The cortical activity of subjects with compromised central vision (eg, amblyopes) is thought to b... more The cortical activity of subjects with compromised central vision (eg, amblyopes) is thought to be much weaker for stimulation of the affected eye than in the fellow eye. Because these subjects are known to exhibit considerable difficulties in keeping steady fixation, we ...
Proceedings of the 2010 Symposium on Eye-Tracking Research & Applications - ETRA '10, 2010
Recent advances in high magnification retinal imaging have allowed for visualization of individua... more Recent advances in high magnification retinal imaging have allowed for visualization of individual retinal photoreceptors, but these systems also suffer from distortions due to fixational eye motion. Algorithms developed to remove these distortions have the added benefit of providing arc second level resolution of the eye movements that produce them. The system also allows for visualization of targets on the retina, allowing for absolute retinal position measures to the level of individual cones. This paper will describe the process used to remove the eye movement artifacts and present analysis of their spectral characteristics. We find a roughly 1/f amplitude spectrum similar to that reported by Findlay (1971) with no evidence for a distinct tremor component.
Persons with amblyopia, especially those with strabismus, are known to exhibit abnormal fixationa... more Persons with amblyopia, especially those with strabismus, are known to exhibit abnormal fixational eye movements. In this paper, we compared six characteristics of fixational eye movements among normal control eyes (n=16), the non-amblyopic fellow eyes and the amblyopic eyes of anisometropic (n=14) and strabismic amblyopes (n=14). These characteristics include the frequency, magnitude of landing errors, amplitude and speed of microsaccades, and the amplitude and speed of slow drifts. Fixational eye movements were recorded using retinal imaging while observers monocularly fixated a 1° cross. Eye position data were recovered using a cross-correlation procedure. We found that in general, the characteristics of fixational eye movements are not significantly different between the fellow eyes of amblyopes and controls, and that the strabismic amblyopic eyes are always different from the other groups. Next, we determined the primary factors that limit fixation stability and visual acuity i...
Visual acuity (VA) measurement by the use of letter charts is the most frequently used method for... more Visual acuity (VA) measurement by the use of letter charts is the most frequently used method for testing foveal function. However, since a decimal visual acuity of 1.0, often used as limit for normality, can be achieved with less than two-thirds of the normal number of optic nerve axons, the letter chart test cannot be regarded as sensitive to low-degree damage. The reason why letter charts are unable to detect small and subtle defects is due to the high receptor density and the proportionally large stimuli used in VA testing. The Rarebit technique was developed with the explicit aim to improve detection of small defects. The method is built on the principle of detection of very small and bright stimuli, corresponding to half the normal minimum angle of resolution in the tested retinal location. The test principle is easy to understand, the results are easy to interpret, and no expensive equipment is required. The system includes two tests Rarebit Perimetry (RBP) and Rarebit Fovea Test (RFT), the first for evaluation of the 30º visual field (with the possibility to test out to 60º), and the second for evaluation of the foveal function i.e., the most central 4º VF. Several studies have shown that conventional perimetry and vision function tests are insensitive to minor neuro-visual damage. In Rarebit improved detection of small defects, is intended to be obtained due to two deliberate deviations from standard procedures. Target information content is minimized (i.e., small dots compared to the receptive field at the tested retinal location, briefly exposed) and thresholding is replaced by simple probing of the completeness of the retino-cortical detector matrix. The aim of the current thesis was twofold. Paper I evaluated the physiologic properties of the test, i.e., the effect of age, binocular summation and different luminance levels in the test stimuli, in normal subjects. In Paper III the fixation stability during the test was studied. In order to determine the potential clinical value, patients with diabetes and cataract were examined. In the patients with diabetes, the results from the RFT were compared with fundus photographs and OCT measurements. The cataract patients were examined with both RBP and RFT prior to and after cataract surgery (Paper II and IV). The findings from the studies support the idea behind the RFT, i.e., that small stimuli might be useful for detecting small defects in the neuro-retinal architecture and can be expected to give additional information about the visual system, compared to findings from conventional tests, e.g., VA and funduscopic examinations. The RFT can be expected to be of clinical value in patients where early identification of damage is relevant and sensitive methods for follow-up are required.
People with central vision loss often prefer boldface print over normal print for reading. Howeve... more People with central vision loss often prefer boldface print over normal print for reading. However, little is known about how reading speed is influenced by the letter-stroke boldness of font. In this study, we examined the reliance of reading speed on stroke boldness, and determined whether this reliance differs between the normal central and peripheral vision. Reading speed was measured using the rapid serial visual presentation paradigm, where observers with normal vision read aloud short single sentences presented on a computer monitor, one word at a time. Text was rendered in Courier at six levels of boldness, defined as the stroke-width normalized to that of the standard Courier font: 0.27, 0.72, 1, 1.48, 1.89 and 3.04× the standard. Testings were conducted at the fovea and 10° in the inferior visual field. Print sizes used were 0.8× and 1.4× the critical print size (smallest print size that can be read at the maximum reading speed). At the fovea, reading speed was invariant f...
The cortical activity of subjects with compromised central vision (e.g., amblyopes) is thought to... more The cortical activity of subjects with compromised central vision (e.g., amblyopes) is thought to be much weaker for stimulation of the affected eye than in the fellow eye. Because these subjects are known to exhibit considerable difficulties in keeping steady fixation, we investigated the effects of anomalous fixation on their multifocal visual-evoked potential (mfVEP) responses using a dual Purkinje image (dPi) eye tracker. Our results show that mfVEP responses to stimulation of the central 5 degrees were depressed in the affected eye compared to those in the normal eye and the magnitude of response reductions was proportional to the degree of visual acuity loss in amblyopic subjects. Fixation was far less stable while viewing with the affected eye than with the fellow eye, some exhibiting jerk nystagmus and/or saccadic oscillations. Normal subjects with artificially imposed nystagmus showed similar reductions of VEP responses. The relative magnitudes of the deficits in mfVEP responses were tightly correlated with the degree of fixation instability. These results suggest that the interpretation of anomalous neural or perceptual processing in amblyopic subjects must take the effects of unsteady fixation during measurements into consideration in order to reveal the true nature and extent of sensory neural deficits.
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Papers by Girish Kumar