Reward magnitude shift effects in rats with hippocampal lesions.

Abstract

Rats with either hippocampal or neocortical lesions and normal unoperated control rats received 60 runway training trials under 12-or 1-pellet reward and then 42 trials under the original reward magnitude for half of the rats or under the opposite magnitude for the remaining half. Over the first 60 trials performance levels increased reliably with reward magnitude for each surgical condition; no reliable differences occurred among conditions. Following reward change neocortical and normal animals showed reliable negative contrast effects under decreased reward and performance similar to 12-pellet controls under increased reward. Hippocampally damaged rats showed nonsignificant performance changes following reward change. Their postshift performance was a reliable direct function of initial reward magnitudes.

The effects of dorsal hippocampal lesions on reinforcement shifts

“…-In marked contrast to both studies, Franchina and Brown (1971) found an almost complete absence of behavioral change following reward shifts. Two major differences between their study and the present one might account for the discrepancy.…”

“…and (2) a negative contrast effect (NCE) for both hippocampal and sham downshift groups. These results are in contrast to earlier fail!lres to find a NCE in hippocampal Ss (Franchina & Brown, 1971;Murphy & Brown, 1970). It was proposed that the immediate feedback inherent in the licking response resulted in the standard contrast effect not found when longer ITIs are introduced.…”

“…These results are in contrast to earlier fail!lres to find a NCE in hippocampal Ss (Franchina & Brown, 1971;Murphy & Brown, 1970). It was proposed that the immediate feedback inherent in the licking response resulted in the standard contrast effect not found when longer ITIs are introduced.The effect of hippocampal lesions has often been characterized as a loss of behavioral inhibition, resulting in increased activity levels (Teitelbaum & Milner, 1963) and slower behavioral adjustment to reinforcement schedule changes (Clark & Isaacson, 1965;Jarrard, 1965;Niki, 1965)_ Consistent with this, hippocampal rats fail to show any behavioral modification of a running response after shifts in incentives (Franchina & Brown, 1971). However, these Ss do show the typical magnitude of reward effects during the preshift-acquisition phase.…”

“…The effect of hippocampal lesions has often been characterized as a loss of behavioral inhibition, resulting in increased activity levels (Teitelbaum & Milner, 1963) and slower behavioral adjustment to reinforcement schedule changes (Clark & Isaacson, 1965;Jarrard, 1965;Niki, 1965)_ Consistent with this, hippocampal rats fail to show any behavioral modification of a running response after shifts in incentives (Franchina & Brown, 1971). However, these Ss do show the typical magnitude of reward effects during the preshift-acquisition phase.…”

“…The lesions used in this study were considerably more localized than the complete hippocampal aspirations used by Franchina and Brown (I 971) and Murphy and Brown (1970)_ Furthermore, this design used a symmetrical shift paradigm, while Murphy and Brown (I970) only shifted their Ss to lower reward levels. It might be expected that response differences due to hippocampal lesions might be symmetrical, as was found by Franchina and Brown (1971)_…”