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Sathyasri Narasimhan, Srimant Tripathy, Brendan Barrett; The temporal dynamics of visual sensory memory while tracking multiple moving dots. Journal of Vision 2007;7(9):690. doi: 10.1167/7.9.690.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the role of visual memory while tracking multiple simultaneous trajectories, several experiments were conducted using variations of the paradigm presented in Tripathy and Barrett (2004). The stimuli consist of several linear, non-parallel, left-to-right moving dots, each travelling at the same speed. At the monitor's midline, halfway through the trajectories, one of the dots (the target) deviated clockwise/counter-clockwise while the remaining trajectories (distractors) continued without deviation. The observer reported the direction of deviation of the target and thresholds were estimated using the method of constant stimuli. When the target and distractor trajectories persisted through the length of the trial, deviation thresholds increased rapidly with the number of trajectories (Tripathy & Barrett, 2004). When a temporal delay was introduced at the mid-point of the trajectories, thresholds increased more rapidly as the delay was increased. These results suggest that observers' performance in multiple object tracking tasks of this kind is primarily influenced by the decay of trajectory traces in visual sensory memory. When the distractors disappeared about a 100ms before the target deviated, thresholds were substantially lower than when the distractors persisted through the length of the trial. In these experiments varying the dot speed identified the critical variable to be the time between the disappearance of the distractors and the deviation of the target, rather than length of the target trajectory between the two events. The earlier the distractors disappeared, the lower the thresholds were; in some observers thresholds continued to improve even when the disappearance of the distractors preceded the deviation of the target by 400ms. Taken together, our results suggest that it takes nearly 400 ms to shift attention to the target; this shift in attention helps to speed up the processing of the deviation which must be completed before substantial decay of the traces has occurred.
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