Abstract
A controversy exists on photic driving in the human visual cortex evoked by intermittent photic stimulation. Frequency entrainment and resonance phenomena are reported for frequencies higher than 12 Hz in some studies while missing in others. We hypothesized that this might be due to different experimental conditions, since both high and low intensity light stimulation were used. However, most studies do not report radiometric measurements, which makes it impossible to categorize the stimulation according to photopic, mesopic, and scotopic vision. Low intensity light stimulation might lead to scotopic vision, where rod perception dominates. In this study, we investigated photic driving for rod-dominated visual input under scotopic conditions. Twelve healthy volunteers were stimulated with low intensity light flashes at 20 stimulation frequencies, leading to rod activation only. The frequencies were multiples of the individual alpha frequency (α) of each volunteer in the range from 0.40 to 2.30∗α. Three hundred and six-channel whole head magnetoencephalography recordings were analyzed in time, frequency, and spatiotemporal domains with the Topographic Matching Pursuit algorithm. We found resonance phenomena and frequency entrainment for stimulations at or close to the individual alpha frequency (0.90–1.10∗α) and half of the alpha frequency (0.40–0.55∗α). No signs of resonance and frequency entrainment phenomena were revealed around 2.00∗α. Instead, on-responses at the beginning and off-responses at the end of each stimulation train were observed for the first time in a photic driving experiment at frequencies of 1.30–2.30∗α, indicating that the flicker fusion threshold was reached. All results, the resonance and entrainment as well as the fusion effects, provide evidence for rod-dominated photic driving in the visual cortex.
Highlights
Intermittent photic stimulation (IPS) is an important method in neurophysiological examinations in clinical practice
The aim of this study is to investigate for the first time photic driving under the condition of scotopic vision in the human visual system
We found that 50–100% of the volunteers showed good to moderate photic driving responses for the stimulation frequencies around alpha (0.90, 0.95, 1.00, 1.05, and 1.10∗α), and 33–42% for an IPS at 0.50 and 0.55∗α
Summary
Intermittent photic stimulation (IPS) is an important method in neurophysiological examinations in clinical practice. Photic driving can be induced by IPS, and is characterized by rhythmic brain activity related to that. Two main measureable phenomena in electroencephalography (EEG) and magnetoencephalography (MEG) are observed: frequency entrainment and resonance effects. Frequency entrainment is characterized by the frequencylocking of a free running brain rhythm to the stimulation frequency of the IPS (Hayashi, 1985; Silberstein, 1995; Pikovsky et al, 2001). In the case of the alpha rhythm, the effects are called alpha frequency entrainment and alpha resonance. The quantization of the photic driving effect is widely used to study differences between healthy brain activity and several neurophysiological diseases, such as schizophrenia, epilepsy, and dementia, e.g., (Drake et al, 1989; Fukami et al, 2008)
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