Evaluation of touch perception abilities in typical and atypical ageing.

The World Health Organization (WHO) currently reports over 55 million cases of dementia worldwide (Shin, 2022). However, in the United Kingdom, only two-thirds of people with dementia commonly receive a formal diagnosis (Ford et al., 2019). This is problematic not only for patients but also for caregivers, families, and by extension for the overall community, consolidating stigmas and barriers in caring for dementia patients.

Ageing is a complex multifactorial process characterised by an overall loss of sensory functions and cognitive abilities (McIntyre et al., 2021). The idea that sensory impairments may serve as early indicators of cognitive impairment was first introduced by Baltes and Lindenberger (1997). According to their "Common cause theory", the overall decline of sensory processing enhances the risk of a decrease in cognitive fluid abilities (Olderbak et al., 2015).

In line with this rationale, it has been observed that hearing loss and reduced visual sensitivity are associated with cognitive decline (Fischer et al., 2016). However, compared to other sensory modalities that have been largely investigated (Tseng et al., 2018) little is known about the progression of tactile function during ageing and its relationship with cognitive abilities (Löffler et al., 2023).

Examining the age-related alterations of touch perception is an important and demanding field of study, as the results are currently unclear.

First of all, the affective component of touch seems to be more preserved during ageing (Sehlstedt et al., 2016), whereas its discriminative counterpart undergoes a progressive deterioration (McIntyre et al., 2021). Specifically, the structural alterations on the peripheral nervous system (e.g., decreased skin elasticity, deteriorations in receptive systems, and denervation of afferent fibres, McIntyre et al., 2021) may account for the majority of the age-related decline in the discriminative component of touch.

These physiological changes are commonly accompanied by modifications in the cortical processing of touch (Tamè et al., 2019). Indeed, tactile information is subjected to various hierarchical stages of analysis, in which a low-level perceptual representation is gradually transformed into a high-level cognitive construct (Tamè & Longo, 2023).

The primary somatosensory cortex (S1) is the main region responsible for the initial processing of somatic stimuli, followed by higher-level processing that allows for stimuli localization on the body surface and perception of space (Longo et al., 2010). The knowledge of the spatial configuration of the body is also mediated by the body's structural representation (BSR, Tamè et al., 2017) which represents the highest form of abstraction of tactile processing.

This construct is built beyond S1 and primarily in the parietal lobe (Longo et al., 2010). Indeed, parietal lesions can cause several neurological conditions in which the body representation is altered (e.g., finger agnosia, Tamè et al., 2017). Furthermore, parietal alterations are known to be a hallmark of the pathophysiological progression of AD (Braak & Braak, 1991).

However, it is unclear how ageing affects the various stages of tactile processing, particularly in pathological conditions. Therefore, developing effective assessment tools to define the progression of somatosensory deficits in healthy ageing can provide novel important insight into the mechanisms subserving these conditions. This knowledge can be applied in clinical practice for the assessment of pathological conditions, by favouring early diagnoses that will improve the quality of life of dementia patients.