this increase is due to the heavy processing requirement incurred either in planning the next chunk of discourse or in the introduction of many new or high perplexity entities. In a sample of academic lecture speech, we find that non-error disfluencies (e.g. filled pauses) occur preferentially shortly after but not right at the beginning of a discourse segment. This suggests that the processing load may not increase just at the boundary onset, i.e. that the speaker can make use of earlier planning during the first portion of the new segment. In contrast, errors of selection or serial ordering of grammatical elements do not show a boundary-related peak in their distribution across a discourse segment, supporting the hypothesis that this second kind of nonfluent event arises at a different point in the speech production planning process.