An electroencephalogram (EEG) is a medical procedure that records the electrical activity of the brain. Electrodes placed on the scalp detect tiny electrical signals, displayed as wavy lines on a screen, providing insights into brain function. During an EEG, patients often observe flashing lights. This article explains their purpose.
The Role of Photic Stimulation in EEG
The technical term for using flashing lights during an EEG is intermittent photic stimulation (IPS). This technique introduces a controlled visual stimulus to the brain. Its purpose is to challenge the brain, revealing underlying patterns or abnormal brain wave activity not visible during resting states. Photic stimulation is a standard component in EEGs when specific diagnostic goals require it.
How Flashing Lights Influence Brain Activity
When flashing lights are presented during an EEG, visual signals travel from the eyes to the brain’s visual cortex. These light flashes are brief and precisely timed, typically ranging from 1 to 30 flashes per second (Hertz). The brain’s electrical activity can synchronize with the frequency of these flashes, a phenomenon known as “photic driving.” This controlled visual input aims to make certain brain patterns, particularly those that are not typical, more apparent.
Photic driving is most pronounced in the occipital region of the brain, which processes visual information. This synchronization helps identify how the brain responds to specific frequencies, providing insights into the integrity of the visual and occipital cortices. The controlled visual stress can reveal latent brain abnormalities that might otherwise remain undetected during a routine EEG.
Identifying Specific Neurological Responses
Neurologists primarily use photic stimulation to identify responses related to photosensitive epilepsy. This condition triggers seizures or specific brainwave patterns from flashing lights or visual patterns. The abnormal EEG response to intermittent light stimulation is known as a photoparoxysmal response (PPR), which can manifest as cortical spikes or spike and wave discharges on the EEG recording.
A PPR indicates an abnormal tendency to develop hypersynchronized neuronal activity. While a photoparoxysmal response does not always mean a person has epilepsy, it provides crucial diagnostic information, particularly for forms of epilepsy where seizures are visually provoked. PPR is a well-recognized phenomenon, occurring in some patients with epilepsy, and is often linked to idiopathic generalized epilepsies.
Patient Experience During Photic Stimulation
During the photic stimulation portion of an EEG, the patient typically sits with electrodes attached to their scalp. A strobe light is placed in front of the patient’s face, flashing into their eyes. The technician varies the flash rate, and the patient might be asked to keep their eyes open or closed.
The procedure is generally considered safe, with medical staff monitoring the patient throughout. While the lights are designed to provoke a response, including potentially triggering a seizure in susceptible individuals, this occurs in a controlled environment with immediate medical attention available. Patients might experience sensations such as seeing colors or patterns, or feel mild discomfort.