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Maximilien Chaumon, Juan R. Vidal, Laurent Hugueville, Catherine Tallon-Baudry; The time course of sensory amplification by feature-based attention: A direct measure on frequency-tagged evoked responses. Journal of Vision 2005;5(8):679. doi: 10.1167/5.8.679.
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Selective attention is controlled by a network of dynamically interacting cortical areas. Attentional commands originating in frontal and parietal cortices act as top-down signals sensitizing the sensory areas, resulting in enhanced sensory responses to attended stimuli. How long does it take for the sensory response to an attended stimulus to be enhanced? Electrophysiological studies showed that the first differences between attended and unattended objects appear as early as 150 ms in extrastriate cortex. However, do these differences reflect the enhanced response to a stimulus, or the reception of attentional top-down commands? Indeed, because attentional top-down commands are received in sensory areas, their electrophysiological signature often merges with sensory driven responses. We used Steady-State Visually Evoked Potentials (SSVEPs) to a flickering object in order to selectively monitor the bottom-up sensory response in extrastriate cortex. We show that 400 ms are necessary for feature-based attention to enhance the sensory response to an attended object. In addition, attentional modulations in the classical evoked potentials, originating in extrastriate cortices, occur earlier than the sensory enhancement. This attentional-related activity could reflect the conversion of attentional top-down commands into a local tuning of extra striate areas. Last, because the attentional sensory enhancement by feature-based attention measured here (400 ms) is shorter than the amplification of the sensory signal by spatial attention (600 ms, Müller et al., 1998a), our results add to the view that feature-based attentional mechanisms precede the deployment of spatial attention.
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