Physiology and Behavior, Vol. 13, pp. 745-748. Brain Research Publications Inc., 1974. Printed in the U.S.A.

Paw Preference Shifts in the Rat Following Forced Practice

DOUGLAS MARTIN AND WILLIAM G. WEBSTER 1

Department o f Psychology, Carleton University, Ottawa, Ontario, Canada

(Received 8 July 1974)

MARTIN, D. AND W. G. WEBSTER. Paw preference shifts in the rat following forced practice. PHYSIOL. BEHAV. 13(6) 745-748, 1974. - Rats which are forced to reach for food pellets with the nonpreferred paw subsequently show a shift in paw preference. The present study demonstrated that this shift occurs regardless of whether the cerebral hemispheres are surgically separated and/or the hemisphere contralateral to the initially preferred paw is functionally depressed during the administration of forced practice training.

Paw preference Handedness Cortical spreading depression

Corpus callosum Cerebral dominance Laterality

MANY nonhuman species manifest lateral preferences, of which forelimb preferences have been the most widely studied. Individual animals show a marked and consistent preference for using one paw or the other when placed into situations allowing for or requiring a unimanual response such as retrieving pieces of food [1, 10, 11, 12]. The study of such preferences and of variables which modify their development or expression derives significance from their being a model or analogue of human handedness and perhaps of human hemispheric specialization, which have proved to be most elusive to experimental study.

Generally, the preferences of animals are stable over time, but they can be reversed by unilateral ablation of the sensorimotor cortex contralateral to the preferred paw [2, 8, 13]. In the rat, complete paw preference shifts can also be effected through repeatedly forcing the animal to reach with its nonpreferred paw [6], although a similar treatment in cats produces a shift apparently restricted to that reaching task on which the forced practice was administered [ 12].

The purpose of the present study was to attempt to effect a shift in paw preference in the rat through administration of forced practice to the nonpreferred paw while the hemisphere contralateral to the perferred paw was temporarily nonfunctional under cortical spreading depression (CSD). If forced practice effects are mediated normally through a shift in the "balance of activity" between the hemispheres [4] resulting from the extinc- tion of a response initiation tendency in the hemisphere contralateral to the preferred paw, forced practice administered while that hemisphere is nonfunctional under CSD should produce posttreatment ambidexterity

and mixed responding, especially if the corpus callosum has been severed as well [9].

The three phases of the study were initial assessment of paw preference, the administration of forced practice with the nonpreferred paw, and the posttreatment reassessment of paw preference. On the basis of the original assessment, the animals were divided into 5 groups so that the distribu- tion of paw preference was similar in them. In the two experimental groups, forced practice with the nonpreferred paw was administered while the hemisphere contralateral to the preferred paw was functionally depressed. In one of these groups, the corpus callosum was intact but in the other it had been severed. Two of the three control groups were to insure that CSD alone or CSD and callosectomy would not produce paw preference shifts without forced practice. The third control group, which was intact surgi- cally and similar to the animals in Peterson's [6] study, was to insure that the number of forced practice trials adminis- tered was sufficient to effect a paw preference shift in normal rats.

METHOD

A n im als

Seventy male albino rats weighing between 180-250 g were divided into 5 groups. The 2 groups which were to have the corpus callosum sectioned were each assigned 20 animals and the remaining 3 groups were assigned 10 animals each. The 5 groups were as follows: (1) an experi- mental group with the corpus callosum left intact (Group EI), which received forced practice with the nonpreferred paw while the hemisphere contralateral to the preferred

Supported by Research Grant APA-0399 from the National Research Council of Canada. Reprints from Dr. William G. Webster at the Department of Psychology, Carleton University, Ottawa, Ontario, K1S 5B6, Canada.

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paw was functionally depressed; (2) an experimental group with the corpus callosum sectioned in the midline (Group EC), which received forced practice while the hemisphere contralateral to the preferred paw was functionally de- pressed; (3) a control group with the corpus callosum intact (Group CI), which received no forced practice but did receive unilateral CSD; (4) a control group with the corpus caUosum severed (Group CC), which was administered unilateral CSD but no forced practice; and (5) a surgically intact control group (Group CF), which received forced practice with the nonpreferred paw.

Apparatus

Paw preferences were assessed in a box approximately 60 cm square and 45 cm high having a clear Plexiglas front across which were mounted three small food cups, 2.5 cm deep, to which the animal had access through 1.5 cm holes in the front. The animal was required to reach through the hole into the foodwell to retrieve 45 mg Noyes pellets placed into the foodwells according to a balanced irregular sequence. On each trial, the paw or paw sequence used by the animal to retrieve the pellet was recorded.

Surgery

A two-stage surgical procedure was used for sectioning the corpus callosum and for implanting the CSD cannula. In the first stage, the corpus callosum was sectioned in Groups EC and CC using a procedure developed and described by McIntyre [5]. This involved inserting a modified hook- shaped scalpel blade through a trephine hole drilled anterior to Bregma on the midline and drawing it through in such a way that the callosum is sectioned.

A CSD cannula was implanted in all animals except those of Group CF. A 3 mm dia. hole was drilled in the skull overlying the parieto-occipital cortex of the hemi- sphere contralateral to the preoperatively preferred paw and the cannula, a 1 cm length of polyethylene tubing was cemented to the skull and held firmly in place with the aid of bone screw anchors. A piece of saline-soaked filter paper was kept in the cannula to prevent drying of the dura.

Procedure

Animals were tested for paw preference under condi- tions of 2 0 - 2 3 hour food deprivation. Before being assigned to groups, all animals were adapted to the handed- ness box and received 50 trials of paw preference assess- ment. The animals were assigned to groups so that the distribution of paw preference with respect to direction and magnitude was similar in all groups.

Animals were anesthetized with Equithesin (3.2 cc/kg) or with sodium pentobartitol (Nembutal, 55 mg/kg)and the corpus callosum was sectioned in those animals in Groups EC and CC; animals in Groups EI, CI, and CF were allowed to simply recover from the anesthetic. Twelve days later, all animals were again anesthetized and those in Groups EC, CC, El, and CI had a cannula implanted over the hemi- sphere contralateral to the preferred paw as initially assessed. Animals in Group CF again just recovered from the anesthetic.

Three days after surgery, the animals received 50 trials of paw preference assessment to insure that the surgical treatments had not induced any paw preference shifts.

On each of the following 2 days, the animals in 2 of the

forced practice conditions (Groups EC and El) received 200 trials of forced practice with the nonpreferred paw with the hemisphere contralateral to the preferred paw being func- tionally depressed and those of the other forced practice condition (Group CF) received 200 trials of forced practice but without spreading depression being applied to the ipsilateral hemisphere. Forced practice was effected through the placement of a glove, made of relatively non- sticky adhesive tape, on the preferred paw. Using its free paw, the animal was allowed to retrieve food pellets placed into the handedness box foodwells according to a balanced irregular sequence. In most cases, a few training trials were necessary before the animal would reliably retrieve the pellets. Animals in the groups which did not receive forced practice (Groups CI and CC) had the glove placed on the preferred paw but remained in their home cages in the experimental room.

CSD was initiated by placing into the cannula a piece of filter paper soaked with a 25% solution of potassium chloride (KC1). Forced practice was not begun until the animal displayed an absence of placing responses with the contralateral forelimb. At the end of each session the dura was flushed with saline. Animals in those groups which received CSD but no forced practice (Groups CI and CC) were maintained under CSD on each of the 2 days for a period of time comparable to what it would take to administer 200 forced practice trials.

All animals then received 300 paw preference assess- ment trials distributed over 3 days at 100 trials per day.

After completion of the study, all operated animals were perfused with saline followed by Formalin and the brains were examined for superficial damage due to the cannulae. The brains of all animals which had the corpus callosum severed (Groups EC and CC) were sliced at 40 u and stained with cresyl violet to allow for histological verifica- tion of the sectioning and for examination of any uninten- tional brain damage. The brains of 2 randomly selected animals from each of Groups EI and CI were also sliced and stained and examined for possible damage around the cannula tip and sites of bone screw placement.

RESULTS

Histology

Five of the 25 surviving animals in the groups which had had the corpus callosum sectioned were excluded from data analysis since portions of the callosum were found to be intact or since the cut had deviated from the midline. Most of the animals with cannula placements were found to have slight cortical indentations of varying severity at the site of implantation and the cresyl violet stain indicated some gliosis around these. In several cases in which perfusion had been delayed for some weeks after testing abscesses were found to have developed, but as no evidence of such abscesses was found in the case of any animal perfused within a few days of the completion of testing, it is unlikely that these abscesses were present during the time of testing or that they influenced the results.

Paw Preference Shifts

Fifty-two animals survived all phases of the study. Of the 47 animals included in the data analysis, 28 had an initial left paw preference, 18 had an initial right paw preference, and 1 was ambidexterous. Ambidexterity was

PAW PREFERENCE SHIFTS IN THE RAT 747

TABLE 1

MEAN PERCENTAGE SHIFTS IN PAW PREFERENCE

Strength of Immediately Postoperative Posttreatment Group Initial Preference* Change in Preference Change in Preference:~

El 94.5 0.4 89.4

EC 98.0 2.8 73.2

CE 96.2 -1.4 t 96.3

CI 94.5 -4.5t -0.1t

CC 97.4 0.6 -0.9t

*Calculated from the percentage use of the more frequently used paw. tNegative signs indicate an increase in the use of the preferred paw. :~Calculated as a change from the preoperative and immediately postoperative paw preferences

combined.

defined as a preference for the more frequently used paw of less than 74%, which on the basis of 50 testing trials repre- sents a statistically significant deviation from chance (p<0.001). The surgical manipulations did not influence paw preference as evidenced by the fact that at the time of the 50 reassessment trials administered following surgery, all animals had the same preference as that initially assessed.

Table 1 shows the change in use of the initially preferred paw from the preoperative assessment session to the immediately postoperative reassessment session and to the posttreatment reassessment sessions. The groups which did not received forced practice (CE and CC) showed no change in paw preference. (The negative shifts shown in the table reflect the fact that a few animals demonstrated a slight increase in the use of the originally preferred paw.) All 3 groups which received forced practice (Groups El, EC, and CF) showed nearly complete paw preference shifts. Group CF, the intact control group, showed the greatest mean change with 96.8%; the experimental group receiving forced practice with the corpus callosum intact (El) had a mean change of 89.4%. The incidence of mixed responses did not increase. The data from Group EC, which had the corpus callosum severed and received forced practice under CSD, were much more variable. Six of the 10 animals showed large shifts (ranging from 83% to 100%) while 4 showed only partial shifts (ranging from 27% to 47%). The mean percentage shift for this group was 73.2.

For purpose of analysis, the 3 forced practice groups, (El, EC, and CF) were subdivided according to pretreat- ment paw preference. There were no overall differences in mean percentage among groups (F = 2.92) nor as a function of the direction of the original preference (F = 0.09). A Newman-Keuls Multiple Comparisons Test confirmed this result. However, the variances among the groups were found to be heterogeneous (Fmax = 56.4, p < 0.001 ).

Throughout all testing, the mean time per trial was recorded on the basis of 10 trial blocks. In all groups,

response rates were significantly higher in the postoperative sessions than in the preoperative sessions (F = 38.6, p<0.01) indicating that at the time of testing the animals had recovered from the surgery and were not hampered in their responding by the surgical intervention(s).

DISCUSSION

The results of the study indicate that extinction of the response initiation tendency by the hemisphere contra- lateral to the preferred paw is not necessary for there to be a shift in preference following forced practice with the nonpreferred paw, as extinction was presumably prevented by the administration of CSD. The extent of shift in Groups EC and EI suggests that the hemisphere contra- lateral to the paw receiving forced practice may acquire a response initiation tendency of its own and may then suppress that tendency in the opposite hemisphere (con- tralateral to the preferred paw). The interpretation how- ever, is complicated by the fact that complete shifts in Group EC occurred in 6 of the 10 animals, suggesting that if reciprocal interhemispheric inhibition [3,4] does indeed occur, it is not necessarily mediated exclusively through the corpus callosum.

A small preliminary study was undertaken to determine the extent to which other commissural structures may mediate this inhibition. The same procedure used in the present study was followed with several additional rats, except that the surgery involved a much deeper split-brain operation. Histological examination of the two animals completing the study indicated that the corpus callosum, hippocampal commissure, anterior commissure, and massa intermedia were all severed, (leaving the posterior com- missure as the only forebrain commissure intact) but behaviorally both showed virtually a 100% shift in paw preference. Accordingly, the question as to which struc- tures actually do play a part in mediating the inhibition remains unanswered.

748 MARTIN AND WEBSTER

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