The role of chromatic texture and 3D shape in colour discrimination, memory colour, and colour constancy of natural objects

The primary goal of this work was to investigate colour perception in a natural environment and to contribute to the understanding of how cues to familiar object identity influence colour appearance. A large number of studies on colour appearance employ 2D uniformly coloured patches, discarding perceptual cues such as binocular disparity, 3D luminance shading, mutual reflection, and glossy highlights are integral part of a natural scene. Moreover, natural objects possess specific cues that help our recognition (shape, surface texture or colour distribution). The aim of the first main experiment presented in this thesis was to understand the effect of shape on (1) memory colour under constant and varying illumination and on (2) colour constancy for uniformly coloured stimuli. The results demonstrated the existence of a range of memory colours associated with a familiar object, the size of which was strongly object-shape-dependent. For all objects, memory retrieval was significantly faster for object-diagnostic shape relative to generic shapes. Based on two successive controls, the author suggests that shape cues to the object identity affect the range of memory colour proportionally to the original object chromatic distribution. The second experiment examined the subject’s accuracy and precision in adjusting a stimulus colour to its typical appearance. Independently on the illuminant, results showed that memory colour accuracy and precision were enhanced by the presence of chromatic textures, diagnostic shapes, or 3D configurations with a strong interaction between diagnosticity and dimensionality of the shape. Hence, more cues to the object identity and more natural stimuli facilitate the observers in accessing their colour information from memory. A direct relationship was demonstrated between chromatic surface representation, object’s physical properties, and identificability and dimensionality of shape on memory colour accuracy, suggesting high-level mechanisms. Chromatic textures facilitated colour constancy. The third and fourth experiments tested the subject’s ability to discriminate between two chromatic stimuli in a simultaneous and successive 2AFC task, respectively. Simultaneous discrimination threshold performances for polychromatic surfaces were only due to low-level mechanism of the stimulus, whereas in the successive discrimination, i.e. when memory is involved, high-level mechanisms were established. The effect of shape was strongly task- dependent and was modulate by the object memory colour. These findings together with the strong interaction between chromatic cues and shape cues to the object identity lead to the conclusion that high level mechanisms linked to object recognition facilitated both tasks. Hence, the current thesis presents new findings on memory colour and colour constancy presented in a natural context and demonstrates the effect of high-level mechanisms in chromatic discrimination as a function of cues to the object identity such as shape and texture. This work contributes to a deeper understanding of colour perception and object recognition in the natural world.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Simulation of thalamic prosthetic vision: reading accuracy, speed, and acuity in sighted humans

The psychophysics of reading with artificial sight has received increasing attention as visual prostheses are becoming a real possibility to restore useful function to the blind through the coarse, pseudo-pixelized vision they generate. Studies to date have focused on simulating retinal and cortical prostheses; here we extend that work to report on thalamic designs. This study examined the reading performance of normally sighted human subjects using a simulation of three thalamic visual prostheses that varied in phosphene count, to help understand the level of functional ability afforded by thalamic designs in a task of daily living. Reading accuracy, reading speed, and reading acuity of 20 subjects were measured as a function of letter size, using a task based on the MNREAD chart. Results showed that fluid reading was feasible with appropriate combinations of letter size and phosphene count, and performance degraded smoothly as font size was decreased, with an approximate doubling of phosphene count resulting in an increase of 0.2 logMAR in acuity. Results here were consistent with previous results from our laboratory. Results were also consistent with those from the literature, despite using naive subjects who were not trained on the simulator, in contrast to other reports

Visual Acuity of Simulated Thalamic Visual Prostheses in Normally Sighted Humans

Simulation in normally sighted individuals is a crucial tool to evaluate the performance of potential visual prosthesis designs prior to human implantation of a device. Here, we investigated the effects of electrode count on visual acuity, learning rate and response time in 16 normally sighted subjects using a simulated thalamic visual prosthesis, providing the first performance reports for thalamic designs. A new letter recognition paradigm using a multiple-optotype two-alternative forced choice task was adapted from the Snellen eye chart, and specifically devised to be readily communicated to both human and non-human primate subjects. Validation of the method against a standard Snellen acuity test in 21 human subjects showed no significant differences between the two tests. The novel task was then used to address three questions about simulations of the center-weighted phosphene patterns typical of thalamic designs: What are the expected Snellen acuities for devices with varying numbers of contacts, do subjects display rapid adaptation to the new visual modality, and can response time in the task provide clues to the mechanisms of perception in low-resolution artificial vision? Population performance (hit rate) was significantly above chance when viewing Snellen 20/200 optotypes (Log MAR 1.0) with 370 phosphenes in the central 10 degrees of vision, ranging to Snellen 20/800 (Log MAR 1.6) with 25 central phosphenes. Furthermore, subjects demonstrated learning within the 1–2 hours of task experience indicating the potential for an effective rehabilitation and possibly better visual performance after a longer period of training. Response time differences suggest that direct letter perception occurred when hit rate was above 75%, whereas a slower strategy like feature-based pattern matching was used in conditions of lower relative resolution. As pattern matching can substantially boost effective acuity, these results suggest post-implant therapy should specifically address feature detection skills

Perceptual learning in a non-human primate model of artificial vision

Visual perceptual grouping, the process of forming global percepts from discrete elements, is experience-dependent. Here we show that the learning time course in an animal model of artificial vision is predicted primarily from the density of visual elements. Three naïve adult non-human primates were tasked with recognizing the letters of the Roman alphabet presented at variable size and visualized through patterns of discrete visual elements, specifically, simulated phosphenes mimicking a thalamic visual prosthesis. The animals viewed a spatially static letter using a gaze-contingent pattern and then chose, by gaze fixation, between a matching letter and a non-matching distractor. Months of learning were required for the animals to recognize letters using simulated phosphene vision. Learning rates increased in proportion to the mean density of the phosphenes in each pattern. Furthermore, skill acquisition transferred from trained to untrained patterns, not depending on the precise retinal layout of the simulated phosphenes. Taken together, the findings suggest that learning of perceptual grouping in a gaze-contingent visual prosthesis can be described simply by the density of visual activation

Validation analysis.

<p>Bland-Altman plot of the differences in visual acuity scores between our letter recognition task and the Snellen chart test. The vertical axis displays the difference between measurements on both tasks, and the horizontal axis displays the mean acuity score value between the tasks. The solid line is the mean difference in LogMAR acuity (−0.02), and the area within the dotted lines represents the limits of agreement between LogMAR scores in the two visual acuity tasks. Each point represents the results from a single subject (blue circles), although some are overlapped (blue circles with black centers).</p

Photographs of the apparatus.

<p>A subject is shown performing the experiment (left) along with a close-up of the goggles with eye tracking camera (right). In use, the head frame is adjusted to the seated height of each subject, and the monitor position adjusted to maintain a consistent distance to each subject's eyes. The base of the head frame is mechanically secured to the desk, but the upper part can be raised or lowered. The camera is mounted on a flexible arm that holds position once adjusted for a close-up view of the subject's eye. Experiment workstations and experimentor controls are not shown.</p

Comparison between paradigms.

<p>Testing using the Snellen chart (left) proceeds top to bottom, and left to right, with the subject calling out each letter on successive rows, or declaring their inability to do so. Testing started with the 20/50 line as shown here, which is the fourth line of standard charts. Testing using our letter recognition task (right) is performed in a balanced, randomly interleaved pattern; the figure, designed to show the equivalence between tests at given acuities, uses shuffle arrows to imply the interleaving. Trial conditions are represented as a cue letter followed by the two alternatives. Red and Green highlights illustrate single letter scoring (correct/incorrect) during an example data collection session where the subject was assessed with 20/18 on the Snellen task and 20/20 on the letter recognition task.</p

Learning effects.

<p>Scatter plots of population hit rate and response time in the first versus last segment of trials. (LEFT) Population hit rates split by condition (stars) and overall mean (bulls eye). Data are above the line of equality as subjects perform more accurately in the last 200 trials (10 per condition) than in the first 200 trials. (CENTER) Population response time for each condition (stars) and overall mean (bulls eye). Nearly every datum is below the line of equality as subjects have faster responses in the last 200 trials than in the first 200. (RIGHT) Combining population response times and hit rates from the two previous graphs with linkages between first (open circles) and last (filled stars) segments reveals two distinct spans of behavior, one at low hit rates that is more disorganized, and one at higher hit rates that displays strong structure. The lower hit rates correspond to the Chance regime where subjects are more likely to be guessing; the higher hit rates correspond to the Mid-Range and High-Performance regimes. The largest improvements, as shown by the longest linkages, cluster around the threshold between Chance and Mid-Range regimes.</p

Stimulus combinations.

<p>The combinations of optotype size F₁–F₅, Snellen 20/100 through 20/1600 (horizontal direction) and phosphene pattern P₁–P₄ (vertical direction) are shown in this collection of snapshots of the center part of the subject display during the Free View phase. Parameter values were selected such that it was impossible for subjects to identify letters with the lowest resolution phosphene pattern viewing the smallest optotypes (lower left subfigure) to provide a negative control, and quite easy with the highest resolution pattern viewing the largest optotypes (upper right subfigure) to provide a positive control. Snapshots shown are for gaze positions at the center of the monitor. Only the centralmost 10° of stimuli are shown; the remainder of the screen would be uniformly gray. Stimuli, animated through the gaze-contingent mechanism, were more readily identifiable than the static images above might suggest.</p

Validation experiment optotype sizes.

<p>Corresponding Snellen and LogMAR visual acuity values of optotypes used in the Validation experiment. These irregularly spaced sizes correspond to the lower eight lines of the standard Snellen chart.</p