No / Rendering packages are used by visual psychophysicists to produce complex stimuli for their experiments, tacitly assuming that the simulation results accurately reflect the light-surface interactions of a real scene. RADIANCE is a physically based, freely available, and commonly used rendering software. We validated the calculation accuracy of this package by comparing simulation results with measurements from real scenes. RADIANCE recovers color gradients well but the results are shifted in color space. Currently, there is no better simulation alternative for achieving physical accuracy than by combining a spectral rendering method with RADIANCE.

Perception relies on the integration of numerous noisy inputs (cues). Cue combination has been relatively understudied in somatosensation, compared to vision and audition. Here, we investigated whether haptic cutaneous and hand configuration cues are combined optimally to discriminate between coin-sized discs of different sizes. When the hand is open such that the thumb and index fingers span the diameter of a disc to contact its perimeter, cutaneous cues occur from the indentation of the skin caused by the curvature of the disc (smaller discs cause greater indentation). Simultaneously, the hand configuration cue (relating to the perceived distance between fingers), provides an additional cue to size. These cues may vary in their reliability. In three experiments involving 34 participants, we measured these cues and considered three hypotheses for how humans may use them: humans rely solely on the least noisy cue (Winner-Take-All Model), humans combine cues based on a simple average (Average-Measurement Model), or humans combine cues via an optimal weighted average (Optimally-Weighted Model). Each experiment tested participants using a two-interval forced-choice (2IFC) paradigm with 3D printed disc stimuli. On each trial, under occluded vision, participants felt two discs sequentially and responded which felt larger. Participants were tested with each finger’s cutaneous cue alone, the hand configuration cue alone, and all cues together. In two experiments, the presented discs were both circular. In a third experiment, unknown to participants, some of the presented discs were oval-like cue conflict stimuli. Participant performance was compared to predictions of the cue combination models. We conclude that humans may combine haptic cutaneous and hand configuration cues optimally to judge the size of held objects. / Thesis / Doctor of Philosophy (PhD) / The sense of touch is understudied compared to the senses of sight and hearing. But simply reaching for a coin without looking involves complex calculations and decision-making. We studied how the brain may approach tasks like this. We were interested in how well the brain deals with multiple sources of information that do not always agree with each other. We investigated these questions in computer simulations and in experiments with undergraduate participants. Using carefully designed 3D-printed discs, we tested dozens of participants across 3 different experiments. Our results show that humans may use information in the best possible way and applications relevant to VR and robot-assisted surgery.

Psychophysics is defined as the scientific study of the relationship between stimuli and sensation. It has been used extensively over the last three decades for evaluation and design of manual materials handling tasks in many industries. Despite this, much is still not known about how subjective ratings, the core of the psychophysical methodology, relate to physical (biomechanical) loads. A fundamental assumption of this method is that humans are capable of estimating biomechanical and physiological loads that are placed on the body. Based on this assumption, estimates that are obtained through the methodology are used as an indicator of physical loads and stresses, and are assumed to be related to injury risk. An experiment was performed to achieve two primary goals:1) determine the correspondence between biomechanical loads (moments at the elbow, shoulder and torso) and subjective ratings of joint loads, as well as subjectively determined maximal loads and 2) determine whether any particular joint (i.e. low back, shoulder, elbow) is the limiting factor when a subject determines a maximally acceptable load. Participants were instructed to pose in four different postures, one serving as a baseline (neutral, or 'familiar') posture, while the remaining three varied moments at the elbow, shoulder and torso. While in each of these postures, participants determined a maximum acceptable static load (MASL). Ratings of perceived exertions for specific joints were also reported, as well as whole body ratings while supporting various fractions of the MASL. Experimental findings indicated that subject and posture effects neared significance as main effects on the magnitude of MASL. Strength was shown to be, at best, a weak predictor of MASL. Though no conclusive evidence was found to indicate that a specific joint is the limiting factor when determining maximum acceptability, trends in the data suggested that the low back and shoulder are possible candidates. Overall, the results of the study indicated that humans consider more than simple joint moments when forming perceptions of efforts and acceptability during static load handling. / Master of Science

Thesis (Ph. D.)--University of California, San Diego, 2003. / Vita. Includes bibliographical references (leaves 211-216).

A connectionist architecture is developed that can be used for modeling choice probabilities and reaction times in psychophysics and word recognition. The network architecture consists of a feed-forward network and a decoding module. Learning is by mean-variance back-propagation, an extension of the standard back-propagation learning algorithm. The new learning procedure is interpreted as a selective attention mechanism, and leads to a better model of learning in simple identification tasks than the standard back-propagation. Choice probabilities are modeled by the input/output relations of the network, and reaction times are modeled by the time taken for the network, particularly the decoding module, to achieve a stable state. The model is applied to both unidimensional and multidimensional identification tasks in psychophysics and to word recognition.

Visual images from the two eyes are transmitted to the brain. Because the eyes are horizontally separated, there is a horizontal disparity between the two images. The amount of disparity between the images of a given point depends on the distance of that point from the viewer's point of fixation. A natural visual environment contains surfaces at many different depths. Therefore, the brain must process a spatial distribution of disparities. How are these disparities spatially put together? Brief (about 200 msec) static Cyclopean random-dot stereograms were used as stimuli for vergence and depth discrimination to answer this question. The results indicated a large averaging region for vergence, and a smaller pooling region for depth discrimination. Vergence responded to the mean disparity of two transparent planes. When a disparate target was present in a fixation plane surround, vergence improved as target size was increased, with a saturation at 3-6 degrees. Depth discrimination thresholds improved with target size, reaching a minimum at 1-3 degrees, but increased for larger targets. Depth discrimination showed a dependence on the extent of a disparity pedestal surrounding the target, consistent with vergence facilitation. Vergence might, therefore, implement a coarse-to-fine reduction in binocular matching noise. Interocular decorrelation can be considered as multiple chance matches at different disparities. The spatial pooling limits found for disparity were replicated when interocular decorrelation was discriminated. The disparity of the random dots also influenced the apparent horizontal. alignment of neighbouring monocular lines. This finding suggests that disparity averaging takes place at an early stage of visual processing. The following possible explanations were considered: 1) Disparities are detected in different spatial frequency channels (Marr and Poggio, 1979). 2) Second-order luminance patterns are matched between the two eyes using non-linear channels. 3) Secondary disparity filters process disparities extracted from linear filters.

Colour constancy is defined as the ability to perceive the surface colours of objects within scenes as approximately constant through changes in scene illumination. Colour constancy in real life functions so seamlessly that most people do not realise that the colour of the light emanating from an object can change markedly throughout the day. Constancy measurements made in simple scenes constructed from flat coloured patches do not produce constancy of this high degree. The question that must be asked is: what are the features of everyday scenes that improve constancy? A novel technique is presented for testing colour constancy. Results are presented showing measurements of constancy in simple and complex scenes. More specifically, matching experiments are performed for patches against uniform and multi-patch backgrounds, the latter of which provide colour contrast. Objects created by the addition of shape and 3-D shading information are also matched against backgrounds consisting of matte reflecting patches. In the final set of experiments observers match detailed depictions of objects - rich in chromatic contrast, shading, mutual illumination and other real life features - within depictions of real life scenes. The results show similar performance across the conditions that contain chromatic contrast, although some uncertainty still remains as to whether the results are indicative of human colour constancy performance or to sensory match capabilities. An interesting division exists between patch matches performed against uniform and multi-patch backgrounds that is manifested as a shift in CIE xy space. A simple model of early chromatic processes is proposed and examined in the context of the results.

The simultaneous application of interlaced innocuous warm and cool stimuli (a thermal grill stimulus, TGS) can elicit sensations of burning heat (the Thermal Grill Illusion, TGI). The TGS is thought to alter the central interactions between somatosensory sub-modalities (i.e. cold-inhibition of pain). Previous psychophysical findings point to body site differences in perceptual thermal thresholds. The primary objective of this study was to evaluate whether, using the same TGS, a TGI can be elicited at body sites other than the upper extremity. The present findings indicate that the TGI can be induced at the palm, back, calf, and foot. Pain and unpleasantness in response to the TGS were more frequent and intense following stimulation of the palm and back than the calf and foot. Lower cold pain thresholds were associated with lower pain intensity ratings in response to the TGS. These two findings may reflect differences in central integrative processes.

The simultaneous application of interlaced innocuous warm and cool stimuli (a thermal grill stimulus, TGS) can elicit sensations of burning heat (the Thermal Grill Illusion, TGI). The TGS is thought to alter the central interactions between somatosensory sub-modalities (i.e. cold-inhibition of pain). Previous psychophysical findings point to body site differences in perceptual thermal thresholds. The primary objective of this study was to evaluate whether, using the same TGS, a TGI can be elicited at body sites other than the upper extremity. The present findings indicate that the TGI can be induced at the palm, back, calf, and foot. Pain and unpleasantness in response to the TGS were more frequent and intense following stimulation of the palm and back than the calf and foot. Lower cold pain thresholds were associated with lower pain intensity ratings in response to the TGS. These two findings may reflect differences in central integrative processes.

Thesis (M.S.)--Rutgers University, 2009. / "Graduate Program in Psychology." Includes bibliographical references (p. 26-27).