Quantitation of photic driving in dementia with normal EEG.

The EEG is frequently normal when early dementia is clinically suspected. The presence or absence of occipital driving with intermittent photic stimulation is not helpful in detecting cerebral pathology, but quantitation of photic driving was shown to distinguish between dementia and depression in geriatric patients. We quantified photic driving in twenty clinically demented patients without diffuse or focal EEG abnormality, and compared this in 20 age-matched normal controls. Intermittent stimulation at odd-numbered frequencies between 1 and 31 Hz was carried out. The product of frequency of stimulation and duration of driving response was determined, and the incidence of absent, subharmonic and supraharmonic photic driving was determined. The quantitative measure of frequency times following duration was significantly greater in the nondemented individuals than in demented patients with normal EEGs. More nondemented individuals demonstrated supraharmonic and subharmonic responses than did demented patients. The range of normal variability in persistence, duration, and symmetry of photic driving is great, and caution in determining abnormality on this basis is appropriate. Subtle but measurable changes in photic driving may occur early in dementing illnesses, however.