Line by line: the ERP correlates of stroke order priming in letters.

The perception of written letters reflects the action sequences that produce them. Faster recognition is observed for letters presented as sequences of strokes in a temporal order consistent with letter writing, compared to an inconsistent order. During a speeded letter identification task, parietal event-related potential (ERP) components were analysed separately for each stroke-frame in action-consistent and inconsistent stimulus sequences, during both passive and active (task-engaged) viewing. Electrophysiological data provided unique insights into stroke order priming by comparing local neural organisation during early, response-independent stages with later response-dependent stages. ERPs over posterior scalp areas revealed speeded visual processing for action-consistent stroke sequences prior to, and upon, letter completion. These signatures of perceptually facilitated letter processing were present in both active and passive viewing conditions, indicating that priming was not response-contingent, but rather an inherent part of visual letter perception. Stroke order priming is discussed in terms of matching stored letter production action codes, which upon activation provide top-down facilitation for visual processing of letters.