Michael Noonan and Seymour Axelrod

Canisius College and The State University of New York at Buffalo

Presented at the Society for Neuroscience annual meeting, Anaheim, California, October, 1992.


Noonan and Axelrod (1991, 1992) showed that split-brained rats acquire a left-right response differentiation faster than intact rats. We interpret this as evidence of an improved sense of left and right in the surviving but isolated cerebral hemispheres.

In the present study, we compared hemidecorticate rats with intact subjects to ask whether a single surviving hemisphere would likewise demonstrate facilitated acquisition of a left-right response differentiation.


The subjects were trained in sequential weeks on two different tasks in a water-filled T-maze (Fig. 1) The order in which these two tests was administered was varied and counterbalanced across groups.

Left-Right Response Differentiation (LRRD): On each trial, both arms of the maze are either lit or unlit, this condition varying randomly from trial to trial. When the arms are lit, the escape ladder is always found on the right. When the arms are unlit, the escape is found on the left.

Brightness-Discrimination (BD): On each trial, only one arm of the maze is lit (the illuminated side varying randomly from trial to trial), and the escape ladder is always found in the lit arm. This test taps the same general performance variables as the LRRD task, but does not require the animal to distinguish left and right.


Our hypothesis was not confirmed. Hemidecortication did not selectively facilitate (nor significantly impair) LRRD, regardless of whether it was administered as a first test or as a second test (Fig. 3)

However, more detailed analyses, taking into account side of lesion and directional bias, revealed a consistent pattern.

1. On either T-maze task, when it was presented first, intact subjects showed no lateral population bias, tending to rightward and leftward in roughly equal proportions. (Fig. 4)

2. The unilateral lesions produced an ipsiversive turning bias, i.e., leftward for left-lesioned rats and rightward for right-lesioned rats. (Fig. 5)

3. On either task, when it was second, both Hemidecorticates and Sham-operates had a tendency to turn left. (Fig. 4)

4. On either task, when it was the second, L-decorticate subjects required more trials to reach criterion than shams and R-lesioned subjects required fewer trials than shams. (Fig. 6)


Two accounts of these finding present themselves, although neither is entirely satisfactory.

A. The first focuses on the interplay of two sources of turning bias. Since mastery of either task requires the rat to ultimately turn left and right in 50:50 proportions, both the turning tendencies induced by the surgical lesions, and the overall tendency for rats to become left-biased on second tests, can be considered as disadvantages, particularly when the two operate in the same direction. L-lesioned subjects are biased leftward to begin with, and so are further disadvantaged on the second test when the leftward second-test bias is added. R-lesioned subjects are biased rightward to begin with; the addition of the second-test left bias opposes this and brings them more readily to 50:50 proportions.

This account is parsimonious. However, it assumes, but lamentably offers no explanation for the origin of, the overall L-bias on second tests.

B. Alternatively, the side-of-lesion data can be interpreted as indicating a hemispheric specialization for the acquisition of second left/right associative tasks--perhaps for sensory/motor re-programming. Specifically, the data suggest a left-brain importance for the mediation of such events.

Such a specialization and a consequent asymmetrical engagement of the hemispheres during second tasks would be compatible with the increased behavioral bias shown on second tasks. However, that rats are biased leftward on second tasks suggests a right-hemisphere dominance at that time, and thus conflicts in direction with the implication of the side-of-lesion data. Concern for this conflict would be ameliorated if we were to speculate further and hypothesize that the hemispheric specialization for the cognitive process of re-programing is independant of the neural asymmetry underlying the leftward motor bias.

Our current efforts continue our exploration of the relationship between behavioral bias and left-right learning, and seek additional data bearing on the notion that the processing of left-right associative learning is asymmetrically mediated in the brain of rats.

Contact Info: Michael Noonan, PhD, Canisius College , 2001 Main St., Buffalo, NY 14208                                                                             noonan@canisius.edu