Right-sided population bias in male rats: role of stress

Recent studies have shown that rats develop a pronounced right-sided population bias when tested repeatedly over several days in a shock-escape T-maze paradigm. In the present study we sought to determine if this bias was the result of learning or of the repeated exposure to a stressor (footshock). Rats were tested in the T-maze for 5 trials on each of 6 consecutive days. One group of rats (HS-C) was allowed a free left or right choice on each trial whereas another group of rats (HS-A) was forced to alternate left and right turns on each trial for the first 5 days of testing and then allowed a free choice on Day 6. The population and individual laterality of the HS-A group did not differ from that of the HS-C group, indicating that learning does not account for the biases. A third group of rats (LS-C) was tested using a lower shock level; this decrease in stressor intensity delayed the development of a right-sided population bias without affecting the development of individual laterality. These results indicate that repeated exposure to a stressor, rather than learning, is primarily responsible for the marked right-sided population bias observed in the T-maze.