunction has no effect upon working memory [cf. ( 16,18)]. Results from recent studies suggest that disturbances in reference memory in rats are probably not related to the hippocampal formation, but to a neighboring structure. the entorhinal cortex (12-14). The entorhinal cortex appears to constitute an important link between the neocortical association areas and the hippocampal formation (20). Within this complex the lateral entorhinal cortex (LEC) seems to play a prominent role in the rat by apparently being in the position of exerting modulatory influence upon both cognition and memory ( 10-14). The cognitive function appears to be mediated in connection with the hippocampal formation and the mnemonic function in connection with the temporal cortex ‘Requests for reprints should be addressed to Trond Myhrer. Norwegian I, Norway.
(TC). In the rat. LEC projects heavily to TC, whereas the contribution from the medial entorhinal cortex is very modest (9). In turn, TC projects predominantly to LEC via the perirhinal cortex (4,26). The fiber connections of TC and LEC in the rat seem to be routed by way of the adjacent white matter [cf. (27)]. Thus. sections through the white matter at a site corresponding to the level of the rhinal fissure can effectively disrupt connections between TC and LEC (cf. Fig. I ). In a recent study of TULEC disruptions and memory. neurochemical relations in TC and LEC were examined ( 13). A reduction in the levels of glutamateiaspartate in layers I-111 in both TC and LEC was seen to accompany impaired acquisition and retention of a visual discrimination task, whereas no changes in the levels of acetylcholine or GABA occurred. Additionally. it was discovered that the concentrations of both glutamate and acetylcholine were higher in LEC than TC. Furthermore. the left LEC contained a larger number of glutamergic nerve endings than the right LEC (indicated in Fig. 1). Such an asymmetrical distribution was not seen for acetylcholine or GABA ( 13). The notion that glutamate is involved in retaining the above visual discrimination task has recently obtained a considerably strengthened position. The profound memory impairment seen to follow TULEC transecDefence Centre of Psychology
293
and Educatton.
Oslo MiliAkershuh.
N-0015 Oslo
MYHHER AND IVERSEN
294
tions ( 13) is completely relieved by systemic administration of glutamergic agonists (15). The purpose of the present study was to investigate whether the uneven dis~ibution of glutamate across the two lateral entorhinal cortices might he related to the behavioral level. More specifically, rats with either left, right or bilateral transections of TC/LEC connections were tested for retroactive memory of a simultaneous brightness discrimination the previous studies cited above (12-15).
(LEFT)
IRIGHT)
TC
TC
task used in
Long-term synaptic potentiation (LTP) has been advanced as a leading candidate for a synaptic mechanism underlying learning and memory in mammals. This mechanism involves the Nmethyl-D-aspartate (NMDA) receptor which is believed to be glutamergic (1). Thus, there are reasons to assume that the left LEC with the higher concentration of glutamate is better able to conduct LTP than the right LEC with fewer glutamergic nerve endings. If this is the case, disruption of TClLEC connections in the left hemisphere would be expected to have more deleterious effect on retention of the ~scriminat~on task than corresponding lesion in the right hemisphere. Dissection of tissue samples for neurochemical analyses is incompatible with appropriate histological preparation of the area of lesion site in the temporal region (13). Because histological verification of lesions would be of vital importance in the present study, analyses of neurochemis~ in TC and LEC had to be omitted. Tissue samples from the intact hemispheres in the case of unilateral lesions would be of minor interest.
LEC
VMEC
FIG. I. Diagram of a horizontal section of the tem~r~-hip~~~~ region showing a simplified version of fiber connections. The higher concentration of glutamate/aspme in left versus right LEC is symbolized with plural nerve terminals in left LEC. Abbreviations: LEC =iateral entorhinal cortex; MEC = medial entorhinal cortex; PC = perirhinal cofiex; TC = temporal cortex.
a site corresponding app~ximately to the level of the rhinal fissure. The control lesions were performed in the same way except that the insertion was made in one stage only with a cannula 3.5 mm long and with a round end.
METHOD
Subjects
Thirty-eight male Wistar rats from a commercial supplier (Dyrlrege M@legaatdsAvlslaboratorium, Demnark), weighing 300340 g at the time of surgery, served as subjects. They were randomly assigned to four groups: 11 animals received transections of fiber connections between TC and LEC in the left hemisphere (designated Left group). 11 received co~s~nding transections in the right hemisphere (designated Right group), 6 received bilateral transections (designated Slat group), and 10 served as controls. Of these control rats 5 received bilateral control lesions and 5 had only their scalp reflected, The rats were housed individually and had free access to commercial rat pellets and water. The rats were handled individually 3 days preoperatively. being allowed to explore a tabletop (80 x 40 cm) for 3 min a day. The climatized (21°C) viva&m was illuminated from 0700 to 1900 hours. Surgery
The rats were anesthetized IP with diazepam (10 mg/kg) and fenatyl fluanisone (2 mg/kg) and placed in a stereotaxic head holder with their skulls horizontal. The lesions were made mechanically by means of the sharp edges of cannulas (dia. 0.5 mm) provided with collar to control for insertion depth. The cannula to be used was mounted on a syringe. The point of insertion was 7.8 mm posterior to bregma and 6.7 mm lateral to midline. Each cannula was inserted into the brain in a position deviating 20” from the vertical in the sagittal plane (tip of cam&a pointing rostrally). From this position the syringe was moved ten times back and forth in an axis deviating about 45” from the frontal plane (opening of angle pointing medially). These maneuvers were carried out in two stages with insertion depths 6 and 8 mm from the top of the skull. In this way, the distal part of the angular bundle was transected at
Histology
Upon termination of testing the brains were removed and frozen. The brains were sectioned horizontally on a CO,-freezing microtome at 30 km, every twelfth section being preserved. The sections were stained with methylene blue. The extent of fibers transected was estimated from the degree to which the white matter between TC and LEC was damaged at the three dorsoventral levels presented in Fig. 2. The white matter (not the alveus) was divided in four equal columns, each column representing 25 percent of the fibers. The occurrence of damage was evaluated under relatively high magnification. The number of columns affected at each dorso-ventral level were counted, and the mean percentage of damage was computed for each animal.
Testing was carried out in a Plexiglas cage (56 X 34 X 20 cm) previously described (14). In brief, a Plexiglas wall with an opening (10 x 10 cm) in the middle divided the apparatus in two equal compartments, start compartment and goal compartment. Three interchangeable aluminium cylinders (3 x 7 cm) with a round well (2x 2 cm) in the top served as ~~~rnin~~. The cylinders were located in fixed positions (equal distance between each) along the wall opposite to the partition wall in the goal compartment. The cylinders were natural grey (aluminium) or painted black (except for the well). The well of the positive cylinder was filled with water. The only light was a 15-W bulb 60 cm above the apparatus. Procedure
During acquisition and retention testing the rats were deprived of water for 23% hr a day. On the first day, each rat was allowed to explore the empty test apparatus for 15 min. On the second day,
MEMORY AND LATERALIZATION
FIG. 2. Examples
of temporo-entorhinal
lesions in the Left group (A), Right group (13). and Bilat group (C).
the subjects were trained to run from the start compartment into the goal compartment in which they were rewarded with some laps of water from the well in the positive cylinder. The rats were given 10 trials, and intertrial interval was 20 set during which they stayed in their home cage. On the third day, the animals were given trials until the occurrence of 5 correct responses in succession. The rats were operated 24 hr after the criterion had been achieved. Twelve days after surgery. the animals were tested for retention of the discrimination task on this single day. Testing was terminated when the previous criterion was reached or if it had not been achieved within 30 trials. The following behavioral data were recorded: number of trials to criterion, numbers and type of errors to criterion. In order to drink or investigate whether the well in a cylinder contained water the rats had to stand on their hind legs with at least one forepaw on top of the cylinder. Error response was scored when a negative cylinder was mounted and found empty of water (e.g., licking the empty well). Approaching or investigating negative cylinders (except the well) was not scored as an error. The positive cylinder was either black or grey and the two cylinders of opposite color were negative. The position of the positive cylinder (left, middle, right) was changed in a prearranged randomized order. One set of randomized positions was used on Day 2 of training and another one on Day 3 and on retention testing. A counterbalanced paradigm was followed in which half of the subjects were trained with black cylinder as positive and the other half with grey cylinder as positive. Statistical overall analyses were made with Kruskal-Wallis one-way ANOVA and group comparisons with two-tailed Mann-
Whitney U-test. Pearson r test.
Correlation
coefficients
were computed
with
RESULTS
Histology The lesions appeared as a section in the distal part of the angular bundle at a site between TC and LEC (Fig. 2). The transections, which often affected the alveus of the hippocampal formation, were 0.5-l .O mm long in rostro-caudal extent and 34 mm long in dorso-ventral extent. Inasmuch as the cannula sections could not follow the exact curvature of the rhinal fissure, TC/LEC connections between a comparatively small part in the caudal end of TC and in the rostra1 end of LEC were probably not accessible for denervation. In the Left group, the mean percentage of fiber lesion was 73 (range 42-100; Fig. 2A). Additional damage to the subicular area in the ventral-most part of the hippocampal formation was seen in five rats. In the Right group, the mean percentage of fiber lesion was 78 (range 50-100; Fig. 2B). Additional damage to the ventral-most subiculum was seen in three rats. In the Bilat group. the mean percentage of fiber lesion was 72 (range 42-84; Fig. 2C). In the latter group, unilateral ventral subicular damage was seen in three rats. The control lesions appeared as a very slight track in the neocortex overlying the temporal region. Corresponding cortical damage was also seen in the experimental animals.
The sham-operated
and lesion-operated
control
rats did not
296
MYHREK AND IVEKSEk TABLE I LEARNING AND RETENTION MEASURES TO CRITERION FOR SIMULTANEOUS BRIGHTNESS DISCRIMINATION Acquisition
Retention
Trials
Trials
Errors
Percentage Saving
Group
N
Median
Range
Median
Cont
10
15.0
15-27
7.5
5-10
1 .o
o-2
81.9
Left
11
16.0
15-21
12.0
5-27
3.0
O-10
37.8
Right
11
17.0
15-20
5.0
5-10
0.0
o-2
Bilat
6
16.0
15-25
21.0
18-30
7.0
5-13
differ reliably in any measures and were treated as a single control group. The experimental groups did not differ significantly from the control group during acquisition of the discrimination task (Table 1). During retention, however, the percentage saving of trials differed among the groups. Both the control group and the Right group displayed high percentage of savings, whereas the Left group and the Bilat group showed low percentage of savings. Because of uneven distribution of data, nonparametric statistics were applied. Km&al-Wallis ANOVA revealed a significant overall effect in number of trials to reach the learning criterion following surgery, H(3) = 19.34, p
Rrsearch
B~rlleiin.
Vol.
25, pp. 293-29X.
c Pergamon
Press
plc,
1990. Printed
II361 -9230190
in the U.S.4
$3 (K) +
.OO
Changes in Retention of a Visual Discrimination Task Following Unilateral and Bilateral Transections of Temporo-Entorhinal Connections in Rats TROND
MYHRER’
AND EVY GRIN1
IVERSEN
Norwe,qian Defence Research Establishment, Division for Environmental N-2007 Kjeller, Norway-
Received
MYHRER,
3 1 January
Toxicoloq
1990
T. AND E. G. IVERSEN.
Changes in retenrion qf a ~.isud di.wriminafion connections in rats. BRAIN RES BULL 25(2) 293-298,
rusk fo//owin,y
unilurernl
trncl hilarerd
1990.-In a previous study it wac shown that the lateral entorhinal cortex (LEC) in the left hemisphere contains higher concentration of glutamate than in the right hemiapherc. The purpose of the present study was to examine potential relationship betwjeen this neurochemical asymmetry and behavior. Rats with either left, right. or bilateral transections of the fiber connections between the temporal cortex and LEC were tested for postoperative retention of a visual discrimination task. Because histological verification of lesions was crucial in this study. analvae\ of neurochemistry had to be omitted. The results showed that both left and bilateral lesions resulted in impaired retention. but the &lateral group was even more impaired than the left group. The rats with lesions in the right hemisphere, however. used fewer trials to reach the learning criterion than the control group, but they did not make fewer errors than the controls. The results are discussed in term\ of lateralization of mnemonic processes. tranwcrions
Temporal
of temporo-entorhinal
region
Lesions
Memory
Hemispheric
asymmetry
DAMAGE to the hippocampal formation has been regarded as critical in bitemporal amnesia in humans (22). However, results from subsequent animal research have led to doubt about whether the hippocampal formation or related structures are critically involved in bitemporal amnesia (24). Bilateral hippocampal lesions in the rat have not been shown to produce a general memory deficit. but a decrement only in a special kind of short-term memory (working memory). A growing body of evidence implies that bilateral hippocampal dysfunction impairs working memory more than reference memory, and that unilateral dysfunction has no effect upon working memory [cf. ( 16,18)]. Results from recent studies suggest that disturbances in reference memory in rats are probably not related to the hippocampal formation, but to a neighboring structure. the entorhinal cortex (12-14). The entorhinal cortex appears to constitute an important link between the neocortical association areas and the hippocampal formation (20). Within this complex the lateral entorhinal cortex (LEC) seems to play a prominent role in the rat by apparently being in the position of exerting modulatory influence upon both cognition and memory ( 10-14). The cognitive function appears to be mediated in connection with the hippocampal formation and the mnemonic function in connection with the temporal cortex ‘Requests for reprints should be addressed to Trond Myhrer. Norwegian I, Norway.
(TC). In the rat. LEC projects heavily to TC, whereas the contribution from the medial entorhinal cortex is very modest (9). In turn, TC projects predominantly to LEC via the perirhinal cortex (4,26). The fiber connections of TC and LEC in the rat seem to be routed by way of the adjacent white matter [cf. (27)]. Thus. sections through the white matter at a site corresponding to the level of the rhinal fissure can effectively disrupt connections between TC and LEC (cf. Fig. I ). In a recent study of TULEC disruptions and memory. neurochemical relations in TC and LEC were examined ( 13). A reduction in the levels of glutamateiaspartate in layers I-111 in both TC and LEC was seen to accompany impaired acquisition and retention of a visual discrimination task, whereas no changes in the levels of acetylcholine or GABA occurred. Additionally. it was discovered that the concentrations of both glutamate and acetylcholine were higher in LEC than TC. Furthermore. the left LEC contained a larger number of glutamergic nerve endings than the right LEC (indicated in Fig. 1). Such an asymmetrical distribution was not seen for acetylcholine or GABA ( 13). The notion that glutamate is involved in retaining the above visual discrimination task has recently obtained a considerably strengthened position. The profound memory impairment seen to follow TULEC transecDefence Centre of Psychology
293
and Educatton.
Oslo MiliAkershuh.
N-0015 Oslo
MYHHER AND IVERSEN
294
tions ( 13) is completely relieved by systemic administration of glutamergic agonists (15). The purpose of the present study was to investigate whether the uneven dis~ibution of glutamate across the two lateral entorhinal cortices might he related to the behavioral level. More specifically, rats with either left, right or bilateral transections of TC/LEC connections were tested for retroactive memory of a simultaneous brightness discrimination the previous studies cited above (12-15).
(LEFT)
IRIGHT)
TC
TC
task used in
Long-term synaptic potentiation (LTP) has been advanced as a leading candidate for a synaptic mechanism underlying learning and memory in mammals. This mechanism involves the Nmethyl-D-aspartate (NMDA) receptor which is believed to be glutamergic (1). Thus, there are reasons to assume that the left LEC with the higher concentration of glutamate is better able to conduct LTP than the right LEC with fewer glutamergic nerve endings. If this is the case, disruption of TClLEC connections in the left hemisphere would be expected to have more deleterious effect on retention of the ~scriminat~on task than corresponding lesion in the right hemisphere. Dissection of tissue samples for neurochemical analyses is incompatible with appropriate histological preparation of the area of lesion site in the temporal region (13). Because histological verification of lesions would be of vital importance in the present study, analyses of neurochemis~ in TC and LEC had to be omitted. Tissue samples from the intact hemispheres in the case of unilateral lesions would be of minor interest.
LEC
VMEC
FIG. I. Diagram of a horizontal section of the tem~r~-hip~~~~ region showing a simplified version of fiber connections. The higher concentration of glutamate/aspme in left versus right LEC is symbolized with plural nerve terminals in left LEC. Abbreviations: LEC =iateral entorhinal cortex; MEC = medial entorhinal cortex; PC = perirhinal cofiex; TC = temporal cortex.
a site corresponding app~ximately to the level of the rhinal fissure. The control lesions were performed in the same way except that the insertion was made in one stage only with a cannula 3.5 mm long and with a round end.
METHOD
Subjects
Thirty-eight male Wistar rats from a commercial supplier (Dyrlrege M@legaatdsAvlslaboratorium, Demnark), weighing 300340 g at the time of surgery, served as subjects. They were randomly assigned to four groups: 11 animals received transections of fiber connections between TC and LEC in the left hemisphere (designated Left group). 11 received co~s~nding transections in the right hemisphere (designated Right group), 6 received bilateral transections (designated Slat group), and 10 served as controls. Of these control rats 5 received bilateral control lesions and 5 had only their scalp reflected, The rats were housed individually and had free access to commercial rat pellets and water. The rats were handled individually 3 days preoperatively. being allowed to explore a tabletop (80 x 40 cm) for 3 min a day. The climatized (21°C) viva&m was illuminated from 0700 to 1900 hours. Surgery
The rats were anesthetized IP with diazepam (10 mg/kg) and fenatyl fluanisone (2 mg/kg) and placed in a stereotaxic head holder with their skulls horizontal. The lesions were made mechanically by means of the sharp edges of cannulas (dia. 0.5 mm) provided with collar to control for insertion depth. The cannula to be used was mounted on a syringe. The point of insertion was 7.8 mm posterior to bregma and 6.7 mm lateral to midline. Each cannula was inserted into the brain in a position deviating 20” from the vertical in the sagittal plane (tip of cam&a pointing rostrally). From this position the syringe was moved ten times back and forth in an axis deviating about 45” from the frontal plane (opening of angle pointing medially). These maneuvers were carried out in two stages with insertion depths 6 and 8 mm from the top of the skull. In this way, the distal part of the angular bundle was transected at
Histology
Upon termination of testing the brains were removed and frozen. The brains were sectioned horizontally on a CO,-freezing microtome at 30 km, every twelfth section being preserved. The sections were stained with methylene blue. The extent of fibers transected was estimated from the degree to which the white matter between TC and LEC was damaged at the three dorsoventral levels presented in Fig. 2. The white matter (not the alveus) was divided in four equal columns, each column representing 25 percent of the fibers. The occurrence of damage was evaluated under relatively high magnification. The number of columns affected at each dorso-ventral level were counted, and the mean percentage of damage was computed for each animal.
Testing was carried out in a Plexiglas cage (56 X 34 X 20 cm) previously described (14). In brief, a Plexiglas wall with an opening (10 x 10 cm) in the middle divided the apparatus in two equal compartments, start compartment and goal compartment. Three interchangeable aluminium cylinders (3 x 7 cm) with a round well (2x 2 cm) in the top served as ~~~rnin~~. The cylinders were located in fixed positions (equal distance between each) along the wall opposite to the partition wall in the goal compartment. The cylinders were natural grey (aluminium) or painted black (except for the well). The well of the positive cylinder was filled with water. The only light was a 15-W bulb 60 cm above the apparatus. Procedure
During acquisition and retention testing the rats were deprived of water for 23% hr a day. On the first day, each rat was allowed to explore the empty test apparatus for 15 min. On the second day,
MEMORY AND LATERALIZATION
FIG. 2. Examples
of temporo-entorhinal
lesions in the Left group (A), Right group (13). and Bilat group (C).
the subjects were trained to run from the start compartment into the goal compartment in which they were rewarded with some laps of water from the well in the positive cylinder. The rats were given 10 trials, and intertrial interval was 20 set during which they stayed in their home cage. On the third day, the animals were given trials until the occurrence of 5 correct responses in succession. The rats were operated 24 hr after the criterion had been achieved. Twelve days after surgery. the animals were tested for retention of the discrimination task on this single day. Testing was terminated when the previous criterion was reached or if it had not been achieved within 30 trials. The following behavioral data were recorded: number of trials to criterion, numbers and type of errors to criterion. In order to drink or investigate whether the well in a cylinder contained water the rats had to stand on their hind legs with at least one forepaw on top of the cylinder. Error response was scored when a negative cylinder was mounted and found empty of water (e.g., licking the empty well). Approaching or investigating negative cylinders (except the well) was not scored as an error. The positive cylinder was either black or grey and the two cylinders of opposite color were negative. The position of the positive cylinder (left, middle, right) was changed in a prearranged randomized order. One set of randomized positions was used on Day 2 of training and another one on Day 3 and on retention testing. A counterbalanced paradigm was followed in which half of the subjects were trained with black cylinder as positive and the other half with grey cylinder as positive. Statistical overall analyses were made with Kruskal-Wallis one-way ANOVA and group comparisons with two-tailed Mann-
Whitney U-test. Pearson r test.
Correlation
coefficients
were computed
with
RESULTS
Histology The lesions appeared as a section in the distal part of the angular bundle at a site between TC and LEC (Fig. 2). The transections, which often affected the alveus of the hippocampal formation, were 0.5-l .O mm long in rostro-caudal extent and 34 mm long in dorso-ventral extent. Inasmuch as the cannula sections could not follow the exact curvature of the rhinal fissure, TC/LEC connections between a comparatively small part in the caudal end of TC and in the rostra1 end of LEC were probably not accessible for denervation. In the Left group, the mean percentage of fiber lesion was 73 (range 42-100; Fig. 2A). Additional damage to the subicular area in the ventral-most part of the hippocampal formation was seen in five rats. In the Right group, the mean percentage of fiber lesion was 78 (range 50-100; Fig. 2B). Additional damage to the ventral-most subiculum was seen in three rats. In the Bilat group. the mean percentage of fiber lesion was 72 (range 42-84; Fig. 2C). In the latter group, unilateral ventral subicular damage was seen in three rats. The control lesions appeared as a very slight track in the neocortex overlying the temporal region. Corresponding cortical damage was also seen in the experimental animals.
The sham-operated
and lesion-operated
control
rats did not
296
MYHREK AND IVEKSEk TABLE I LEARNING AND RETENTION MEASURES TO CRITERION FOR SIMULTANEOUS BRIGHTNESS DISCRIMINATION Acquisition
Retention
Trials
Trials
Errors
Percentage Saving
Group
N
Median
Range
Median
Cont
10
15.0
15-27
7.5
5-10
1 .o
o-2
81.9
Left
11
16.0
15-21
12.0
5-27
3.0
O-10
37.8
Right
11
17.0
15-20
5.0
5-10
0.0
o-2
Bilat
6
16.0
15-25
21.0
18-30
7.0
5-13
differ reliably in any measures and were treated as a single control group. The experimental groups did not differ significantly from the control group during acquisition of the discrimination task (Table 1). During retention, however, the percentage saving of trials differed among the groups. Both the control group and the Right group displayed high percentage of savings, whereas the Left group and the Bilat group showed low percentage of savings. Because of uneven distribution of data, nonparametric statistics were applied. Km&al-Wallis ANOVA revealed a significant overall effect in number of trials to reach the learning criterion following surgery, H(3) = 19.34, p
