Figure 10

Effects of hippocampal lesions on choice between immediate, small reward and large, delayed reward. (a) Pattern of choice in the last three sessions before surgery; the sham and lesion groups were matched for performance. Rats' preference for the large reinforcer declined with delay (p < .001). (b) Choice in the first seven postoperative sessions. Although there was a change in behaviour in the lesioned group (lesion × pre/postop., p = .021), the difference between the two groups was not significant in its own right for these sessions (p = .08). (c) Effects of omitting all delays in alternating sessions (error bar, 2 SED for the three-way interaction). H-lesioned rats remained sensitive to the contingencies, altering their behaviour in response to delay omission, as shams did. (d) Last of six further consecutive sessions in which delays were omitted. Both groups preferred the large reinforcer strongly when it was not delayed, with no differences between sham and H-lesioned rats. (e) First three sessions following reintroduction of delays. Hippocampal-lesioned rats were impulsive, choosing the large, delayed reinforcer less often than shams (* p = .027). (f) Next three sessions following reintroduction of delays. Hippocampal-lesioned rats remained impulsive (** p = .007), and generalization between trial blocks occurred, reducing their preference for the large reinforcer in the zero-delay block as well (see text).