. Significance Dementia presents a global healthcare crisis, and neuroimaging is the main method for developing effective diagnoses and treatments. Yet currently, there is a lack of sensitive, portable, and low-cost neuroimaging tools. As dementia is associated with vascular and metabolic dysfunction, near-infrared spectroscopy (NIRS) has the potential to fill this gap. Aim This future perspective aims to briefly review the use of NIRS in dementia to date and identify the challenges involved in realizing the full impact of NIRS for dementia research, including device development, study design, and data analysis approaches. Approach We briefly appraised the current literature to assess the challenges, giving a critical analysis of the methods used. To assess the sensitivity of different NIRS device configurations to the brain with atrophy (as is common in most forms of dementia), we performed an optical modeling analysis to compare their cortical sensitivity. Results The first NIRS dementia study was published in 1996, and the number of studies has increased over time. In general, these studies identified diminished hemodynamic responses in the frontal lobe and altered functional connectivity in dementia. Our analysis showed that traditional (low-density) NIRS arrays are sensitive to the brain with atrophy (although we see a mean decrease of 22% in the relative brain sensitivity with respect to the healthy brain), but there is a significant improvement (a factor of 50 sensitivity increase) with high-density arrays. Conclusions NIRS has a bright future in dementia research. Advances in technology – high-density devices and intelligent data analysis—will allow new, naturalistic task designs that may have more clinical relevance and increased reproducibility for longitudinal studies. The portable and low-cost nature of NIRS provides the potential for use in clinical and screening tests.

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Objectives. This review aimed to evaluate previous studies using Near-infrared spectroscopy (NIRS) in dementia by summarising the results, determining the consensus in the literature, and delineating if, and how, NIRS experimental and analysis methods may be improved for future studies in dementia. Methods. Three databases (PsychINFO, Medline, Embase) were searched for original research studies using NIRS in dementia and prodromal disease stages. We included both observational and randomised control trials, and studies published in English. Animal studies, conference abstracts, and reviews were excluded. Results. From 759 identified records, 80 studies using NIRS in dementia and prodromal populations across a range of activation tasks testing memory (28), word retrieval (22), and motor (7) and visuo-spatial function (4), as well as in the resting state (29) were evaluated. Across these cognitive domains, dementia patients generally showed a blunted haemodynamic response, often localised to frontal regions of interest, and a lack of task-appropriate frontal lateralisation. Prodromal stages, such as Mild Cognitive Impairment, revealed mixed results and were associated with either diminished responses or hyperactivity, accompanied by reduced cognitive function, the latter suggesting a possible compensatory neural response which is not present at the dementia stage. Conclusion. There is clear evidence of alterations in brain oxygenation in both dementia and prodromal stages across a range of cognitive domains and in the resting state, indicating an ability of NIRS to distinguish dementia from healthy ageing, or at-risk populations. A consensus as to the nature of these changes, however, is difficult to reach due to a lack of standardisation in optical techniques and processing methods. Further studies are required exploring more naturalistic settings and in a wider range of dementia subtypes.

Syllables are an essential building block of speech. We recently showed that tactile stimuli linked to the perceptual centers of syllables in continuous speech can improve speech comprehension. The rate of syllables lies in the theta frequency range, between 4 and 8 Hz, and the behavioural effect appears linked to multisensory integration in this frequency band. Because this neural activity may be oscillatory, we hypothesized that a behavioural effect may also occur not only while but also after this activity has been evoked or entrained through vibrotactile pulses. Here we show that audiotactile integration regarding the perception of single syllables, both on the neural and on the behavioural level, is consistent with this hypothesis. We first stimulated subjects with a series of vibrotactile pulses and then presented them with a syllable in background noise. We show that, at a delay of 200 ms after the last vibrotactile pulse, audiotactile integration still occurred in the theta band and syllable discrimination was enhanced. Moreover, the dependence of both the neural multisensory integration as well as of the behavioral discrimination on the delay of the audio signal with respect to the last tactile pulse was consistent with a damped oscillation. In addition, the multisensory gain is correlated with the syllable discrimination score. Our results therefore evidence the role of the theta band in audiotactile integration and provide evidence that these effect may involve oscillatory activity that still persists after the tactile stimulation.