Cellular mechanisms underlying activity patterns in the monkey thalamus during visual behavior

We show for the first time with in vitro recording that burst firing in tha lamic relay cells of the monkey is evoked by activation of voltage-dependen t, low threshold Ca2+ spikes (LTSs), as has been described in other mammals . Due to variations in LTS amplitude, the number of action potentials evoke d by an LTS could vary between 1 and 8. These data confirm the presence of two modes of firing in the monkey for thalamic relay cells, tonic and burst , the latter related to the activation of LTSs. With these details of the c ellular processes underlying burst firing, we could account for many of the firing patterns we recorded from the lateral geniculate nucleus of the tha lamus in behaving monkeys. In particular, we found clear evidence of burst firing during alert wakefulness, which had been thought to occur only durin g sleep or certain pathological states. This makes it likely that the burst firing seen in awake humans has the same cellular basis of LTSs, and this supports previous suggestions that burst firing represents an important rel ay mode for visual processing.