Selection in vision : a study of visual search

Abstract

This thesis is concerned with the selective processing of information which arrives via the visual system. The introduction discusses the problem of stimulus control during selection and specifies the aim of the thesis. Nine experiments are reported. The first six examine the relationship between selection accuracy and stimulus exposure-time. It is suggested that a plausible explanation of the obtained results can be found in a two-stage model of information processing (e.g., Neisser, 1967). The remaining three experiments address the question of whether a letter-name can direct the selection process, and the results, while somewhat indecisive, do not indicate that it can. Specifically, a) in Experiment 1 accuracy of search for a single target declined as the number of irrelevant items, and their physical (shape)similarity to the target, increased. These results could be explained either by sophisticated parallel-serial models of information processing, or by a two-stage model; Experiment 2 suggested that selection was not based on a limited retinal region, thereby negating the possibility of the distance separating I terns influencing the results of Experiment 1; Experiment 3 varied stimulus exposure-time along with similarity and number of items. The results replicated the effects obtained in Experiment 1, and the negatively accelerated accuracy/time curves demonstrated a plateau effect. The level of the plateau differed for each curve and these differences were attributed to errors during initial selection; Experiment 4 found the long term course of the accuracy/time curves (i.e., exposure-time varied from 100 to 500 msec). This gave subjects the opportunity to begin a second processing run, but, possibly due to image-decay or to the absence of eye movements, no evidence of a second run was detected; Experiment 5 supplemented the time course data obtained in Experiments 3 and 4 by sampling behaviour over an exposure-time range of 15 to 400 msec. The results showed that both image-decay and the opportunity for eye movements had no effect; the results of Experiment 6 replicated those obtained in Experiment 3, and indicated that while the relationship between target and irrelevant items affected accuracy of selection, it did not influence the time required for this selection. b)Using the PEST procedure Experiment 7 firstly, demonstrated that the process of building up a structural representation of an alphabetic character requires time, and secondly, indicated that the prior development of object structure may be necessary for naming; Experiment 8 showed that the name of a target letter need not interfere with its selection; In Experiment 9 a confounding of shape and name information made a straight-forward interpretation of the results difficult. The data, however, did not suggest that nominal information was used to direct the selection of the target. Finally, the discussion speculates further on the nature of the information processing system underlying the findings.