Case Studies in Invertebrate Visual Processing: I. Spectral and Spatial Processing in the Early Visual System of Drosophila melanogaster II. Binocular Stereopsis in Sepia officinalis

This thesis addresses two aspects of visual processing in two different invertebrate organisms. The fruit fly, Drosophila melanogaster, has emerged as a key model for invertebrate vision research. Despite extensive characterisation of motion vision, very little is known about how flies process colour information, or how the spectral content of light affects other visual modalities. With the aim to accurately dissect the different components of the Drosophila visual system responsible for processing colour, I have developed a versatile visual stimulation setup to probe for the combinations of spatial, temporal and spectral visual response properties. Using flies that express neural activity indicators, I can track visual responses to a colour stimulus (i.e. narrow bands of light across the spectrum) via a two-photon imaging system. The visual stimulus is projected on a specialised screen material that scatters wavelengths of light across the spectrum equally at all locations of the screen, thus enabling presentation of spatially structured stimuli. Using this setup, I have characterised spectral responses, intensity-response relationships, and receptive fields of neurons in the early visual system of a variety of genetically modified strains of Drosophila. Specifically, I compared visual responses in the medulla of flies expressing either a subset or all photoreceptor opsins, with differing levels of screening pigment present in the eye. I found layer-specific shifts of spectral response properties correlating with projection regions of photoreceptor terminals. I also 3 found that a reduction in screening pigment shifts the general spectral response in the neuropil towards the longer wavelengths of light. I have also mapped receptive fields across the different layers of the medulla for the peak spectral response wavelength. My results suggest that receptive field dimensions match the expected size predicted by the conservation of a columnar organisation in the medulla, with little variation from layer to layer. In a subset of these cells, we see an elongated receptive field suggestive of static orientation selectivity with an apparent split in the preferred axis of orientation of these receptive fields, with a near-orthogonal angle between the summed vectors of the split populations. The camera type eyes of vertebrates and cephalopods exhibit remarkable convergence, but it is currently unknown if the mechanisms for visual information processing in these brains, which exhibit wildly disparate architecture, is also shared. I chose to investigate the visual processing mechanism known as stereopsis in the cuttlefish Sepia officinalis. Stereoscopic vision is used to assess depth information by comparing the disparity between left and right visual fields. This strategy is commonplace in vertebrates having evolved multiple times independently but has only been demonstrated in one invertebrate: the praying mantis. Cuttlefish require precise distance estimation during their predatory hunt when they extend two tentacles in a ballistic strike to catch their target. Using a 3D perception paradigm whereby the cuttlefish were fitted with anaglyph glasses, I show that these animals use stereopsis to resolve distance to their prey. Although this is not an exclusive depth perception mechanism for hunting, it does shorten the time and distance covered prior to striking at a target. Furthermore, stereopsis in cuttlefish works differently to vertebrates, as cuttlefish can extract stereopsis cues from anti-correlated stimuli.BBSRC Doctoral Training Partnershi

Vision Sensors and Edge Detection

Vision Sensors and Edge Detection book reflects a selection of recent developments within the area of vision sensors and edge detection. There are two sections in this book. The first section presents vision sensors with applications to panoramic vision sensors, wireless vision sensors, and automated vision sensor inspection, and the second one shows image processing techniques, such as, image measurements, image transformations, filtering, and parallel computing

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

Modelling chromatic processes in human vision

This investigation programme has concentrated on a number of topics concerning visual function in humans, with particular emphasis on the processing of chromatic information. The programme included an investigation of colour constancy which was carried out using a computer - controlled colour display. A new dynamic colour matching method was developed and the experimental conditions arranged so as to test the constancy of colour appearance. The technigue makes possible the definition of an index of colour constancy which simplifies the interpretation of experimental results. Various spatial, temporal and chromatic parameters of the stimulus configuration were investigated and the results show the importance of the stimulus boundary and near surround in determining the magnitude of the constancy effects observed. Pupillary function was investigated in normal and amblyopic observers. The results suggest that the pupil light reflex is essentially of normal amplitude in amblyopic eyes, although a latency anomaly does exist. Pupillary responses to achromatic, sinusoidal grating stimuli were anomalous in the affected eyes of many amblyopic subjects. Interestingly, certain response parameters were found to be anomalous in the normal eyes of amblyopic observers. Pupillary responses in normal observers were measured following stimulation with isoluminant, red - green chromatic gratings. Responses were found to vary systematically with grating spatial frequency, and broadly reflect the way in which detection thresholds for the same stimuli vary with spatial frequency. Response latencies were approximately 80 ms longer than for equivalent responses to achromatic gratings. No responses were observed when the same experiments were carried out with a protanope. Chromatic discrimination thresholds were measured using a spatio - temporal luminance masking technique. The technique is implemented on a carefully calibrated colour monitor and obviates the requirement of stimulus isoluminance. Preliminary experiments employing psychophysical and pupillometric procedures illustrate the use of the technique in the subjective and objective assessment of colour vision. The rapid changes in visual sensitivity which occur following an intense flash of light were investigated by measuring increment thresholds for a small, brief test stimulus. Results are presented which show the dependence of the sensitivity changes on various spatial, temporal and chromatic parameters. An attempt is made to relate the experimental findings to the properties of retinal neurones

Chromatic processing in the zebrafish (Danio rerio) inner retina: bipolar cell physiology and open hardware designs for spectrally accurate stimulation under two-photon

Colour vision describes the ability of animals to differentiate objects based on their spectral reflectance properties independent of light intensity. It is an essential evolutionary trait that allows species to efficiently forage for food, avoid predation, break camouflage, communicate with conspecifics, or to find mates. Zebrafish is a powerful model for studying colour vision as it possesses four cone-photoreceptor types which can be categorised as Red-, Green-, Blue- and UV-sensitive. From first principles, its retina therefore holds the potential to process diverse chromatic computations. In the presented work, the focus was on retinal bipolar cells (BC). These are the retina’s first projection neurons. They receive inputs from the photoreceptors in the outer retina, and send their axon terminals to the inner retina, the inner plexiform layer (IPL). Diverse types within this class of interneuron shape light responses collected by the photoreceptor array into parallel channels with diverse spectral properties. BCs also make connections with all other neuron types within the retina, including horizontal cells in the outer retina, and amacrine as well as retinal ganglion cells in the inner retina. This makes them a central hub for spectral processing within the retina. By combining genetically encoded calcium indicator and two-photon microscopy, light-driven activity from larval zebrafish BC synaptic terminals was systematically recorded in vivo. Synaptic responses to tetrachromatic light stimulation unveiled an unprecedented degree of visual specialisation, including retinal regions dedicated to distinct light-guided behaviours. These regional characteristics were further correlated to functional BC types which were strongly associated with specific retinal positions and axonal stratification depths. Overall, BC projections to the inner plexiform layer displayed a sophisticated level of organisation, structured into chromatic and achromatic functional layers which systematically adjusted their response profiles across the eye to match natural spectral input statistics. Together, these findings bolster our understanding of “colour-processing” in this animal’s inner retina and suggest that unlike in mammals, teleost fish BCs already encode complex chromatic responses in the inner plexiform layer before driving retinal ganglion cells. Additionally, the study of colour vision from an organism requires precise control over the light stimuli’s temporal, spatial and spectral features. Therefore, chromatic stimulators, designed to be combined with two-photon microscopy, were developed throughout this work. These devices allowed circumventing experimental limitations, such as spectral crosstalk between the microscope and the stimulus light. Furthermore, they were conceived as open source projects to be easily replicated and adapted to any organism’s retina with different spectral sensitivities through the free control over the number and spectra of stimulation light sources. These open source projects originated from the desire to set up a stimulation standard for the field of visual neuroscience

The spatial organization of movement detecting mechanisms in human vision

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Fourier Transforms

The 21st century ushered in a new era of technology that has been reshaping everyday life, simplifying outdated processes, and even giving rise to entirely new business sectors. Today, contemporary users of products and services expect more and more personalized products and services that can meet their unique needs. In that sense, it is necessary to further develop existing methods, adapt them to new applications, or even discover new methods. This book provides a thorough review of some methods that have an increasing impact on humanity today and that can solve different types of problems even in specific industries. Upgrading with Fourier Transformation gives a different meaning to these methods that support the development of new technologies and have a good projected acceleration in the future

Some aspects of the pupil response in relation to stimulus movement and colour

Recent investigations have shown that the activity of the visual system in the processing of some stimulus attributes such as spatial structure and stimulus colour are reflected in the pupil response in the form of a small transient constriction at the stimulus onset. The response amplitude appears to be proportional to the level of activity generated and varies systematically with the properties of the visual stimulus. In this report, it has been shown that the processing of coherent motion information can also produce pupillary changes in the form of pupil motion responses (PMRs). The results show that PMRs are present for both foveal and peripheral stimulus presentations and are not affected significantly by degradation of retinal image quality. It has been demonstrated that PMRs are also elicited to sudden changes in stimulus speed and direction, and that the response amplitudes vary systematically with the percentage change involved. There are extensive psychophysical data showing that blindsight subjects are sensitive to movement information when the stimulus is presented in their blind field. In this study, pupillometric parallels for the psychophysical findings have been demonstrated. Data which reveal the existence of residual chromatic discrimination in the absence of VI have also been obtained in two blindsight subjects. It has been shown that their psychophysical performance improves with the level of chromatic saturation of the stimulus. The pupil colour responses have been investigated under similar conditions. Results show that the pupil responses to coloured stimuli can also be elicited in the blind field and that these responses parallel the psychophysical findings. The results obtained have been discussed in relation to the properties of the neuronal structures and visual pathways which are likely to mediate the observed pupillometric and psychophysical findings

The assessment of visual behaviour and depth perception in surgery

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A STUDY OF ILLUMINANT ESTIMATION AND GROUND TRUTH COLORS FOR COLOR CONSTANCY

Ph.DDOCTOR OF PHILOSOPH