The effects of dorsal hippocampal lesions on reinforcement shifts

Abstract

This experiment attempted to clarify the effects of dorsal hippocampal lesions on a lick-rate response to reinforcement shifts. Rats received preshift solutions of either 4% or 32% sucrose and were then shifted to the opposite concentration. The results indicated (1) that Ss shifted to the increased concentration quickly adjusted to the control levels. 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.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. In contrast, Murphy and Brown (1970), using a licking response, found rapid behavioral adjustment to a decreased reward, but failed to find any evidence of the well-documented negative contrast effect (NCE; Black, 1968;Vogel, Mikulka, & Spear, 1968)_ Murphy and Brown also found that the same Ss did not display any preference for varying sucrose concentrations in preference tests, but did respond appropriately during the preshift phase.The present experiment was conducted fo examine the effects of dorsal hippocampal lesions on lick rates after reward shifts. 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)_ METHOD SubjectsThe Ss were 40 adult male hooded rats of the Long-Evans strain, approximately 180 days old at the start of the experiment. SurgeryAll Ss were anesthetized with an intraperitoneal injection of *All reprint requests should be sent to the second author, Psychology Department, Old Dominion University, Norfolk, Virginia 23508. sodium pentobarbital (50 mg/kg), and 20 rats received hippocampal radio-frequency lesions using the bregma as the zero point. The lesion coordinates were: AP -3.4. ML 3.0, DV 3.5. The remaining 20 rats received sham operations consisting of opening the scalp and drilling two holes in the skull. All Ss were injected with .1 cc of atropine prior to surgery to reduce congestion. After surgery, the Ss were given a .1O-cc injection of pe...

Effects of hippocampal damage on reward threshold and response rate during self-stimulation of the ventral tegmental area in the rat

“…It has been shown that reward thresholds are lower during the presentation of an ascending current series, in comparison to random or descending current series [12,28]. Rats with hippocampal lesions have previously been described as being 'extremely sensitive' to contrast effects engendered by shifts in reward [31], and it has recently been reported that rats with extensive hippocampal damage were more sensitive than controls to 'increases' in the qualitative aspects of reward [48]. The current results confirm these earlier observations, and, in the case of ICSS, suggest that lesioned animals may be more sensitive to positive, rather than negative, contrasts.…”

The hippocampus and reward: Effects of hippocampal lesions on progressive-ratio responding.

“…Contrast effects have been reported in another paradigm. The effects of aspiration lesions (Murphy & Brown, 1970) or radio-frequency lesions (Kramarcy, Mikulka, & Freeman, 1973) of the dorsal hippocampus on lick-rate response to reinforcement shifts have been investigated. Food deprived, lesioned animals in both experiments exhibited rapid decreases in licking when sucrose solution was changed from 32% to 4%.…”

Effects of intrahippocampal administration of colchicine on incentive contrast and on radial maze performance.

“…The effects of hippocampal lesions on contrast in consummatory behavior are not clear. One study found that hippocampal lesions had no effect on the negative contrast that occurs when rats are shifted from 32% to 4% sucrose (Kramarcy, Mikulka, & Freeman, 1973), but another study with a nearly identical procedure found that such lesions prevented the occurrence of negative contrast (Murphy & Brown, 1970). There were differences in the size of the lesions reported in these studies, small lesions restricted to the dorsal hippocampus in the Kramarcy et al experiment and larger lesions in the Murphy and Brown experiment, but there was still some sparing of the ventral tips of the hippocampus in the latter study.…”