Capacity limitations in the perception of relative position Endel P

1
Capacity limitations in the perception of
relative positionEndel PõderTallinn Pedagogical
University, 25 Narva Road, Tallinn 10120,
EstoniaE-mail ep_at_tpu.ee
Examples of stimuli (set-size 8, target present)
Purpose 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 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).

3D shape
2
Effect of training (relative position stimuli)

• Results
• Little effect of set-size for orientation and 3D
  shape stimuli.
• Large effect of set-size for relative position
  stimuli
• Training did not eliminate the large set-size
  effect with relative position stimuli
• Mainly the asymptotic (high contrast) performance
  improved as the result of training.
• Psychometric functions saturated far below 100
  percent correct for relative position stimuli and
  set-size 8.

• Conclusions
• The perception of relative position of stimulus
  components needs usually some capacity limited
  processing. Exceptions are stimuli with simple 3D
  interpretation.
• SDT-based search models (e.g. Palmer et al.,
  2000) need further modification in order to
  account for the present data.

References Heathcote, A., Mewhort, D. J. K.
(1993). Representation and selection of relative
position. Journal of Experimental Psychology
Human Perception and Performance, 19,
488-516. Palmer, J., Verghese, P., Pavel, M.
(2000). The psychophysics of visual search.
Vision Research, 40, 1227-1268. Põder, E.
(1999). Search for feature and for relative
position Measurement of capacity limitations.
Vision Research, 39, 1321-1327.