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Scientific Understanding of Consciousness |
Attention Gateway in Visual Thalamus
Nature 456, 391-394 (20 November 2008) Guarding the gateway to cortex with attention in visual thalamus Kerry McAlonan, James Cavanaugh & Robert H. Wurtz Laboratory of Sensorimotor Research, National Eye Institute, National Institutes of Health, Bethesda, Maryland 20892, USA (paraphrase) The massive visual input from the eye to the brain requires selective processing of some visual information at the expense of other information, a process referred to as visual attention. Increases in the responses of visual neurons with attention have been extensively studied along the visual processing streams in monkey cerebral cortex, from primary visual areas to parietal and frontal cortex. Here we show, by recording neurons in attending macaque monkeys (Macaca mulatta), that attention modulates visual signals before they even reach cortex by increasing responses of both magnocellular and parvocellular neurons in the first relay between retina and cortex, the lateral geniculate nucleus (LGN). At the same time, attention decreases neuronal responses in the adjacent thalamic reticular nucleus (TRN). Crick argued for such modulation of the LGN by observing that it is inhibited by the TRN, and suggested that "if the thalamus is the gateway to the cortex, the reticular complex might be described as the guardian of the gateway", a reciprocal relationship we now show to be more than just hypothesis. The reciprocal modulation in LGN and TRN appears only during the initial visual response, but the modulation of LGN reappears later in the response, suggesting separate early and late sources of attentional modulation in LGN. We find that attention modulates thalamic visual responses in two phases: an initial modulation that attenuates TRN responses and enhances LGN responses, followed by a slowly building later enhancement limited to LGN. The initial LGN modulation might provide a substantial fraction of the modulation seen subsequently in cortical area V1. Later attentional effects in LGN, and effects others have reported in higher cortical visual areas, might be more closely related to goal-directed attention, which frequently also develops later in the visual response particularly in higher cortical areas. This later modulation in LGN might in fact reflect feedback from cortex onto the LGN via the established connections from V1 layer 6, whereas the initial modulation in LGN by way of TRN may have its origins in subcortical structures, possibly including the superior colliculus. Although obviously separate in time course, the two phases of modulation may represent two distinct attentional influences, and may be early indicators for identifying and distinguishing feed-forward and feedback visual attentional mechanisms. (end of paraphrase)
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