Parallel versus serial processing in visual search: Further evidence from subadditive effects of visual quality

Egeth, H., & Dagenbach, D. (1991). Parallel versus serial processing in visual search: Further evidence from subadditive effects of visual quality. Journal of Experimental Psychology: Human Perception and Performance, 17(2), 551-560.

Abstract:
The authors propose a diagnostic for distinguishing between serial and parallel processing in visual search; it is based on testing for subadditive effects of a within-trial visual quality manipulation on target-absent trials. It was evaluated in 2 experiments wherein parallel and serial processing might be expected on the basis of previous work and was then applied to a more uncertain situation in a 3rd experiment. The diagnostic indicates parallel processing of stimuli that differ from each other on a featural basis (Xs and Os) and canonical letters that differ in line arrangement (Ts and Ls) but serial processing when Ts and Ls are randomly rotated. These results form a coherent pattern that is understandable in terms of the literature on visual search, and thus they suggest that the diagnostic may be a useful addition to the methodology used to distinguish between serial and parallel processes.

Note:
This paper discussed the issue of parallel/ serial processing in visual search. Traditionally, researchers used the set size effect to define the processing architecture (e.g., the search time increases as a function of the number of distractors, showing a serial processing; a flat slope indicates the parallel processing). For the serial search processing, the ratio between the slope of target-absence and target-presence condition should be approximately 2: 1. Also, both parallel/ serial search occur in visual search at different stage. In the pre-attentive stage, visual search was processed in parallel to gain the simple features. Then, serial search is required for more details.
However, this measurement may be wrong at some time. Searching conjunction features can be parallel with limited capacity. A limited-capacity parallel model can also show the set-size effect (Townsend,1974).
In this paper, they manipulated the visual quality of letters and asked participants to search X among Os (Exp 1), rotated T among Ls (Exp 2), and canonical T among Ls (Exp 3). The manipulation allows different predictions for parallel and serial models in visual search with the target-absence (Table 1) and target0precsence (Appendix). For the target-absence trial, the sub-additive effect indicates the parallel processing. But, the violation of the sub-additive did not necessarily mean the serial processing.
Results showed the sub- additivity in Exp 1and 3 (suggesting parallel processing), and additivity in Exp 2 (suggesting serial processing). More interesting, findings from Exp 3 implied that searching conjunction features can be parallel according to target-distractor similarity, distractor homogeneity, and set-size.
The computation of the additivity by manipulating the information load, and visual quality is a direct test for parallel/ serial processing. The authors also mentioned a possibility of individual differences in visual search.