Mapping the natural visual world of the zebrafish (Danio rerio): from sensory input to behavioural output

Vision is one of the most crucial senses for animals to catch prey, find mates and stay alive. The tetrachromatic zebrafish (Danio rerio) is a widely used model animal in visual neuroscience with four cone photoreceptors sensitive to UV, blue, green and red light. However, a detailed understanding of how their visual system is adapted to the natural environment, and what is important for the fish to see in their shallow freshwater habitats of the Indian subcontinent, has been missing. Therefore, it also has not been possible to carefully assess the importance of different parts of the light spectrum for their natural behaviours. In this thesis I introduce a new method for natural imaging, characterise the spectral composition of zebrafish’s natural visual world and demonstrate the role of UV light in their prey capture behaviours. To characterise the light conditions in natural environments, I developed and built two hyperspectral scanners to take spectrally detailed light measurements in shallow ponds and slowly moving streams in North-East India. As expected, the spectral profile becomes increasingly monochromatic and red shifted when moving from surface to the bottom. However, the short wavelength dominated surface and long wavelength dominated bottom are separated with colour-rich horizon. These spectral statistics match rather perfectly with the cone densities and colour processing abilities of the bipolar cells in the larval zebrafish retina. Previous work has demonstrated how prey capture behaviours on larval zebrafish can be triggered by small, bright spots. The short wavelength dominated upper part of the visual field projects light from UV bright prey items perfectly to the ventro-temporal part of the retina (“strike zone”) with high density of UV cones. Finally, with my behaviour experiments I demonstrate how prey capture behaviours are strongly driven by UV bright paramecia detected with the strike zone

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

The contribution of psychophysical spatial frequency channels to the discrimination of broadband contrast.

The design and function of the human visual system is thought to have been shaped by the environment and tasks humans have performed throughout evolution and experience. Thus, it is important to establish the association between the properties of natural scenes and the responses of the visual system to these features at a behaviourally relevant level (e.g., psychophysically). A relevant property of natural scenes - which human observers are sensitive to - is the slope of the orientation averaged Fourier amplitude spectrum (α). It characterizes the decrease in amplitude as a function of spatial frequency (1/fα), and has an average value of 1.0 (on logarithmic axes) in natural scenes. The process of detecting a change in α is thought to stem from the activity of one or more spatial frequency channels, but their exact contribution remains to be defined. The overarching goal of this dissertation was to assess the contribution of spatial frequency channels to the detection, and discrimination, of a change in α. We first set out to obtain psychophysical evidence that α discrimination thresholds were dependent on the spatial frequency content of noise stimuli with 1/f amplitude spectra. Our results showed that while all spatial frequency channels contribute to α discrimination, the channel(s) of most influence seem to correlate in peak spatial frequency to the dominant perceptual scale of the noise image. While the contribution of low spatial frequency channels discrimination of steep αs was evident from our data, the influence of higher spatial frequency channels to the discrimination of shallower αs was not. In an attempt to better isolate the role of high spatial frequency channels, we attempted to modulate their activity with trans-cranial Direct Current Stimulation (tDCS) and change α discrimination thresholds. However, while this technique is capable of modulating channels to alter contrast detection, we found tDCS to be an ill-suited technique to alter the response characteristics of channels under suprathreshold conditions (i.e., broadband noise discrimination). As a final effort to isolate the contribution of spatial frequency channels to the discrimination of broadband contrast, we used a classification image paradigm to understand how different spatial frequencies contribute to the identification of a change in α. Interestingly, this method revealed task dependent contributions of spatial frequency channels to the discrimination of α. The identification of α is specific to α, in that increments and decrements in contrast in specific spatial frequency bands signal for particular αs, while the identification of a change in α (i.e., discrimination) was not specific to α. Regardless of the reference α, observers used an increment in contrast in low spatial frequency bands and decrement of contrast in higher spatial frequency bands to identify the odd stimulus. Taken together, these findings demonstrate that the discrimination of α is unspecific to α. Observers to rely on differences in contrast between low and high spatial frequency bands to detect a change in α, but may not be particularly tuned to certain α values as has previously been argued

Estimating the subjective perception of object size and position through brain imaging and psychophysics

Perception is subjective and context-dependent. Size and position perception are no exceptions. Studies have shown that apparent object size is represented by the retinotopic location of peak response in V1. Such representation is likely supported by a combination of V1 architecture and top-down driven retinotopic reorganisation. Are apparent object size and position encoded via a common mechanism? Using functional magnetic resonance imaging and a model-based reconstruction technique, the first part of this thesis sets out to test if retinotopic encoding of size percepts can be generalised to apparent position representation and whether neural signatures could be used to predict an individual’s perceptual experience. Here, I present evidence that static apparent position – induced by a dot-variant Muller-Lyer illusion – is represented retinotopically in V1. However, there is mixed evidence for retinotopic representation of motion-induced position shifts (e.g. curveball illusion) in early visual areas. My findings could be reconciled by assuming dual representation of veridical and percept-based information in early visual areas, which is consistent with the larger framework of predictive coding. The second part of the thesis sets out to compare different psychophysical methods for measuring size perception in the Ebbinghaus illusion. Consistent with the idea that psychophysical methods are not equally susceptible to cognitive factors, my experiments reveal a consistent discrepancy in illusion magnitude estimates between a traditional forced choice (2AFC) task and a novel perceptual matching (PM) task – a variant of a comparison-of-comparisons (CoC) task, a design widely seen as the gold standard in psychophysics. Further investigation reveals the difference was not driven by greater 2AFC susceptibility to cognitive factors, but a tendency for PM to skew illusion magnitude estimates towards the underlying stimulus distribution. I show that this dependency can be largely corrected using adaptive stimulus sampling

A Unified Cognitive Model of Visual Filling-In Based on an Emergic Network Architecture

The Emergic Cognitive Model (ECM) is a unified computational model of visual filling-in based on the Emergic Network architecture. The Emergic Network was designed to help realize systems undergoing continuous change. In this thesis, eight different filling-in phenomena are demonstrated under a regime of continuous eye movement (and under static eye conditions as well). ECM indirectly demonstrates the power of unification inherent with Emergic Networks when cognition is decomposed according to finer-grained functions supporting change. These can interact to raise additional emergent behaviours via cognitive re-use, hence the Emergic prefix throughout. Nevertheless, the model is robust and parameter free. Differential re-use occurs in the nature of model interaction with a particular testing paradigm. ECM has a novel decomposition due to the requirements of handling motion and of supporting unified modelling via finer functional grains. The breadth of phenomenal behaviour covered is largely to lend credence to our novel decomposition. The Emergic Network architecture is a hybrid between classical connectionism and classical computationalism that facilitates the construction of unified cognitive models. It helps cutting up of functionalism into finer-grains distributed over space (by harnessing massive recurrence) and over time (by harnessing continuous change), yet simplifies by using standard computer code to focus on the interaction of information flows. Thus while the structure of the network looks neurocentric, the dynamics are best understood in flowcentric terms. Surprisingly, dynamic system analysis (as usually understood) is not involved. An Emergic Network is engineered much like straightforward software or hardware systems that deal with continuously varying inputs. Ultimately, this thesis addresses the problem of reduction and induction over complex systems, and the Emergic Network architecture is merely a tool to assist in this epistemic endeavour. ECM is strictly a sensory model and apart from perception, yet it is informed by phenomenology. It addresses the attribution problem of how much of a phenomenon is best explained at a sensory level of analysis, rather than at a perceptual one. As the causal information flows are stable under eye movement, we hypothesize that they are the locus of consciousness, howsoever it is ultimately realized

Blickpunktabhängige Computergraphik

Contemporary digital displays feature multi-million pixels at ever-increasing refresh rates. Reality, on the other hand, provides us with a view of the world that is continuous in space and time. The discrepancy between viewing the physical world and its sampled depiction on digital displays gives rise to perceptual quality degradations. By measuring or estimating where we look, gaze-contingent algorithms aim at exploiting the way we visually perceive to remedy visible artifacts. This dissertation presents a variety of novel gaze-contingent algorithms and respective perceptual studies. Chapter 4 and 5 present methods to boost perceived visual quality of conventional video footage when viewed on commodity monitors or projectors. In Chapter 6 a novel head-mounted display with real-time gaze tracking is described. The device enables a large variety of applications in the context of Virtual Reality and Augmented Reality. Using the gaze-tracking VR headset, a novel gaze-contingent render method is described in Chapter 7. The gaze-aware approach greatly reduces computational efforts for shading virtual worlds. The described methods and studies show that gaze-contingent algorithms are able to improve the quality of displayed images and videos or reduce the computational effort for image generation, while display quality perceived by the user does not change.Moderne digitale Bildschirme ermöglichen immer höhere Auflösungen bei ebenfalls steigenden Bildwiederholraten. Die Realität hingegen ist in Raum und Zeit kontinuierlich. Diese Grundverschiedenheit führt beim Betrachter zu perzeptuellen Unterschieden. Die Verfolgung der Aug-Blickrichtung ermöglicht blickpunktabhängige Darstellungsmethoden, die sichtbare Artefakte verhindern können. Diese Dissertation trägt zu vier Bereichen blickpunktabhängiger und wahrnehmungstreuer Darstellungsmethoden bei. Die Verfahren in Kapitel 4 und 5 haben zum Ziel, die wahrgenommene visuelle Qualität von Videos für den Betrachter zu erhöhen, wobei die Videos auf gewöhnlicher Ausgabehardware wie z.B. einem Fernseher oder Projektor dargestellt werden. Kapitel 6 beschreibt die Entwicklung eines neuartigen Head-mounted Displays mit Unterstützung zur Erfassung der Blickrichtung in Echtzeit. Die Kombination der Funktionen ermöglicht eine Reihe interessanter Anwendungen in Bezug auf Virtuelle Realität (VR) und Erweiterte Realität (AR). Das vierte und abschließende Verfahren in Kapitel 7 dieser Dissertation beschreibt einen neuen Algorithmus, der das entwickelte Eye-Tracking Head-mounted Display zum blickpunktabhängigen Rendern nutzt. Die Qualität des Shadings wird hierbei auf Basis eines Wahrnehmungsmodells für jeden Bildpixel in Echtzeit analysiert und angepasst. Das Verfahren hat das Potenzial den Berechnungsaufwand für das Shading einer virtuellen Szene auf ein Bruchteil zu reduzieren. Die in dieser Dissertation beschriebenen Verfahren und Untersuchungen zeigen, dass blickpunktabhängige Algorithmen die Darstellungsqualität von Bildern und Videos wirksam verbessern können, beziehungsweise sich bei gleichbleibender Bildqualität der Berechnungsaufwand des bildgebenden Verfahrens erheblich verringern lässt