This article applies to:
Experiment Author: Adapted from STEP and used with permission of Brian MacWhinney
This experiment asks participants to look for a particular letter (presented either as a large letter composed of other letters, or as the letters that make up the larger letter). For example, if the target were "a", both of these would be permissible:
The experiment hopes to show that large-scale letters are recognized more quickly and easily than the smaller-scale letters that make them up. Note that this particular experiment does not exactly replicate experiment 3 of Navon's paper, which involved H and S as targets. Subjects there were asked to report whether they saw an H or an S at a particular level (global or local) and the item of interest was how much the opposite target at the other level interfered with processing the actual target.
There are many different theories to explain how features and figures interact to produce visual perception. The Gestalt view is that all features are perceived and integrated at once. This view, however, is not consistent with the finding that people extract more information from images the longer they view those images. The interaction between top-down and bottom-up processing is not entirely clear.
The aim of this experiment was to show that the perceptual system processes every scene starting with the global features and progressing to the local features that compose them. In order to get at this processing, Navon attempted to find figures for which the global-local relation was immediately apparent. The letters composed of other letters were ideal because they were easily constructed and recognized as well as having hierarchically obvious features.
The experiment succeeded in showing the effect that global features take precedence over local features in speed of perception
In this experiment the stimuli are large letters composed of small letters. Whenever there is an "A" or an "H" press the corresponding letter on the keyboard. There will always be either an A or an H present, and it can be either the big letter or the little letters. Press the correct button as soon as you see which letter is present. The letters will be presented slightly to the left or right of the center of the screen. There are 16 trials in the experiment.
Navon (1977). Forest before trees: The precedence of global features in visual perception. Cognitive Psychology, 9, 353-383.
Cited Experiment Abstract
The idea that global structuring of a visual scene preceded analysis of local features is suggested, discussed, and tested. In the first two experiments subjects were asked to respond to an auditorily presented name of a letter while looking at a visual stimulus that consisted of a large character (the global level) made out of small characters (the local level). The subjects' auditory discrimination responses were subject to interference only by the global level and not by the local one. In Experiment 3 subjects were presented with large characters made out of small ones, and they had to recognize either just the large characters or just the small ones. Whereas the identity of the small characters had no effect on recognition of the large ones, global cues which conflicted with the local ones did inhibit the responses to the local level. In Experiment 4 subjects were asked to judge whether pairs of simple patterns of geometrical forms which were presented for a brief duration were the same or different. The patterns within a pair could differ either at the global or at the local level. It was found that global differences were detected more often than local differences.
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