Altered Frontal Lateralization Underlies the Category Fluency Deficits in Older Adults with Mild Cognitive Impairment: A Near-Infrared Spectroscopy Study.

Michael K Yeung,Sophia L Sze,Ruby Yu,Timothy Kwok,Agnes S Chan,Jean Woo,David H K Shum

doi:10.3389/fnagi.2016.00059

Michael K Yeung, Sophia L Sze + Show 5 more

Open Access

https://doi.org/10.3389/fnagi.2016.00059

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Abstract

Individuals with mild cognitive impairment (MCI) have been consistently found to have category fluency deficits. However, little is known about the neural basis of these deficits. A diversity of neuroimaging studies has revealed left-lateralized prefrontal activations due to verbal processing and control functions during the performance of category fluency tasks. Given the reports of structural and functional abnormalities in the prefrontal cortices in individuals with MCI, it is conceivable that these individuals would also exhibit altered prefrontal activation patterns during a category fluency task. The present study aimed to investigate the prefrontal dynamics during the category fluency task in older adults with MCI by using near-infrared spectroscopy (NIRS). Twenty-six older adults with MCI were compared with 26 older adults with normal cognition (NC) who were matched in age, gender, handedness, and educational level. All participants performed a category fluency task while the prefrontal dynamics were recorded. The results showed that the MCI group generated fewer unique words, made fewer switches between subcategories, and generated fewer new subcategories than did the NC group. Importantly, the NIRS results showed that the NC group exhibited a left lateralization of frontal activations during the category fluency task, while the MCI group did not exhibit such a lateralization. Furthermore, there was a significant positive correlation between the category fluency performance and the extent of lateralization, suggesting that the category fluency deficits in the MCI group could be related to frontal dysfunction. That is, the rightward shift of frontal activations in the MCI group may reflect the presence of cortical reorganization in which the contralateral regions (i.e., the right hemisphere) are recruited to take over the function that is declining in the specialized regions (i.e., the left hemisphere). Our lateralization finding may serve as an objective neural marker for distinguishing between normal aging and MCI. Our study highlights that an alteration of neural functioning is already present at the prodromal stage of dementia.

Highlights

Mild cognitive impairment (MCI) is considered to be a transitional stage between healthy aging and dementia
The mild cognitive impairment (MCI) group performed significantly worse than the normal cognition (NC) group on almost all of the measures
For speed/executive functions, the MCI group was significantly slower on both conditions of the SST and had significantly poorer organization in copying on the Rey-Osterrieth Complex Figure Test (RCFT) than did the NC group

Summary

Introduction

Mild cognitive impairment (MCI) is considered to be a transitional stage between healthy aging and dementia. Individuals with MCI are concerned about a cognitive decline and exhibit objective cognitive impairment but have essentially normal activities of daily living and are not demented (Albert et al, 2011; Petersen et al, 2014). The conversion rate of MCI to dementia has been estimated to be approximately 10–20% per annum (Petersen et al, 1999; Amieva et al, 2004; Venneri et al, 2011; Wang et al, 2011). Because early detection of individuals at the prodromal stage of dementia offers the best potential for intervention (Ernst et al, 1997), it is important to identify objective markers, such as neuropsychological and neurophysiological characteristics that can reliably distinguish MCI from normal aging

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