The thalamic reticular nucleus (TRN) is thought to function in the attentional searchlight. We analyzed the detection of deviant acoustic stimuli by TRN neurons and the consequences of deviance detection on the TRN target, the medial geniculate body (MGB) of the rat. TRN neurons responded more strongly to pure-tone stimuli presented as deviant stimuli (low appearance probability) than those presented as standard stimuli (high probability) (deviance-detection index = 0.321). MGB neurons also showed deviance detection in this procedure, albeit to a smaller extent (deviance-detection index = 0.154). TRN neuron deviance detection either enhanced (14 neurons) or suppressed (27 neurons) MGB neuronal responses to a probe stimulus. Both effects were neutralized by inactivation of the auditory TRN. Deviance modulation effects were cross-modal. Deviance detection probably causes TRN neurons to transiently deactivate surrounding TRN neurons in response to a fresh stimulus, altering auditory thalamus responses and inducing attention shift. © 2009 Nature America, Inc. All rights reserved.