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

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

Background Figure vs. Ground Rubin’s illusion

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

Background Processing Differences “Figure” aids detection of: –Contour discontinuity –Retinal image displacement –Line orientation Possible Reasons: –Differential attention? –Differential resolution? –Differential sensitivity to spatial frequency?

The Experiment Assumptions & 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

The Experiment Research 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.

The Experiment Design & 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!

The Experiment Design & 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

The Experiment Design & 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

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

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