Attentional mechanisms are important for selecting relevant and filtering out irrelevant information from cluttered visual scenes. Selective attention has been shown to affect neural activity in extrastriate and even in striate visual cortex. This chapter reviews evidence from functional magnetic resonance imaging studies (fMRI) demonstrating that attentional response modulation is not confined to cortical processing, but can occur as early as at the thalamic level. Neural activity in the human lateral geniculate nucleus (LGN) was modulated by attention in multiple ways. The LGN, traditionally viewed as the gateway to visual cortex, may play an important role as a gatekeeper in controlling attentional response gain. In the human pulvinar, a dorso-medial region was identified that responded to attended but not to unattended visual stimuli. Taken together, these studies demonstrate that fMRI can be used effectively to study attentional mechanisms of thalamo-cortical circuits in the human brain.
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