  Language is a valuable source of clinical information in Alzheimer's Disease,
as it declines concurrently with neurodegeneration. Consequently, speech and
language data have been extensively studied in connection with its diagnosis.
This paper summarises current findings on the use of artificial intelligence,
speech and language processing to predict cognitive decline in the context of
Alzheimer's Disease, detailing current research procedures, highlighting their
limitations and suggesting strategies to address them. We conducted a
systematic review of original research between 2000 and 2019, registered in
PROSPERO (reference CRD42018116606). An interdisciplinary search covered six
databases on engineering (ACM and IEEE), psychology (PsycINFO), medicine
(PubMed and Embase) and Web of Science. Bibliographies of relevant papers were
screened until December 2019. From 3,654 search results 51 articles were
selected against the eligibility criteria. Four tables summarise their
findings: study details (aim, population, interventions, comparisons, methods
and outcomes), data details (size, type, modalities, annotation, balance,
availability and language of study), methodology (pre-processing, feature
generation, machine learning, evaluation and results) and clinical
applicability (research implications, clinical potential, risk of bias and
strengths/limitations). While promising results are reported across nearly all
51 studies, very few have been implemented in clinical research or practice. We
concluded that the main limitations of the field are poor standardisation,
limited comparability of results, and a degree of disconnect between study aims
and clinical applications. Attempts to close these gaps should support
translation of future research into clinical practice.
