Different Markers of Semantic-Lexical Impairment Allow One to Obtain Different Information on the Conversion from MCI to AD: A Narrative Review of an Ongoing Research Program

Abstract

Background: In this narrative review, we have surveyed results obtained from a research program dealing with the role of semantic memory disorders as a predictor of progression from mild cognitive impairment (MCI) to Alzheimer's disease (AD). Objectives: In this research program, we have taken into account many different putative markers, provided of a different complexity in the study of the semantic network. These markers ranged from the number of words produced on a semantic fluency task to the following: (a) the discrepancy between scores obtained on semantic vs. phonemic word fluency tests; (b) the presence, at the single-word level, of features (such as a loss of low typical words on a category verbal fluency task) typical of a degraded semantic system; or (c) the presence of more complex phenomena (such as the semantic distance between consecutively produced word pairs) concerning the organization of the semantic network. In the present review, all these studies have been presented, providing separate subsections for (a) methods, (b) results, and (c) a short discussion. Some tentative general conclusions have been drawn at the end of the review. We found that at baseline all these markers are impaired in MCI patients who will later convert to AD, but also that they do not necessarily show a linear worsening during the progression to AD and allow one to make different predictions about the time of development of AD. Our conclusions were that, rather than searching for the best marker of conversion, we should use a range of different markers allowing us to obtain the information most appropriate to the goal of our investigation.

Keywords: conversion from MCI to AD; phonemic-semantic fluency discrepancy; semantic and phonemic fluency tasks; semantic memory disorders; word typicality.