Purpose

1. Capacity limitations in the perception of relative positionEndel PõderTallinn Pedagogical University, 25 Narva Road, Tallinn 10120, EstoniaE-mail: ep@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.