Behavioral and Electrophysiological (P300) Correlates for Visual and Auditory Process in Alzheimer’s Disease

Abstract

Working memory deficits are a recognized feature of Alzheimer’s disease (AD). The current study aimed to examine the visual and auditory processes of working memory capacity in individuals with dementia (IWD) and neurotypical individuals (NTI) through the use of behavioral and electrophysiological measures. The behavioral task employed in the present study was a visual n-back (4-back) task for five different stimuli categories, viz., common objects, fruits, vehicles, numbers, and alphabets using the E-Prime 2.0 software. The outcomes of the behavioral task in terms of reaction time and threshold of performance for each category of the stimulus were extracted and was subjected for analysis. The auditory electrophysiological measures were obtained using Electrical Geodesics Inc. (EGI) NetStation 5.4 and were further subjected to post-processing and analysis using the EEGLAB plugin on the MATLAB software. The primary auditory electrophysiological measure was the P300 elicited using both speech (/da/-frequent and /ga/- infrequent) and tone (1kHz pure tone-frequent and 2kHz- infrequent) stimuli. An oddball paradigm (80:20) was employed to elicit P300 from the participants. Further, the P1-N1-P2 complex was also obtained and subjected to analysis. The latency and amplitude of these evoked potentials were considered for further analysis. NTI group demonstrated shorter reaction times and higher thresholds compared to the IWD group for various categories of stimuli. The threshold of performance of the NTI group was observed to be 4-back level for various categories of stimuli whereas the performance of the IWD group was scattered at different levels. The majority of the participants of the IWD group could only reach a threshold of 2-back level for various categories of stimuli. In general, the performance of the IWD group was more variable and scattered compared to the NTI group which was more consistent and stable. Significant between-group differences were noted at almost all levels of the n-back task for all categories of stimuli. Comparison of reaction time at the threshold level of performance also revealed significant differences across the groups with superior performance by the NTI group. The performance of the NTI group was superior to the IWD group in the electrophysiological measures as well. Prolonged latencies and diminished amplitudes were observed for the IWD group for both speech and tone stimuli with some exceptions. Between-group differences revealed that P300 elicited using both speech and tone stimuli from various cortical regions were different in terms of their latency and amplitude, especially at the parietal region. Within-group differences were also observed in the NTI group for P300 elicited using both speech and tone stimuli. In contrast, within-group differences were minimal in the IWD group. Statistically, it was observed that shorter latencies and higher amplitudes were present for speech evoked P300 compared to tone evoked P300 at the majority of cortical regions. Further, the P1-N1-P2 complex was examined for both tone and speech stimuli. These also revealed a superior performance by the NTI group than the IWD group. Latencies and amplitude of P300 elicited from various cortical regions showed a fair to moderate correlation with a threshold reaction time of the n-back task in the NTI group. Whereas, the IWD group demonstrated strong to perfect negative correlations across the latencies and amplitudes of P300 elicited from different cortical regions with the threshold reaction time of the n-back task. Further studies are warranted to validate the findings of the current study.