Flicker-light induced visual phenomena: frequency dependence and specificity of whole percepts and percept features

Real-time fMRI neurofeedback modulates induced hallucinations and underlying brain mechanisms

Hallucinations can occur in the healthy population, are clinically relevant and frequent symptoms in many neuropsychiatric conditions, and have been shown to mark disease progression in patients with neurodegenerative disorders where antipsychotic treatment remains challenging. Here, we combine MR-robotics capable of inducing a clinically-relevant hallucination, with real-time fMRI neurofeedback (fMRI-NF) to train healthy individuals to up-regulate a fronto-parietal brain network associated with the robotically-induced hallucination. Over three days, participants learned to modulate occurrences of and transition probabilities to this network, leading to heightened sensitivity to induced hallucinations after training. Moreover, participants who became sensitive and succeeded in fMRI-NF training, showed sustained and specific neural changes after training, characterized by increased hallucination network occurrences during induction and decreased hallucination network occurrences during a matched control condition. These data demonstrate that fMRI-NF modulates specific hallucination network dynamics and highlights the potential of fMRI-NF as a novel antipsychotic treatment in neurodegenerative disorders and schizophrenia.

Flicker light stimulation enhances the emotional response to music: a comparison study to the effects of psychedelics

Flicker light stimulation (FLS) is a non-pharmacological method of inducing altered states of consciousness (ASCs), producing hallucination-like phenomena as well as effects extending beyond the visual modality, including emotional effects. Research into the psychological and neural mechanisms of FLS is still in its infancy, but can be informed by research into other methods of inducing ASCs. For instance, research on classic psychedelics has reported enhancement of emotional responses to music. Here, we test to what degree FLS might also enhance the emotional response to music, using a study protocol designed to resemble a previous study on the effects of LSD as closely as possible, to allow for comparison of effect sizes across modalities and inform future research into FLS as an ASC-induction method. Twenty participants listened to emotionally evocative music in two conditions - with and without FLS - and reported on their emotional response to the music. FLS showed a significant enhancing effect on reported music-evoked emotion, especially emotions relating to "Joyful Activation"; additionally, we found that the experienced intensity of FLS correlated with reports of higher levels of emotional arousal. These findings motivate further research into FLS as a method for inducing ASCs and into the interactions between visual phenomena and music-evoked emotion.

A novel model of divergent predictive perception

Predictive processing theories state that our subjective experience of reality is shaped by a balance of expectations based on previous knowledge about the world (i.e. priors) and confidence in sensory input from the environment. Divergent experiences (e.g. hallucinations and synaesthesia) are likely to occur when there is an imbalance between one's reliance on priors and sensory input. In a novel theoretical model, inspired by both predictive processing and psychological principles, we propose that predictable divergent experiences are associated with natural or environmentally induced prior/sensory imbalances: inappropriately strong or inflexible (i.e. maladaptive) high-level priors (beliefs) combined with low sensory confidence can result in reality discrimination issues, a characteristic of psychosis; maladaptive low-level priors (sensory expectations) combined with high sensory confidence can result in atypical sensory sensitivities and persistent divergent percepts, a characteristic of synaesthesia. Crucially, we propose that whether different divergent experiences manifest with dominantly sensory (e.g. hallucinations) or nonsensory characteristics (e.g. delusions) depends on mental imagery ability, which is a spectrum from aphantasia (absent or weak imagery) to hyperphantasia (extremely vivid imagery). We theorize that imagery is critically involved in shaping the sensory richness of divergent perceptual experience. In sum, to predict a range of divergent perceptual experiences in both clinical and general populations, three factors must be accounted for: a maladaptive use of priors, individual level of confidence in sensory input, and mental imagery ability. These ideas can be expressed formally using nonparametric regression modeling. We provide evidence for our theory from previous work and deliver predictions for future research.

Robotically-induced auditory-verbal hallucinations: combining self-monitoring and strong perceptual priors

BACKGROUND

Inducing hallucinations under controlled experimental conditions in non-hallucinating individuals represents a novel research avenue oriented toward understanding complex hallucinatory phenomena, avoiding confounds observed in patients. Auditory-verbal hallucinations (AVH) are one of the most common and distressing psychotic symptoms, whose etiology remains largely unknown. Two prominent accounts portray AVH either as a deficit in auditory-verbal self-monitoring, or as a result of overly strong perceptual priors.

METHODS

In order to test both theoretical models and evaluate their potential integration, we developed a robotic procedure able to induce self-monitoring perturbations (consisting of sensorimotor conflicts between poking movements and corresponding tactile feedback) and a perceptual prior associated with otherness sensations (i.e. feeling the presence of a non-existing another person).

RESULTS

Here, in two independent studies, we show that this robotic procedure led to AVH-like phenomena in healthy individuals, quantified as an increase in false alarm rate in a voice detection task. Robotically-induced AVH-like sensations were further associated with delusional ideation and to both AVH accounts. Specifically, a condition with stronger sensorimotor conflicts induced more AVH-like sensations (self-monitoring), while, in the otherness-related experimental condition, there were more AVH-like sensations when participants were detecting other-voice stimuli, compared to detecting self-voice stimuli (strong-priors).

CONCLUSIONS

By demonstrating an experimental procedure able to induce AVH-like sensations in non-hallucinating individuals, we shed new light on AVH phenomenology, thereby integrating self-monitoring and strong-priors accounts.

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

Visual hallucinations can be phenomenologically divided into those of a simple or complex nature. Both simple and complex hallucinations can occur in pathological and non-pathological states, and can also be induced experimentally by visual stimulation or deprivation-for example using a high-frequency, eyes-open flicker (Ganzflicker) and perceptual deprivation (Ganzfeld). Here we leverage the differences in visual stimulation that these two techniques involve to investigate the role of bottom-up and top-down processes in shifting the complexity of visual hallucinations, and to assess whether these techniques involve a shared underlying hallucinatory mechanism despite their differences. For each technique, we measured the frequency and complexity of the hallucinations produced, utilising button presses, retrospective drawing, interviews, and questionnaires. For both experimental techniques, simple hallucinations were more common than complex hallucinations. Crucially, we found that Ganzflicker was more effective than Ganzfeld at eliciting simple hallucinations, while complex hallucinations remained equivalent across the two conditions. As a result, the likelihood that an experienced hallucination was complex was higher during Ganzfeld. Despite these differences, we found a correlation between the frequency and total time spent hallucinating in Ganzflicker and Ganzfeld conditions, suggesting some shared mechanisms between the two methodologies. We attribute the tendency to experience frequent simple hallucinations in both conditions to a shared low-level core hallucinatory mechanism, such as excitability of visual cortex, potentially amplified in Ganzflicker compared to Ganzfeld due to heightened bottom-up input. The tendency to experience complex hallucinations, in contrast, may be related to top-down processes less affected by visual stimulation.

Effect of frequency and rhythmicity on flicker light-induced hallucinatory phenomena

Flicker light stimulation (FLS) uses stroboscopic light on closed eyes to induce transient visual hallucinatory phenomena, such as the perception of geometric patterns, motion, and colours. It remains an open question where the neural correlates of these hallucinatory experiences emerge along the visual pathway. To allow future testing of suggested underlying mechanisms (e.g., changes in functional connectivity, neural entrainment), we sought to systematically characterise the effects of frequency (3 Hz, 8 Hz, 10 Hz and 18 Hz) and rhythmicity (rhythmic and arrhythmic conditions) on flicker-induced subjective experiences. Using a novel questionnaire, we found that flicker frequency and rhythmicity significantly influenced the degree to which participants experienced simple visual hallucinations, particularly the perception of Klüver forms and dynamics (e.g., motion). Participants reported their experience of geometric patterns and dynamics was at highest intensity during 10 Hz rhythmic stimulation. Further, we found that frequency-matched arrhythmic FLS strongly reduced these subjective effects compared to equivalent rhythmic stimulation. Together, these results provide evidence that flicker rhythmicity critically contributes to the effects of FLS beyond the effects of frequency alone, indicating that neural entrainment may drive the induced phenomenal experience.

Neuroscience robotics for controlled induction and real-time assessment of hallucinations

Although hallucinations are important and frequent symptoms in major psychiatric and neurological diseases, little is known about their brain mechanisms. Hallucinations are unpredictable and private experiences, making their investigation, quantification and assessment highly challenging. A major shortcoming in hallucination research is the absence of methods able to induce specific and short-lasting hallucinations, which resemble clinical hallucinations, can be elicited repeatedly and vary across experimental conditions. By integrating clinical observations and recent advances in cognitive neuroscience with robotics, we have designed a novel device and sensorimotor method able to repeatedly induce a specific, clinically relevant hallucination: presence hallucination. Presence hallucinations are induced by applying specific conflicting (spatiotemporal) sensorimotor stimulation including an upper extremity and the torso of the participant. Another, MRI-compatible, robotic device using similar sensorimotor stimulation permitted the identification of the brain mechanisms of these hallucinations. Enabling the identification of behavioral and a frontotemporal neural biomarkers of hallucinations, under fully controlled experimental conditions and in real-time, this method can be applied in healthy participants as well as patients with schizophrenia, neurodegenerative disease or other hallucinations. The execution of these protocols requires intermediate-level skills in cognitive neuroscience and MRI processing, as well as minimal coding experience to control the robotic device. These protocols take ~3 h to be completed.

Phosphene Attributes Depend on Frequency and Intensity of Retinal tACS

Phosphene is the experience of light without natural visual stimulation. It can be induced by electrical stimulation of the retina, optic nerve or cortex. Induction of phosphenes can be potentially used in assistive devices for the blind. Analysis of phosphene might be beneficial for practical reasons such as adjustment of transcranial alternating current stimulation (tACS) frequency and intensity to eliminate phosphene perception (e.g., tACS studies using verum tACS group and sham group) or, on the contrary, to maximize perception of phosphenes in order to be more able to study their dynamics. In this study, subjective reports of 50 healthy subjects exposed to different intensities of retinal tACS at 4 different frequencies (6, 10, 20 and 40 Hz) were analyzed. The effectiveness of different tACS frequencies in inducing phosphenes was at least 92 %. Subject reported 41 different phosphene types; the most common were light flashes and light circles. Changing the intensity of stimulation often induced a change in phosphene attributes. Up to nine phosphene attributes changed when the tACS intensity was changed. Significant positive correlation was observed between number of a different phosphene types and tACS frequency. Based on these findings, it can be concluded that tACS is effective in eliciting phosphenes whose type and attributes change depending on the frequency and intensity of tACS. The presented results open new questions for future research.

Evidences of emerging pain consciousness during prenatal development: a narrative review

BACKGROUND

The study of consciousness has always been considered a challenge for neonatologists, even more when considering the uterine period. Our review aimed to individuate at what gestational age the fetus, which later became a premature infant, can feel the perception of external stimuli. Therefore, the aim of our review was to study the onset of consciousness during the fetal life.

MATERIALS AND METHODS

A literature search was performed in Medline-PubMed database. We included all papers found with the following MeSH words: "consciousness or cognition or awareness or comprehension or cognitive or consciousness of pain" in combination with "embryo or fetus or fetal life or newborn." Studies were selected if titles and/or abstracts suggested an association between formation of consciousness (the basics of neurodevelopment) and preterm infant or fetus. Titles and abstracts were first screened by three independent reviewers according to Cochrane Collaboration's recommendations.

RESULTS

From the literature review, we found only 8 papers describing the onset of consciousness in the transition period from fetus to premature newborn. Therefore, according to these papers, we temporally analyzed the formation of the thalamocortical connections that are the basis of consciousness.

CONCLUSIONS

We can conclude that from a neuroanatomical point of view, it is rather unlikely that the infant can be seen as a conscious human before 24 weeks of gestational age, thus before all the thalamocortical connections are established. Further literature data have to confirm this hypothesis.

Electrokinesis hypothesis of neuron selection for synapse formation and pruning

What neurons are connected and what neurons are disconnected? These are crucial questions since they determine the structural connectivity of the brain. We address this matter in question and propose a speculative hypothesis. We claim that the neuron selection for synapse formation and pruning process is an electrokinesis-based stochastic physical process. Propagating input signals generate a varying electromagnetic field in the interstitial fluid, which is filled with ions. Varying electromagnetic field exerts Lorentz force on the ions and causes local fluid flows. These fluid flows alter the position of presynaptic boutons and dendritic spines, and hence the likelihood of making contact. Above a certain level of field strength, the likelihood of pruning increases. If the proposed hypothesis is correct, in addition to explaining the natural process of neuron selection for synapse formation and pruning, it can also explain why and why not brain stimulation techniques work. Additionally, it will provide a controllable mechanism to alter brain connectivity as desired, which may have a profound impact on many connectivity-based psychiatric and neurological diseases.

Anomalous visual experience is linked to perceptual uncertainty and visual imagery vividness

An imbalance between top-down and bottom-up processing on perception (specifically, over-reliance on top-down processing) can lead to anomalous perception, such as illusions. One factor that may be involved in anomalous perception is visual mental imagery, which is the experience of "seeing" with the mind's eye. There are vast individual differences in self-reported imagery vividness, and more vivid imagery is linked to a more sensory-like experience. We, therefore, hypothesized that susceptibility to anomalous perception is linked to individual imagery vividness. To investigate this, we adopted a paradigm that is known to elicit the perception of faces in pure visual noise (pareidolia). In four experiments, we explored how imagery vividness contributes to this experience under different response instructions and environments. We found strong evidence that people with more vivid imagery were more likely to see faces in the noise, although removing suggestive instructions weakened this relationship. Analyses from the first two experiments led us to explore confidence as another factor in pareidolia proneness. We, therefore, modulated environment noise and added a confidence rating in a novel design. We found strong evidence that pareidolia proneness is correlated with uncertainty about real percepts. Decreasing perceptual ambiguity abolished the relationship between pareidolia proneness and both imagery vividness and confidence. The results cannot be explained by incidental face-like patterns in the noise, individual variations in response bias, perceptual sensitivity, subjective perceptual thresholds, viewing distance, testing environments, motivation, gender, or prosopagnosia. This indicates a critical role of mental imagery vividness and perceptual uncertainty in anomalous perceptual experience.

Altered states phenomena induced by visual flicker light stimulation

Flicker light stimulation can induce short-term alterations in consciousness including hallucinatory color perception and geometric patterns. In the study at hand, the subjective experiences during 3 Hz and 10 Hz stroboscopic light stimulation of the closed eyes were assessed. In a within-subjects design (N = 24), we applied the Positive and Negative Affect Schedule (mood state), time perception ratings, the Altered State of Consciousness Rating Scale, and the Phenomenology of Consciousness Inventory. Furthermore, we tested for effects of personality traits (NEO Five-Factor Inventory-2 and Tellegen Absorption Scale) on subjective experiences. Such systematic quantification improves replicability, facilitates comparisons between pharmacological and non-pharmacological techniques to induce altered states of consciousness, and is the prerequisite to study their underlying neuronal mechanisms. The resulting data showed that flicker light stimulation-induced states were characterized by vivid visual hallucinations of simple types, with effects strongest in the 10 Hz condition. Additionally, participants' personality trait of Absorption scores highly correlated with the experienced alterations in consciousness. Our data demonstrate that flicker light stimulation is capable of inducing visual effects with an intensity rated to be similar in strength to effects induced by psychedelic substances and thereby support the investigation of potentially shared underlying neuronal mechanisms.

The Ganzflicker experience: High probability of seeing vivid and complex pseudo-hallucinations with imagery but not aphantasia

There are considerable individual differences in visual mental imagery ability across the general population, including a "blind mind's eye", or aphantasia. Recent studies have shown that imagery is linked to differences in perception in the healthy population, and clinical work has found a connection between imagery and hallucinatory experiences in neurological disorders. However, whether imagery ability is associated with anomalous perception-including hallucinations-in the general population remains unclear. In the current study, we explored the relationship between imagery ability and the anomalous perception of pseudo-hallucinations (PH) using rhythmic flicker stimulation ("Ganzflicker"). Specifically, we investigated whether the ability to generate voluntary imagery is associated with susceptibility to flicker-induced PH. We additionally explored individual differences in observed features of PH. We recruited a sample of people with aphantasia (aphants) and imagery (imagers) to view a constant red-and-black flicker for approximately 10 min. We found that imagers were more susceptible to PH, and saw more complex and vivid PH, compared to aphants. This study provides the first evidence that the ability to generate visual imagery increases the likelihood of experiencing complex and vivid anomalous percepts.

Hallucinations on demand: the utility of experimentally induced phenomena in hallucination research

Despite the desire to delve deeper into hallucinations of all types, methodological obstacles have frustrated development of more rigorous quantitative experimental techniques, thereby hampering research progress. Here, we discuss these obstacles and, with reference to visual phenomena, argue that experimentally induced phenomena (e.g. hallucinations induced by flickering light and classical conditioning) can bring hallucinations within reach of more objective behavioural and neural measurement. Expanding the scope of hallucination research raises questions about which phenomena qualify as hallucinations, and how to identify phenomena suitable for use as laboratory models of hallucination. Due to the ambiguity inherent in current hallucination definitions, we suggest that the utility of phenomena for use as laboratory hallucination models should be represented on a continuous spectrum, where suitability varies with the degree to which external sensory information constrains conscious experience. We suggest that existing strategies that group pathological hallucinations into meaningful subtypes based on hallucination characteristics (including phenomenology, disorder and neural activity) can guide extrapolation from hallucination models to other hallucinatory phenomena. Using a spectrum of phenomena to guide scientific hallucination research should help unite the historically separate fields of psychophysics, cognitive neuroscience and clinical research to better understand and treat hallucinations, and inform models of consciousness. This article is part of the theme issue 'Offline perception: voluntary and spontaneous perceptual experiences without matching external stimulation'.

Sensory dynamics of visual hallucinations in the normal population

Hallucinations occur in both normal and clinical populations. Due to their unpredictability and complexity, the mechanisms underlying hallucinations remain largely untested. Here we show that visual hallucinations can be induced in the normal population by visual flicker, limited to an annulus that constricts content complexity to simple moving grey blobs, allowing objective mechanistic investigation. Hallucination strength peaked at ~11 Hz flicker and was dependent on cortical processing. Hallucinated motion speed increased with flicker rate, when mapped onto visual cortex it was independent of eccentricity, underwent local sensory adaptation and showed the same bistable and mnemonic dynamics as sensory perception. A neural field model with motion selectivity provides a mechanism for both hallucinations and perception. Our results demonstrate that hallucinations can be studied objectively, and they share multiple mechanisms with sensory perception. We anticipate that this assay will be critical to test theories of human consciousness and clinical models of hallucination.

Human Visual Cortex Responses to Rapid Cone and Melanopsin-Directed Flicker

Signals from cones are recombined in postreceptoral channels [luminance, L + M; red-green, L - M; blue-yellow, S - (L + M)]. The melanopsin-containing retinal ganglion cells are also active at daytime light levels and recent psychophysical results suggest that melanopsin contributes to conscious vision in humans. Here, we measured BOLD fMRI responses to spectral modulations that separately targeted the postreceptoral cone channels and melanopsin. Responses to spatially uniform (27.5° field size, central 5° obscured) flicker at 0.5, 1, 2, 4, 8, 16, 32, and 64 Hz were recorded from areas V1, V2/V3, motion-sensitive area MT, and the lateral occipital complex. In V1 and V2/V3, higher temporal sensitivity was observed to L + M + S (16 Hz) compared with L - M flicker (8 Hz), consistent with psychophysical findings. Area MT was most sensitive to rapid (32 Hz) flicker of either L + M + S or L - M. We found S cone responses only in areas V1 and V2/V3 (peak frequency: 4-8 Hz). In addition, we studied an L + M modulation and found responses that were effectively identical at all temporal frequencies to those recorded for the L + M + S modulation. Finally, we measured the cortical response to melanopsin-directed flicker and compared this response with control modulations that addressed stimulus imprecision and the possibility of stimulation of cones in the shadow of retinal blood vessels (penumbral cones). For our stimulus conditions, melanopsin flicker did not elicit a cortical response exceeding that of the control modulations. We note that failure to control for penumbral cone stimulation could be mistaken for a melanopsin response.

SIGNIFICANCE STATEMENT

The retina contains cone photoreceptors and ganglion cells that contain the photopigment melanopsin. Cones provide brightness and color signals to visual cortex. Melanopsin influences circadian rhythm and the pupil, but its contribution to cortex and perception is less clear. We measured the response of human visual cortex with fMRI using spectral modulations tailored to stimulate the cones and melanopsin separately. We found that cortical responses to cone signals vary systematically across visual areas. Differences in temporal sensitivity for achromatic, red-green, and blue-yellow stimuli generally reflect the known perceptual properties of vision. We found that melanopsin signals do not produce a measurable response in visual cortex at temporal frequencies between 0.5 and 64 Hz at daytime light levels.

A bidirectional link between brain oscillations and geometric patterns

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 (<10 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.

Selective stimulation of penumbral cones reveals perception in the shadow of retinal blood vessels

In 1819, Johann Purkinje described how a moving light source that displaces the shadow of the retinal blood vessels to adjacent cones can produce the entopic percept of a branching tree. Here, we describe a novel method for producing a similar percept. We used a device that mixes 56 narrowband primaries under computer control, in conjunction with the method of silent substitution, to present observers with a spectral modulation that selectively targeted penumbral cones in the shadow of the retinal blood vessels. Such a modulation elicits a clear Purkinje-tree percept. We show that the percept is specific to penumbral L and M cone stimulation and is not produced by selective penumbral S cone stimulation. The Purkinje-tree percept was strongest at 16 Hz and fell off at lower (8 Hz) and higher (32 Hz) temporal frequencies. Selective stimulation of open-field cones that are not in shadow, with penumbral cones silenced, also produced the percept, but it was not seen when penumbral and open-field cones were modulated together. This indicates the need for spatial contrast between penumbral and open-field cones to create the Purkinje-tree percept. Our observation provides a new means for studying the response of retinally stabilized images and demonstrates that penumbral cones can support spatial vision. Further, the result illustrates a way in which silent substitution techniques can fail to be silent. We show that inadvertent penumbral cone stimulation can accompany melanopsin-directed modulations that are designed only to silence open-field cones. This in turn can result in visual responses that might be mistaken as melanopsin-driven.

Recent advances in the neuropsychopharmacology of serotonergic hallucinogens

Serotonergic hallucinogens, such as (+)-lysergic acid diethylamide, psilocybin, and mescaline, are somewhat enigmatic substances. Although these drugs are derived from multiple chemical families, they all produce remarkably similar effects in animals and humans, and they show cross-tolerance. This article reviews the evidence demonstrating the serotonin 5-HT2A receptor is the primary site of hallucinogen action. The 5-HT2A receptor is responsible for mediating the effects of hallucinogens in human subjects, as well as in animal behavioral paradigms such as drug discrimination, head twitch response, prepulse inhibition of startle, exploratory behavior, and interval timing. Many recent clinical trials have yielded important new findings regarding the psychopharmacology of these substances. Furthermore, the use of modern imaging and electrophysiological techniques is beginning to help unravel how hallucinogens work in the brain. Evidence is also emerging that hallucinogens may possess therapeutic efficacy.

Parametric alpha- and beta-band signatures of supramodal numerosity information in human working memory

Numerosity can be assessed by analog estimation, similar to a continuous magnitude, or by discrete quantification of the individual items in a set. While the extent to which these two processes rely on common neural mechanisms remains debated, recent studies of sensory working memory (WM) have identified an oscillatory signature of continuous magnitude information, in terms of quantitative modulations of prefrontal upper beta activity (20-30 Hz). Here, we examined how such parametric oscillatory WM activity may also reflect the abstract assessment of the numerosity of discrete items. We recorded EEG while participants (n = 24) processed the number of stimulus pulses presented in the visual, auditory, or tactile modality, under otherwise identical experimental conditions. Behavioral response profiles showed varying degrees of analog estimation and of discretized quantification in the different modalities. During sustained processing in WM, the amplitude of posterior alpha oscillations (8-13 Hz) reflected the increased memory load associated with maintaining larger sets of discrete items. In contrast, earlier numerosity-dependent modulations of right prefrontal upper beta (20-30 Hz) specifically reflected the extent to which numerosity was assessed by analog estimation, both between and within presentation modalities. Together, the analog approximation-but not the discretized representation-of numerosity information exhibited a parametric oscillatory signature akin to a continuous sensory magnitude. The results suggest dissociable oscillatory mechanisms of abstract numerosity integration, at a supramodal processing stage in human WM.

Supramodal parametric working memory processing in humans

Previous studies of delayed-match-to-sample (DMTS) frequency discrimination in animals and humans have succeeded in delineating the neural signature of frequency processing in somatosensory working memory (WM). During retention of vibrotactile frequencies, stimulus-dependent single-cell and population activity in prefrontal cortex was found to reflect the task-relevant memory content, whereas increases in occipital alpha activity signaled the disengagement of areas not relevant for the tactile task. Here, we recorded EEG from human participants to determine the extent to which these mechanisms can be generalized to frequency retention in the visual and auditory domains. Subjects performed analogous variants of a DMTS frequency discrimination task, with the frequency information presented either visually, auditorily, or by vibrotactile stimulation. Examining oscillatory EEG activity during frequency retention, we found characteristic topographical distributions of alpha power over visual, auditory, and somatosensory cortices, indicating systematic patterns of inhibition and engagement of early sensory areas, depending on stimulus modality. The task-relevant frequency information, in contrast, was found to be represented in right prefrontal cortex, independent of presentation mode. In each of the three modality conditions, parametric modulations of prefrontal upper beta activity (20-30 Hz) emerged, in a very similar manner as recently found in vibrotactile tasks. Together, the findings corroborate a view of parametric WM as supramodal internal scaling of abstract quantity information and suggest strong relevance of previous evidence from vibrotactile work for a more general framework of quantity processing in human working memory.

The dynamics of visual experience, an EEG study of subjective pattern formation

Background

Since the origin of psychological science a number of studies have reported visual pattern formation in the absence of either physiological stimulation or direct visual-spatial references. Subjective patterns range from simple phosphenes to complex patterns but are highly specific and reported reliably across studies.

Methodology/Principal Findings

Using independent-component analysis (ICA) we report a reduction in amplitude variance consistent with subjective-pattern formation in ventral posterior areas of the electroencephalogram (EEG). The EEG exhibits significantly increased power at delta/theta and gamma-frequencies (point and circle patterns) or a series of high-frequency harmonics of a delta oscillation (spiral patterns).

Conclusions/Significance

Subjective-pattern formation may be described in a way entirely consistent with identical pattern formation in fluids or granular flows. In this manner, we propose subjective-pattern structure to be represented within a spatio-temporal lattice of harmonic oscillations which bind topographically organized visual-neuronal assemblies by virtue of low frequency modulation.

A model for the origin and properties of flicker-induced geometric phosphenes

We present a model for flicker phosphenes, the spontaneous appearance of geometric patterns in the visual field when a subject is exposed to diffuse flickering light. We suggest that the phenomenon results from interaction of cortical lateral inhibition with resonant periodic stimuli. We find that the best temporal frequency for eliciting phosphenes is a multiple of intrinsic (damped) oscillatory rhythms in the cortex. We show how both the quantitative and qualitative aspects of the patterns change with frequency of stimulation and provide an explanation for these differences. We use Floquet theory combined with the theory of pattern formation to derive the parameter regimes where the phosphenes occur. We use symmetric bifurcation theory to show why low frequency flicker should produce hexagonal patterns while high frequency produces pinwheels, targets, and spirals.