Simultaneous and successive presentations of single-featured and multi-featured visual forms: Implications for the parallel processing hypothesis

The parallel processing hypothesis predicts no difference in hit rate (HR) for identical forms presented simultaneously or successively. This was tested in two experiments differing only in the number of features distinguishing stimulus forms: (1) one feature (Landolt Cs); (2) multiple features (the graphemes A, T, U). Each experiment had three conditions: (1) single form, (2) four simultaneous forms, (3) four successive forms. The major finding was a lack of HR increase for four simultaneous Cs over one C. HRs for successive Cs and both multiple letter conditions sharply increased over the one form condition. Results call into question the level at which parallel processing occurs. Three decision models, all assuming perceptual independence, were tested. None fit all the data.