The ability to speak coherently, maintaining focus on the topic at hand, is critical for effective communication and is commonly impaired following brain damage. Recent data suggests that executive processes that regulate access to semantic knowledge (i.e., semantic control) are critical for maintaining coherence during speech. To test this hypothesis, we assessed speech coherence in a case-series of fluent stroke aphasic patients with deficits in semantic control. Patients were asked to speak about a series of topics and their responses were analysed using computational linguistic methods to derive measures of their global coherence (the degree to which they spoke about the topic given) and local coherence (the degree to which they maintained a topic from one moment to the next). Compared with age-matched controls, patients showed severe impairments to global coherence and milder impairments to local coherence, suggesting that semantic control deficits give rise to being "led up the garden path", i.e., one sentence automatically cueing another, with the topic becoming increasingly less relevant to the original question. Global coherence was strongly correlated with the patients performance on tests of semantic control, with poorer ability to maintain top-down global coherence being associated with greater semantic control deficits. Other aspects of speech production were also impaired but were not correlated with semantic control deficits. This is the first study to investigate the impact of semantic control impairments within a naturalistic setting, and indicates patients with these impairments are likely to find maintaining focus in everyday conversation difficult.

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