An electrophysiological study of the selection of the color and shape of alphanumeric characters in response choice.

We examined the functional and temporal organization of task-directed selective processing of the color and shape of alphanumeric characters. Dependent variables were measures derived from event-related brain potentials (onset latency of selection negativity (SN) and the lateralized readiness potential). We found SNs related to the relevance for correct task-performance of the color, the global shape and the local shape of the characters. The onset latencies of these SNs suggested that color and global shape were identified at the same time and earlier than local shape. The absence of an SN to global shape if the color was irrelevant suggested that selection on the basis of the global shape depended hierarchically on that of color. Selection of the local shape was independent of color relevance. The lateralized readiness potentials suggested that response selection occurred when the conjunction of color, global shape and local shape had been identified. No evidence was found for temporal overlap between stimulus identification and response selection. These findings show that selective task-directed processing can be studied at multiple levels in a single paradigm. They are inconsistent with the central assumption of several continuous information transmission models that relevant partial information is used to activate associated responses as soon as it becomes available.