A Bidirectional Link between Brain Oscillations and Geometric Patterns

Abstract

Like hallucinogenic drugs, full-field flickering visual stimulation produces regular, geometric hallucinations such as radial or spiral patterns. Computational and theoretical models have revealed that the geometry of these hallucinations can be related to functional neuro-anatomy. However, while experimental evidence links both visual flicker and hallucinogenic drugs to upward and downward modulations of brain oscillatory activity, the exact relation between brain oscillations and geometric hallucinations remains a mystery. Here we demonstrate that, in human observers, this link is bidirectional. The same flicker frequencies that preferentially induced radial (Ͻ10 Hz) or spiral (10 -20 Hz) hallucinations in a behavioral experiment involving full-field uniform flicker without any actual shape displayed, also showed selective oscillatory EEG enhancement when observers viewed a genuine static image of a radial or spiral pattern without any flicker. This bidirectional property constrains the possible neuronal events at the origin of visual hallucinations, and further suggests that brain oscillations, which are strictly temporal in nature, could nonetheless act as preferential channels for spatial information.

Increased spontaneous EEG signal diversity during stroboscopically-induced altered states of consciousness

“…Previous work has shown that the content of simple, but not complex, visual hallucinations (i.e. radial, grid or spiral patterns), can be modulated by the specific stroboscopic stimulation frequency 18,24 . As previously mentioned, stroboscopic stimulation at 3 Hz caused entrainment of this frequency, SSVEP, along with its harmonics, leading to a significant increase in theta power for 3 Hz relative to the other conditions ( Fig.…”

“…Further exploratory analyses were carried out to compare signal diversity measures with more standard electrophysiological measures. We computed the average spectral power profile for delta (1-4 Hz), theta (4-8 Hz), alpha (8-13 Hz) and beta (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) Hz) frequency bands, for each condition and participant. As for LZs, for each participant and condition the 10 min recording was divided into non-overlapping 10 s segments.…”

“…Next, we computed the average spectral power profile, using original data, for the delta (1-4 Hz), theta (4-8 Hz), alpha (8)(9)(10)(11)(12)(13) and beta (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) Hz) frequency bands for each condition. As shown in Fig 6, we observed a substantial decrease in alpha was converted to the dB scale using a log10 conversion.…”

“…SIVH occur with eyes closed and are characterised by intense visual experiences of amorphous colours, geometric patterns and colourful kaleidoscopic imagery [18][19][20][21][22][23] . The specific form of SIVH appears to depend on the stroboscopic frequency, for example, stroboscopic stimulation at <10 Hz tends to induce radial patterns, whereas stimulation between 10-20 Hz tends to induce spiral patterns 18,24 . The most vivid visual experiences are usually reported to occur at a stimulation frequency close to the dominant alpha frequency (8)(9)(10)(11)(12), with stimulation frequencies < 5 Hz or > 30 Hz inducing subjectively weaker visual experiences 18,[24][25][26] .…”

“…The specific form of SIVH appears to depend on the stroboscopic frequency, for example, stroboscopic stimulation at <10 Hz tends to induce radial patterns, whereas stimulation between 10-20 Hz tends to induce spiral patterns 18,24 . The most vivid visual experiences are usually reported to occur at a stimulation frequency close to the dominant alpha frequency (8)(9)(10)(11)(12), with stimulation frequencies < 5 Hz or > 30 Hz inducing subjectively weaker visual experiences 18,[24][25][26] . The contents of SIVH remain outside of voluntary control, but the individual usually retains insight into their non-veridical nature 25 .…”