We recently found that neurons in the cerebellar dentate nucleus carry temporally-specific signals when monkeys predicted the upcoming stimulus timing (Ohmae et al. 2013). To understand how these signals are processed further in the thalamocortical pathways, we examined neuronal activity in the anterior intralaminar and paralaminar nuclei of the thalamus. In the missing oddball task, a brief (35 ms) audiovisual stimulus was presented repeatedly at a fixed interstimulus interval (ISI, 100-600 ms). Monkeys were trained to make an immediate saccade to a visible target when one stimulus in series was omitted, so that they predicted next stimulus timing throughout the trial.
So far, we have recorded from 53 task-related neurons which were classified into 3 groups; 1) Entrainment-type neurons (38%) exhibited a gradual elevation of inhibitory response to each stimulus as the repetition progressed, similarly to neurons in the cerebellum. 2) Sensory-type neurons (28%) regularly responded to each stimulus but reduced the response gain for the repeated stimuli. 3) Switch-type neurons (34%) initially showed excitatory response to each stimulus, but in trials with long ISIs the direction of firing modulation gradually reversed, resulting in a shift of the timing of peak activity that eventually synchronized with the stimulus onset.
To explore how these signals were generated, neuronal activity was examined in the non-target trials, in which the animals maintained fixation throughout the trial and were not required to predict next stimulus timing. Like neurons in the cerebellum, both the Entrainment-type and Switch-type neurons decreased the firing modulation in the non-target trials compared to the missing oddball trials. Furthermore, for the Switch-type neurons, the response reversal and synchronization of peak activity to stimulus onset found in the missing oddball trials disappeared. These results suggest that neurons in the central thalamus might integrate signals from sensory areas and the cerebellum, thereby generating a predictive code for the next stimulus timing.