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

Neural basis of a visuo-motor transformation in the fly

How the outputs of populations of sensory neurons are used by motor systems to generate appropriate behaviour is a long standing question in neuroscience. I address this problem by studying a comparatively simple model system. In the fly, Neck Motor Neurons control gaze-stabilising head movements that occur during wholebody rotations. These motor neurons receive several sensory inputs including one from well-characterized visual interneurons, Tangential Cells (TCs), which respond to panoramic image shifts induced during self-motion. In chapter one, I provide a general introduction to sensory-motor circuits and the fly gaze-stabilisation system. In chapter two, I report that the visual receptive fields of Neck Motor Neurons are similar to those of the TCs. Using this result, I show an alignment between the coordinate systems used by the visual and the neck motor systems to process visual information. Thus, TCs encode visual inputs in a manner already closely matched to the requirements of the neck motor neurons, considerably facilitating the visual-motor transformation In chapter three, I analyse the gating of neck motor neuron visual responses by convergent mechanosensory inputs from the halteres. Some neck motor neurons do not fire action potentials in response to visual stimuli alone, but they will in response to haltere movements. I show that visual stimuli produce sustained sub-threshold depolarisations in these neurons. These visual depolarisations increase the proportion of haltere-induced action potentials in neck motor neurons. Thus, visual inputs can only affect the spiking output if the halteres are moving. This simple mechanism could explain why flies only make visually induced head movements during walking or flight: behaviours that involve beating the halteres. By analysing how the outputs of a model sensory system are used, I have shown a novel alignment between sensory and motor neuron populations and a simple mechanism underlying multisensory fusion

A survey of visual preprocessing and shape representation techniques

Many recent theories and methods proposed for visual preprocessing and shape representation are summarized. The survey brings together research from the fields of biology, psychology, computer science, electrical engineering, and most recently, neural networks. It was motivated by the need to preprocess images for a sparse distributed memory (SDM), but the techniques presented may also prove useful for applying other associative memories to visual pattern recognition. The material of this survey is divided into three sections: an overview of biological visual processing; methods of preprocessing (extracting parts of shape, texture, motion, and depth); and shape representation and recognition (form invariance, primitives and structural descriptions, and theories of attention)

Dynamic Signal Compression for Robust Motion Vision in Flies

Sensory systems need to reliably extract information from highly variable natural signals. Flies, for instance, use optic flow to guide their course and are remarkably adept at estimating image velocity regardless of image statistics. Current circuit models, however, cannot account for this robustness. Here, we demonstrate that the Drosophila visual system reduces input variability by rapidly adjusting its sensitivity to local contrast conditions. We exhaustively map functional properties of neurons in the motion detection circuit and find that local responses are compressed by surround contrast. The compressive signal is fast, integrates spatially, and derives from neural feedback. Training convolutional neural networks on estimating the velocity of natural stimuli shows that this dynamic signal compression can close the performance gap between model and organism. Overall, our work represents a comprehensive mechanistic account of how neural systems attain the robustness to carry out survival-critical tasks in challenging real-world environments

A comparison of visual and haltere-mediated equilibrium reflexes in the fruit fly Drosophila melanogaster

Flies exhibit extraordinary maneuverability, relying on feedback from multiple sensory organs to control flight. Both the compound eyes and the mechanosensory halteres encode angular motion as the fly rotates about the three body axes during flight. Since these two sensory modalities differ in their mechanisms of transduction, they are likely to differ in their temporal responses. We recorded changes in stroke kinematics in response to mechanical and visual rotations delivered within a flight simulator. Our results show that the visual system is tuned to relatively slow rotation whereas the haltere-mediated response to mechanical rotation increases with rising angular velocity. The integration of feedback from these two modalities may enhance aerodynamic performance by enabling the fly to sense a wide range of angular velocities during flight

Shift in lateralization during illusory self‐motion: EEG responses to visual flicker at 10 Hz and frequency‐specific modulation by tACS

Self‐motion perception is a key aspect of higher vestibular processing, suggested to rely upon hemispheric lateralization and alpha‐band oscillations. The first aim of this study was to test for any lateralization in the EEG alpha band during the illusory sense of self‐movement (vection) induced by large optic flow stimuli. Visual stimuli flickered at alpha frequency (approx. 10 Hz) in order to produce steady state visually evoked potentials (SSVEP s), a robust EEG measure which allows probing the frequency‐specific response of the cortex. The first main result was that differential lateralization of the alpha SSVEP response was found during vection compared with a matched random motion control condition, supporting the idea of lateralization of visual–vestibular function. Additionally, this effect was frequency‐specific, not evident with lower frequency SSVEP s. The second aim of this study was to test for a causal role of the right hemisphere in producing this lateralization effect and to explore the possibility of selectively modulating the SSVEP response. Transcranial alternating current stimulation (tACS ) was applied over the right hemisphere simultaneously with SSVEP recording, using a novel artefact removal strategy for combined tACS ‐EEG . The second main result was that tACS enhanced SSVEP amplitudes, and the effect of tACS was not confined to the right hemisphere. Subsequent control experiments showed the effect of tACS requires the flicker frequency and tACS frequency to be closely matched and tACS to be of sufficient intensity. Combined tACS ‐SSVEP s are a promising method for future investigation into the role of neural oscillations and for optimizing tACS