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Steven Franconeri, Todd Handy; Rapid shifts of attention between two objects during spatial relationship judgments. Journal of Vision 2007;7(9):582. doi: 10.1167/7.9.582.
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© ARVO (1962-2015); The Authors (2016-present)
The ability to judge spatial relationships among objects in the world is a fundamental part of visual processing. Despite the frequency of these judgments, they are attentionally demanding, as demonstrated by inefficient visual search for a targets defined by a spatial relationship. However, there is little evidence of *why* these judgments are difficult. We show that reporting a spatial relationship may require rapid shifts of spatial attention between two objects being judged.
Participants reported whether displays contained a red square on the left and a green square on the right, or the opposite arrangement. Using ERP, we measured a high temporal resolution trace of horizontal shifts of attention, by examining the difference over time between left and right electrode sites over visual cortex. A shift of attention to one side of the visual field is accompanied by greater negativity in the electrode sites on the contralateral side.
Five participants performed the task over two 2-hour sessions. Each participant's results suggest that shifts of attention (lasting 50–100ms) began 100ms after display onset. Individuals showed preferences for initial shifts toward an object on one side (e.g. on the left, as measured by the difference between the left and right electrodes), and/or for a given color (e.g. the green object, as measured by the difference between electrodes contralateral to green and those contralateral to red ). These strategies varied among subjects and may reflect differences in the way that people implement visual routines.
Despite the feeling that we apprehend the relationship between two objects by attending to both at once, these results suggest that spatial relationship judgments require a serial shift of attention. We speculate that this shift may actually be the mechanism that produces the spatial relation judgment, perhaps through a form of efference copy of the direction of the shift.
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