Malleability in vision: investigations into the temporal integration of motion and functional blindness around the physiological blind spot

Field: Neuroscience (Life Sciences)

The human visual system is notably malleable, with representational plasticity demonstrated at multiple levels of the processing hierarchy. The organisation of early visual cortex, for example, can be influenced by the visual ‘diet’ during development. Moreover, in cases of congenital blindness swathes of visual cortex can be recruited for auditory or somatosensory processing. The present thesis demonstrates two, more subtle, examples of visual flexibility at distinct time scales: a short term malleability in our perception of object motion, and a long-term malleability in sensitivity to inputs encoded by underutilised regions of retina.Across the first three chapters, I investigate malleability in our perception of object motion. Functional interactions with our environment necessitate an accurate model of motion. To catch, or safely avoid, a flying ball we must rapidly and accurately estimate its trajectory, and plan movements accordingly. As catching is an action we all appear to perform with relative ease, it may be surprising to note that we tend to make systematic errors if asked to estimate the momentary position of a moving object. The nature of these errors has been of great interest, and driven a considerable body of research. The first three chapters focus on one such perceptual error – the Frohlich effect - where observers perceive the onset position of a moving object as shifted forward in the direction of motion (Frohlich, 1923).The first empirical chapter (Chapter Two) demonstrates an exaggeration of the Frohlich effect following adaptation to rapid motion (Miller & Arnold, 2015). From these findings, I theorise that judgments of onset position can be described by a mechanism that accumulates positional information over multiple variably weighted time windows (i.e. a temporal integration of position, see: Eagleman & Sejnowski, 2000; Krekelberg & Lappe, 2000). Furthermore, the exaggerations observed following adaptation suggest that temporal integration in this context might be a malleable process, with adaptation serving to reweight the contributions of different integrating channels to better suit the prevailing, dynamic, conditions. This malleability may provide some ecological benefits, such as enhancing judgments of trajectory in dynamic environments.Chapter Three compares the temporal integration model with a competing theory – perceptual extrapolation (Nijihawan, 1994). Perceptual extrapolation theory proposes that positional distortions reflect on mechanisms that generate perceptually explicit predictive signals based on trajectory, thereby causing people to ‘see’ moving objects in advanced positions. Notably, perceptual extrapolation predicts that moving objects can be perceived in positions that they never physically occupy, whereas temporal integration does not. I examine these possibilities by studying movement into and out from the physiological blind spot, and find no evidence for an extrapolation of momentary position, providing further support for a mechanism that integrates positional information over time. The final empirical chapter (Chapter Four) turns to an investigation of the impact of training individuals to detect motion direction in the area directly surrounding the physiological blind spot, to investigate if this training can modulate visual sensitivity. A notable feature of the retinal blind spot is that it is surrounded by a limited region of tissue with reduced, but not absent photosensitivity (Armaly, 1969). In Chapter Four I leverage this physiology to investigate if training individuals to discern motion direction in the area immediately surrounding the blind spot can heighten sensitivity in this minimally photosensitive region. Following extended training on a motion discrimination task, I found a reduction in the size of the functionally-defined blind spot (Miller, Wallis, Bex, & Arnold, 2015). Importantly, these reductions held not only for the trained task, but also for standard measures of perimetry, and for a colour discrimination task, thereby suggesting a generalised improvement in sensitivity at the edges of the blind spot. These findings suggest that simple behavioural methods could be developed to benefit clinical cases of localised blindness.Outside of the individual conclusions of each chapter, the work presented provides a demonstration of the malleability of motion perception. Over short timescales estimates of momentary position suggest malleable contributions from multiple mechanisms, with different integration windows. Over longer periods, estimates of sensitivity suggest a malleable contribution from underlying neural substrates, which can be beneficially trained to enhance sensitivity.