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Studies examining the effect of social isolation on cognitive function typically involve older adults and/or specialist groups (e.g., expeditions). We considered the effects of COVID-19-induced social isolation on cognitive function within a representative sample of the general population. We additionally considered how participants 'shielding' due to underlying health complications, or living alone, performed. We predicted that performance would be poorest under strictest, most-isolating conditions. At five timepoints over 13 weeks, participants (N = 342; aged 18-72 years) completed online tasks measuring attention, memory, decision-making, time-estimation, and learning. Participants indicated their mood as 'lockdown' was eased. Performance typically improved as opportunities for social contact increased. Interactions between participant sub-groups and timepoint demonstrated that performance was shaped by individuals' social isolation levels. Social isolation is linked to cognitive decline in the absence of ageing covariates. The impact of social isolation on cognitive function should be considered when implementing prolonged pandemic-related restrictive conditions.

Semantic ambiguity has been shown to slow comprehension, although it is unclear whether this ambiguity disadvantage is attributable to competition in semantic activation or difficulties in response selection. We tested the two accounts by examining semantic relatedness decisions to homonyms, or words with multiple unrelated meanings (e.g., "football/electric fan"). Our behavioral results showed that the ambiguity disadvantage arises only when the different meanings of words are of comparable frequency, and are thus activated in parallel. Critically, this effect was observed regardless of response-selection difficulties, both when the different meanings triggered inconsistent responses on related trials (e.g., "fan-breeze") and consistent responses on unrelated trials (e.g., "fan-snake"). Our electrophysiological results confirmed that this effect arises during semantic activation of the ambiguous word, indexed by the N400, not during response selection. Overall, the findings show that ambiguity resolution involves semantic competition and delineate why and when this competition arises.