Susana Chung

The effect of stimulus contrast on pre-saccadic orientation discrimination

Task: Whether the target Gabor is tilted clockwise or counterclockwise. Participants: Seven observers with normal or corrected-to-normal vision. Apparatus: Eyelink 1000. Display++ monitor. 120 Hz at 1920x1080. 76cm distance. Stimulus conditions: Gabor patches: 2 cpd, 0.5 deg SD. Flanker-target distance: 4 deg. Flankers: Always vertical. Target orientation: A value corresponding to ~80% discrimination performance Blocked: 1) Fixation : 600 trials per observer 2) Saccade : 3000 trials per observer Randomized: i) Unflanked – high contrast (100%) ii) Unflanked – low contrast (25%) iii) Both target and flankers high contrast iv) Target low contrast, flankers high contrast v) Target high contrast, flankers low contrast vi) Both target and flankers low contrast Procedure: Conclusions

Effects of Flankers Within the Crowding Zone.

Crowding refers to the inability to recognize an object that separates from its neighbors (flankers) less than the critical spacing. One account of crowding is that features from the target and flankers are combined erroneously within the crowding zone. Currently, little is known about what happens within the crowding zone. Here, we examined whether the adverse effect of flankers is combined linearly within the crowding zone. We measured the identification accuracy of a target letter presented at 10° nasal field or 10° lower field, with two flanking letters presented along the radial meridian. We first measured the identification accuracy of the target letter for six target-flanker separations, when the separation between the inner flanker and the target, and the separation between the outer flanker and the target, were yoked. A cumulative-Gaussian function was used to fit this set of data, from which we derived the critical spacings corresponding to 30%, 50%, 70% and 90% identification accuracies. Next, we measured the identification accuracy of the target by fixing the inner[outer] flanker at one of the critical spacings determined above, but varying the separation between the target and the outer[inner] flanker. Identification accuracy was also measured when only the inner or the outer flanker was present. In general, performance was better with only one, than two flankers. When two flankers were present with one at a fixed separation from the target, the critical spacing of the movable flanker was consistently smaller than the critical spacing for the yoked condition, by 51% and 34% for the inner and outer flanker, respectively. Our results suggest that within the crowding zone, the effectiveness of a flanker on the target depends on the presence of, and the location of other flankers. The overall crowding effect is not a linear combination of the effects of individual flankers. Meeting abstract presented at VSS 2015.