Abstract
Introduction and Motivation. The considerable evidence of the saccadic system's plastic nature has led some researchers to investigate the subsequent effects of saccadic adaptation on the conscious perception of visual stimuli. These studies found perceptual misjudgments of stimulus location after the amplitude of a saccadic eye movement had been adapted to compensate for an artificially induced, post-saccadic visual error. However, the overall effect on perception remains unclear, as only one saccade vector was adapted, the perceptually judged stimuli were flashed only briefly, and misjudgments were usually found only in the presence of a saccadic eye movement - a situation where perceptual illusions have also been demonstrated in the absence of saccadic adaptation. Here, we studied the effect of saccadic adaptation on the perception of a persistent object during fixation.
Methods. Three adaptation blocks (75 trials) were interleaved with four perceptual judgment blocks (60 trials). During adaptation trials, participants freely scanned a search display for a pre-specified target item. To reduce the amplitude of participants' saccades in only one dimension (horizontal or vertical), the search display was continuously shifted along that dimension during every saccade using a novel, whole-field saccadic adaptation paradigm (Garaas, Nieuwenhuis & Pomplun, 2008). Participants' perception of object size was measured by having them fixate a cross figure and indicate which bar (horizontal or vertical) they perceived to be longer. Cross figures of varying aspect ratios were presented using a staircase procedure which ‘narrowed in’ on participants' perceptual bias.
Results. The adapted component of participants' saccades was reduced by an average of 31%, while the unadapted component remained precise. Perceptual judgments were progressively biased toward perceiving greater relative height or width during the reduction of horizontal or vertical saccade components, respectively, r = 0.61, p Conclusion. Saccadic adaptation is accompanied by changes in perceived object size.