ASYMMETRICAL EFFECTS OF LESIONS IN FR2 ON LEFT-RIGHT RESPONSE DIFFERENTIATION IN THE RAT
Michael Noonan, Dennis Chmiel, Jr. and Seymour Axelrod
Canisius College and The State University of New York at Buffalo
Supported in part by NSF IBN-9209551.
Presented at Society for Neuroscience, Miami Beach, Florida, November, 1994.
This study focused on the behavior of rats obliged sequentially to learn two different
left-right associative learning tasks in the same maze. In an earlier study, Noonan and
Axelrod (1992) examined the effects of complete left or right hemidecortication on such
learning, and found two suggestions in the behavior of the rats of lateral asymmetry in
the neural mediation of the learning. First, the rats demonstrated a consistent leftward
turning bias, which was greater in strength on the second of the two tasks. Second, rats
with cortical ablation on opposite sides of the brain showed dramatically different
facility with second-task acquisition: rats with left hemidecortication acquired second
tasks more slowly, and rats with right hemidecortication more quickly, than controls.
The present study investigated the consequences of cortical lesions confined to the left
or right FR2 area (Zilles nomenclature) in an effort to learn if these asymmetric effects
could be localized to a limited subdivision of the cortical hemisphere.
Long-Evans rats were prepared with either left- or right-FR2 ablation. Six weeks later the
subjects were trained in sequential weeks on two different tasks in a water-filled M-maze
1. Brightness Discrimination (BD). On each trial, only one side of the maze was
illuminated, the side varying randomly from trial to trial. The escape ramp was always
located in the illuminated arm.
2. Left-Right Response Differentiation (LRRD). On each trial, both arms of the maze were
either lit or unlit, this condition varying randomly from trial to trial. When the arms
were illuminated, the escape ramp was always in the right arm. When the maze was unlit,
the escape ramp was in the left arm.
On each task, testing continued for 25 trials per day until the rat made 10 successive
Turning Bias. Turning bias was computed by tabulating direction of the first turn made on
each trial and computing for each rat the ratio R/(L+R). On the first task (BD), our
animals were about as often left-biased as right-biased, regardless of the side of lesion.
On the second task (LRRD), both groups showed a left-turning bias, particularly the
left-lesioned rats (Fig. 2).
Acquisition Scores. Trials-to-criterion was taken as the index of proficiency on each
task. The left- and right-lesioned rats did not differ on the first (BD) task, but did
differ on the second (LRRD) task (Fig. 3): rats with right-sided lesions learned the
second task faster than rats with left-sided lesions.
The tendency of the rats to develop leftward behavioral biases on the second of these two
tasks replicates and confirms earlier observations on total hemidecortication. It seems to
imply an attempted right-hemisphere dominance under these circumstances.
That right-lesioned rats handle second learning tasks better than left-lesioned ones is
also consistent with our earlier observations, and implies that the right hemisphere, when
present, interferes in some way with the transition to the new associations of the second
task. That this effect is seen when the cortical damage is limited to FR2 suggests that
the relevant asymmetry is motor and/or attentional in nature.
Both findings indicate that there is asymmetrical involvement of the two sides of the rat
brain in second-task learning, and that the direction (sidedness) of this asymmetry is
consistent across the population.