Mechanisms of Attentional Selection in Human Visual Cortex

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Mechanisms of Attentional Selection in Human Visual CortexAbstract With National Science Foundation support, Dr. Somers will conduct a three-year study to investigate the mechanisms of attentional selection in human visual cortex using functional MRI combined with psychophysics and computational modeling. Studies are proposed that will investigate 1) the spatial distribution of attention under different forms of selection, 2) the role of suppression in selection and its relationship to facilitation, and 3) the computational mechanisms by which attention facilitates or suppresses a stimulus representation in visual cortex. The human mind regularly encounters complex environments that contain more information than it can process at one instant. Attention acts as the critical gateway to perception and cognition by selecting the stimuli that will be processed. The metaphor of the spotlight, selecting a single contiguous region of space, has long been employed to describe visual attention (James, 1890). In addition to spatial selection, object-based and feature-based selection have also been inferred behaviorally. This proposal seeks to investigate the spatial patterns of modulations these differing forms of selection can exert on human visual cortex. Using fMRI, we have observed that under appropriate task demands, the attentional spotlight may be split to attend to two distinct regions of space simultaneously while leaving the intermediate region unselected (see preliminary results). The capabilities and limitations in splitting the attentional spotlight will be investigated in this proposal. Investigations of how feature-based and object-based attention operate across space in retinotopic visual cortex are also proposed. Further studies will investigate the relationship between the spatial limits of attentional resolution and the processing capabilities of different visual cortical areas. Attentional selection is a competitive process with winners, the selected stimuli, and losers, unselected stimuli. It has been argued that selection is comprised not only of facilitative processes, but also of suppressive processes. Indeed, attentional suppression of unattended stimuli in visual cortex has been observed (e.g., Somers et al, 1999). Here, it is proposed to investigate how suppression acts across space and its relationship to facilitation. Is suppression purely a passive process (e.g., withdrawal) that results when attention is directed elsewhere? Or is there an active component to suppression? Is active suppression required to split the spotlight? Is suppression uniform across non-attended space or do robust distractors receive targeted suppression? To what degree does suppression vary with task form and/or task demands? Attentional selection modulates activity even in primary visual cortex. This implies that attention directly interacts with low-level, stimulus representations. What are the computational mechanisms by which attention selects stimuli without disrupting stimulus encoding in visual cortex? Does attention primarily modulate the gain (multiplicative effect) of stimulus-driven responses? Or does attention primarily bias (additive effect) stimulus responses to give them a competitive advantage in gaining access to cognitive processes? To investigate these issues a series of parametric fMRI/psychophysical studies is proposed in which physical stimulus properties will be varied while attention is directed toward or away from the stimulus. These data will be used to construct fMRI stimulus response functions both with and without attention and will allow us to infer the primary computational effect of attention on a neuronal population. In concert with these experiments, computational modeling studies will be performed. The modeling will provide a framework to integrate not only the present experimental results, but also the primate database on single unit responses in visual cortex. It is expected that the modeling will refine understanding of selection mechanisms and help to generate future experimental questions. The broader impact of the proposed studies includes increased understanding of cognitive brain mechanisms in normal humans. This knowledge about the relationship between mind and brain will inform future clinical studies, particularly for populations with attentional dysfunction. Computational understanding of how the brain implements attention may also aid design of future computer architectures. This research also offers training opportunities for undergraduate and graduate students, including those from underrepresented groups.