Perception of wholes and of their component parts: Some configural superiority effects

1. Perception of wholes and of their component parts: Some configural superiority effects Pomerantz, J. R., Sager, L. C., and Stoever, R. J. (1977). Journal of Experimental Psychology: Human Perception and Performance, 3, 422-435.

2. Introduction • Feature detectors have sometimes been presumed to operate independently of one another. • But the independent assumption has difficulty in explaining certain context effects in perception, e.g. Gestalt phenomenon. • In Gestalt, there were 2 claims • The appearance of one element in the visual field was influenced heavily by elements nearby • The perception in general could not be characterized as the simple sum of independent features or sensations

3. Introduction – context aiding perception • Two sides of stories • Interference: context acts to impair perception • Context may adds noise to the perceptual system, overloading or otherwise disrupting its normal operation. • Explanation of word superiority effect: • Perceptual interpretation • Postperceptual interpretation (preferred) • Aiding: context could aid perception itself • Context could aid perception itself, rather than some postperceptual process. • Context influenced perception, but always in a harmful way. (Homa, Haver, & Schwartz, 1976) • Schendel and Shaw (1976) found a beneficial effect of context, which parallel the result in word superiority effects. • Context improving discrimination, even though it does not show that the effect is necessarily perceptual in origin.

4. Purpose • To explore in more detail the possibility that context can improve perception itself. • That is, clarifying the conditions under which context might aid perception and with localizing the stage of processing at which context would have its effects

5. Experiment 1 • Pomerantz and Garner (1973) showed that subjects perceived two curvature elements as a whole. • Failure of selective attention was therefore proposed as an operational measure of perceptual grouping. • Pomerantz and Schwaitzberg (1975) found that two-choice RTs increased when the elements were moved further. • In this experiment, they tested whether having a constant element nearby actually improved performance.

6. Experiment 1 – method • Context condition: ( ) ‧ ) ) • No-context condition: (‧) • Two-alternative RT task • 8subjects • 3 blocks, order of conditions within blocks was counterbalanced across subjects • Each condition consisted of 72 stimulus, divided equally between two stimulus. () () )) Is the target “(“ or “)”? Auditory warning signal 200 ms

7. Experiment 1 – result • Mean correct RT for the context condition was 421 ms. • Mean correct RT for the no-context condition was 444 ms. • The difference is significant. (p < .05) • Configural superiority effect was obtained even knew both the location and the identity of the stimulus alternatives beforehand.

8. Experiment 1 – discussion • Stimulus must be held in memory and recognized, and then selected and executed the proper response. • Next step is to clarify whether the effect is a perceptual one? • Recognize faster? • Easier to associate with the required response? • What is the contexts that impair perception instead of facilitating it? • If all contexts help, … • It could be explained by lateral enhancement and the claim that context converts an absolute judgment into a relative one. • Any context providing a similar positional anchor should be helpful. • Otherwise, … • It can’t be explained by lateral enhancement.

9. Experiment 2 • Perceptual alone? • Employed task with minimal memory demands • To indicate the location in the field of the odd stimuli, and neither the set of possible stimuli nor an arbitrary response code need be remembered. • If effect remains, it can be assured that the effect is perceptual in origin. • All context? • The possible important factors of previous stimuli are: • Mirror image • Bilaterally symmetrical • Experiment 2 use different stimulus to control the two factors.

10. Experiment 2 – method • 12 subjects were used. • Decide which quadrant contained the disparate elements. • Each array was presented 4 times, with the first time always be treated as practice and not scored.

11. Experiment 2 – result • Replicate the result from experiment 1

12. Experiment 2 – result • Destroy the original grouping structure. • No superiority effects. • It is unlikely that the effect found in A, B and C was due to lateral enhancement.

13. Experiment 2 – result • Basic superiority effect did not depend on regular spacing of the stimuli. • Mirror image and bilateral symmetry were not the factors that improve the emergent of context.

14. Experiment 2 • Unusually long reaction times were obtained in experiment 2. • Simply difficult to see? • Performance may be process limited rather than state limited. • Can the effect be obtained with other types of simple perceptual discrimination?

15. Experiment 3 • Different discrimination: • Orientation of a curved line segment • Position of a line relative to a fixed point • Positive v.s. negative diagonal line • Horizontal v.s. vertical line • Line length • Larger size to eliminate limitation in processing.

16. Experiment 3 – result Baseline condition • Context per se does not automatically help or hinder perception. • Certain contexts help, while others hinders.

17. Experiment 4 • Word superiority effects may vanish when the subject can prepare himself for the kind of discrimination he will have to make. • Whether the same would be true for the this superiority effect?

18. Experiment 4 – method • The method and procedure are similar to experiment 2 and 3. • Block designed with counterbalanced order • 2 blocks with 32 stimulus exposure each  practice trials • 3 more replications of the 2 blocks were counted

19. Experiment 4 – result RT : 1,480 ms 641 ms • Configural superiority can emerge even when the subject has foreknowledge of the exact discrimination he will have to make on each trial. • The long RTs were reduced.

20. Experiment 4 – discussion • We can obtain the effect with a number of elementary perceptual discriminations and in both classification and oddity tasks. • Seemed to be free from memory involvement. • It is not obvious what component processes are involved in performing an oddity task • array may be processed serially or in parallel model.

21. Experiment 4 – discussion • To test the processing model of superiority effect, the set size should be alternated. • In serial model, … • The time required to locate the odd stimulus should grow with the size of the array to be processed. • In parallel model, …  The disparate stimulus spontaneously segregates regardless of the number of background stimuli.

22. Experiment 5 • Measured the time required to determine whether any stimulus in an array is different from the others, as a function of the number of the stimuli in the array. • Detection task with stimulus same as experiment 4, and press when detect a disparity • 3 conditions: • No context(NC) • Good context (GC) • Poor context (PC) • 3 set sezes: • 2, 4, and 6 • All intermixed • All position has equal chance • 50% absent, 50% detection

23. Experiment 5 • GC has fastest RT, while PC show the lowest. • Display size had virtually no effect with the GC arrays, while had large effect on the PC arrays. • Within NC arrays, for all subjects, the RTs were longest when the arrays contained just four stimuli.

24. Experiment 5 • When an array contains just diagonal lines with no context, the lines appear to interact to form larger configurations.

25. Experiment 5 – discussion • GC arrays: parallel feature-inhibition model • PC arrays: serial-memory model • Subjects processed the no-context arrays as holistic configurations, which is itself another instance of configural dominance.

26. Conclusion • Context can improve discriminability. • Context appears to have its effect directly on perceptual components of discrimination. • How does context improve discriminability? • Context changes an absolute judgment into a relative one by providing an anchor • Context interacts with the target to form emergent features

27. Conclusion • Novel features happen to be more discriminable than the features contained by the targets without context. • Emergent features could be detected indirectly by integrating the output of detectors for simple parts. • Emergent features serve as functional units in perception, regardless of whether they are detected directly or are derived from simple parts.