Limited flexibility in the filter underlying saccadic targeting.

Abstract

The choice of where to look in a visual scene depends on visual processing of information from potential target locations. We examined to what extent the sampling window, or filter, underlying saccadic eye movements is under flexible control and adjusted to the behavioural task demands. Observers performed a contrast discrimination task with systematic variations in the spatial scale and location of the visual signals: small (sigma=0.175 degrees ) or large (sigma=0.8 degrees ) Gaussian signals were presented 4.5 degrees , 6 degrees , or 9 degrees away from central fixation. In experiment 1, we measured the accuracy of the first saccade as a function of target contrast. The efficiency of saccadic targeting decreased with increases in both scale and eccentricity. In experiment 2, the filter underlying saccadic targeting was estimated with the classification image method. We found that the filter (1) had a center-surround organisation, even though the signal was Gaussian; (2) was much too small for the large scale items; (3) remained constant up to the largest measured eccentricity of 9 degrees . The filter underlying the decision of where to look is not fixed, and can be adjusted to the task demands. However, there are clear limits to this flexibility. These limits reflect the coding of visual information by early mechanisms, and the extent to which the neural circuitry involved in programming saccadic eye movements is able to appropriately weigh and combine the outputs from these mechanisms.