Abstract
Studies of the contextual effects of prior probability on saccades show latency changes when subjects make saccades to one of only two high-probability locations, compared to blocks with many locations. However, in this study we asked, does a second target location in the same block influence saccadic accuracy? We had subjects perform antisaccades in blocks containing two equally probable target locations, all at 8.5° eccentricity. In all blocks one target location was on the horizontal meridian. In half of the blocks, this was paired with a target above the meridian, while in the other half it was paired with a target below the meridian. We explored the effects of proximity by varying the directional angle of the second target from the horizontal meridian (20°, 40°, 60° or 80°) in different blocks. Last, we included two blocks of prosaccades, one with above- and one with below-meridian second targets with a directional angle of 20°. We analyzed the vertical and horizontal coordinates of saccadic endpoints to the horizontal-meridian target as a function of 1) the location (above or below the meridian) of the second target, and 2) the directional angle of the second saccadic goal. While prosaccades were not influenced by the location of the second target, antisaccades were significantly displaced towards the second target. This effect on antisaccades was greater the closer the second target was to the first target. These findings show that targets at other locations in prior trials can cause antisaccade goal coordinates to shift in the current trial. This new context effect can be modeled as the effects of summation between neural activity at current goal coordinates and reduced but persistent activity at the second location.
Acknowledgments: This work was supported by an operating grant from the CIHR (MOP-81270). AR was supported by a Pratt-Johnson fellowship, and WvZ by a post-doctoral award from the Michael Smith Foundation for Health Research. JJSB was supported by a Canada Research Chair and a Senior Scholarship from the Michael Smith Foundation for Health Research.