HIPPOCAMPAL ENSEMBLE ACTIVITY DURING SPATIAL DELAYED-NONMATCH-TO-SAMPLE PERFORMANCE IN RATS

Multiple-cell recording from specially designed arrays of microwire el ectrodes allowed analysis of anatomically defined ensemble activity fr om 10 different locations within the hippocampus of rats (n = 7) perfo rming a two-lever operant version of a spatial delayed-nonmatch-to-sam ple task (DNMS). Application of population analysis procedures to ense mbles of single-neuron activity within the CA1 and CA3 fields revealed firing patterns related to task-relevant events within a DNMS trial. The patterns were extracted via a canonical discriminant analysis in t he form of ''roots'' that represented sources of variance in firing wi thin the ensemble, such as phase of the task (Sample or Nonmatch), spa tial position of the lever press response (left or right), and correct versus error trials. Comparison of the ensemble firing on correct ver sus error trials revealed important insight into ensemble information encoding, such as ''miscoding'' of the response position and lack of d istinct encoding of the response in the Sample phase, which became inc reasingly vulnerable to error as a function of the duration of delay i nterval. The extracted discriminant scores were reflective of multiple representations within ensembles and suggested that ''conjunctions'' of task-relevant features could be represented effectively by small nu mbers of hippocampal neurons. The findings support the long-held suppo sition that hippocampal neurons play a critical role in the encoding a nd retrieval of information in recognition memory tasks.