Photic stimulation

A routine EEG can be helpful if epileptiform discharges are seen. However, the EEG may be normal between seizures, and most routine EEGs are not performed during a seizure. Maneuvers such as sleep deprivation, photic stimulation, hyperventilation, or prolonged monitoring can help reveal EEG changes consistent with epilepsy. 

Patients with epilepsy may have completely normal findings in these assessments. Many of the tests are done to rule out other causes of seizures (e.g., infection or electrolyte imbalance). Often the EEG appears normal between seizures.20 Several manipulations can be done in an attempt to capture seizure or seizure-like activity on the EEG. These include sleep deprivation, photic stimulation, prolonged (greater than 20 minutes) EEG recording, and 24-hour EEG monitoring with video correlation. 

Jacobsen et al, (2002) 9 Smokers fMRI/photic stimulation IV nic lOmcg kg vs. saline No effect on visual cortex... 

Provocative stimuli (e.g., photic stimulation with flashing strobe light)... 

Renshaw PF, Yurgelun-Todd DA, Cohen BM. 1994. Greater hemodynamic response to photic stimulation in schizophrenic patients An echo planar MRI study. Am J Psychiatry 151 1493-1495. 

Electroencephalography (EEG) records the electrical activity of the brain. The record is interpreted by observing the basic rhythms and waveforms, the symmetry of the recording, and abnormal electrical discharges. It also may be used to assess the response to photic stimulation or hyperventilation. It is used primarily in the diagnosis of seizures and may be helpful in the evaluation of patients with altered mental status. EEG also may be used to measure evoked potentials (EPs). The EPs are the EEG response to repetitive stimuli (visual, auditory, or tactile) and provide information about the presence of abnormalities and disturbances (but not the cause) in the specific pathways tested. 

Photic stimulation—Stimulation of the visual cortex through visual stimulation with bright and alternating light. 

Ali M. Pattern of EEG recovery under photic stimulation by light of different colors. Electroencephalogr Clin Neurophysiol 1972 33(3) 332—5. 

Various animals show spontaneous epilepsy or seizures that can be readily induced by sensory stimulation (see Jobe et al. 1991). Tottering mice display seizures that resemble absence attacks behaviourally, in their EEG pattern and response to drugs. DBA/2 mice show reflex seizures to audiogenic stimuli while photically-induced seizures can be obtained in the Senegalese baboon, Papiopapio, which are similar to generalised tonic-clonic epilepsy. 

Lesions of serotonergic neurons in laboratory animals have been reported by some, but not all, investigators to disrupt locomotor rhythms or to result in the loss of the daily rhythm of corticosterone. There is evidence that, in the hamster, the median raphe nucleus projects to the SCN whereas the dorsal raphe nucleus innervates the IGL furthermore, the serotonergic innervation to the SCN and not the IGL is necessary for the photic entrainment of locomotor activity [22]. It appears then, that the SCN circadian pacemaker or clock is modulated by stimulation of serotonergic receptors in the SCN and that serotonergic projections to the SCN may modulate the phase of the SCN in intact animals. 

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