Eva Wong and Noami Weisstein, 1983

1. Sharp Targets Are Detected Better Against a Figure, and Blurred Targets Are Detected Better Against a Background Eva Wong and Noami Weisstein, 1983

2. Overview • Background • Figure vs. Ground • Reversal • Processing Differences • Experiments • Assumptions & Hypothesis • Research Question • Design & Measures • Results • Discussion

3. BackgroundFigure vs. Ground • Rubin’s illusion

4. BackgroundFigure-Ground Reversals • When “foreground” becomes “background” and/or vice versa • Widespread in art (according to Douglas Hofstadter, anyway)

5. BackgroundProcessing Differences • “Figure” aids detection of: • Contour discontinuity • Retinal image displacement • Line orientation • Possible Reasons: • Differential attention? • Differential resolution? • Differential sensitivity to spatial frequency?

6. The ExperimentAssumptions & Hypothesis • “Figure” and “Ground” represent different channels in the visual system • The channels have different functions: • “Figure” responsible for detail • “Ground” responsible for ‘global information’ • Therefore: • “Figure” channel more sensitive to high spatial frequencies • “Ground” channel more sensitive to low spatial frequencies

7. The ExperimentResearch Question • So, is the detection threshold: • lower in the figural regions for high spatial frequencies (such as a sharp target?) • lower in the ground region for low spatial frequencies (such as a blurred target?) vs.

8. The ExperimentDesign & Measures • First Experiment • Purpose • Find observers who hold their (monocular) gaze regardless of what’s figure or ground • Procedure • Half of subjects initiate trial when the faces are figure; the other half initiate the trial only when the goblet is figure • The stimulus then appears in the blind spot at 50% probability • Measure detection accuracy; if different than chance, they’re not fixating!

9. The ExperimentDesign & Measures (cont.) • Second Experiment • Purpose • Establish luminance level where TP = 70% for both blurred and sharp targets • Procedure • Display sharp target at fixation cross at 50% probability • Change luminance until 70% accuracy is achieved for each of three blocks • Measure the final luminance value for each observer • Repeat for blurred target

10. The ExperimentDesign & Measures (cont.) • Third Experiment: • Purpose • Determine accuracy of target detection against figure and against ground regions • Procedure • Target has a 50% probability of being presented • If target is presented (20 msec), it has • A 50% probability of being in the “goblet region” • A 50% probability of being in a “face region” • Measure TP and FP to estimate d’ and plot ROC

11. Results Discrimination improves: • When sharp targets displayed in figure • When blurred targets displayed in ground Off-fixation attenuates d’ by a “fixed magnitude” • Reflects an early processing constraint: retinal eccentricity • Caused by decreasing resolution with increasing distance from fovea

12. Discussion • Conclusions: • Different visual processes mediate the analysis of figure and ground • Accuracy not determined solely by attention, as defined by gaze or what is perceived as figure) • [Accuracy is also not determined solely by photoreceptor density] • “Global information extraction” may proceed faster than figure analysis