What Geometric Visual Hallucinations Tell Us about the Visual Cortex

Abstract

Many observers see geometric visual hallucinations after taking hallucinogens such as LSD, cannabis, mescaline or psilocybin; on viewing bright flickering lights; on waking up or falling asleep; in "near-death" experiences; and in many other syndromes. Klüver organized the images into four groups called form constants: (I) tunnels and funnels, (II) spirals, (III) lattices, including honeycombs and triangles, and (IV) cobwebs. In most cases, the images are seen in both eyes and move with them. We interpret this to mean that they are generated in the brain. Here, we sum- nectivity between hypercolumns exhibits symmetries, rendering it invariant under the action of the Euclidean group E(2), composed of reflections and translations in the plane, and a (novel) shift-twist action. Using this symmetry, we show that the various patterns of activity that spontaneously emerge when V1's spatially uniform resting state becomes unstable correspond to the form constants when transformed to the visual field using the retino-cortical map. The results are sensitive to the detailed specification of the lateral connectivity and suggest that the cortical mechanisms that generate geometric visual hallucinations are closely related to those used to process edges, contours, surfaces, and textures.

Effects of the anesthetic agent propofol on neural populations

“…This contrasts to the Liley-model, that considers a volume conduction mechanism for the activity spread along the axonal branch. It has been shown in previous theoretical studies that the choice of the axonal connection probability functions can significantly alter spatio-temporal dynamics of the neural population (Hutt 2008;Hutt and Atay 2005;Laing and Troy 2003;Bressloff 2001;Bressloff et al 2002;Coombes 2005). This model of axonal activity spread has been shown to extend the damped activity wave considered in the model of Liley et al Hutt 2007) to nonlocal interactions.…”

Neural interactions between flicker-induced self-organized visual hallucinations and physical stimuli

“…Spontaneous percepts have preferred spatial and temporal scales, which can be manipulated by stimulus and cognitive conditions. Fan-shaped mescaline hallucinations have a spatial frequency of up to Ϸ15 features per hemifield, suggesting a spatial scale of Ϸ2 mm on primary visual cortex, comparable to the size of an orientation hypercolumn (15,19,20). This spatial scale can be altered by physical interactions; e.g., the illusions driven by fractal noises or apparent motion are highly dependent on the spatial and temporal statistics of the noise or the speed of the apparent movement.…”

“…(ii) If so, can they interact with neural activity evoked by physical stimuli to affect perception? To tackle these questions we consider stereotyped geometric hallucinations, often triggered by migraine, drug intoxications, and empty-field flicker (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20); these phenomena are thought to arise from autonomous activity in visual cortex. Although little is known about how spontaneous activity affects the perception of physical stimuli, it is possible to approach the second question from another direction: How do physical stimuli affect perception of spontaneous activity?…”

“…1). We regard these hallucinations as spontaneous states because they are apparently self-organized by visual cortex (14)(15)(16)(17)(18)(19)(20) and can arise without visual stimulation; these perceptible states may be analogous to Kenet et al's (3) subliminal states (9). Their geometric forms are predicted by neural network models that generate stripe patterns of neural activity whose spatial orientation on cortex combines with the nonlinear retinocortical mapping to determine the perceptual effect (14)(15)(16)(17)(18)(19)(20) (Fig.…”

Theoretical understanding of the early visual processes by data compression and data selection

“…Equations (11) and (12) specify how the activities are initialized by external inputs and then modified by the contextual influences via the neural connections. This model (Li 1999a) has a translation invariant structure, such that all neurons of the same type have the same properties, and the neural connections J iθ,jθ (or W iθ,jθ ) have the same structure from all the pre-synaptic neuron jθ except for a translation and rotation to suit jθ (Bressloff et al 2002). The dynamics of this model are such that (1) model response does not spontaneously break input translation symmetry when I iθ is independent of i (otherwise, the model would hal- lucinate salient locations when there is none); (2) when the inputs are not translation invariant, the model manifests these variant locations by response highlights whose magnitudes reflect, with sufficient sensitivity, the degrees of input variances; and (3) particularly the phenomena of iso-orientation suppression and colinear facilitation, etc outlined in section (3).…”