Seeing by exploring

The classical notion of how things are seen is that perception is passive, that the eyes are windows, and in floods reality. Physiological work of the 19th century cast doubt on this view that perception is passive acceptance of reality. Perception is not at the present time a popular topic for philosophers. This must be partly because scientific accounts of perception have now gone a long way away from appearances. They depend on physiological and psycho-physical experiments which require technical investigation and do not fall within traditional concepts of philosophy. Theories of visual perception are examined, both from a physical and psycho-physical standpoint

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

Temporal features of size constancy for perception and action in a real-world setting: A combined EEG-kinematics study

A stable representation of object size, in spite of continuous variations in retinal input due to changes in viewing distance, is critical for perceiving and acting in a real 3D world. In fact, our perceptual and visuo-motor systems exhibit size and grip constancies in order to compensate for the natural shrinkage of the retinal image with increased distance. The neural basis of this size-distance scaling remains largely unknown, although multiple lines of evidence suggest that size-constancy operations might take place remarkably early, already at the level of the primary visual cortex. In this study, we examined for the first time the temporal dynamics of size constancy during perception and action by using a combined measurement of event-related potentials (ERPs) and kinematics. Participants were asked to maintain their gaze steadily on a fixation point and perform either a manual estimation or a grasping task towards disks of different sizes placed at different distances. Importantly, the physical size of the target was scaled with distance to yield a constant retinal angle. Meanwhile, we recorded EEG data from 64 scalp electrodes and hand movements with a motion capture system. We focused on the first positive-going visual evoked component peaking at approximately 90 ms after stimulus onset. We found earlier latencies and greater amplitudes in response to bigger than smaller disks of matched retinal size, regardless of the task. In line with the ERP results, manual estimates and peak grip apertures were larger for the bigger targets. We also found task-related differences at later stages of processing from a cluster of central electrodes, whereby the mean amplitude of the P2 component was greater for manual estimation than grasping. Taken together, these findings provide novel evidence that size constancy for real objects at real distances occurs at the earliest cortical stages and that early visual processing does not change as a function of task demands

Stereoacuity in processing near or far stimuli

Abstract The experiment compared stereoacuity with Chinese characters when they appeared at different visual field, depth, and time duration. Character in front of the horopter was presented in LoVF, which induces crossed retinal disparities (CRD). In contrast, character behind the horopter was presented in UVF, which induces uncrossed retinal disparities (URD). The results showed that males were superior to the information presented on the UVF, while females did not show significant bias. Moreover, males were more sensitive to the size constancy illusion in which a far thing appears larger (e.g., character behind the horopter) under short and long timescales, while females were sensitive to character in front of the horopter under long timescales. The results supported earlier claims that female brains were less lateralized than male brains, and two genders showed different strategies in processing the stereoscopic stimuli

stereoacuity in processing near and far stimuli

Abstract The experiment compared stereoacuity with Chinese characters when they appeared at different visual field, depth, and time duration. Character in front of the horopter was presented in LoVF, which induces crossed retinal disparities (CRD). In contrast, character behind the horopter was presented in UVF, which induces uncrossed retinal disparities (URD). The results showed that males were superior to the information presented on the UVF, while females did not show significant bias. Moreover, males were more sensitive to the size constancy illusion in which a far thing appears larger (e.g., character behind the horopter) under short and long timescales, while females were sensitive to character in front of the horopter under long timescales. The results supported earlier claims that female brains were less lateralized than male brains, and two genders showed different strategies in processing the stereoscopic stimuli

The temporal dynamics of size perception in adults and children

In a series of experiments, the influence of familiar object size on the speed of processing was investigated in adults and children, using a simple reaction time (RT) approach. In chapter 2, we demonstrated that children exhibited size-constancy-like responses starting from the age of five, although this conclusion was limited by task accuracy (experiment 1). The influence of symbolic and familiar size on simple RT was explored in 3 to 6-year-olds and adults (experiment 2). The task was conducted under reduced viewing conditions to enhance the contribution of familiar size as a visual cue. Although, we were unable to observe an effect of familiar or symbolic size on RT, we attributed this result to important methodological issues. In chapter 3, we report six experiments where we tested the influence of familiar size on simple RTs, measured under regular and reduced viewing conditions. The effect of animacy on RTs was also examined. We found that RTs were affected by familiar size in a manner that reflected the level of congruency between the physical size and the stored representation of size, such that congruent stimuli were responded to faster than incongruent stimuli. We also observed an animacy effect on RT: participants reacted faster to animals than non-animals. Finally, in chapter 4 we report an ERP study that investigated the electrophysiological correlates of familiar size. Results showed that the visual system processes familiar size around 100 ms after stimulus onset. Taken together, our findings demonstrate that familiar size is an automatic property of visual processing that can affect speeded motor responses. Future research could investigate the neural mechanisms underlying familiar object size and animacy, and specifically whether these mechanisms develop with age

Modelling the perceptual similarity of facial expressions from image statistics and neural responses

The ability to perceive facial expressions of emotion is essential for effective social communication. We investigated how the perception of facial expression emerges from the image properties that convey this important social signal, and how neural responses in face-selective brain regions might track these properties. To do this, we measured the perceptual similarity between expressions of basic emotions, and investigated how this is reflected in image measures and in the neural response of different face-selective regions. We show that the perceptual similarity of different facial expressions (fear, anger, disgust, sadness, happiness) can be predicted by both surface and feature shape information in the image. Using block design fMRI, we found that the perceptual similarity of expressions could also be predicted from the patterns of neural response in the face-selective posterior superior temporal sulcus (STS), but not in the fusiform face area (FFA). These results show that the perception of facial expression is dependent on the shape and surface properties of the image and on the activity of specific face-selective regions

A cross-modal investigation into the relationships between bistable perception and a global temporal mechanism

When the two eyes are presented with sufficiently different images, Binocular Rivalry (BR) occurs. BR is a form of bistable perception involving stochastic alternations in awareness between distinct images shown to each eye. It has been suggested that the dynamics of BR are due to the activity of a central temporal process and are linked to involuntary mechanisms of selective attention (aka exogenous attention). To test these ideas, stimuli designed to evoke exogenous attention and central temporal processes were employed during BR observation. These stimuli included auditory and visual looming motion and streams of transient events of varied temporal rate and pattern. Although these stimuli exerted a strong impact over some aspects of BR, they were unable to override its characteristic stochastic pattern of alternations completely. It is concluded that BR is subject to distributed influences, but ultimately, is achieved in neural processing areas specific to the binocular conflict

Phenomenology, Quantity, and Numerosity.

There are many situations in which we interact with collections of objects, from a crowd of people to a bowl of blackberries. There is an experience of the quantity of these items, although not a precise number, and we have this impression quickly and effortlessly. It can be described as an expressive property of the whole. In the literature, the study of this sense of numerosity has a long history, which is reviewed here with examples. I argue that numerosity is a direct perceptual experience, and that all experiences of numerosity, not only estimations, are affected by perceptual organisation