Hippocampal afterdischarge interferes with storage of spatial information in a working memory test

In an attempt to impair spatial working memory by reversible functional blockade rather than by irreversible lesion of the hippocampus, eight male hooded rats were trained to asymptotic performance of 1.2 to 1.4 errors per trial in the spatial 12-choice apparatus ( Bure s et al. 1982), formally similar to the radial maze. The rats were implanted with hippocampal stimulating and recording electrodes, which were used for eliciting and monitoring hippocampal afterdischarge (HAD) lasting for at least 20 s. In Experiment 1, HAD elicited 1 or 10 min before testing increased the incidence of errors to 2.75 or 2.50 per trial, respectively, but the performance still remained above chance level (4.18). In Experiment 2, interruption of the trial by 1, 10, 20 and 30 min intervals inserted between choices 6 and 7 increased the incidence of errors in choices 7 to 12 to 1.0, 1.5, 2.1 and 2.5, respectively. HAD elicited immediately after choice 6 increased error incidence in the subsequent 6 choices performed after 1-min or 10-min intervals to 3.1 or 2.75, respectively, i.e. to the chance level of 3 errors in 6 choices. It is concluded that HAD elicits transient shortening of the memory span for newly acquired spatial information (anterograde effect) and erases the current spatial working memory record (retrograde effect).