Processing resources and interplay among sensory modalities: an EEG investigation

The primary aim of the present thesis was to investigate how the human brain handles and distributes limited processing resources among different sensory modalities. Two main hypothesis have been conventionally proposed: (1) common processing resources shared among sensory modalities (supra-modal attentional system) or (2) independent processing resources for each sensory modality. By means of four EEG experiments, we tested whether putative competitive interactions between sensory modalities – regardless of attentional influences – are present in early sensory areas. We observed no competitive interactions between sensory modalities, supporting independent processing resources in early sensory areas. Consequently, we tested the influence of top-down attention on a cross-modal dual task. We found evidence for shared attentional resources between visual and tactile modalities. Taken together, our results point toward a hybrid model of inter-modal attention. Attentional processing resources seem to be controlled by a supra-modal attentional system, however, in early sensory areas, the absence of competitive interactions strongly reduces interferences between sensory modalities, thus providing a strong processing resource independence

Psilocybin Induces Aberrant Prediction Error Processing of Tactile Mismatch Responses—A Simultaneous EEG–FMRI Study

As source of sensory information, the body provides a sense of agency and self/non-self-discrimination. The integration of bodily states and sensory inputs with prior beliefs has been linked to the generation of bodily self-consciousness. The ability to detect surprising tactile stimuli is essential for the survival of an organism and for the formation of mental body representations. Despite the relevance for a variety of psychiatric disorders characterized by altered body and self-perception, the neurobiology of these processes is poorly understood. We therefore investigated the effect of psilocybin (Psi), known to induce alterations in self-experience, on tactile mismatch responses by combining pharmacological manipulations with simultaneous electroencephalography-functional magnetic resonance imaging (EEG-fMRI) recording. Psi reduced activity in response to tactile surprising stimuli in frontal regions, the visual cortex, and the cerebellum. Furthermore, Psi reduced tactile mismatch negativity EEG responses at frontal electrodes, associated with alterations of body- and self-experience. This study provides first evidence that Psi alters the integration of tactile sensory inputs through aberrant prediction error processing and highlights the importance of the 5-HT2A system in tactile deviancy processing as well as in the integration of bodily and self-related stimuli. These findings may have important implications for the treatment of psychiatric disorders characterized by aberrant bodily self-awareness

The Tactile Motion Aftereffect

The tactile motion aftereffect (tMAE) is a perceptual phenomenon in which illusory motion is reported following adaptation to a unidirectionally moving tactile stimulus. Unlike its visual counterpart, relatively little is known about the tMAE. For that reason, the purpose of this dissertation was to gain a better understanding of the tMAE using both psychophysical and neuroimaging techniques. In a series of five experiments the skin was adapted using a plastic cylinder with a square-wave patterned surface. Chapter 2 consists of two experiments, both of which adapted the glabrous surface of the right hand. Experiment 1 showed that the prevalence, duration, and vividness of the tMAE did not differ between the fingers (thumb excluded), palm and fingers (thumb included), and palm and fingers (thumb excluded). Thus, the divergent prevalence rates of two previous studies (Hollins & Favorov, 1994; Lerner & Craig, 1994) cannot be explained by the inclusion of the thumb in the latter study. Experiment 2 showed that as adapting speed increased from 15 to 75 rpm so did the prevalence, duration, and vividness of the tMAE. Previously it has been shown that the tMAE duration increases with adapting duration (Hollins & Favorov, 1994). Given that speed * duration = distance, increasing either adapting speed or duration also increases distance. As such, it was unclear which parameter(s) caused the observed increase in prevalence, duration, and vividness. Chapter 3 manipulated adapting duration (1, 2, and 4 min) and speed (30 and 60 rpm) in the same experiment, thereby allowing the effect of distance to be assessed in the interaction. The results showed that the prevalence, duration, and vividness of the tMAE increased with adapting speed. There was also a positive relationship between adapting duration and prevalence, but not duration or vividness, of the illusion. Distance was only a factor when it came to the tMAE duration. To gain insight into the peripheral neural basis of the tMAE, Chapter 4 measured the prevalence, duration, and vividness of the tMAE on skin areas that differ in their composition of fast adapting (FA) mechanoreceptive units, namely the right cheek, volar surface of the forearm, and glabrous surface of the hand. While there was no difference in duration or vividness between the skin surfaces tested, the tMAE was reported twice as often on the hand than the cheek and forearm, which did not differ significantly from one another. This finding suggests that the tMAE can be induced by adapting FA type I (FA I) units in the glabrous skin (hand) and the hair follicle units (cheek and forearm) and/or the FA I (cheek) and field (forearm) units in the hairy skin. Chapter 5 investigated the central neural basis of the tMAE using functional magnetic resonance imaging (fMRI). Of the areas shown to be responsive to tactile motion on the glabrous surface of the right hand, namely the contralateral (left) thalamus, postcentral gyrus (PCG), and parietal operculum, only the PCG showed evidence of the tMAE; that is, there was a sustained fMRI response following the offset of the illusion trials (cylinder rotating at 60 rpm), but not the control trials (cylinder rotating at 15 rpm), presumably reflecting illusory motion perception. Taken together, the experiments described herein expand our knowledge of the tMAE. Using a cylinder adapting apparatus, it was shown that: prevalence is the best measure of tMAE strength; the tMAE is not as robust as its visual counterpart; adapting duration and speed positively affect the prevalence of the tMAE; the tMAE is twice as prevalent on the glabrous than the hairy skin; the FAI and hair follicle units likely underlie the tMAE; the tMAE is likely caused by adapting direction selective neurons in the contralateral PCG

Top-down effects on early visual processing in humans: a predictive coding framework

An increasing number of human electroencephalography (EEG) studies examining the earliest component of the visual evoked potential, the so-called C1, have cast doubts on the previously prevalent notion that this component is impermeable to top-down effects. This article reviews the original studies that (i) described the C1, (ii) linked it to primary visual cortex (V1) activity, and (iii) suggested that its electrophysiological characteristics are exclusively determined by low-level stimulus attributes, particularly the spatial position of the stimulus within the visual field. We then describe conflicting evidence from animal studies and human neuroimaging experiments and provide an overview of recent EEG and magnetoencephalography (MEG) work showing that initial V1 activity in humans may be strongly modulated by higher-level cognitive factors. Finally, we formulate a theoretical framework for understanding top-down effects on early visual processing in terms of predictive coding

Feeling better:Tactile verbs speed up tactile detection

International audienceEmbodiment of action-related language into the motor system has been extensively documented. Yet the case of sensory words, especially referring to touch, remains overlooked. We investigated the influence of verbs denoting tactile sensations on tactile perception. In Experiment 1, participants detected tactile stimulations on their forearm, preceded by tactile or non-tactile verbs by one of three delays (170, 350, 500ms) reflecting different word processing stages. Results revealed shorter reaction times to tactile stimulations following tactile than non-tactile verbs, irrespective of delay. To ensure that priming pertained to tactile, and not motor, verb properties, Experiment 2 compared the impact of tactile verbs to both action and non-tactile verbs, while stimulations were delivered on the index finger. No priming emerged following action verbs, therefore not supporting the motor-grounded interpretation. Facilitation by tactile verbs was however not observed, possibly owing to methodological changes. Experiment 3, identical to Experiment 2 except that stimulation was delivered to participants’ forearm, replicated the priming effect. Importantly, tactile stimulations were detected faster after tactile than after both non-tactile and action verbs, indicating that verbs’ tactile properties engaged resources shared with sensory perception. Our findings suggest that language conveying tactile information can activate somatosensory representations and subsequently promote tactile detection

Dopamine-induced dissociation of BOLD and neural activity in macaque visual cortex

Neuromodulators determine how neural circuits process information during cognitive states such as wakefulness, attention, learning, and memory [1]. fMRI can provide insight into their function and dynamics, but their exact effect on BOLD responses remains unclear [2, 3 and 4], limiting our ability to interpret the effects of changes in behavioral state using fMRI. Here, we investigated the effects of dopamine (DA) injections on neural responses and haemodynamic signals in macaque primary visual cortex (V1) using fMRI (7T) and intracortical electrophysiology. Aside from DA’s involvement in diseases such as Parkinson’s and schizophrenia, it also plays a role in visual perception [5, 6, 7 and 8]. We mimicked DAergic neuromodulation by systemic injection of L-DOPA and Carbidopa (LDC) or by local application of DA in V1 and found that systemic application of LDC increased the signal-to-noise ratio (SNR) and amplitude of the visually evoked neural responses in V1. However, visually induced BOLD responses decreased, whereas cerebral blood flow (CBF) responses increased. This dissociation of BOLD and CBF suggests that dopamine increases energy metabolism by a disproportionate amount relative to the CBF response, causing the reduced BOLD response. Local application of DA in V1 had no effect on neural activity, suggesting that the dopaminergic effects are mediated by long-range interactions. The combination of BOLD-based and CBF-based fMRI can provide a signature of dopaminergic neuromodulation, indicating that the application of multimodal methods can improve our ability to distinguish sensory processing from neuromodulatory effects

Tactile numerosity is coded in external space

Humans, and several non-human species, possess the ability to make approximate but reliable estimates of the number of objects around them. Alike other perceptual features, numerosity perception is susceptible to adaptation: exposure to a high number of items causes underestimation of the numerosity of a subsequent set of items, and vice versa. Several studies have investigated adaptation in the auditory and visual modality, whereby stimuli are preferentially encoded in an external coordinate system. As tactile stimuli are primarily coded in an internal (body-centered) reference frame, here we ask whether tactile numerosity adaptation operates based on internal or external spatial coordinates as it occurs in vision or audition. Twenty participants performed an adaptation task with their right hand located either in the right (uncrossed) or left (crossed) hemispace. Tactile adaptor and test stimuli were passively delivered either to the same (adapted) or different (non-adapted) hands. Our results show a pattern of over- and under-estimation according to the rate of adaptation (low and high, respectively). In the uncrossed position, we observed stronger adaptation effects when adaptor and test stimuli were delivered to the “adapted” hand. However, when both hands were aligned in the same spatial position (crossed condition), the magnitude of adaptation was similar irrespective of which hand received adaptor and test stimuli. These results demonstrate that numerosity information is automatically coded in external coordinates even in the tactile modality, suggesting that such a spatial reference frame is an intrinsic property of numerosity processing irrespective of the sensory modality