Meredith Shafto, PhD

The input/output asymmetry in language and aging. One of most consistent findings in research on language and aging is that while older adults tend to show declines in language production, they do not show declines in comprehension. This “input/output asymmetry” (MacKay et al., 1999) is a challenge to models of neurocognitive aging, as we must explain both impaired and preserved language ability in the language system where production and comprehension share many underlying processes and representations, and are carried out in the brain in largely overlapping systems. I have demonstrated this asymmetry in a spelling paradigm where older adults are impaired at error correction (output), but not at error detection (input) (Shafto, 2010). This asymmetry highlights that age-related declines have more to do with changes to how we access knowledge than changes to the integrity of underlying knowledge.

Language production: Word finding failures. One of the biggest worries of older adults are word finding problems such as “tip of the tongue states” (TOTs).  People fear that these “memory” failures signal general cognitive decline and encroaching dementia, but behavioral evidence suggests that these problems are normal and reflect temporary failures in language production. During production, word meaning (semantics) must be mapped onto word form (phonology) and word finding problems occur when the connections between semantics and phonology become too weak. Our research using structural and functional MRI supports behavioural evidence that age-related declines in neural integrity impairs phonological access (Shafto et al., 2007; Shafto et al., 2010; Stamatakis et al., 2011).  However, we also find evidence that during TOTs, a frontal “salience” network is activated that is associated with general cognitive control and error detection (Shafto et al 2010). Our ongoing research is using behavioral experiments, structural and functional MRI, and MEG to distinguish the temporary language failures characteristic of normal ageing from more general cognitive failures that may be related to pathological aging.

Language comprehension: Preservation of automatic processing. Older adults remain preserved at many of the core processes underpinning comprehension, including accessing the meaning of words and processing syntax during online sentence comprehension. Using fMRI, we have explored the possibilities that older adults maintain comprehension by reorganization of the component processes underpinning comprehension (Shafto et al., 2012), or by recruiting additional neural resources, especially from the frontal lobes (Tyler et al., 2010). Recent evidence from research from the Centre for Speech, Language and the Brain (Davis et al., 2014) suggests that despite age-related loss of grey and white matter, core automatic language processing is preserved. This is in keeping with previous behavioral evidence (see Burke and Shafto, 2008 for a review), and in keeping with recent evidence I have found from a proofreading study that older adults are preserved at error detection when processing is automatic, but impaired when it is attentionally-demanding (Shafto et al., 2015).

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