Cognition - Lecture 15 (Selective Attention pt.2)

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  • Lecture 15 - Selective Attention
    • Feature-integration theory
      • Regions of visual cortex are specialised for local analysis of different attributes: colour, movement, texture
        • Each provides a "map" of that attribute's values over the visual field
      • A discontinuity within one of these maps is detected automatically
        • Hence "pop-out": no effect of number of objects with disjunctive feature target
      • To bind features of the same object from different maps into an object description, we need focal attention to a location
      • Implication 1: to find a target identified only by conjunction of features will require focal attention to each object in turn - serial search required
      • Implication 2: features are "free-floating" in relation to each other until attended - hence illusory conjunctions
    • Functions of selective attention
      • Defensive filtering: protecting higher-level limited-capacity systems from overload (Broadbent)
      • Positive selection-for-action prioritising one of several possible objects for action or further processing (Allport)
      • Feature integration or "binding": combining the properties of an object analysed in different cortical "maps"
    • Visual selection (as for auditory attention)
      • Is not all-or-none; there is a gradient of enhancement / suppression across the visual field
      • Is an optional process: the size of the attended area is under voluntary control: "zooming the spotlight"
    • Early selection in primary visual cortex and even LGN
      • While fixation maintained on central point, series of digits appears at fixation, and high or low contrast checkerboards appear in left + right periphery
        • P either counts digits at fixation, or detects random luminance changes in left (or right) checkerboard
      • fMRI BOLD signal in LGN/V1 voxels that react to checkerboard luminance change is greater with attention directed to that size than with attention to fixation
        • So: at least some selection (for regions in the visual field) occurs very early in processing
    • Voluntary attention to a spatial locus modulates early components of the ERP in extra-striate visual cortex
    • Endogenous vs. exogenous shifts
      • RT faster just after sudden onset/change at the stimulus location, although does not predict which location stimulus appear at
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