Takahashi T (in Niedermeyer E & Lopes da Silva F, 1999) refers that intermittent photic stimulation with a strobe light may determine three main changes in the EEG: 1. Photoparoxysmal, with epileptiform abnormalities characterized by spikes-and-wave or multiple spike-and-wave complexes, correlated to epilepsy; 2. Frontal or diffuse photomyogenic response, characterized by repetitive muscle potentials, non-epileptic, sometimes correlated to abstinence (alcohol, drugs, etc.); 3. PDR – Photic Driving Response, alleged old alpha rhythm driving, physiological activity elicited over posterior head regions time-locked to the stimulus. Actually, PDR represent a physiological visual response – a visual evoked potential (VEP).
VEP always occur with permeable optic pathways, its recording depending of the equipment, adequate stimulus parameters, and signal averaging. There are two type of VEP responses: 1. Transient VEPs, occurring after low stimulus frequency, under 3 Hz (figure 1). 2. Fast-Following Responses (FFR) or steady-state potentials, when higher frequency stimulus, above 4 – 5 Hz, generate almost senoidal curves, guiding harmonically the central nervous system to oscillate accordingly to the used frequency (figure 2). VEP-flash recording is rather simple to obtain in normal circumstances and obligatory when using the proper equipment.
In certain cases a slightly higher amplitude flash response, either isolated or stationary, may be observed in the EEG (figures 1 e 2). Important: the giant pathological flash-VEP is not under consideration here. Usually, EEG sensitivity (SNS) is not adequate to record flash visual responses. EEG SNS varies into 5 – 7 – 10 µV/mm; VEP SNS is in the order of 0.5 µV/mm, signaling a 10 – 20 times increasing. In summary, PDR is a physiological visual response – the flash-VEP.
Reference: Takahashi, T – Activation Methods. Intermittent Photic Stimulation. Chapter 14. in Electroencephalography. Basic Principles, Clinical Applications and Related Fields. Niedermeyer, E & Lopes da Silva, F. ed, 4th ed, Lippincott Williams & Wilkins, Philadelphia, p 261-284, 1999.