The effects of noise on binocular rivalry waves: a stochastic neural field model

We analyse the effects of extrinsic noise on traveling waves of visual perception in a competitive neural field model of binocular rivalry. The model consists of two one-dimensional excitatory neural fields, whose activity variables represent the responses to left-eye and right-eye stimuli, respectively. The two networks mutually inhibit each other, and slow adaptation is incorporated into the model by taking the network connections to exhibit synaptic depression. We first show how, in the absence of any noise, the system supports a propagating composite wave consisting of an invading activity front in one network co-moving with a retreating front in the other network. Using a separation of time scales and perturbation methods previously developed for stochastic reaction-diffusion equations, we then show how multiplicative noise in the activity variables leads to a diffusive–like displacement (wandering) of the composite wave from its uniformly translating position at long time scales, and fluctuations in the wave profile around its instantaneous position at short time scales. The multiplicative noise also renormalizes the mean speed of the wave. We use our analysis to calculate the first passage time distribution for a stochastic rivalry wave to travel a fixed distance, which we find to be given by an inverse Gaussian. Finally, we investigate the effects of noise in the depression variables, which under an adiabatic approximation leads to quenched disorder in the neural fields during propagation of a wave

Seeing the invisible: The scope and limits of unconscious processing in binocular rivalry

When an image is presented to one eye and a very different image is presented to the corresponding location of the other eye, they compete for conscious representation, such that only one image is visible at a time while the other is suppressed. Called binocular rivalry, this phenomenon and its deviants have been extensively exploited to study the mechanism and neural correlates of consciousness. In this paper, we propose a framework, the unconscious binding hypothesis, to distinguish unconscious processing from conscious processing. According to this framework, the unconscious mind not only encodes individual features but also temporally binds distributed features to give rise to cortical representation, but unlike conscious binding, such unconscious binding is fragile. Under this framework, we review evidence from psychophysical and neuroimaging studies, which suggests that: (1) for invisible low level features, prolonged exposure to visual pattern and simple translational motion can alter the appearance of subsequent visible features (i.e. adaptation); for invisible high level features, although complex spiral motion cannot produce adaptation, nor can objects/words enhance subsequent processing of related stimuli (i.e. priming), images of tools can nevertheless activate the dorsal pathway; and (2) although invisible central cues cannot orient attention, invisible erotic pictures in the periphery can nevertheless guide attention, likely through emotional arousal; reciprocally, the processing of invisible information can be modulated by attention at perceptual and neural levels

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

Measurment of spatial orientation using a biologically plausible gradient model

A Thesis submitted for the degree of Doctor of Philosophy

Mechanisms and Perceptual Consequences of Experience-Dependent Somatosensory Plasticity

Experience can alter neural responses at early stages of cortical processing. This has been demonstrated in the primary somatosensory cortex (SI), where neural responses undergo plasticity following consistent tactile training. Specifically, animals trained to detect the sequence of simultaneous tactile stimuli delivered across several digits exhibit multi-digit receptive fields (RFs) in SI, area 3b, where RFs are normally confined to a single digit. This finding indicates that neural circuits in primary sensory areas may conform to the statistical properties of stimuli used in training. However, 3b RFs in these studies were quantified using inconsistent hand held stimuli, and the function of such RFs for task performance was unknown. In this thesis we conducted a series of experiments in humans and non-human primates. We sought to understand how similar continuous sensory experience modifies neuronal properties of 3b cells and plasticity’s function for tactile perception, as well as the role of attention signals in facilitating these plastic changes in sensory cortex. We characterized 3b RFs with well controlled bar stimuli on individual digits in a naïve animal and in animals trained to detect the temporal pattern of multi-digit tactile stimuli. In the trained animal, we additionally quantified responses while the animal attended to multi-digit stimuli or while its attention was directed to the visual modality. We explored the function of such plasticity for tactile perception, hypothesizing that the features of multi-digit tactile stimuli confer changes in RF properties and tactile acuity. We tested in humans if presumed RF expansion as a result of multi-digit tactile training accounts for improvements in tactile spatial acuity across fingers at the expense of single-digit spatial acuity or temporal acuity between digits. We observed that training subjects on a multi-digit task interfered with single digit spatial acuity in an orientation and location specific manner and increased temporal acuity across the trained digits. We found that 3b RFs in the trained animal were enlarged, but feature selectivity (e.g. orientation tuning) was unchanged following training. These data suggest that stimulus properties may specify perceptual changes but not 3b plasticity following multi-digit tactile training. We describe many cells, even in a naïve animal and particularly for those with inhibited responses to tactile stimuli, with classical RFs extending over several digits. At the same time, we do not observe that 3b cells exhibit similar feature selectivity across digits, supporting the paradigm that 3b primarily represents tactile features on a single digit. We find that tactile attention modifies the firing rate of 3b cells with RFs covering both attended digits, enhancing responses following stimuli that match cells’ RF location. We conclude that cognitive state can alter responses early in sensory processing. Finally, we suggest future experiments to further determine how tactile spatial attention alters 3b neural processing, and its relationship to behavior and experience-dependent plasticity

Stimulus and task-dependent gamma activity in monkey V1

The single unit doctrine proposes that each one of our percepts and sensations is represented by the activity of specialized high-level cells in the brain. A common criticism applied to this proposal is the one referred to as the "combinatorial problem". We are constantly confronted with unlimited combinations of elements and features, and yet we face no problem in recognizing patterns and objects present in visual scenes. Are there enough neurons in the brain to singly code for each one of our percepts? Or is it the case that perceptions are represented by the distributed activity of different neuronal ensembles? We lack a general theory capable of explaining how distributed information can be efficiently integrated into single percepts. The working hypothesis here is that distributed neuronal ensembles signal relations present in the stimulus by selectively synchronizing their spiking responses. Synchronization is generally associated with oscillatory activity in the brain. Gamma oscillations in particular have been linked to various integrative processes in the visual system. Studies in anesthetized animals have shown a conspicuous increase in power for the gamma frequency band (30 to 60 Hz) in response to visual stimuli. Recently, these observations have been extended to behavioral studies which addressed the role of gamma activity in cognitive processes demanding selective attention. The initial motivation for carrying out this work was to test if the binding-by-synchronization (BBS) hypothesis serves as a neuronal mechanism for perceptual grouping in the visual system. To this aim we used single and superimposed grating stimuli. Superimposed gratings (plaids) are bi-stable stimuli capable of eliciting different percepts depending on their physical characteristics. In this way, plaids can be perceived either as a single moving surface (pattern plaids), or as two segregated surfaces drifting in different directions (component plaids). While testing the BBS hypothesis, we performed various experiments which addressed the role of both stimulus and cortical architecture on the properties of gamma oscillations in the primary visual cortex (V1) of monkeys. Additionally, we investigated whether gamma activity could also be modulated by allocating attention in time. Finally, we report on gamma-phase shifts in area V1, and how they depend on the level of neuronal activation. ...Einleitung: Die visuelle Hirnforschung hat eine große Informationsmenge über die analytischen Fähigkeiten des Nervensystems zusammengetragen. Die Einführung von Einzelzellableitungen ermöglichte eine detaillierte Beschreibung der Eigenschaften rezeptiver Felder im Sehsystem. Konzentrische rezeptive Felder in der Netzhaut antworten optimal auf einen Luminanzkontrast in ihren On- und Off-Regionen. Antworteigenschaften entwickeln sich schrittweise entlang der Sehbahn, indem zunehmend komplexere Eigenschaften des visuellen Reizes extrahiert werden. Die Pionierarbeiten von David Hubel und Torsten Wiesel beschrieben zunächst Orientierung- und Richtungsselektivität von Neuronen in frühen visuellen Kortexarealen. Später fand man Einzelzellen im medialen Temporallappen, die auf komplexe Objekte wie Hände und Gesichter antworten. Die Hirnforschung ist daher lange davon ausgegangen, dass die Repräsentation komplexer Objekte eine natürliche Entfaltung von Konvergenz entlang der Sehbahn darstellt. Zellen, welche auf elementare Merkmale des Stimulus antworteten, bildeten so durch ihr Muster anatomischer Verbindungen schrittweise die spezialisierten Neurone in höheren visuellen Arealen. Diese Sichtweise zeigt allerdings Limitationen auf. Eine beständige Kritik, die an der "Einzelzelldoktrin" geübt wird, ist das sogenannte kombinatorische Problem. Obwohl wir ständig mit einer unbegrenzten Fülle an Kombinationen verschiedener Elemente und Merkmale konfrontiert sind, laufen wir selten Gefahr, Muster und Objekte in einer visuellen Szene nicht zu erkennen. Ist es überhaupt möglich, dass jedes unserer möglichen Perzepte durch die Antwort eines einzelnen hoch spezialisierten Neurons im Hirn kodiert wird? Falls nicht, welcher Mechanismus könnte einen relationalen Code darstellen, der es ermöglicht, die Aktivität verschiedener neuronaler Ensembles zu integrieren? Die Anforderungen an einen solchen Mechanismus treten besonders hervor, wenn man sich die verteilte Struktur der visuellen Verarbeitung verdeutlicht. Die Merkmalsextraktion entlang der Sehbahn führt unvermeidbar zu einer räumlich verstreuten Repräsentation eines visuellen Reizes. Zusätzlich kommen parallele Bahnen neuronaler Verarbeitung im Hirn häufig vor. Es fehlt eine universale Theorie darüber, wie die verteilte Information effizient in eine einzige Wahrnehmung integriert wird. Die Arbeitshypothese hier lautet, dass das Hirn die Zeitdomäne benutzt, um visuelle Informationen zu integrieren und zu verarbeiten. Konkret würden neuronale Ensemble die aus dem Stimulus hervorgehenden Beziehungen durch eine selektive Synchronisation ihrer Aktionspotenziale signalisieren. Synchronisation ist normalerweise mit oszillatorischer Hirnaktivität assoziiert. Besonders die Oszillationen im Gamma Frequenzband sind mit verschiedensten integrativen Prozessen im Sehsystem in Verbindung gebracht worden. Arbeiten an anästhesierten Tieren haben einen auffälligen Anstieg von Energie im Gamma Frequenzband (30-60 Hz) unter visueller Stimulation gezeigt. Kürzlich sind diese Beobachtungen auf Verhaltensstudien ausgeweitet worden, welche die Rolle von Gamma Aktivität bei der für kognitive Prozesse erforderlichen gerichteten Aufmerksamkeit untersuchen. Die ursprüngliche Motivation dieser Arbeit war es, die von Wolf Singer und Mitarbeitern formulierte "binding-bysynchronization (BBS)" Hypothese zu testen. Dies wurde durch die Ableitung neuronaler Antworten in V1 bei Darbietung eines Paars übereinander gelegter Balkengitter ("Plaid" Stimulus) angegangen. Physikalische Manipulationen der Luminanz in Unterregionen des Plaid-Stimulus können die Wahrnehmung zugunsten der Bewegung der Einzelkomponenten (zwei Objekte, die sich übereinander schieben) oder der Bewegung des Gesamtmusters (ein einziges sich in eine gemeinsame Richtung bewegendes Objekt) beeinflussen. Die gleichzeitige Ableitung von zwei Neuronen, die jeweils nur selektiv auf eines der beiden Balkengitter antworteten, ermöglichte es uns, zwei Vorhersagen der BBS Hypothese zu testen. Falls beide V1 Neurone auf dasselbe Balkengitter antworteten, sollten sie ihre Aktivität unabhängig davon, ob das Plaid in Einzelkomponenten oder als Gesamtmuster wahrgenommen würde, synchronisieren. Der Grund dafür wäre, dass beide Neurone auf dasselbe Objekt reagierten. Im zweiten Fall antworten beide Ableitstellen auf jeweils eine der beiden Balkengitterkomponenten. Hier sagt die BBS Hypothese voraus, dass beide ihre Aktivität nur bei Gesamtmusterbewegung synchronisieren würden, da sie nur in dieser Bedingung auf dasselbe Objekt antworten würden. ..

Dorsal stream : from algorithm to neuroscience

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 173-195).The dorsal stream in the primate visual cortex is involved in the perception of motion and the recognition of actions. The two topics, motion processing in the brain, and action recognition in videos, have been developed independently in the field of neuroscience and computer vision. We present a dorsal stream model that can be used for the recognition of actions as well as explaining neurophysiology in the dorsal stream. The model consists of a spatio-temporal feature detectors of increasing complexity: an input image sequence is first analyzed by an array of motion sensitive units which, through a hierarchy of processing stages, lead to position and scale invariant representation of motion in a video sequence. The model outperforms or on par with the state-of-the-art computer vision algorithms on a range of human action datasets. We then describe the extension of the model into a high-throughput system for the recognition of mouse behaviors in their homecage. We provide software and a very large manually annotated video database used for training and testing the system. Our system outperforms a commercial software and performs on par with human scoring, as measured from the ground-truth manual annotations of more than 10 hours of videos of freely behaving mice. We complete the neurobiological side of the model by showing it could explain the motion processing as well as action selectivity in the dorsal stream, based on comparisons between model outputs and the neuronal responses in the dorsal stream. Specifically, the model could explain pattern and component sensitivity and distribution [161], local motion integration [97], and speed-tuning [144] of MT cells. The model, when combining with the ventral stream model [173], could also explain the action and actor selectivity in the STP area. There exists only a few models for the motion processing in the dorsal stream, and these models were not be applied to the real-world computer vision tasks. Our model is one that agrees with (or processes) data at different levels: from computer vision algorithm, practical software, to neuroscience.by Hueihan Jhuang.Ph.D

Disruption of spatio-temporal processing in human vision using transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a non-invasive technique used to reversibly modulate the activity of cortical neurons using time-varying magnetic fields. Recently TMS has been used to demonstrate the functional necessity of human cortical areas to visual tasks. For example, it has been shown that delivering TMS over human visual area V5/MT selectively disrupts global motion perception. The temporal resolution of TMS is considered to be one of its main advantages as each pulse has a duration of less than 1 ms. Despite this impressive temporal resolution, however, the critical period(s) during which TMS of area V5/MT disrupts performance on motion-based tasks is still far from clear. To resolve this issue, the influence of TMS on direction discrimination was measured for translational global motion stimuli and components of optic flow (rotational and radial global motion). The results of these experiments provide evidence that there are two critical periods during which delivery of TMS over V5/MT disrupts performance on global motion tasks: an early temporal window centred at 64 ms prior to and a late temporal window centred at 146 ms post global motion onset. Importantly, the early period cannot be explained by a TMS-induced muscular artefact. The onset of the late temporal window was contrast-dependent, consistent with longer neural activation latencies associated with lower contrasts. The theoretical relevance of the two epochs is discussed in relation to feedforward and feedback pathways known to exist in the human visual system, and the first quantitative model of the effects of TMS on global motion processing is presented. A second issue is that the precise mechanism behind TMS disruption of visual perception is largely unknown. For example, one view is that the “virtual lesion” paradigm reduces the effective signal strength, which can be likened to a reduction in perceived target visibility. Alternatively, other evidence suggests that TMS induces neural noise, thereby reducing the signal-to-noise ratio, which results in an overall increase in threshold. TMS was delivered over the primary visual cortex (area V1) to determine whether its influence on orientation discrimination could be characterised as a reduction in the visual signal strength, or an increase in TMS-induced noise. It was found that TMS produced a uniform reduction in perceived stimulus visibility for all observers. In addition, an overall increase in threshold (JND) was also observed for some observers, but this effect disappeared when TMS intensity was reduced. Importantly, susceptibility to TMS, defined as an overall increase in JND, was not dependent on observers’ phosphene thresholds. It is concluded that single-pulse TMS can both reduce signal strength (perceived visibility) and induce task-specific noise, but these effects are separable, dependent on TMS intensity and individual susceptibility

Optogenetic interrogation of primary visual cortex and its impact on neural coding and behavior

Understanding the mechanism by which the brain transforms simple sensory inputs into rich perceptual experiences is one of the great mysteries of systems neuroscience. Undoubtedly this involves the activity of large populations of interconnected neurons, but while the responses of individual neurons to a variety of sensory stimuli have been well-characterized, how populations of such neurons organize their activity to create our sensory perceptions is almost entirely unknown. To investigate this complex circuitry requires the ability to causally manipulate the activity of neural populations and monitor the resultant effects. Here we focus on primary visual cortex (V1), which has been shown to be crucial for visual perception, and utilize optogenetic tools to render the activity of genetically- defined neural populations sensitive to light. By simultaneously recording and modulating (either driving or silencing) the activity of excitatory (glutamatergic) neurons, we are able to causally examine their role in visual perception. Here we report 3 major findings. First, we show that activating subpopulations of excitatory neurons can improve visual perception under certain conditions and that information in V1 used for perceptual decisions is integrated across spatially-limited populations of neurons. Further, we show that a key signature of this information integration is a reduction in correlated variability between neurons. Correlated variability has been implicated as a major source of behavioral choice related activity in the cortex, and theorized to be a major factor limiting information in cortical populations. However, until now, there has not been a way to manipulate correlations without altering firing rates or other task related variables. Here we demonstrate a novel method using optogenetic stimulation to causally manipulate correlated variability between cortical neurons without altering their firing rates. Lastly, with the goal of expanding the currently limited repertoire of optogenetic tools for non-human primates, we establish the viability of a novel optogenetic construct capable of dramatically silencing neural populations using a recently discovered anion conducting channelrhodopsin