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pentobarital anesthesia. The hippocampal lesions were made by aspira-
tion using the techniques described by Isaacson and Woodruff (1975).
These procedures resulted in bilateral lesions, which involve removing
60-90% of the hippocampus as well as removing part of the overlying
neocortex. At the end of the experiment the rats were perfused with saline
and 10% formalin solution. The brains were embedded in celloidin, and
2()-μm sections were cut and stained in thionin. The size and location of
the lesions were found to be comparable with those produced in many
studies originating in this laboratory (e.g., Woodruff and Isaacson, 1972).
Five-minute open field activity tests were given from the day after
surgery through the fifth day and on every other day thereafter for a total
of 10 postoperative sessions. The open field measured Ixlm and it was
subdivided into 49 squares of equal area. The measure of activity was the
number of squares crossed in the 57-min test period. The open field arena
was illuminated by one 40-W lamp 80 cm above the floor of the arena. The
rest of the room was dark. Rearing, grooming, and defecation were also
recorded by an observer on a multichannel recorder in the darkened
room.
Beginning on the 14th day after surgery, all animals were placed on a
23-hr water deprivation schedule. On the fourth day of deprivation each
rat was given a 10-min adaptation period in the passive avoidance ap-
paratus. This was a wooden box (60 × 21 × 31 cm) painted gray with a
wooden door through the middle, which could be raised to allow move-
ment between the two halves of the box. The floor was a metal grid.
During the adaptation period, the door was raised and water was available
at one end.
On the following day (the first day of training) 10 trials were given.
Three minutes were allowed for each trial. A maximum time of 10 sec of
drinking was permitted on each trial. The latencies to enter the second
compartment to drink were recorded. The intertrial interval between
removal from the drinking compartment, replacement in the starting
compartment, and raising of the door between compartments was about
15 sec. One the second day of training, five more trials were given. On
trials 6-10 the rat received an electric footshock of 0.5 mA on entering the
second compartment.
All between-group comparisons were made by Mann-Whitney U tests,
and probabilities are for two-tailed tests on independent samples. All
intragroup probabilities were determined according to the Wilcoxon
matched-pairs test.
Both groups of animals with hippocampal damage became hyperactive
in the open field by the second week after surgery compared to their
preoperative levels of activity (P < 0.01, Fig. 1). Activity levels were
higher for animals with hippocampal lesions than for those with neocorti-
cal damage (P < 0.05) and for unoperated controls (P < 0.002) in this
second week. Rearing was depressed in animals with hippocampal dam-