Figure 10.

Water maze learning in fully hippocampectomized mice shortly after operation and in the chronic state. (A) Brain sections of mice in which the hippocampi were destroyed by bilateral infusion of excitotoxic substances. (B) Standardized water maze used for many years in Zürich to study thousands of mice. The procedure included 3 days with totally 18 trials to measure acquisition, followed by 2 days (12 trials) after reversal of the platform position. The first reversal trial was taken as a probe trial, analyzing the approaches to the former platform position (the classic proof of spatial memory), the rest of the reversal time served to assess the flexibility of the mice in adapting to a changed situation. The trial length was 120 s. Note that for better presentation, values for two subsequent trials were averaged, thus showing only 9 and 6 data points for acquisition and reversal. (C) Severe impairment (as expected) of lesioned mice to reach the platform, chiefly because of thigmotaxis (wall hugging). The reversal phase showed a clear memory effect in the controls followed by learning a new platform position. (D) Comparison of probe trial scores (red circled trials), showing random place preferences in the lesioned animals and significant differences to the controls. (E) After different recovery times, lesioned mice showed still significant differences in learning the task as compared to the controls (chiefly because of stronger thigmotaxis), but they show distinct learning curves, and a strong increase in search time after platform reversal. Thereafter, however, they appeared to be unable to orient toward the new platform position. (F) The “the gold standard” score for assessing spatial memory did not show any differences between the groups, both groups scoring significantly above chance levels. Unpublished data. Lesioning was performed in Zürich by Rob Deacon and Giovanni Colacicco.