Novelty detection and context dependent processing of sky-compass cues in the brain of the desert locust Schistocerca gregaria

NERVOUS SYSTEMS facilitate purposeful interactions between animals and their environment, based on the perceptual powers, cognition and higher motor control. Through goal-directed behavior, the animal aims to increase its advantage and minimize risk. For instance, the migratory desert locust should profit from being fast in finding a fresh habitat, thus minimizing the investment of bodily resources in locomotion as well as the risk of starvation or capture by a predator en route. Efficient solutions to this and similar tasks – be it finding your way to work, the daily foraging of worker bees or the seasonal long-range migration of monarch butterflies - strongly depend on spatial orientation in local or global frames of reference. Local settings may include visual landmarks at stable positions that can be mapped onto egocentric space and learned for orientation, e.g. to remember a short route to a source of benefit (e.g. food) that is distant or visually less salient than the landmarks. Compass signals can mediate orientation to a global reference-frame (allothetic orienation), e.g. for locomotion in a particular compass direction or to merely ensure motion along a straight line. Whilst spatial orientation is a prerequisite of doing the planned in such tasks, animal survival in general depends on the ability to adequately respond to the unexpected, i.e. to unpredicted events such as the approach of a predator or mate. The process of identifying relevant events in the outside world that are not predictable from preceding events is termed novelty detection. Yet, the definition of ‘novelty’ is highly contextual: depending on the current situation and goal, some changes may be irrelevant and remain ´undetected´. The present thesis describes neuronal representations of a compass stimulus, correlates of novelty detection and interactions between the two in the minute brain of an insect, the migratory desert locust Schistocerca gregaria. Experiments were carried out in tethered locusts with legs and wings removed. More precisely, adult male subjects in the gregarious phase (see phase theory, Uvarov 1966) that migrates in swarms across territories in North Africa and the Middle East were used. The author performed electrophysiological recordings from single neurons in the locust brain, while either the compass stimulus (Chapter I) or events in the visual scenery (Chapter II) or combinations of both (Chapter III) were being presented to the animal. Injections of a tracer through the recording electrode, visualized by means of fluorescent-dye coupling, allowed the allocation of cellular morphologies to previously described types of neuron or the characterization of novel cell types, respectively. Recordings were focused on cells of the central complex, a higher integration area in the insect brain that was shown to be involved in the visually mediated control of goal-directed locomotion. Experiments delivered insights into how representations of the compass cue are modulated in a manner suited for their integration in the control of goal-directed locomotion. In particular, an interaction between compass-signaling and novelty detection was found, corresponding to a process in which input in one sensory domain (object vision) modulates the processing of concurrent input to a different exteroceptive sensory system (compass sense). In addition to deepening the understanding of the compass network in the locust brain, the results reveal fundamental parallels to higher context-dependent processing of sensory information by the vertebrate cortex, both with respect to spatial cues and novelty detection

Asymmetries between achromatic and chromatic extraction of 3D motion signals

Motion in depth (MID) can be cued by high-resolution changes in binocular disparity over time (CD), and low-resolution interocular velocity differences (IOVD). Computational differences between these two mechanisms suggest that they may be implemented in visual pathways with different spatial and temporal resolutions. Here, we used fMRI to examine how achromatic and S-cone signals contribute to human MID perception. Both CD and IOVD stimuli evoked responses in a widespread network that included early visual areas, parts of the dorsal and ventral streams, and motion-selective area hMT+. Crucially, however, we measured an interaction between MID type and chromaticity. fMRI CD responses were largely driven by achromatic stimuli, but IOVD responses were better driven by isoluminant S-cone inputs. In our psychophysical experiments, when S-cone and achromatic stimuli were matched for perceived contrast, participants were equally sensitive to the MID in achromatic and S-cone IOVD stimuli. In comparison, they were relatively insensitive to S-cone CD. These findings provide evidence that MID mechanisms asymmetrically draw on information in precortical pathways. An early opponent motion signal optimally conveyed by the S-cone pathway may provide a substantial contribution to the IOVD mechanism

Beyond Rehabilitation of Acuity, Ocular Alignment, and Binocularity in Infantile Strabismus

Infantile strabismus impairs the perception of all attributes of the visual scene. High spatial frequency components are no longer visible, leading to amblyopia. Binocularity is altered, leading to the loss of stereopsis. Spatial perception is impaired as well as detection of vertical orientation, the fastest movements, directions of movement, the highest contrasts and colors. Infantile strabismus also affects other vision-dependent processes such as control of postural stability. But presently, rehabilitative therapies for infantile strabismus by ophthalmologists, orthoptists and optometrists are restricted to preventing or curing amblyopia of the deviated eye, aligning the eyes and, whenever possible, preserving or restoring binocular vision during the critical period of development, i.e., before ~10 years of age. All the other impairments are thus ignored; whether they may recover after strabismus treatment even remains unknown. We argue here that medical and paramedical professionals may extend their present treatments of the perceptual losses associated with infantile strabismus. This hypothesis is based on findings from fundamental research on visual system organization of higher mammals in particular at the cortical level. In strabismic subjects (as in normal-seeing ones), information about all of the visual attributes converge, interact and are thus inter-dependent at multiple levels of encoding ranging from the single neuron to neuronal assemblies in visual cortex. Thus if the perception of one attribute is restored this may help to rehabilitate the perception of other attributes. Concomitantly, vision-dependent processes may also improve. This could occur spontaneously, but still should be assessed and validated. If not, medical and paramedical staff, in collaboration with neuroscientists, will have to break new ground in the field of therapies to help reorganize brain circuitry and promote more comprehensive functional recovery. Findings from fundamental research studies in both young and adult patients already support our hypothesis and are reviewed here. For example, presenting different contrasts to each eye of a strabismic patient during training sessions facilitates recovery of acuity in the amblyopic eye as well as of 3D perception. Recent data also demonstrate that visual recoveries in strabismic subjects improve postural stability. These findings form the basis for a roadmap for future research and clinical development to extend presently applied rehabilitative therapies for infantile strabismus

The analysis of complex motion patterns in primate cortex

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 1995.Includes bibliographical references.by Bard J. Geesaman.Ph.D

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

Neuronal encoding of natural imagery in dragonfly motion pathways

Vision is the primary sense of humans and most other animals. While the act of seeing seems easy, the neuronal architectures that underlie this ability are some of the most complex of the brain. Insects represent an excellent model for investigating how vision operates as they often lead rich visual lives while possessing relatively simple brains. Among insects, aerial predators such as the dragonfly face additional survival tasks. Not only must aerial predators successfully navigate three-dimensional visual environments, they must also be able to identify and track their prey. This task is made even more difficult due to the complexity of visual scenes that contain detail on all scales of magnification, making the job of the predator particularly challenging. Here I investigate the physiology of neurons accessible through tracts in the third neuropil of the optic lobe of the dragonfly. It is at this stage of processing that the first evidence of both wide-field motion and object detection emerges. My research extends the current understanding of two main pathways in the dragonfly visual system, the wide-field motion pathway and target-tracking pathway. While wide-field motion pathways have been studied in numerous insects, until now the dragonfly wide-field motion pathway remains unstudied. Investigation of this pathway has revealed properties, novel among insects, specifically the purely optical adaptation to motion at both high and low velocities through motion adaptation. Here I characterise these newly described neurons and investigate their adaptation properties. The dragonfly target-tracking pathway has been studied extensively, but most research has focussed on classical stimuli such as gratings and small black objects moving on white monitors. Here I extend previous research, which characterised the behaviour of target tracking neurons in cluttered environments, developing a paradigm to allow numerous properties of targets to be changed while still measuring tracking performance. I show that dragonfly neurons interact with clutter through the previously discovered selective attention system, treating cluttered scenes as collections of target-like features. I further show that this system uses the direction and speed of the target and background as one of the key parameters for tracking success. I also elucidate some additional properties of selective attention including the capacity to select for inhibitory targets or weakly salient features in preference to strongly excitatory ones. In collaboration with colleagues, I have also performed some limited modelling to demonstrate that a selective attention model, which includes switching best explains experimental data. Finally, I explore a mathematical model called divisive normalisation which may partially explain how neurons with large receptive fields can be used to re-establish target position information (lost in a position invariant system) through relatively simple integrations of multiple large receptive field neurons. In summary, my thesis provides a broad investigation into several questions about how dragonflies can function in natural environments. More broadly, my thesis addresses general questions about vision and how complicated visual tasks can be solved via clever strategies employed in neuronal systems and their modelled equivalents.Thesis (Ph.D.) -- University of Adelaide, Adelaide Medical School, 201

The time-course of colour vision

Four experiments are presented, each investigating temporal properties of colour vision processing in human observers. The first experiment replicates and extends an experiment by Stromeyer et al. (1991). We look for a phase difference between combined temporal modulations in orthogonal directions in colour space, which might null the often-claimed latency of signals originating from the short-wavelength sensitive cones (S-cones). We provide another estimate of the magnitude of this latency, and give evidence to suggest that it originates early in the chromatic pathway, before signals from S-cones are combined with those that receive opposed L- and M-cone input. In the second experiment we adapt observers to two stimuli that are matched in the mean and amplitude of modulation they offer to the cone classes and to the cardinal opponent mechanisms, but that differ in chromatic appearance, and hence their modulation of later colour mechanisms. Chromatic discrimination thresholds after adaptation to these two stimuli differ along intermediate directions in colour space, and we argue that these differences reveal the adaptation response of central colour mechanisms. In the third experiment we demonstrate similar adaptation using the same stimuli, measured with reaction times rather than thresholds. In the final experiment, we measure the degree to which colour constancy is achieved as a function of time in a simulated stimulus environment in which the illuminant changes periodically. We find that perfect constancy is not achieved instantaneously after an illuminant chromaticity shift and that constancy of colour appearance judgements increases over several seconds

Mechanisms of visual feature binding

Visual feature binding is the method by which coherent objects and scenes are perceived. Advances in the science of perception have indicated that visual features such as colour, motion, and orientation are to some extent, processed separately in primate early visual cortex. However, the mechanism by which these features are integrated remains unclear. Phenomenologically, the process of binding features to form objects appears to be an efficient and automatic process. Some research also shows a high temporal resolution for binding features together, in addition to populations of neurons that jointly code for features. However, dominant theories of feature binding and the majority of the binding literature indicate that the feature binding process is severely limited by a relatively low temporal resolution, especially when compared to other perceptual properties such as feature detection. To identify and resolve the discrepancy in the feature binding literature, I investigate the feature binding process and its inter-relationship with perceptual surface segregation. Surface segregation has been postulated as the method by which features can be rapidly bound together, giving them impression of a high temporal resolution. In Chapter 2, displays are used that alternate between two arrays of differently coloured, oppositely moving dots. The alternation frequency is modified in order to gauge the temporal resolution of binding. This is combined with surface segregation cues such as coherent motion, consistency of dot configuration, and colour. In Chapter 3, coloured, oriented gratings are used to investigate colour-orientation binding. Angular separation, spatial and temporal coincidence, and stimulus presentation duration are varied. Across these experiments, a number of these surface segregation cues are manipulated in order to measure the corresponding effects on feature binding, perceptual interpretation of the stimulus, and its neural representation. The results of the psychophysical experiments indicate that feature binding, surface segregation, and temporal integration are inextricably linked. These findings are reinforced by data gathered through functional magnetic resonance imaging (fMRI) of human subjects. Both surface segregation and feature pairs were found to modulate neural activity in early visual cortex, providing evidence that similar neural substrates are recruited for both feature binding and surface segregation. Overall, the two complementary sets of experiments using stimulus conjunctions of colour-motion and colour-orientation stimuli provide converging evidence and insight into the dynamics of the underlying binding mechanisms. A discussion of the implications of the research follows, concluding that rapidly formed surface representations can be maintained across presentation intervals by temporal integration. Attentional selection of one feature (e.g. orientation) can then be used to boost the response to the paired feature (colour) in order to identify and extract the correct feature pairing. Based on the known properties of the visual system, several potential neural mechanisms are proposed that are consistent with both the psychophysical and neural data, in addition to suggested future directions for the study of visual feature binding

Modeling and Computational Framework for the Specification and Simulation of Large-scale Spiking Neural Networks

Recurrently connected neural networks, in which synaptic connections between neurons can form directed cycles, have been used extensively in the literature to describe various neurophysiological phenomena, such as coordinate transformations during sensorimotor integration. Due to the directed cycles that can exist in recurrent networks, there is no well-known way to a priori specify synaptic weights to elicit neuron spiking responses to stimuli based on available neurophysiology. Using a common mean field assumption, that synaptic inputs are uncorrelated for sufficiently large populations of neurons, we show that the connection topology and a neuron\u27s response characteristics can be decoupled. This assumption allows specification of neuron steady-state responses independent of the connection topology. Specification of neuron responses necessitates the creation of a novel simulator (computational framework) which allows modeling of large populations of connected spiking neurons. We describe the implementation of a spike-based computational framework, designed to take advantage of high performance computing architectures when available. We show that performance of the computational framework is improved using multiple message passing processes for large populations of neurons, resulting in a worst-case linear relationship between the number of neurons and the time required to complete a simulation. Using the computational framework and the ability to specify neuron response characteristics independent of synaptic weights, we systematically investigate the effects of Hebbian learning on the hemodynamic response. Changes in the magnitude of the hemodynamic responses of neural populations are assessed using a forward model that relates population synaptic currents to the blood oxygen dependant (BOLD) response via local field potentials. We show that the magnitude of the hemodynamic response is not a accurate indicator of underlying spiking activity for all network topologies. Instead, we note that large changes in the aggregate response of the population (\u3e50%) can results in a decrease in the overall magnitude of the BOLD signal. We hypothesize that the hemodynamic response magnitude changed due to fluctuations in the balance of excitatory and inhibitory inputs in neural subpopulations. These results have important implications for mean-field models, suggesting that the underlying excitatory/inhibitory neural dynamics within a population may need to be taken into account to accurately predict hemodynamic responses