Abstract
The primary function of saccades is to sequentially orient the high-acuity fovea toward informative locations in the scene. This process also yields luminance transients on the retina, which are commonly substituted in the laboratory by an abrupt onset of the stimulus (a contrast or luminance step) while the subject maintains fixation. However, these two types of transients differ considerably, as the retinal image follows stereotypical velocity and acceleration profiles during saccades. To investigate the functional relevance of this difference, we compared contrast sensitivity in the two conditions in which the same stimulus was brought into the fovea by a 6.75 deg saccade and by a contrast step during fixation. We measured contrast thresholds in N=9 emmetropic observers when exposed for 100 ms to gratings at 1, 4, and 8 cycles/degrees. While individual variability existed, on average across observers the saccades selectively enhanced the mid-range of frequency: contrast sensitivity was significantly higher at 4 cpd following a saccade than in the step condition (p< 0.01; Wilcoxon), whereas no differences were observed at the other two spatial frequencies. The resulting shape of the contrast sensitivity function was significantly more band-pass following saccades than following a contrast transient at fixation (sensitivity ratio 4/1 cpd: 1.37 saccades; 0.89 fixation; p< 0.02; Wilcoxon). These results are consistent with the responses of a model of transients V1 complex cells that were exposed to reconstructions of the retinal stimulations experienced by subjects in the two conditions. In the model, the average responses of arrays of cells with receptive fields at different phases and locations were multiplied by a biphasic gain function to model saccadic suppression and enhancement. These results suggest important visual contributions from the specific dynamics of saccade transients and caution against equating these modulations to stimulus steps in laboratory experiments.
Acknowledgement: NIH EY18363, NSF 1457238, 1420212 and EU grant H2020-MSCA-734227 – PLATYPUS