Sharp Targets Are Detected Better Against a Figure, and Blurred Targets Are Detected Better Against a Background

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Sharp Targets Are Detected Better Against a
Figure, and Blurred Targets Are Detected Better
Against a Background

• Eva Wong and Noami Weisstein, 1983

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Overview

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

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BackgroundFigure vs. Ground

• Rubins illusion

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BackgroundFigure-Ground Reversals

• When foreground becomes background and/or
  vice versa
• Widespread in art (according to Douglas
  Hofstadter, anyway)

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BackgroundProcessing Differences

• Figure aids detection of
• Contour discontinuity
• Retinal image displacement
• Line orientation
• Possible Reasons
• Differential attention?
• Differential resolution?
• Differential sensitivity to spatial frequency?

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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

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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.

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The ExperimentDesign Measures

• First Experiment
• Purpose
• Find observers who hold their (monocular) gaze
  regardless of whats 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, theyre not fixating!

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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

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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

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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

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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