Dysfunction of the magnocellular stream in Alzheimer's disease evaluated by pattern electroretinograms and visual evoked potentials

Abstract

Background: Visuo-spatial disturbances could represent a clinical feature of early stage Alzheimer's disease (AD). The magnocellular (M) pathway has anatomo-physiological characteristic which make it more suitable for detecting form, motion and depth compared with parvocellular one (P).

Objective: Aim of our study was to evaluate specific visual subsystem involvement in a group of AD patients, recording isoluminant chromatic and luminance pattern electroretinograms and pattern visual evoked potentials.

Material and methods: data were obtained from 15 AD patients (9 females and 6 males, mean age+/-1SD: 77.6+/-4.01 years) not yet undergoing any treatment, and from 10 age-matched healthy controls. Diagnosis of probable AD was clinically and neuroradiologically established. PERGs were recorded monocularly in response to equiluminant red-green (R-G), blue-yellow (B-Y) and luminance yellow-black (Y-Bk) horizontal square gratings of 0.3c/deg and 90% contrast, reversed at 1Hz. VEPs were recorded in response to full-field (14 deg) equiluminant chromatic R-G, B-Y and luminance Y-Bk sinusoidal gratings of 2c/deg, presented in onset (300ms)-offset (700ms) mode, at the contrast levels of 90%.

Results: All data were retrieved in terms of peak-amplitude and latency and assessed using the Student's t-test for paired data. Temporal differences of PERGs and VEPs, evoked by Y-Bk grating in AD patients compared with controls, suggest a specific impairment of the magnocellular stream.

Conclusions: Our study support the hypothesis that the impairment of the PERGs and VEPs arising from the magnocellular streams of visual processing may indicate a primary dysfunction of the M-pathways in AD.

Fig. 1

Transient PERGs in response to either Y-Bk luminance grating (A and D), equiluminant chromatic R-G (B and E) and B-Y (C and F) in controls (top traces) and AD patients (bottom traces). For each inset, individual waveform are represented by dotted line, the grand mean by bold line. Note the amplitude loss for Y-Bk stimuli (A and D) and B-Y (C and F) stimuli.

Fig. 2

Transient VEPs in response to either Y-Bk luminance grating (A and D), R-G (B and E) and B-Y (C and F) in controls (top traces) and AD patients (bottom traces). As in previous figure, for each inset, individual waveform are represented by dotted line, the grand mean by bold line. Note the latency delay for Y-Bk and B-Y stimuli, whereas R-G are relatively spared.