Change of information represented by thalamic mediodorsal neurons during the delay period

The thalamic mediodorsal (MD) nucleus plays an important role in transforming visual information into motor information during spatial working-memory performances. To understand the neural mechanism of this transformation process, we examined whether or not the information represented in individual MD neuron's activity changes during a trial of the task. Two monkeys performed two oculomotor delayed-response tasks (an ordinary and a rotatory oculomotor delayed-response task). As MD neurons show directional delay-period activity, we compared the directional selectivity of the same MD neuron between these two tasks and determined whether the activity represented the cue direction or the saccade direction. Among the 26 MD neurons that showed directional delay-period activity, representing information of 27% of neurons gradually altered from the visual domain to the oculomotor domain, while the remaining neurons kept holding either the visual or the oculomotor information throughout the delay period. These results indicate that gradual alteration of information representing in individual MD neurons during the delay period plays an important role in spatial working-memory performance. This alteration could be performed by interactions among MD neurons representing different information during the delay period.