18 Language Predicts Verbal Learning in Parkinson's Disease

Abstract

Objective:Cognitive impairment in Parkinson's disease (CIPD) is present in approximately 40% of patients. Language deficits, evidenced by poor word- retrieval, have historically characterized memory weaknesses in PD. That is, the "retrieval deficit hypothesis," suggests successful memory encoding, but poor retrieval subsequent to language and executive dysfunction, another prominent area of CIPD. However, recent studies suggest that memory impairments in PD are instead at the level of learning. At present, several suggested etiologies to explain learning impairments in PD exist that are not related to language, for example that processing speed deficits (another characteristic of CIPD) impact learning; however, other studies present evidence against this theory. Therefore, we hypothesize that deficits in language continue to be a primary component of memory impairment in PD, but at the level of learning rather than retrievalParticipants and Methods:85 adults (age M = 61.54, SD = 10.00; %female = 26.7; Dementia Rating Scale M = 137.77, SD = 5.63) diagnosed with Parkinson's disease according to the UK Brain Bank criteria for idiopathic PD, completed a neuropsychological test battery when "off" levodopa medication. The battery included the Boston Naming Test (BNT), verbal fluency tests (Controlled Oral Word Association [COWA] and category fluency), the California Verbal Learning Test, 2nd Edition (CVLT-II), and the Oral Symbol Digit Modalities Test (SDMT). Separate linear regression models were used to examine BNT, COWA, category fluency, and SDMT performance as predictors of total learning (sum of trials 1-5), short-delay free recall, long-delay free recall, and recognition discriminability on the CVLT-II. Analyses were adjusted for age, sex, education, and disease severity (MDS-Unified Parkinson's Disease Rating Scale, part 3 score). Follow up analyses adjusted for processing speed (oral SDMT).Results:Adjusted linear regression models revealed that both verbal fluencies predicted verbal learning (letter: ß = .37, p < .01; category: ß = .45, p < .01), long-delay free recall (letter: ß = .25, p = .05; category: ß = .34, p = .01), and recognition discriminability (letter: ß = .36, p = .02; category: ß =.33, p = .03) on the CVLT-II. Confrontation naming significantly predicted only long-delay free recall (ß =.31, p = .01). Processing speed predicted verbal learning (ß = .51, p < .01), short-delay free recall (ß = .35, p = .03), and long-delay free recall (ß = .44, p < .01). After adjusting for processing speed, letter fluency significantly predicted learning (ß = .23, p = .05) and discriminability (ß = .33, p = .04). Category fluency significantly predicted learning only (ß = .28, p = .04). Finally, confrontation naming significantly predicted only long-delay free recall (ß= .28, p = .01).Conclusions:While processing speed was associated with verbal learning and recall, components of language predicted variance in verbal learning in PD that was not accounted for by speed. Additionally, discriminability was related to aspects of language that are more reliant on executive functioning. It is therefore suggested that verbal memory in PD is interpreted within the context of one's language ability. Other potential mechanisms and clinical implications are discussed.