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Oris Shenyan: Visual hallucinations induced by Ganzflicker and Ganzfeld differ in frequency, complexity, and content

Hallucinations are commonly defined as perception in the absence of external stimuli1,2.

In the case of visual hallucinations, these false percepts can range from simple—such as geometric forms3,4,5, to more complex, such as objects, human figures, or landscapes6,7. As well as occurring in pathological states such as schizophrenia8, Parkinson’s disease9, epilepsy10, migraine11 and vision impairment12, visual hallucinations can also be induced in non-pathological states—through the use of hallucinogens and psychedelics13,14, and via experimental manipulations such as states of sensory15 and perceptual16 deprivation (known as a ‘Ganzfeld’), and the use of flicker over closed17,18, or open19,20 eyes (also referred to as ‘Ganzflicker’ when viewed with open eyes). While experimentally-induced hallucinations are often referred to as ‘pseudo-hallucinations’ due to the participants’ awareness that the hallucinatory experience is not real, the percepts experienced can be very vivid and vision-like19,20. Experimental methods use different means of visual stimulation to induce hallucinations, and it is presently unclear to what extent these different approaches rely on different mechanisms and lead to differences in the nature and complexity of the resulting hallucinations19,20.

To interrogate this, we compared two experimental methods that differ vastly in the degree of bottom-up stimulation they involve: a salient, eyes-open visual flicker at a frequency appropriate for inducing visual hallucination (Ganzflicker), and visual and auditory perceptual deprivation (Ganzfeld). We tested whether increased bottom-up stimulation would alter the frequency and complexity of the hallucinations induced, and also whether the underlying mechanisms of the resulting hallucinations are shared across these methods despite the variations in visual stimulation.

Visual hallucinations can be divided into those of a ‘simple’ or ‘complex’ nature. Simple hallucinations contain abstract content such as colour alterations, elementary shapes, or geometric patterns. During a systematic study of the subjective effects of mescaline21, Heinrich Klüver outlined four key examples of simple hallucinations including tunnels, spirals, honeycombs and cobwebs. These form constants are cross-cultural and present across a multitude of hallucinatory states such as during psychedelic visual imagery22, migraine11, hypnagogia23 and flicker17, suggesting there may be a shared hallucinatory mechanism of relatively low-level origin underlying their occurrence. Indeed, neural network simulations suggest that form constants could arise from increased excitation in early visual cortices. When aberrant waves of excitation spread across early visual cortices, resulting in stripes on visual cortex, the transformation of these stripes from cortical space to retinal would result in characteristic geometric patterns resembling the aforementioned form constants3,4,5.

Complex hallucinations are those containing figurative elements such as faces, objects, or scenes. The representation of complex hallucinations has been associated with higher-order visual regions7,24,25. However, in psychedelic visual hallucinations, complex percepts resembling figurative constructs often also incorporate geometric elements, suggesting that there may not be a binary distinction between simple and complex hallucinations, and that lower-level visual areas may be concurrently active with higher-level visual areas during complex imagery14. This could suggest that simple hallucinations arise when activity is strongest in lower-level areas, whereas complex hallucinations arise when this activity travels up the visual hierarchy, either alone or in conjunction with top-down interpretative influences18,26,27. Another explanation is that complex hallucinations are more akin to mental imagery than to veridical vision20—i.e., they involve a top-down, or retro-hierarchical process27,28,29.

Our aim was to better understand both the nature of simple and complex hallucinations, and their underlying mechanisms. There are challenges in understanding the nature of these experiences, however, due to the difficulty in both eliciting and measuring hallucinations in a controlled laboratory environment with objective and methodologically sound techniques30. For instance, hallucinations experienced in pathological states can be uncommon and unpredictable—especially in clinical or laboratory settings. This is partly demonstrated by the number of single-subject case studies within the field, suggesting the difficulty of recruiting large numbers of patients experiencing visual hallucinations which can be measured on demand31. Similarly, psychedelics elicit a multitude of changes in conscious experience32 that make it difficult to draw conclusions about their specific effects on the visual system and the hallucinatory state. Greater control over the induction of hallucinations can instead be achieved in a laboratory environment using visual (and auditory) stimulation.

Two methods of interest which are known to induce both simple and complex pseudo-hallucinations are high-frequency, eyes-open visual flicker (Ganzflicker) and perceptual deprivation (known as the ‘Ganzfeld’ effect). These methods have different stimulation conditions and have been suggested to rely on different underlying mechanisms17,33,34,35, based on which different predictions can be made surrounding the different types of hallucinations that may be elicited, and their complexity.

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