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Limb movement-related neurons in the cerebellar nuclei (CN) typically produce bursts of discharge in association with movement. Consequently, given the inhibitory nature of the Purkinje cell (PC) projection to CN, it is puzzling that only a minority of movement-related PCs pause; the majority burst. Some of the movement-related CN activity may be the result of excitation from collaterals of mossy and climbing fiber projections to the cerebellar cortex. The only other input to CN is diffuse and neuromodulatory, from locus ceruleus and raphe nuclei. To investigate the role of the excitatory mossy fiber input, single units in CN were recorded in macaque monkeys during the performance of reaching and manipulation tasks, before and after blocking the PC input with local microinjections of GABA(A) antagonists (bicuculline or SR95531). After these injections, the movement-related modulation of CN discharge was greater and began earlier, compared with the modulation in the preinjection group of neurons. These observations indicate that an important excitatory drive is provided by extracerebellar inputs to CN, most likely from collaterals of mossy fibers. PCs may serve primarily to regulate this activity, by either pausing or bursting as necessary.

The lateral geniculate nucleus (LGN) receives input from the retina that is spatially organized into a receptive-field center and surround. It maintains this organization in the signal that it sends to the visual cortex. Previous studies have focused on changes in the receptive-field 'surround' that are generated at the LGN, possibly as a local contrast enhancement mechanism. The present study suggests a role for the LGN in regulating the receptive-field center sensitivity under the control of GABAergic circuitry. Local microiontophoresis of the GABAA receptor antagonist bicuculline increased the contrast sensitivity of LGN relay cells to many spatial frequencies. Difference of Gaussians analysis showed that the increased was due to an increased sensitivity of the receptive-field center. Similar increases in receptive-field center sensitivity may be produced during behavioral arousal by the action of pontine and mesencephalic pathways upon the activity of the LGN GABAergic circuitry.

The lateral geniculate nucleus conveys the center-surround organized retinal receptive fields to the cortex in a way that does not significantly alter their spatial structure. However, non-retinal influences may change the 'strength' (detectability) of the signal under conditions of anesthesia, arousal and attention. A previous analysis of receiver operating characteristic curves in cat suggests that a reduction in signal detectability occurs in lateral geniculate nucleus (LGN) relay cells in anesthetized animals in comparison to the retinal afferents. In the present study, it was found that antagonism of GABAA receptors with bicuculline (BIC) increased signal detectability in LGN relay cells in the tree shrew (Tupaia belangeri). This change is consistent with the hypothesis that feedforward and/or feedback GABAergic circuits in the LGN differentially affect the retinogeniculate transfer ratio for visually driven activity versus maintained (spontaneous) activity. Under conditions of arousal or attention, signal detectability may be increased by brainstem activation, thus increasing the flow of information in the visual system.