Response To Intermittent Photic Stimulation

Similar to hyperventilation, intermittent photic stimulation is performed to "activate" the EEG. Photic stimulation is performed by using a commercial stroboscopic stimulator placed approx 1 m from the patient's eyes. They are asked to keep their eyes closed and look straight ahead while the ambient room lighting is turned down. The test is performed by alternating flashes varying from 1 to 35 Hz and lasting for 10 s and interrupted by 10 to 30 s with no stimulation. The variables that need to be controlled for are the distance from the subject to the strobe, the luminance of the strobe, direction of gaze, and level of consciousness. The most common abnormal activation is to produce epileptic activity in relationship to the photic stimulation. In healthy people, one may see any level of "driving." This phenomena appears in the occipital leads and is the result of a "flash" visual evoked response. Therefore, the background rhythm gets linked to the timing of the photic stimulator (Fig. 3). The first response appears shortly after the stimulator goes on (<100 ms) and stops when the stimulator shuts off. It is more likely to occur around the baseline background frequency (±2-4 Hz). One may also see what is referred to as a photomyogenic response as a normal variant. In this condition, widespread muscle twitching appears, which is timed to the stimulator. This reaction is felt to

Photomyogenic Response

Fig. 3. Midway through this 20-s portion of a routine EEG is the 10-s period of intermittent photic stimulation noted by the artifact on the last channel. The frequency of the photic stimulator is eight flashes per second. There is a prominent evoked response noted over the occipital leads at the same frequency. This is a normal reaction.

Fig. 3. Midway through this 20-s portion of a routine EEG is the 10-s period of intermittent photic stimulation noted by the artifact on the last channel. The frequency of the photic stimulator is eight flashes per second. There is a prominent evoked response noted over the occipital leads at the same frequency. This is a normal reaction.

represent a heightened brainstem-mediated reflex reaction to the photic stimulator. Additional enhanced levels of photic driving reactions are recognized and defined in a review by Waltz et al. (5).

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