What influences the accessibility of conceptual knowledge? Evidence from Experimental Psychology, Neuropsychology and Brain Stimulation

Previous studies have shown that accessibility of conceptual information declines when sets of semantically-related items are presented repeatedly, although the underlying basis of this effect is debated – it is unclear if comprehension can decline without massed repetition of individual items, or if this effect is restricted to lexical retrieval in picture naming. Furthermore, declining comprehension has been characterised as arising from both ‘too much activation’ (i.e., on-going strong activation of competitors) and ‘too much inhibition’ (i.e., a failure to overcome inhibition which may facilitate the earlier retrieval of semantically-related targets). The thesis explored the impact of experimental manipulations (speed of presentation; strength of association between category and target item; modality of presentation; type of semantic decision required), on the magnitude of declining comprehension in healthy young adults. Comprehension declined even without individual item repetition, especially for strongly-associated targets (which may have accrued more competition or inhibition). The effect was found irrespective of presentation modality and more strongly at fast presentation speeds (when there was less time to overcome competition/inhibition). Next, the thesis examined the impact of ageing and semantic aphasia on changes in comprehension within the continuous categorisation paradigm. In these populations, controlled retrieval of conceptual information is thought to be weakened (relative to younger adults and healthy controls without aphasia). This should exaggerate declines in comprehension that reflect difficulty overcoming competition, but reduce the effect if it arises from the inhibition of competitors on earlier trials. The results were in line with the second hypothesis, since older adults and patients with semantic aphasia maintained their performance throughout the categories, unlike younger adults. Lastly, the thesis examined how this effect is modulated by transcranial electrical stimulation delivered to a key brain region implicated in semantic control – left inferior frontal gyrus (LIFG). Stimulation of LIFG attenuated the effect of declining comprehension, perhaps because initial retrieval was facilitated (potentially reducing the inhibition of related information), and/or because subsequent target selection was strengthened. Together, these results provide a more comprehensive account of what drives declining performance in continuous categorisation in healthy young adults who have the capacity to strongly engage semantic control.