AN ELECTROPHYSIOLOGICAL STUDY OF VISUAL PROCESSING IN ALZHEIMERS-DISEASE

Visual processing of sinusoidally modulated gratings was studied in a group of patients (n = 11) with Alzheimer's disease (AD) and an elderl y normal control group (n = 9). Spatial square wave gratings (1.47 c/d ) were reversed at a temporal frequency of 4 or 8 Hz. EEG recordings a t rest and during visual stimulation were obtained from 20 channels us ing the 10/20 international system. The power spectrum of the 2nd and 4th harmonic of the stimulation frequency was calculated by Fast Fouri er Transform (FFT) at a resolution of 0.25 Hz. Association of activity between occipital, temporal, parietal and central regions was measure d by intra- and inter-hemispheric coherence and phase at harmonics of the stimulation frequency. A significant difference (P < 0.01) in evok ed activity of the 4th harmonic at O1 and O2 was found between the two groups with less activity in the AD patients. In the AD group there w as a significant correlation (P < 0.05) between evoked activity at the 2nd harmonic of the 8 Hz visual stimulation and Mini-Mental State (MM S) score. This correlation was independent of the age effect on MMS. R esponse phase between O1 and O2 for both 4 and 8 Hz stimuli was close to 0-degrees and coherence had similar values in both groups. Occipita l and central regions showed a phase reversal for all harmonic respons es to both visual stimuli. The AD patients showed statistically signif icant (P < 0.05) phase dispersion at O1-P3 and O2-P4 not seen in the c ontrol group. These data suggest that many functions of primary visual cortex are preserved in AD patients, whereas occipital to parietal co nnectivity and processing is disrupted.