Effects of visual distractors on discourse coherence in young and older adults: A test of the inhibitory deficit hypothesis

Producing coherent discourse requires us to regulate the content of our speech and avoid interference from discourse-irrelevant concepts that become active in semantic memory. The inhibitory deficit hypothesis proposes that coherence declines in later life are due to a reduced ability to inhibit these irrelevant ideas. However, existing evidence in support of this view is correlational. We performed an experimental test of the hypothesis by asking young (18-25) and older (70-90) participants to produce discourse on a range of topics while attending to two types of visual distractors: images of meaningful concepts and meaningless abstract patterns. The overall global coherence of responses was lower when participants were distracted (cf. no distraction) but this effect was not larger for meaningful distractors. Participants also spoke more slowly under distraction. These effects did not differ between age groups. Critically, however, in the meaningful distractor condition, responses diverged from the original topic more quickly than in the other conditions. This effect was only present in older participants. These results suggest two underlying effects at play. First, performing a concurrent task has a general effect on the speed and coherence of discourse, which in this study was age-invariant. Second, for older people, tasks that activate a series of irrelevant semantic representations have an additional cumulative effect on discourse content, causing it to deviate off topic more rapidly. Our results support the inhibitory deficit hypothesis and suggest that older people can improve their coherence by avoiding semantically-laden environmental distractors like TV or radio programmes.