Capacity limitations in the perception of relative position
Endel Põder
Tallinn Pedagogical University, 25 Narva Road, Tallinn 10120, Estonia
E-mail: ep@tpu.ee
Examples of stimuli
Purpose
(set-size 8, target present)
To study the capacity limitations in the perception of
objects defined by the relative position of the same
components. Relative position
Background
• Heathcote and Mewhort (1993) have demonstrated in
the reaction time experiments that after a short training
observers were able to discriminate simple stimuli from
their mirror images in parallel (independent of set size).
• Põder (1999) found large set size effects with similar
stimuli and percentage correct as measure of
performance
Orientation
Methods
Stimuli:
• Squares divided in bright and dark halves
• Shaded circles (“bumps” and “holes” in 3D
interpretation)
Procedure:
Visual search, brief presentation, percentage correct vs.
contrast measured.
Three target-distractor combinations:
• Relative position (target mirror image of distractors)
• Orientation (target rotated 90 degrees in respect to 3D shape
distractors)
• 3D shape (target with reversed shading in respect to
distractors)
Set sizes 1 and 8.
Additional training experiment with relative position
stimuli.
Subjects:
16 undergraduate students with normal or corrected-to-
normal vision (4-5 per each target-distractor
combination, and 3 for additional training experiment).
Effect of training
Results (relative position stimuli)
• Little effect of set-size for orientation and 3D
shape stimuli. Session 1
• Large effect of set-size for relative position 100
stimuli
90
Percent correct
• Training did not eliminate the large set-size Set-size
effect with relative position stimuli 80
1
• Mainly the asymptotic (high contrast) 70
8
performance improved as the result of training.
60
• Psychometric functions saturated far below 100
percent correct for relative position stimuli and 50
set-size 8. 0 10 20 30 40 50
Contrast (%)
Relative position
100 Session 4
90 100
Percent correct
Set-size
80 90
Percent correct
1 Set-size
80
70 8 1
70
60 8
60
50
0 10 20 30 40 50 60 50
Contrast (%) 0 10 20 30 40 50
Contrast (%)
Orientation
100
90
Percent correct
Set-size
80 Conclusions
1
70 8 • The perception of relative position of stimulus
components needs usually some capacity
60
limited processing. Exceptions are stimuli with
50 simple 3D interpretation.
0 10 20 30 40 50 60 • SDT-based search models (e.g. Palmer et al.,
Contrast (%) 2000) need further modification in order to
account for the present data.
3D shape
100
90 References
Percent correct
Set-size
Heathcote, A., Mewhort, D. J. K. (1993). Representation and selection
80 of relative position. Journal of Experimental Psychology: Human
1
Perception and Performance, 19, 488-516.
70 Palmer, J., Verghese, P., & Pavel, M. (2000). The psychophysics of
8
visual search. Vision Research, 40, 1227-1268.
60 Põder, E. (1999). Search for feature and for relative position:
Measurement of capacity limitations. Vision Research, 39, 1321-
50 1327.
0 10 20 30 40 50 60
Contrast (%)