Acquisition and influence of expectations about visual speed

It has been long hypothesized that due to the inherent ambiguities of visual input and the limitations of the visual system, vision is a form of “unconscious inference” whereby the brain relies on assumptions (aka expectations) to interpret the external world. This hypothesis has been recently formalized into Bayesian models of perception (the “Bayesian brain”) that represent these expectations as prior probabilities. In this thesis, I focus on a particular kind of expectation that humans are thought to possess – that objects in the world tend to be still or move slowly – known as the “slow speed prior”. Through a combination of experimental and theoretical work, I investigate how the speed prior is acquired and how it impacts motion perception. The first part of my work consists of an experiment where subjects are exposed to simple "training" stimuli moving more often at high speeds than at low speeds. By subsequently testing the subjects with slow-moving stimuli of high uncertainty (low contrast), I find that their perception gradually changes in a manner consistent with the progressive acquisition of an expectation that favours progressively higher speeds. Thus subjects appear to gradually internalize the speed statistics of the stimulus ensemble over the duration of the experiment. I model these results using an existing Bayesian model of motion perception that incorporates a speed prior with a peak at zero, extending the model so that the mean gradually shifts away from zero. Although the first experiment presents evidence for the plasticity of the speed prior, the experimental paradigm and the constraints of the model limit the accuracy and precision in the reconstruction of observers’ priors. To address these limitations, I perform a different experiment where subjects compare the speed of moving gratings of different contrasts. The new paradigm allows more precise measurements of the contrast-dependent biases in perceived speed. Using a less constrained Bayesian model, I extract the priors of subjects and find considerable interindividual variability. Furthermore, noting that the Bayesian model cannot account for certain subtleties in the data, I combine the model with a non-Bayesian, physiologically motivated model of speed tuning of cortical neurons and show that the combination offers an improved description of the data. Using the paradigm of the second experiment, I then explore the role of visual experience on the form of the speed prior. By recruiting avid video gamers (who are routinely exposed to high speeds) and nongamers of both sexes, I study the differences in the prior among groups and find, surprisingly, that subjects’ speed priors depend more on gender than on gaming experience. In a final series of experiments similar to the first, I also test subjects on variations of the trained stimulus configuration – namely different orientations and motion directions. Subjects’ responses suggest that they are able to apply the changed prior to different orientations and, furthermore, that the changed prior persists for at least a week after the end of the experiment. These results provide further support for the plasticity of the speed prior but also suggest that the learned prior may be used only across similar stimulus configurations, whereas in sufficiently different configurations or contexts a “default” prior may be used instead

Performance-monitoring integrated reweighting model of perceptual learning

Perceptual learning (PL) has been traditionally thought of as highly specific to stimulus properties, task and retinotopic position. This view is being progressively challenged, with accumulating evidence that learning can generalize (transfer) across various parameters under certain conditions. For example, retinotopic specificity can be diminished when the proportion of easy to hard trials is high, such as when multiple short staircases, instead of a single long one, are used during training. To date, there is a paucity of mechanistic explanations of what conditions affect transfer of learning. Here we present a model based on the popular Integrated Reweighting Theory model of PL but departing from its one-layer architecture by including a novel key feature: dynamic weighting of retinotopic-location-specific vs location-independent representations based on internal performance estimates of these representations. This dynamic weighting is closely related to gating in a mixture-of-experts architecture. Our dynamic performance-monitoring model (DPMM) unifies a variety of psychophysical data on transfer of PL, such as the short-vs-long staircase effect, as well as several findings from the double-training literature. Furthermore, the DPMM makes testable predictions and ultimately helps understand the mechanisms of generalization of PL, with potential applications to vision rehabilitation and enhancement

Changing expectations about speed alters perceived motion direction

SummaryOur perceptions are fundamentally altered by our knowledge of the world. When cloud-gazing, for example, we tend spontaneously to recognize known objects in the random configurations of evaporated moisture. How our brains acquire such knowledge and how it impacts our perceptions is a matter of heated discussion. A topic of recent debate has concerned the hypothesis that our visual system ‘assumes’ that objects are static or move slowly [1] rather than more quickly [1–3]. This hypothesis, or ‘prior on slow speeds’, was postulated because it could elegantly explain a number of perceptual biases observed in situations of uncertainty [2]. Interestingly, those biases affect not only the perception of speed, but also the direction of motion. For example, the direction of a line whose endpoints are hidden (as in the ‘aperture problem’) or poorly visible (for example, at low contrast or for short presentations) is more often perceived as being perpendicular to the line than it really is — an illusion consistent with expecting that the line moves more slowly than it really does. How this ‘prior on slow speeds’ is shaped by experience and whether it remains malleable in adults is unclear. Here, we show that systematic exposure to high-speed stimuli can lead to a reversal of this direction illusion. This suggests that the shaping of the brain's prior expectations of even the most basic properties of the environment is a continuous process

Contrast dependency and prior expectations in human speed perception

AbstractThe perceived speed of moving objects has long been known to depend on image contrast. Lowering the contrast of first-order motion stimuli typically decreases perceived speed – the well-known “Thompson effect”. It has been suggested that contrast-dependent biases are the result of optimal inference by the visual system, whereby unreliable sensory information is combined with prior beliefs. The Thompson effect is thought to result from the prior belief that objects move slowly (in Bayesian terminology, a “slow speed prior”). However, there is some evidence that the Thompson effect is attenuated or even reversed at higher speeds. Does the effect of contrast on perceived speed depend on absolute speed and what does this imply for Bayesian models with a slow speed prior? We asked subjects to compare the speeds of simultaneously presented drifting gratings of different contrasts. At low contrasts (3–15%), we found that the Thompson effect was attenuated at high speeds: at 8 and 12deg/s, perceived speed increased less with contrast than at 1 and 4deg/s; however, at higher contrasts (15–95%), the situation was reversed. A semi-parametric Bayesian model was used to extract the subjects’ speed priors and was subsequently improved by combining it with a model of speed tuning. These novel findings regarding the dual, contrast-dependent effect of high speeds help reconcile existing conflicting literature and suggest that physiologically plausible mechanisms of representation of speed in the visual cortex may need to be incorporated into Bayesian models to account for certain subtleties of human speed perception

Perceptual learning in visual hyperacuity: A reweighting model

AbstractImprovements of visual hyperacuity are a key focus in research of perceptual learning. Of particular interest has been the specificity of visual hyperacuity learning to the particular features of the trained stimuli as well as disruption of learning that occurs in some cases when different stimulus features are trained together. The implications of these phenomena on the underlying learning mechanisms are still open to debate; however, there is a marked absence of computational models that explore these phenomena in a unified way. Here we implement a computational learning model based on reweighting and extend it to enable direct comparison, by means of simulations, with a variety of existing psychophysical data. We find that this very simple model can account for a diversity of findings, such as disruption of learning of one task by practice on a similar task, as well as transfer of learning across both tasks and stimulus configurations under certain conditions. These simulations help explain existing results in the literature as well as provide important insights and predictions regarding the reliability of different hyperacuity tasks and stimuli. Our simulations also shed light on the model’s limitations, for example in accounting for temporal aspects of training procedures or dependency of learning with contextual stimuli, which will need to be addressed by future research

Urban and rural dietary patterns in Greece in the years 1957- 2008; an economic analysis

This paper attempts to describe and study in detail the evolution of urban and rural dietary patterns in Greece during the period 1957-2008, in terms of natural and technical features. Natural features refer to the consumption of animal versus plant products, while technical features pertain to agricultural and industrial products consumption. The analysis leads to a conclusion that the dietary patterns obtain internationalized, industrial origin and growing share of animal products, while the weight of tradition (i.e. Mediterranean plant products and services) weakens in Greece after World War II. Additionally, during this period the urban and rural dietary patterns, by comparison, have evolved at different paces and characteristics; the rural dietary patterns converged to urban ones with ‘asymmetric’ way, time lag and significant particularities

Urban and rural nutritional standards in Greece: 1957-2008

Greece during the period 1957-2008, in terms of natural and technical features. Natural features refer to the consumption of animal versus plant products, while technical features pertain to agricultural and industrial products consumption. The analysis leads to a conclusion that the dietary patterns obtain internationalized, industrial origin and growing share of animal products, while theweight of tradition (i.e. Mediterranean plant products and services) weakens in Greece after World War II. Additionally, during this period the urban and rural dietary patterns, by comparison, have evolved at different paces and characteristics; the rural dietary patterns converged to urban ones with ‘asymmetric’ way, time lag and significant particularities