Task difficulty increases thresholds of rewarding brain stimulation

The results suggest that the effort required to make the operant response contributes to the position of rate-frequency curves and that shifts in rate- frequency functions must be interpreted with caution when such shifts are obtained by CNS lesions or drug injections.

Abstract

The effect of increasing task difficulty on the threshold of rewarding, electrical brain stimulation was evaluated. Rats were trained to press a lever to obtain a brief burst of pulses to the lateral hypothalamus. The threshold was psychophysically scaled using a descending method of limits in which the pulse frequency was varied to yield a maximum to minimum range of self-stimulation rates. As expected, weighting the lever with 0, 15, 30, or 45 g produced progressive decreases in maximal rates, but it also caused a weight-related shift to the right of the rate-frequency ogives in each of the 7 rats. Although the degree of shift varied from rat to rat, it did not matter whether criterion performance was defined as half-maximum rate or as a constant rate of 5 responses. These results suggest that the effort required to make the operant response contributes to the position of rate-frequency curves and, further, that shifts in rate-frequency functions must be interpreted with caution when such shifts are obtained by CNS lesions or drug injections.