Thalamic reticular nucleus activation reflects attentional gating during classical conditioning

All senses, except olfaction, are routed through the thalamus to cerebral c ortex. Thus, the thalamus is often referred to as the sensory gateway to co rtex. Located between thalamus and cortex is a thin lamina of neurons calle d the thalamic reticular nucleus, which may function as an attentional gate . The phenomenon of blocking in classical conditioning provides an opportun ity to test whether an attended stimulus activates the thalamic reticular n ucleus more than an unattended stimulus: when a second stimulus is presente d together with a previously conditioned stimulus, conditioned responding t o the second stimulus is inhibited. Different groups of rats were given conditioning sessions with a single sti mulus, a light or a tone, and then given conditioning sessions with compoun d (light and tone) stimuli. Blocking was confirmed using probe trials of si ngle stimulus presentations. After a final test session of compound stimulu s presentations, the brains were processed for the presence of Fos protein. Here we show that Fos-positive neurons were more numerous in the sector of the thalamic reticular nucleus associated with the attended conditioned st imulus than in the sector associated with the unattended stimulus. Thus, we provide evidence for an involvement of the thalamic reticular nucleus in s elective attention.