Yueying Dong¹, Ana Chkhaidze¹, Lana Gaspariani¹, Sihan Yang¹, Anastasia Kiyonaga¹; ¹UC San Diego

The seemingly hidden content of the mind is sometimes expressed in observable eye movements. For instance, systematically biased saccade patterns can reveal the visual stimuli that are either imagined or maintained in working memory. However, it is unclear what underlying signal these saccadic signatures reflect, and how much information is contained in them. Here, we use induced visual hallucinations to probe the boundaries of the mental representations that modulate oculomotor activity. Specifically, exposure to bright and periodically flashing light is known to induce hallucinatory visual percepts – often simple geometric motifs and colored shapes. These hallucinations may provide a valuable window into the visual system, as the experience of more or less complex hallucinations is associated with engagement of distinct cortical substrates as well as individual variability in the reported capacity to generate mental images. Here, participants were exposed to a form of stroboscopic stimulation known as Ganzflicker, where a computer monitor alternated at 7.5 Hz between uniform black and red color fields. During the 10-minute exposure period, participants used a joystick to indicate the onset and offset of any hallucinations they experienced, and we tracked their eye movements continuously. After the exposure was complete, participants provided hand-drawn reports of each unique hallucinated percept, along with open-ended narrative recalls and a battery of self-report measures. We examined both content-general eye movements metrics, like saccade frequency and magnitude, as well as content-specific biases, like the spatial patterning of fixations during hallucinations. We found that saccade frequency gradually ramped up as the onset of a hallucination approached, and also ramped down as hallucination intensity dwindled. Moreover, fixation hotspots during stimulation overlapped with the spatial layout of hallucinated content, as reported in the drawings. Therefore, distinct oculomotor markers track the temporal and spatial properties of experimentally-induced visual hallucinatory percepts.