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. © 1989.