Novel assays were used to assess inter alia whether the hippocampus
is involved in detecting novelty per se or in an
associative mismatch process. During training, rats received
two audiovisual sequences (tone–left constant light and click–
left flashing light). In both sham-operated control rats and those
with excitotoxic hippocampal lesions, novel visual targets provoked
an orienting response that habituated during training.
Moreover, like sham-operated rats, rats with hippocampal lesions
acquired associations between the elements of two audiovisual
sequences. However, subsequent test trials in which
the auditory stimuli preceding the visual targets were switched
(click–left constant light and tone–left flashing light) provoked
renewed orienting to the visual targets in sham-operated rats
but not in hippocampal rats. These results support the view that
hippocampal damage results in a failure to detect (or act on)
mismatches that are generated when an auditory stimulus associatively
evokes the memory of one visual stimulus and a
different (familiar) visual stimulus is present in the environment
