Nathan H. Heller¹ (); ¹Johns Hopkins Medicine, Center for Psychedelic and Consciousness Research

Stroboscopic (strobe) hallucinations are dynamic, kaleidoscopic patterns induced by flickering light in front of a person’s closed eyes. They resemble geometric hallucinations reported during various clinical and pharmacological contexts (Billock & Tsou, 2012; Heller et al., 2023). Computational models attribute these percepts to waves of cortical excitation in V1 (Bressloff et al., 2001) and indicate that higher temporal frequency (TF) strobe corresponds to lower spatial frequency (SF) patterns (Rule, Stoffregen, & Ermentrout, 2011). However, anecdotal reports suggest the opposite relationship between TF and SF exists (Smythies, 1960). Here, we present the first attempt to quantify this relationship. First, participants reported which of 4 strobe frequency pairs (5&10Hz, 10&15Hz, 15&20Hz, and 20&25Hz) induced higher spatial frequency patterns, along with their confidence level (1-10). Then, by adjusting the spatial frequency of a checkerboard, participants matched the size of the checkerboard elements (dva) to the elements comprising strobe hallucinations for foveal (central 4 dva) and peripheral regions. Consistent with anecdotal reports, pairwise judgements revealed strobe TF and hallucination SF correspond positively. Confidence was moderate for the 20&25Hz pair (M=6.5, SEM=1.4) and otherwise high (M=9.0, SEM=0.65). Checkerboard adjustment showed that the size of hallucination elements (dva) decreased steadily until the two highest strobe TFs (5Hz: M= 4.19, SEM=1.28; 10Hz: M=1.14, SEM=0.32; 15Hz: M=0.62, SEM=0.16; 20Hz: M=0.33, SEM=0.09; 25Hz: 0.33, SEM=0.11). A repeated-measures ANOVA was conducted to assess the effects of visual region (foveal vs. peripheral) and strobe-stimulation frequency (5-25 Hz) on hallucination element size. There was a significant main effect of frequency (F[4,12]=15.58, p<.001, ηp2 =0.839), corroborating the strong positive relationship between strobe TF and hallucination SF. The effect of visual region was marginal (F[1,3]=9.64, p=.053, ηp2 =0.763), though there was a significant interaction between frequency and region (F[4,12]=4.88, p=.014, ηp2 =0.619). Computational models of strobe hallucination should be amended to reflect these results.