[Clinical significance of increased reaction to photostimulation in the alpha frequency range]

When evaluating responses in the EEG to intermittent photic stimulation usually the emphasis is placed on the differentiation of photoparoxysmal and photomyogenic responses as well as asymmetric driving effects such as locally increased activation of slow frequencies or locally reduced activation of the basic rhythm indicating a focal disturbance. Apart from these responses, however, sometimes an increased rhythmic response within the alpha-frequency range can be observed. This response is characterized by a bilateral, unproportionately strong activation or increase in amplitude of the particular rhythm, as well as a growing synchronized activation of beta-waves. There is, however, no evidence of paroxysmal characteristics neither directly by spike components, nor indirectly by spread to other regions of the brain. The occurrence of this pattern is studied in a mixed clinical population of 3110 patients whose EEGs included photic stimulation as a routine activating procedure. The described response occurred in 1.4% with a remarkable predominance in female patients. Particularly middle and higher age groups were represented. There were significant differences in age, sex and types of illness compared to other in-patient groups as well as a matched control group with a similarly unremarkable resting activity in the EEG, but without the increased response to photic stimulation. In the young age group traumatic or inflammatory diseases prevailed, in middle and higher age groups cerebrovascular disturbances, vasomotor headaches or migraine and peripheral vascular diseases, e.g. with coronary manifestation. The symptoms include a high proportion of vegetative regulatory dysfunctions and neurasthenic-depressive complaints. There is no obvious relationship to disturbances of the occipital lobes or to seizure disorders whereas regarding not only the symptomatology but also the characteristics of the resting activity in the EEG there are definite relationships to vertebrobasilar insufficiency. Experimental findings and clinical observations support the assumption that a functional alteration located in subcortical, mesodiencephalic areas is the condition responsible for the increased rhythmic response to photic stimulation, however without a specific etiologic significance. Based on neurophysiological models regarding the regulation of thalamo-cortical rhythmicity one can consider a disturbance of mesodiencephalic reticular systems as the mechanism responsible for a shift in the regulation of synchronization-desynchronization-levels with a desinhibitory effect. This leads to the conclusion that not only a unilateral but also a bilateral increase in the response to photic stimulation has to be taken into account as an indicator of a functional alteration. Such an indicator can be particularly valuable when there are only mild symptoms of an apparently unspecific nature.