Bifurcation study of a neural field competition model with an application to perceptual switching in motion integration.

Perceptual multistability is a phenomenon in which alternate interpretations of a fixed stimulus are perceived intermittently. Although correlates between activity in specific cortical areas and perception have been found, the complex patterns of activity and the underlying mechanisms that gate multistable perception are little understood. Here, we present a neural field competition model in which competing states are represented in a continuous feature space. Bifurcation analysis is used to describe the different types of complex spatio-temporal dynamics produced by the model in terms of several parameters and for different inputs. The dynamics of the model was then compared to human perception investigated psychophysically during long presentations of an ambiguous, multistable motion pattern known as the barberpole illusion. In order to do this, the model is operated in a parameter range where known physiological response properties are reproduced whilst also working close to bifurcation. The model accounts for characteristic behaviour from the psychophysical experiments in terms of the type of switching observed and changes in the rate of switching with respect to contrast. In this way, the modelling study sheds light on the underlying mechanisms that drive perceptual switching in different contrast regimes. The general approach presented is applicable to a broad range of perceptual competition problems in which spatial interactions play a role

Physiological Mechanisms Underlying Motion-Induced Blindness

Visual disappearance illusions - such as motion-induced blindness (MIB) - are commonly used to study the neural underpinnings of visual perception. In such illusions a salient visual target becomes perceptually invisible. Previous studies are inconsistent regarding the role of primary visual cortex (V1) in these illusions. Here we provide physiological and psychophysical evidence supporting a role for V1 in generating MIB

Brain maturation during adolescence and young adulthood - an EEG study

Objective: Adolescence is a period of major maturational changes in the brain. It is particularly the maturation of the frontal cortex and its interactions with other brain regions that dominates this late stage of development and has important implications for the cognitive abilities during the entire adult life. Methods: In this thesis, developmental changes in the delta (0 - 4 Hz), theta (4 - 7 Hz) and gamma (28 - 48 Hz) frequency bands were investigated in adolescents (13 - 15 years) and adults (20 - 27 years) during resting and cognitive states. Four tasks were conducted: the spontaneous EEG (eyes-open), the ambiguous task, the unambiguous task and the visual oddball task. During the ambiguous task, participants were exposed to the Stroboscopic Ambiguous Motion (SAM) paradigm. SAM is a multistable stimulus that, when viewed continuously, induces internally-generated changes in the perceived direction of motion. For the unambiguous task, a modified version of the SAM was used. In this case, the perceived direction of motion was controlled externally. In the visual oddball task, randomly interspersed targets had to be identified in a train of frequent non-targets. Single-trial amplitudes of delta, theta and gamma activity obtained during the spontaneous EEG were compared between the groups to assess general maturational changes in the developing brain. The task-related enhancement of the oscillatory activity was compared between the groups in order to assess developmental changes in task performance. Results: The previous findings of delta and theta amplitudes decreasing with age during the spontaneous EEG were replicated. Similarly to previous studies, our results also indicate that for spontaneous EEG gamma amplitudes do not profoundly differ between adolescents and adults. Moreover, the present study supports previous findings from adult research which have established a functional relationship between delta, theta and gamma activities and higher cognitive processes. It has further been found that the developmental changes correlated with task difficulty. Only for the - most challenging - ambiguous task the gamma oscillations were found to reflect a change in task-related cortical processing during adolescence, whereas no age-related differences of gamma responses were detected for less demanding tasks (the unambiguous and visual oddball tasks). The event-related theta responses were found to increase with age for the ambiguous and visual oddball tasks and did not depend on age for the unambiguous task. Furthermore, the event-related delta responses increased with age for all tasks. Conclusions: The main finding of this study is that functional networks of delta, theta and gamma activity undergo maturational changes during adolescence. The found differences in the task-related activations may indicate a protracted development of higher-order cognitive processes during adolescence. The developmental changes of task-related activations seem to vary with task difficulty and frequency band. Significance: Systematic studies on developmental changes of brain oscillations in cognitive tasks are still infrequent and specifically the time period of adolescence has been rarely investigated. The typical age of onset for mental disorders, such as schizophrenia, is the late stages or shortly after the onset of adolescence, rendering the outcome of brain maturation during this period of immense importance for life-long mental health. Thus, it is critical to extend our understanding of the mechanism behind the appearance of cognitive functions during adolescence

Feedback information transfer in the human brain reflects bistable perception in the absence of report

In the search for the neural basis of conscious experience, perception and the cognitive processes associated with reporting perception are typically confounded as neural activity is recorded while participants explicitly report what they experience. Here, we present a novel way to disentangle perception from report using eye movement analysis techniques based on convolutional neural networks and neurodynamical analyses based on information theory. We use a bistable visual stimulus that instantiates two well-known properties of conscious perception: integration and differentiation. At any given moment, observers either perceive the stimulus as one integrated unitary object or as two differentiated objects that are clearly distinct from each other. Using electroencephalography, we show that measures of integration and differentiation based on information theory closely follow participants' perceptual experience of those contents when switches were reported. We observed increased information integration between anterior to posterior electrodes (front to back) prior to a switch to the integrated percept, and higher information differentiation of anterior signals leading up to reporting the differentiated percept. Crucially, information integration was closely linked to perception and even observed in a no-report condition when perceptual transitions were inferred from eye movements alone. In contrast, the link between neural differentiation and perception was observed solely in the active report condition. Our results, therefore, suggest that perception and the processes associated with report require distinct amounts of anterior-posterior network communication and anterior information differentiation. While front-to-back directed information is associated with changes in the content of perception when viewing bistable visual stimuli, regardless of report, frontal information differentiation was absent in the no-report condition and therefore is not directly linked to perception per se.</p

Conflict monitoring and attentional adjustment during binocular rivalry

First published: 06 December 2021To make sense of ambiguous and, at times, fragmentary sensory input, the brain must rely on a process of active interpretation. At any given moment, only one of several possible perceptual representations prevails in our conscious experience. Our hypothesis is that the competition between alternative representations induces a pattern of neural activation resembling cognitive conflict, eventually leading to fluctuations between different perceptual outcomes in the case of steep competition. To test this hypothesis, we probed changes in perceptual awareness between competing images using binocular rivalry. We drew our predictions from the conflict monitoring theory, which holds that cognitive control is invoked by the detection of conflict during information processing. Our results show that fronto-medial theta oscillations (5–7 Hz), an established electroencephalography (EEG) marker of conflict, increases right before perceptual alternations and decreases thereafter, suggesting that conflict monitoring occurs during perceptual competition. Furthermore, to investigate conflict resolution via attentional engagement, we looked for a neural marker of perceptual switches as by parieto-occipital alpha oscillations (8–12 Hz). The power of parieto-occipital alpha displayed an inverse pattern to that of fronto-medial theta, reflecting periods of high interocular inhibition during stable perception, and low inhibition around moments of perceptual change. Our findings aim to elucidate the relationship between conflict monitoring mechanisms and perceptual awareness.H2020 Marie Skłodowska-Curie Actions, Grant/Award Number: 794649; Universitat Pompeu Fabra; FEDER Operative Programme for Catalunya 2014–2020; IkerBasque Research Fellowships; Ramon y Cajal, Grant/Award Number: RYC2019-027538-I; University Pompeu Fabra; AGAUR Generalitat de Catalunya, Grant/Award Numbers: 2017 SGR 1545, FI-DGR 2019; Ministerio de Ciencia e Innovaci on, Grant/Award Number: PID2019-108531GB-I00 AEI/FEDE

Bifurcation analysis applied to a model of motion integration with a multistable stimulus

A computational study into the motion perception dynamics of a multistable psychophysics stimulus is presented. A diagonally drifting grating viewed through a square aperture is can be perceived as moving in the actual grating direction or in line with the aperture edges (horizontally or vertically). The different percepts are the product of interplay between ambiguous contour cues and specific terminator cues. We present a dynamical model of motion integration that performs direction selection for such a stimulus and link the different percepts to coexisting steady-states of the underlying equations. We apply the powerful tools of bifurcation analysis and numerical continuation to study the changes to the model's solution structure under the variation of parameters. Indeed, we apply these tools in a systematic way, taking into account biological and mathematical constraints, in order to fix model parameters. A region of parameter space is identified for which the model reproduces the qualitative behaviour observed in experiments. The temporal dynamics of motion integration are studied within this region; specifically, the effect of varying the stimulus gain is studied, which allows for qualitative predictions to be made

Perceptual Rivalry: Reflexes Reveal the Gradual Nature of Visual Awareness

Rivalry is a common tool to probe visual awareness: a constant physical stimulus evokes multiple, distinct perceptual interpretations (“percepts”) that alternate over time. Percepts are typically described as mutually exclusive, suggesting that a discrete (all-or-none) process underlies changes in visual awareness. Here we follow two strategies to address whether rivalry is an all-or-none process: first, we introduce two reflexes as objective measures of rivalry, pupil dilation and optokinetic nystagmus (OKN); second, we use a continuous input device (analog joystick) to allow observers a gradual subjective report. We find that the “reflexes” reflect the percept rather than the physical stimulus. Both reflexes show a gradual dependence on the time relative to perceptual transitions. Similarly, observers' joystick deflections, which are highly correlated with the reflex measures, indicate gradual transitions. Physically simulating wave-like transitions between percepts suggest piece-meal rivalry (i.e., different regions of space belonging to distinct percepts) as one possible explanation for the gradual transitions. Furthermore, the reflexes show that dominance durations depend on whether or not the percept is actively reported. In addition, reflexes respond to transitions with shorter latencies than the subjective report and show an abundance of short dominance durations. This failure to report fast changes in dominance may result from limited access of introspection to rivalry dynamics. In sum, reflexes reveal that rivalry is a gradual process, rivalry's dynamics is modulated by the required action (response mode), and that rapid transitions in perceptual dominance can slip away from awareness

The relative contribution of noise and adaptation to competition during tri-stable motion perception

Animals exploit antagonistic interactions for sensory processing and these can cause oscillations between competing states. Ambiguous sensory inputs yield such perceptual multi-stability. Despite numerous empirical studies using binocular rivalry or plaid pattern motion, the driving mechanisms behind the spontaneous transitions between alternatives remain unclear. In the current work, we used a tri-stable barberpole motion stimulus combining empirical and modelling approaches to elucidate the contributions of noise and adaptation to underlying competition. We first robustly characterised the coupling between perceptual reports of transitions and continuously recorded eye direction, identifying a critical window of 480ms before button presses within which both measures were most strongly correlated. Second, we identified a novel non monotonic relationship between stimulus contrast and average perceptual switching rate with an initially rising rate before a gentle reduction at higher contrasts. A neural fields model of the underlying dynamics introduced in previous theoretical work and incorporating noise and adaptation mechanisms was adapted, extended and empirically validated. Noise and adaptation contributions were confirmed to dominate at the lower, and higher, contrasts respectively. Model simulations with two free parameters, controlling adaptation dynamics and direction thresholds, captured the measured mean transition rates for participants. We verified the shift from noise dominated towards adaptation-driven in both the eye direction distributions and inter-transition duration statistics. This work combines modelling and empirical evidence to demonstrate the signal strength dependent interplay between noise and adaptation during tri- stability. We propose that the findings generalise beyond the barberpole stimulus case to ambiguous perception in continuous feature space