Event-Related Potential Measures of the Passive Processing of Rapidly and Slowly Presented Auditory Stimuli in MCI

Abstract

Much research effort is currently devoted to the development of a simple, low-cost method to determine early signs of Alzheimer's disease (AD) pathology. The present study employs a simple paradigm in which event-related potentials (ERPs) were recorded to a single auditory stimulus that was presented rapidly or very slowly while the participant was engaged in a visual task. A multi-channel EEG was recorded in 20 healthy older adults and 20 people with mild cognitive impairment (MCI). In two different conditions, a single 80 dB sound pressure level (SPL) auditory stimulus was presented every 1.5 s (fast condition) or every 12.0 s (slow condition). Participants were instructed to watch a silent video and ignore the auditory stimuli. Auditory processing thus occurred passively. When the auditory stimuli were presented rapidly (every 1.5 s), N1 and P2 amplitudes did not differ between the two groups. When the stimuli were presented very slowly, the amplitude of N1 and P2 increased in both groups and their latencies were prolonged. The amplitude of N1 did not significantly differ between the two groups. However, the subsequent positivity was reduced in people with MCI compared to healthy older adults. This late positivity in the slow condition may reflect a delayed P2 or a summation of a composite P2 + P3a. In people with MCI, the priority of processing may not be switched from the visual task to the potentially much more relevant auditory input. ERPs offer promise as a means to identify the pathology underlying cognitive impairment associated with MCI.

Keywords: ERPs; MCI; biomarker; event-related potentials; mild cognitive impairment.

Figure 1

Grand averaged event-related potentials (ERPs) from healthy older adults and people with mild cognitive impairment (MCI) in the fast rate of presentation condition. N1 and P2 amplitude did not differ between the two groups.

Figure 2

Grand averaged ERPs from healthy older adults and people with MCI in the slow rate of presentation condition. The amplitude of N1 did not differ between the groups. The amplitude of P2 was larger for healthy older adults than people with MCI at both the frontal and central regions of interest (ROIs).

Figure 3

Pirateplots of N1 and P2 data providing both descriptive and inferential statistics. Data are collapsed across all central electrode (C3, Cz, C4) sites. The grand averages and SDs (shaded) are illustrated in panel (A). The mean amplitudes of N1 and P2 (thick, solid horizontal line), 95% confidence intervals (CIs; light horizontal box), smooth frequency distribution (shaded area), and individual data points (jittered) are presented in panels (B,C) respectively. The mean amplitude of N1 did not differ between the groups in the fast condition. On the other hand, P2 was larger for the healthy older adults when stimuli were presented slowly.

Figure 4

Healthy older adult and MCI grand averaged ERP waveforms across the four quarters of the slow rate of presentation condition. Data presented are from the Cz electrode site. Note the N1 and P2 did not change across the four quarters for either healthy older adults or people with MCI.