Biomedical subjects

Gordon E Legge

Publications and source records attributed to Gordon E Legge.

11 recordsLinked to original sources

PubMed2006

Wayfinding with words: spatial learning and navigation using dynamically updated verbal descriptions.

This work investigates whether large-scale indoor layouts can be learned and navigated non-visually, using verbal descriptions of layout geometry that are updated, e.g. contingent on a participant's location in a building. In previous research, verbal information has been used to facilitate route following, not to support free exploration and wayfinding. Our results with blindfolded-sighted participants demonstrate that accurate learning and wayfinding performance is possible using verbal descriptions and that it is sufficient to describe only local geometric detail. In addition, no differences in learning or navigation performance were observed between the verbal study and a control study using visual input. Verbal learning was also compared to the performance of a random walk model, demonstrating that human search behavior is not based on chance decision-making. However, the model performed more like human participants after adding a constraint that biased it against reversing direction.

Adolescent↗

PubMed2006

Lost in virtual space: studies in human and ideal spatial navigation.

The authors describe 3 human spatial navigation experiments that investigate how limitations of perception, memory, uncertainty, and decision strategy affect human spatial navigation performance. To better understand the effect of these variables on human navigation performance, the authors developed an ideal-navigator model for indoor navigation whose optimizing algorithm uses a partially observable Markov decision process. The model minimizes the number of actions (translations and rotations) required to move from an unknown starting state to a specific goal state in indoor environments that have perceptual ambiguity. The authors compared the model's performance with that of the human observer to measure human navigation efficiency. Experiment 1 investigated the effect of increasing the layout size on spatial way-finding efficiency and found that participants' efficiencies decreased as layout size increased. The authors investigated whether this reduction in navigation efficiency was due to visual perception (Experiment 2), memory, spatial updating strategy, or decision strategy (Experiment 3).

Adult↗

PubMed2006

Low-vision reading speed: influences of linguistic inference and aging.

PURPOSE: Reading is a dynamic task involving both linguistic and visual analysis. In this study, we asked how two types of linguistic information--characters used in segmenting words from one another, and sentence context--differ in their usefulness for people with normal and low vision. Given evidence for age-related differences in some forms of cognitive processing, we also investigated the effect of age. METHODS: There were four groups of 10 participants: vision status (normal, low) crossed with age (young, <35 years; old, >65 years). Reading speeds were compared for regularly spaced text and text in which the spaces were removed, a manipulation intended to eliminate local cues for text segmentation and force attention to clusters of letters or whole words. We also evaluated the effect of sentence context by comparing reading speeds for regular sentences and sentences in which word order was scrambled. RESULTS: Removal of spaces had a greater impact on low vision than normal vision, reducing average speeds to 45% and 66% of speeds for regularly spaced text, respectively. We interpret this to mean that people with low vision have less access to spatially distributed linguistic regularities of text such as prefixes, suffixes, or word length. Removal of sentence context through scrambling had a greater impact on normal vision than low vision, reducing mean reading speed to 53% and 66%, respectively. Finally, comparison of our young and old readers showed no major differences in the use of sentence context or in the impact of removing spaces between words. CONCLUSIONS: People with low vision appear to rely more on spacing information in sentences, whereas people with normal vision appear to make better use of sentence context, irrespective of age.

Adult↗

PubMed2005

The effect of print size on reading speed in dyslexia.

This article details a study which predicted that across a wide range of print sizes dyslexic reading would follow the same curve shape as skilled reading, with constant reading rates across large print sizes and a sharp decline in reading rates below a critical print size. It also predicted that dyslexic readers would require larger critical print sizes to attain their maximum reading speeds, following the letter position coding deficit hypothesis. Reading speed was measured across twelve print sizes ranging from Snellen equivalents of 20/12 to 20/200 letter sizes for a group of dyslexic readers in Grades 2 to 4 (aged 7 to 10 years), and for non-dyslexic readers in Grades 1 to 3 (aged 6 to 8 years). The groups were equated for word reading ability. Results confirmed that reading rate-by-print size curves followed the same two-limbed shape for dyslexic and non-dyslexic readers. Dyslexic reading curves showed higher critical print sizes and shallower reading rate-by-print size slopes below the critical print size, consistent with the hypothesis of a letter-position coding deficit. Non-dyslexic reading curves also showed a decrease of critical print size with age. A developmental lag model of dyslexic reading does not account for the results, since the regression of critical print size on maximum reading rate differed between groups.

Journal Article↗

PubMed2004

Letter-recognition and reading speed in peripheral vision benefit from perceptual learning.

Visual-span profiles are plots of letter-recognition accuracy as a function of letter position left or right of the midline. Previously, we have shown that contraction of these profiles in peripheral vision can account for slow reading speed in peripheral vision. In this study, we asked two questions: (1) can we modify visual-span profiles through training on letter-recognition, and if so, (2) are these changes accompanied by changes in reading speed? Eighteen normally sighted observers were randomly assigned to one of three groups: training at 10 degrees in the upper visual field, training at 10 degrees in the lower visual field and a no-training control group. We compared observers' characteristics of reading (maximum reading speed and critical print size) and visual-span profiles (peak amplitude and bits of information transmitted) before and after training, and at trained and untrained retinal locations (10 degrees upper and lower visual fields). Reading speeds were measured for six print sizes at each retinal location, using the rapid serial visual presentation paradigm. Visual-span profiles were measured using a trigram letter-recognition task, for a letter size equivalent to 1.4x the critical print size for reading. Training consisted of the repeated measurement of 20 visual-span profiles (over four consecutive days) in either the upper or lower visual field. We also tracked the changes in performance in a sub-group of observers for up to three months following training. We found that the visual-span profiles can be expanded (bits of information transmitted increased by 6 bits) through training with a letter-recognition task, and that there is an accompanying increase (41%) in the maximum reading speed. These improvements transferred, to a large extent, from the trained to an untrained retinal location, and were retained, to a large extent, for at least three months following training. Our results are consistent with the view that the visual span is a bottleneck on reading speed, but a bottleneck that can be increased with practice.

Analysis of Variance↗

PubMed2003

Is word recognition different in central and peripheral vision?

Peripheral vision plays an important role in normal reading, but its role becomes larger for visually impaired people with central-field loss. This experiment studied whether lexical processing differs in central and peripheral vision through the analysis of word-frequency effects in lexical decisions. We asked two main questions: (1) Do central and peripheral vision differ in the time course of lexical processing? and (2) do central and peripheral vision differ in the quality of lexical processing? To address the first question, we examined the time course of frequency effects in central and peripheral vision over a range from 25 to 500 ms. We found that significant frequency effects occurred for the shortest exposures, 25-50 ms, in central vision, whereas significant frequency effects did not occur in peripheral vision until 100 ms. To address the second question, we used word-frequency effects as a marker for the nature of lexical processing. We compared frequency effects in central and peripheral vision for data within matched ranges of percent accuracy (0-20%, 20-40%, 40-60%, 60-80%, and 80-100%). We found that there was no difference in the pattern of frequency effects in central and peripheral vision at equivalent performance levels. We conclude that lexical processing is slower in peripheral vision, but the quality of lexical processing is similar in central and peripheral vision.

Adolescent↗

PubMed2002

Spatial-frequency properties of letter identification in amblyopia.

Amblyopia is characterized by a deficit in identifying small letters (acuity deficit) in the absence of identifiable ocular pathology. One explanation for this deficit is that the amblyopic visual system lacks appropriate channels tuned to high spatial frequencies for identifying small letters. The purpose of this study was to examine the spatial-frequency properties of letter identification in the amblyopic visual system. To do so, we measured contrast thresholds for identifying letters that were band-pass filtered to different bands of spatial frequencies, for letter sizes ranging from 2x to 19.2x larger than acuity letters. Letters were digitally filtered using a set of band-pass filters, with peak object spatial frequencies ranging from 0.88 to 10 c/letter. The bandwidth of the filters was 1 octave. For any given letter size, contrast sensitivity for identifying letters exhibits a spatial-tuning function. The shape of these tuning functions was found to be similar between amblyopic and non-amblyopic eyes, and across all letter sizes. The peak of these functions shifted progressively toward lower object spatial frequency when the letter size became smaller. When compared with the non-amblyopic eyes, the amblyopic eyes have a limited range of tuning functions sensitive to letters. However, when scaled with respect to acuity, the relationship between the peak frequency of the tuning functions and letter size becomes essentially identical in the amblyopic and non-amblyopic eyes. An ideal-observer analysis that takes into account spectral information about letter identity and the contrast-sensitivity function of the observer, but does not invoke narrow-band channels, also shows that the properties of the tuning functions for letter identification are similar between the amblyopic and non-amblyopic eyes. We conclude that the deficit in identifying small letters in amblyopes is not attributable to differences in the shape or selection of "channels", when compared with the normal visual system. Rather, it is a consequence of the difference in the resolution limit between the amblyopic and the non-amblyopic eyes.

Adolescent↗

PubMed2002

Spatial-frequency characteristics of letter identification in central and peripheral vision.

Spatial-frequency characteristics of letter identification are much better understood in the fovea than in the periphery. The purpose of this study was to compare the spatial-frequency characteristics of letter identification in central and peripheral vision. We measured contrast thresholds for identifying single, Times-Roman lower-case letters that were spatially band-pass filtered. Each of the 26 letters was digitally filtered with a set of nine cosine log filters, with peak object spatial frequencies ranging from 0.63 to 10 c/letter, in half-octave steps. Bandwidth of the filters was 1 octave. Three observers with normal vision were each tested monocularly at the fovea, and at 5 degrees and 10 degrees in the inferior visual field. Letter sizes were 0.2, 0.4 and 0.6 log units larger than high contrast, unfiltered acuity letters. Plots of contrast sensitivity for letter identification vs. frequency of the band-pass filters exhibit spatial tuning. In general, the spatial-frequency characteristics of letter identification are fundamentally identical between central and peripheral vision. These characteristics include the scaling of the peak frequency of the spatial-tuning functions with letter size and the bandwidth of the tuning functions. The only difference between the fovea and the periphery is that for the same physical letter size, peak sensitivity of the spatial-tuning functions occurs at a higher retinal frequency at the fovea than in the periphery. To test whether or not the contrast sensitivity function (CSF) can account for the differences in the spatial-frequency characteristics of letter identification between central and peripheral vision, we incorporated a human CSF into an ideal-observer model, and tested the performance of this ideal-observer on the same letter identification task used with the human observers. Data from this CSF-ideal-observer resemble closely those of human observers, suggesting that the spatial-frequency characteristics of human letter identification can be accounted for by the CSF and the letter-identity information, without invoking selection among narrow-band spatial-frequency channels.

Contrast Sensitivity↗

PubMed2002

Mr. Chips 2002: new insights from an ideal-observer model of reading.

The integration of visual, lexical, and oculomotor information is a critical part of reading. Mr. Chips is an ideal-observer model that combines these sources of information optimally to read simple texts in the minimum number of saccades. This model provides a computational framework for interpreting human reading saccades in both normal and low vision. The purpose of this paper is to report performance of the model for conditions emulating reading with normal vision--a visual span of nine characters, multiplicative saccade noise with a standard deviation of 30%, and texts based on three full-length children's books. Comparison of fixation locations by humans and Mr. Chips revealed: (1) that both exhibit very similar word-skipping behavior; (2) both show initial fixations near the center of words, but with a systematic difference suggestive of an asymmetry in the human visual span; and (3) differences in the pattern of refixations within words that may uncover non-optimal lexical inference by human readers. A human context effect--30% difference in mean saccade size between continuous text and random sequences of words--was very similar to the 25% effect for the model associated with a corresponding difference in the predictability of text words. Overall, our findings show that many of the complicated aspects of human reading saccades can be explained concisely by early information-processing constraints.

Computer Simulation↗

PubMed2002

Preneural limitations on letter identification in central and peripheral vision.

We created a sequential ideal-observer model that could address the question, How much of letter identification performance and its change with eccentricity can be accounted for by preneural factors? The ideal-observer model takes into account preneural factors including the stimulus rendering properties of a CRT display, the optical imaging quality of the eye, and photon capture and sampling characteristics of the cones. We validated the formulation of the model by comparing its performance on simple psychophysical tasks with that of previous sequential ideal-observer models. The model was used to study properties of the image rendering of letters. For example, the model's identification of high-resolution letters (i.e., many pixels per letter), but not low-resolution letters, is largely immune to changes in pixel width. We compared human and ideal-observer letter-identification acuity for the lowercase alphabet at 0 degrees, 5 degrees, and 20 retinal eccentricity. Acuity of the ideal observer for high-contrast letters is approximately seven times better than that of the human observers at 0 degrees. Acuity decreased with eccentricity more rapidly for human observers than for the ideal observer such that the thresholds differed by a factor of 50 at 20 degrees. A decrease in stimulus duration from 100 to 33 ms resulted in no decrease in relative threshold size between the human and ideal observers at all eccentricities, indicating that humans effectively integrate stimulus information over this range. Decreasing contrast from 75% to 25%, however, reduced the difference in acuities twofold at all eccentricities between humans and the ideal-observer model, consistent with the presence a compressive nonlinearity only in the human observers. The gap between human and ideal acuity in central vision means that there are substantial limitations in human letter recognition beyond the stage of photoreceptor sampling. The increasing performance gap between human and ideal-observer performance with eccentricity implicates an increasing role of neural limitations with eccentricity in limiting human letter identification.

Adult↗

PubMedDate not supplied

Functional and cortical adaptations to central vision loss.

Age-related macular degeneration (AMD), affecting the retina, afflicts one out of ten people aged 80 years or older in the United States. AMD often results in vision loss to the central 15-20 deg of the visual field (i.e. central scotoma), and frequently afflicts both eyes. In most cases, when the central scotoma includes the fovea, patients will adopt an eccentric preferred retinal locus (PRL) for fixation. The onset of a central scotoma results in the absence of retinal inputs to corresponding regions of retinotopically mapped visual cortex. Animal studies have shown evidence for reorganization in adult mammals for such cortical areas following experimentally induced central scotomata. However, it is still unknown whether reorganization occurs in primary visual cortex (V1) of AMD patients. Nor is it known whether the adoption of a PRL corresponds to changes to the retinotopic mapping of V1. Two recent advances hold out the promise for addressing these issues and for contributing to the rehabilitation of AMD patients: improved methods for assessing visual function across the fields of AMD patients using the scanning laser ophthalmoscope, and the advent of brain-imaging methods for studying retinotopic mapping in humans. For the most part, specialists in these two areas come from different disciplines and communities, with few opportunities to interact. The purpose of this review is to summarize key findings on both the clinical and neuroscience issues related to questions about visual adaptation in AMD patients.

Adaptation, Biological↗