Neurons in the thalamic CM-Pf complex supply striatal neurons with information about behaviorally significant sensory events

The projection from the thalamic centre median-parafascicular (CM-Pf) compl ex to the caudate nucleus and putamen forms a massive striatal input system in primates. We examined the activity of 118 neurons in the CM and 62 neur ons in the Pf nuclei of the thalamus and 310 tonically active neurons (TANs ) in the striatum in awake behaving macaque monkeys and analyzed the effect s of pharmacologic inactivation of the CM-Pf on the sensory responsiveness of the striatal TANs. A large proportion of CM and Pf neurons responded to visual (53%) and/or auditory beep (61%) or click (91%) stimuli presented in behavioral tasks, and many responded to unexpected auditory, visual, or so matosensory stimuli presented outside the task context. The neurons fell in to two classes: those having short-latency facilitatory responses (SLF neur ons, predominantly in the Pf) and those having long-latency facilitatory re sponses (LLF neurons, predominantly in the CM). Responses of both types of neuron appeared regardless of whether or not the sensory stimuli were assoc iated with reward. These response characteristics of CM-Pf neurons sharply contrasted with those of TANs in the striatum, which under the same conditi ons responded preferentially to stimuli associated with reward. Many CM-Pf neurons responded to alerting stimuli such as unexpected handclaps and nois es only for the first few times that they occurred; after that, the identic al stimuli gradually became ineffective in evoking responses. Habituation o f sensory responses was particularly common for the LLF neurons. Inactivati on of neuronal activity in the CM and Pf by local infusion of the GABA(A) r eceptor agonist, muscimol, almost completely abolished the pause and reboun d facilitatory responses of TANs in the striatum. Such injections also dimi nished behavioral responses to stimuli associated with reward. We suggest t hat neurons in the CM and Pf supply striatal neurons with information about behaviorally significant sensory events that can activate conditional resp onses of striatal neurons in combination with dopamine-mediated nigrostriat al inputs having motivational value.