Effects of unilateral and bilateral training in a reaching task on dendritic branching of neurons in the rat motor-sensory forelimb cortex

BEHAVIORAL AND NEURAL BIOLOGY44, 301--314 (1985)

Effects of Unilateral and Bilateral Training in a Reaching Task on Dendritic Branching of Neurons in the Rat MotorSensory Forelimb Cortex WILLIAM T . GREENOUGH, JOHN R . LARSON, 1 AND GINGER S. WITHERS 2

Departments of Psychology and Anatomical Sciences and Neural and Behavioral Biology Program, University of Illinois, Urbana-Champaign, Illinois 61820 Effects of motor training on a neocortical nerve ceil population involved in performance of the motor task were assessed by measuring Layer V pyramidal neuron apical dendritic branching in motor-sensory forelimb cortex of rats trained to reach into a tube for food. Rats were trained to reach with the forepaw they preferred to use (group PRAC), the nonpreferred forepaw (REV), both forepaws (ALT), or neither forepaw (CONT). Across groups, hemispheres opposite trained forepaws had larger apical dendritic fields, in terms of total dendritic length, number of oblique branches from the apical shaft, and length of terminal branches. Similar, although somewhat less consistent, effects were seen when results were analyzed for between- (CONT vs ALT) and within-subject comparisons (trained vs nontrained hemispheres of REV and PRAC). This finding is compatible with the hypothesis that altered dendritic patterns, with associated synapses, are involved in storage of information from the training experience. The withinsubject effects mitigate suggestions that these differences arise from generally acting hormonal or metabolic consequences of the training experience, although the possibility that these effects result from neural activity per se and are unrelated to information storage cannot be excluded. © 1985AcademicPress, Inc.

A number of lines of evidence have begun to point to changes in the number, and presumably the pattern, of brain synapses as a possible substrate or aspect of the adult memory process. Visual cortex neurons of animals reared from weaning in complex environments have higher spine frequency (Globus, Rosenzweig, Bennett, & Diamond, 1973) and more extensive dendritic ramification (Greenough & Volkmar, 1973; Greenough, Volkmar, & Juraska, 1973; Juraska, 1984) in various brain I NOW at Center for the Neurobiology of Learning and Memory, University of California, Irvine, CA 92717. 2 Research supported by National Science Foundation Grant BNS 82 16916. We thank Ingrid Reynolds, Randy Firfer, Christine Collins, and Anita Sirevaag for assistance with various aspects of this project. We thank Michel Baudry for helpful comments on the manuscript. Send requests for reprints to William T. Greenongh, Department of Psychology, University of Illinois, 603 E. Daniel--Rm. 825, Champaign, IL 61820. 301 0163-1047/85 $3.00 Copyright © 1985by AcademicPress, Inc. All rightsof reproductionin any form reserved.

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areas relative to laboratory cage-reared animals. Electron microscopic work has indicated that the visual cortex dendritic data are paralleled by differences in the numbers of synapses per neuron (Turner & Greenough, 1985). Several studies have reported changes in dendritic branching quite similar to those in the rearing studies after a period of complex environment housing in adults (e.g., Green, Greenough, & Schlumpf, 1983; Juraska, Greenough, Elliott, Mack, & Berkowitz, 1980; Uylings, Kuypers, & Veltman, 1978). This adult structural plasticity has led to suggestions that experience-dependent formation of new synapses could be involved in adult long-term memory (e.g., Greenough, 1984). This view has been supported by experiments in which alterations in visual cortex dendritic branching were observed following maze training (Greenough, Juraska & Volkmar, 1979; Chang & Greenough, 1982). There are, however, a number of alternative explanations of changes following a training procedure. In particular, nonspecific aspects of the experience such as stress, physical activity, and sensory stimulation may be responsible for any subsequent anatomical change. One way to examine the importance of such variables is to use a procedure in which sensory input from the training is restricted to one side of the brain, which can then be compared with the contralateral homologous region in the same animal. One such experiment (Chang & Greenough, 1982) indicated that, when visual aspects of maze training were lateralized by means of a contact occluder in split-brain rats, dendritic ramification of neurons previously found sensitive to maze training was greater opposite the open eye. Such results suggest that the structural changes observed are not a consequence of generally acting hormonal or metabolic aspects of the subjects' response to the training procedure. In that experiment, however, there was no direct evidence to indicate that the altered neurons were involved in the learning or performance of the maze task. It is thus important to determine whether training-produced alterations in dendritic structure occur in a class of neurons known to be involved in performing the learned behavior. Considerable evidence from lesion, stimulation, and cellular recording studies indicates that fine forelimb movement including reaching is governed by the forelimb area of the frontal cortex in rats (Castro, 1972a, 1972b; Dolbakyan, Hernandez-Mesa, & Bures, 1977; Donoghue & Wise, 1982; Hall & Lindholm, 1974; Sapienza, Talbi, Jacquemin, & Albe-Fessard, 1981; Stashkevich & Bures, 1981). When confronted with a reaching task, most rats prefer to use one or the other forepaw (Peterson, 1934). This preference can be reversed, apparently permanently, by extensive reaching practice with the nonpreferred forepaw (Peterson, 1951). In addition, our observations indicate that considerable improvement in reaching dexterity occurs over the course of training. In early work on neurochemical correlates of learning, Hyden and Egyhazi (1964) reported

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that handedness reversal training altered measures of RNA in neurons of rat motor cortex. This training procedure has several advantages over most others used in previous experiments on biological sequelae of training: handedness is easy to test, lesion and electrophysiological experiments (Castro, 1972b; Donoghue & Wise, 1982; Peterson & Devine, 1963; Stashkevich & Bures, 1981) indicate that neurons in a circumscribed, identifiable region of rat frontal cortex are involved in the performance of reaching and other forelimb movements, and unilateral handedness training allows comparison within a subject of hemispheres governing the trained and untrained paws. It was thus of interest to examine the effects of reach training on the dendritic branching of neurons in the motor-sensory forelimb region of the neocortex. Layer V pyramidal neurons were selected for this study because they include the projection neurons whose axons terminate in the forelimb region of the rat spinal cord (Donoghue & Wise, 1982). Apical dendrites were examined because a preliminary study (Larson & Greenough, 1981) had indicated that dendritic branching of this region was altered in reach-trained animals.

METHODS

Subjects. Subjects were seven sets of male littermate quadruplet LongEvans hooded rats (28 individual animals) born in the laboratory (stock source: Simonsen Laboratories, Gilroy, CA). Due to inadequate staining, data were not collected from two animals, one CONT and one ALT (see below). Rats were reared socially in groups of three to six in standard laboratory group cages (48 x 26 x 15 cm high) with food and water available ad lib until they began the training procedure at 60-80 days of age. Thereafter, they were housed in pairs in identical cages. Animals were maintained in a temperature-controlled (23 _+ 1.5°C) animal room on a diurnal light cycle of 16:8. Procedure. Rats were trained and tested for paw preference in reaching into a clear Plexiglas tube (1 cm i.d. ; 2.5 or 3.75 cm long) which projected at a 30° upward angle into a clear Plexiglas compartment (30 cm x 20 cm x 23 cm high, with the tube centered 3 cm above a grid floor on the shorter wall; see Fig. 1A). The apparatus and procedure were similar to those described by Hyden and Egyhazi (1964). For training, an additional wall was positioned within the compartment adjacent to the tube such that it forced rats to reach with one forepaw. Prior to the experiment, members of littermate sets were semirandomly assigned to four groups, one member of each set to each of the training conditions. The training conditions were: (i) reversal training (REV): rats were trained to reach with the nonpreferred paw for pieces of chocolate chip cookies; (ii) practice training (PRAC): rats were trained on the preferred paw; (iii) alternation training (ALT): rats were trained on alternate paws on successive

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A.

F

FIG. 1. (A) Reach training apparatus. Biasing wall W forces rat to use forelimb nearest the wall to reach into tube T for cookie bits. (B) 2-DG autoradiograms of brains of rats injected prior to a reaching session or an equivalent control period. Motor-sensory forelimb (MSF) cortex shows a band of increased uptake opposite the reaching forelimb. Asymmetries are also evident in other regions such as superior colliculus (SC), inferior colliculus (IC), and cerebellar hemispheres (CH). (From Fuchs et al., 1983; Figure lb is modified from Greenough, 1984, copyright 1984, Elsevier Science Publishers. Reprinted by permission.) d a y s ; a n d (iv) c o n t r o l ( C O N T ) : no r e a c h training, b u t a l l o w e d to e a t a s i m i l a r a m o u n t o f c o o k i e bits in a s i m i l a r b o x with no t u b e . All r a t s w e r e r e d u c e d to 80% o f b o d y w e i g h t o v e r a 6 - d a y p e r i o d a n d m a i n t a i n e d at a p p r o x i m a t e l y t h a t l e v e l t h r o u g h o u t training b y f e e d i n g t h e m an a m o u n t b a s e d u p o n t h e i r w e i g h t p r i o r to e a c h d a y ' s training. A t t h e o u t s e t , r a t s in t h e t h r e e t r a i n i n g c o n d i t i o n s w e r e t e s t e d for p a w p r e f e r e n c e . T h i s w a s

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done by placing the rat in the training chamber without the biasing wall. Typically, food chunks (60-90 mg bits of Keebler 100's) were placed close to the opening of the 2.5-cm tube and the rat was allowed, initially, to remove them by mouth several times. Rats were induced to reach by withdrawing the tube to a point outside the hole in the wall at which removal by mouth was not possible, then returning it as reaching began. Preference was defined in terms of the paw with which 20 of the first 30 reaches were made; if this criterion was not met, an equivalent proportion of a larger number of trials was used. Two rats, one REV and one ALT, did not meet criterion and were classified as to preference based upon the paw with which the majority of reaches were made. When preference was determined, the rats were trained to use a particular paw by placing the wall next to the tube to prevent reaching with the other paw, As soon as they began to reach reliably, animals for which it had not been done were switched to the longer tube. Training consisted of 33 50-reach training sessions over a 16-day period. During training the control subjects were allowed to eat in a similar apparatus while the other rats were being trained. Following training all rats were tested for paw preference in the manner described above, with the biasing wall removed; the control rats were first tested for preference at this time. We also designed several other tests of handedness which were given after training: (a) reach down--rats were tested for preference on reaching through the wire-rod floor of a cage for food; (b) ladder--the paw used to grip the top rung of a ladder descending from a small elevated platform was tested; (c) step out--the paw used to step through a hole in a box into the home cage was observed; and (d) wire--rats were suspended by the forepaws from a wire and tested for paw preference in hanging on before falling (onto a pillow). Histology and quantitative morphology. Following training, the animals were anesthetized with ketamine, and the dural surface of each hemisphere was exposed for brief electrical stimulation in connection with another experiment (Larson & Greenough, 1981). The animals were then exsanguinated and their brains removed and stained with the Golgi-Cox method (Glaser & Van der Loos, 1981). Test sections from other cortical areas were used to assess completeness of staining. Coronal slabs approximately 7 mm thick containing the motor-sensory forelimb cortex (see below) were dehydrated and embedded in celloidin. Sections were taken at 150 p.m, alkalinized, fixed, and mounted with Permount. Slides were then coded so that the experimenter was unaware of individual subjects' training condition. For this study, the region of anterior cortex involved in performance of the reaching task was identified from projection tracings of the region of highest 2-deoxyglucose (2-DG) uptake in other trained animals performing

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the reaching task (Fuchs, Bajjalieh, Hoffman, & Greenough, 1983). Subjects were injected intraperitoneally with 10/xCi 2-[~4C]DG and immediately placed in the reach-training apparatus, where they were allowed to reach for 45 min or until they stopped, whichever came first. (The average subject reached for 30-40 min.) Brains were cryosectioned at 20 ~m and contact exposed to Kodak X-Omat R film for about 3 weeks. The area of greatest cortical uptake appears as a prominent band (see Fig. 1B) overlapping regions architectonically and electrophysiologically defined as both somatomotor and somatosensory (Donoghue & Wise, 1982; Sanderson, Welker, & Shambes, 1984); hence the term motor-sensory forelimb (MSF) cortex was adopted to describe this region. The location of the region, which was remarkably consistent across well-performing subjects, was traced from five subjects at five anterior-posterior levels, and the most conservative of these boundaries was projected onto the corresponding Golgi-stained sections and marked with small pieces of tape. Apical dendritic fields of Layer V pyramidal neurons in the identified region of cortex of each hemisphere were quantified by tracing them with a 3-dimensional computer-assisted tracking system (modified from that described by DeVoogd, Chang, Floeter, Jencius, & Greenough, 1981) and analyzed in terms of length and number of branches at each order away from the apical dendrite (see Chang & Greenough, 1982). The concentric ring analysis used in many prior studies is not reported here, due to its relative loss of power as one ascends the apical dendrite and the curvature of the rings becomes more transverse to the apical shaft. (Results of those analyses that were run parallel the dendritic number and length analyses, but with weaker effects in distal apical regions.) Stratified sampling of neurons was used. The boundaries of Layer V (determined from Nissl sections from other animals) were translated into a ratio (proportion of distance between pia and white matter). The experimenter then entered coordinates of the tape-indicated boundaries, and the pial surface and the white matter within the sample area, using the computer microscope. The computer selected two equally spaced sample sites within Layer V. The experimenter was required to select the weU-stained pyramidal cell nearest the point indicated by the computer, alternating between somata in the upper third (used as an arbitrary definition of Va) and in the lower two-thirds of this layer. If no well-stained neuron appeared at the appropriate level within I00 /xm to either side of the sample zone, that sample was excluded. This procedure minimizes potential selection bias of the experimenter. Data were collected from 12 serial sections through the MSF region as indicated in the 2-DG study. A total of 1524 apical dendrites, approximately 15 from each laminar level of each hemisphere per subject, was analyzed. Statistical analysis was performed using the SAS general linear model.

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RESULTS Behavioral Measures Of the seven REV rats, using a criterion of 75% of reaches (23/30), only one animal failed to show a clear preference reversal (sign test p < .05). This rat had initially not met the strict criterion for preference. All seven rats in the PRAC group and five of the six rats analyzed in the ALT group maintained their original preference. Correlations between initial reach preference and the other measures of handedness were only sporadically significant, and there was no clear indication that any of these measures was differently affected in the reversal group. Thus the effects of training upon forepaw preference appeared to be quite specific to the task upon which the animals were trained. Dendritic Branching Analyses Data were analyzed in four separate ways. To maximize power in detecting effects of training, data from all trained hemispheres (i.e., both ALT hemispheres; hemispheres opposite trained forelimbs in REV and PRAC) were grouped and compared to data from all nontrained hemispheres (both CONT; hemispheres ipsilateral to trained forelimbs in REV and PRAC). To assess within- and between-subject effects of training, trained vs nontrained hemispheres were analyzed in the unilaterally trained groups, and the combined hemispheres in the ALT group were compared to both in CONT. To assess specific group effects, hemispheres within subjects were compared. Finally, nontrained hemispheres of unilaterally trained subjects (REV and PRAC) were compared with hemispheres of nontrained (CONT) subjects to assess effects of training not specific to the trained hemisphere. Although there were some main effects of position of the soma within Layer V (upper neurons, in general, had less oblique branching from the apical dendrite than lower neurons), there were no interactions between layer position and training condition, so the two cell categories were pooled for analyses of training effects. Trained vs Nontrained Hemispheres The number of branches at each of the first six orders of branching from the apical dendrite is presented in Fig. 2, and the lengths of complete (i.e., not obscured by other stained tissue or truncated by sectioning) terminal and bifurcating branches, as well as summed total measured dendritic length (excluding the apical shaft), is presented in Table 1. It is clear that neurons from trained hemispheres had larger apical dendritic fields in general, as indicated by the difference in total dendritic length. The overall increase reflects two underlying trends, which have previously been described in studies of effects of complex environments on cortical dendritic field structure (Juraska et al., 1980; Juraska, 1984; Uylings et

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GREENOUGH, LARSON, AND WITHERS

.,t2(k@ i.'* % ~*~ Q. co Ec-O 7/~

~.

~.

Trained Nontrained

0 ~ 0

0

4

m

d

"%._

oo

I

0/

1

I

2

3

I

4

I

I

5

6

O r d e r of B r a n c h

FIG. 2. Mean number of branches at each order of bifurcation from the apical dendrite for combined groups. Trained group includes both hemispheres of ALT, hemispheres opposite trained forelimbs in REV and PRAC. Nontrained group includes both hemispheres of CONT, hemispheres opposite nontrained forelimbs in REV and PRAC. ** p < .0001, • p < .001, by analysis of variance.

al., 1978). First, as Fig. 2 indicates, there was an overall tendency for a greater number of branches to appear at all orders other than Order 3 away from the apical dendrite in hemispheres opposite trained forelimbs. (Relatively few neurons had branching beyond Order 5, as is typical.) Second, as seen in Table 1, terminal branches (nonobscured branches TABLE 1 All Trained vs All Nontrained Hemispheres Combined Order of branch 1

Terminal length (gm) Trained 72.7**** Nontrained 64.4 Bifurcating length (/zm) Trained 30.1 Nontrained 28.3 Total Dendrite Trained Length

2

3

4

5

6

64.6** 58.6

62.9 58.9

63.0 62.8

70.3 61.4

59.5 57.5

32.1 30.2 736.7****

40.6 37.8 Nontrained

50.5 46.8 607.1

52.7 40.2

49.8 43.8

Note. For this analysis, data for all nontrained hemispheres (both hemispheres of CONT; hemispheres opposite nontrained forelimbs of REV and PRAC) were pooled for comparison with data for all trained hemispheres (both hemispheres of ALT; hemispheres opposite trained forelimbs of REV and PRAC). ** p < .01. **** p < .0001.

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REACHING AND DENDRITIC BRANCHING

that do not again bifurcate) tended to be longer in hemispheres opposite trained forelimbs. There were no obvious length differences associated with training on the bifurcating branches, a pattern also typical of prior studies of complex environment effects.

Between- and within-Subject Comparisons In general, these same effects could be seen when comparisons were made between trained and nontrained hemispheres across the unilaterally trained groups (REV and PRAC, comparing trained to nontrained hemispheres) or between the bilaterally trained and nontrained animals, pooling across hemispheres within subjects (CONT vs ALT). As shown in Table 2, total dendritic length was significantly greater in ALT than in CONT subjects and in the trained hemispheres of REV and PRAC, relative to TABLE 2

Between- and within-Subject Comparisons CONT vs ALT

Order of branch 1

2

3

4

5

6

Terminal length (tzm) ALT CONT

74.5**** 61.3

69.0*** 58.0

66.1 * 57.6

64.0 63.0

59.8 59.6

60.7 57.6

Number of branches ALT CONT

6.8**** 4.5

Total Dendrite Length

Trained

3.71'***

1.48"

0.82****

0.54****

2.85

1.21

0.37

0.19

758.1"***

Nontrained

0.34 0.18

577.8

R E V / P R A C - - T r a i n e d vs Nontrained

Terminal length Trained 70.6 Nontrained 67.5 Number of branches Trained 5.99 Nontrained 5.52 Total Dendrite Length Trained

60.0 59.2 3.46*** 2.86

715.4"*

60.4 60.3 1.48 1.28

Nontrained

62.4 62.4 0.70 0.67

75.3 63.5 0.47 0.45

61.3 55.8 0.37 0.36

638.0

Note. This table presents data from between subject comparisons (both nontrained hemispheres of C O N T vs both trained hemispheres of ALT), and data from within-subject comparisons (trained hemispheres of REV and PRAC vs nontrained hemispheres of REV and PRAC). * ** *** ****

p p p p

< < < <

.05. .01. .001. .0001.

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GREENOUGH, LARSON, AND WITHERS

their untrained hemispheres. The number of branches at each of the first 6 orders was significantly greater in ALT than in CONT, while differences were directionally similar but only significant at the second order of branching in the within-subject comparison in REV plus PRAC. Terminal branch lengths differed statistically across the first three orders in the ALT vs CONT comparison, while there were no significant terminal length differences in the directionally similar results in the combined REV and PRAC groups. First-order bifurcating branches were significantly longer in ALT than in CONT, but the failure of other bifurcating branch comparisons to yield statistical differences in this study, as well as the general tendency for bifurcating branch lengths not to differ in environmental complexity studies, suggests that this may be an anomalous result.

Hemispheres within Subjects Within-subject comparisons in the unilaterally trained groups showed the same general patterns, although the effects were somewhat weaker in these smaller samples. In the REV group, both total dendritic length and the total number of branches (summed across all orders) were significantly greater opposite the trained forepaw, as shown in Table 3. Within orders, only the number of branches at Order 2 differed significantly between hemispheres in the REV group. In the PRAC group, the number of branches at Order 2 was also significantly greater opposite the trained forelimb, but differences in total dendritic length and in total number of branches fell short of significance. There were no significant differences in the lengths of terminating or bifurcating branches at any order in either of these groups. There were no significant differences between hemispheres on any of these measures in the bilaterally trained ALT group. Surprisingly, in light of the prior result (Larson & Greenough, 1981), there were no significant differences between the preferred and the nonpreferred hemisphere in the nontrained CONT group. TABLE 3 Within-Group C o m p a r i s o n s

N u m b e r of B r a n c h e s Trained/Pref a Nontrained Total Dendrite L e n g t h Trained/Pref ° Nontrained

Control

Reversal

Practice

10.92 12.14

13.19 * * 11.30

13.28 12.63

546.5 610.9

658.4* 566.6

773.3 712.9

Alternation

13.55 12.43 773.6 743.0

Note. In this analysis, h e m i s p h e r e s within groups (trained vs nontrained in R E V and P R A C , or preferred vs nonpreferred in C O N T and ALT) are compared. ° Trained for reversal and practice, preferred for control and alternation. * p < .05. ** p < ,01.

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Nontrained Hemispheres in Unilaterally Trained vs Control Subjects Differences between nontrained hemispheres in the unilaterally trained groups (REV and PRAC) and hemispheres in the nontrained CONT group indicated that there were some effects of having been trained that were not specific to hemispheres opposite trained forelimbs. Total dendritic length was greater in the nontrained hemispheres of trained groups (639.9 vs 577.8/zm, p < .05), and there were more first-order branches in the trained groups (5.52 vs 4.50, p < .001). Sporadic additional differences in other variables further suggested a general effect of the training experience. DISCUSSION The results of the analyses of the combined trained hemispheres vs the combined nontrained hemispheres make it clear that an effect of reach training is to increase the relative size of apical dendritic fields of Layer V pyramidal neurons in the contralateral MSF cortex. This replicates the previous study of these animals (Larson & Greenough, 1981) and is compatible with the hypothesis that altered numbers and/or patterns of synaptic connections are involved in the storage of behaviorally relevant information arising from a training experience, An alternative explanation of this result is that the dendritic field effects arise from some general effect of the training situation, such as stress or other general metabolic factors. As with a previous study of the effects of unilaterally projected visual input from maze training (Chang & Greenough, 1982), the present results argue against this possibility. There is a clear within-animal trend in the REV and PRAC groups for the hemispheres opposite trained forelimbs to have larger apical dendritic fields than those ipsilateral to trained forelimbs, although the statistical results are not as strong as for the combined hemispheres. Percentage differences are quite similar for all trained vs nontrained comparisons, averaging about 8-9%. The differences between the nontrained hemispheres of unilaterally trained groups (REV and PRAC) and the hemispheres of CONT suggests that there may be some nonspecific effects of the training experience. While some of this effect might be attributable to reaching with the nontrained forelimb, which occurs relatively frequently in the early stages of training, the fact that the ALT group had the largest apical dendritic fields suggests that there may additionally be some sort of potentiating interaction between the trained and nontrained hemispheres, or an effect of the training procedure in general upon the hemisphere not being trained. The effect was not due merely to being placed in the apparatus, since this was also done with the CONT animals. While the training procedure used here was sufficient to reverse forelimb preference for reaching in the REV group, it should be noted that this

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study provides no direct evidence that the structural effects of training are related to preference, as opposed to other behavioral consequences of training such as improved dexterity. In fact, the single R E V animal that did not show preference reversal did have larger apical dendritic measures in the hemisphere opposite the trained forepaw. The present results do not rule out the possibility that the dendritic field differences were due to differential m o t o r activity and associated frontal cortical neuronal activity, independent of training or acquired information, upon dendritic field characteristics. Indeed, this possibility cannot be ruled out with the design used here. It should be noted, h o w e v e r , that, in other brain regions, synaptogenesis has been reported following patterns of electrical stimulation that induce physiologically m e a s u r e d synaptic efficacy change but not after patterns involving equivalent or greater amounts of stimulation that do not induce efficacy change (Chang & Greenough, 1984; Lee, Schottler, Oliver, & Lynch, 1980; Lee, Oliver, Schottler, & Lynch, 1981). T h e s e results present an intriguing parallel to recent reports by Jenkins, Merzenich, & Ochs (1984) that the somatosensory cortical representation of forelimb digits in primates becomes enlarged, relative to and apparently at the e x p e n s e of, fields of other digits, following manipulative training procedures. It seems plausible that structural changes of the sort reported here m a y be involved in the s o m a t o s e n s o r y cortical reorganization processes described following training, disuse, and peripheral deafferentation.

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