The minimum duration signal necessary to identify a set of spoken words was
established by the gating technique; most words could be identified before
their acoustic offset. Gated words were used as congruous and incongruous
sentence completions, and isolation points established in the gating experi
ment were compared with the time course of semantic integration evident in
event-related brain potentials. Differential N400 responses to contextually
appropriate and inappropriate words were observed about 200 ms before the
isolation point. Semantic processing was evident before the acoustic signal
was sufficient to identify the words uniquely. Results indicate that seman
tic integration can begin to operate with only partial, incomplete informat
ion about word identity Influences of semantic constraint, word frequency,
and rate of presentation are described.