An automated segmentation approach to calibrating infantile nystagmus waveforms.

Infantile nystagmus (IN) describes a regular, repetitive movement of the eyes. A characteristic feature of each cycle of the IN eye movement waveform is a period in which the eyes are moving at minimal velocity. This so-called "foveation" period has long been considered the basis for the best vision in individuals with IN. In recent years, the technology for measuring eye movements has improved considerably, but there remains the challenge of calibrating the direction of gaze in tracking systems when the eyes are continuously moving. Identifying portions of the nystagmus waveform suitable for calibration typically involves time-consuming manual selection of the foveation periods from the eye trace. Without an accurate calibration, the exact parameters of the waveform cannot be determined. In this study, we present an automated method for segmenting IN waveforms with the purpose of determining the foveation positions to be used for calibration of an eye tracker. On average, the "point of regard" was found to be within 0.21° of that determined by hand-marking by an expert observer. This method enables rapid clinical quantification of waveforms and the possibility of gaze-contingent research paradigms being performed with this patient group.

Spatial summation across the visual field in strabismic and anisometropic amblyopia.

Ricco's area (the largest area of visual space in which stimulus area and intensity are inversely proportional at threshold) has previously been hypothesised to be a result of centre/surround antagonism in retinal ganglion cell receptive fields, but recent evidence suggests a sizeable cortical contribution. Here, Ricco's area was measured in amblyopia, a condition in which retinal receptive fields are normal, to better understand its physiological basis. Spatial summation functions were determined at 12 visual field locations in both eyes of 14 amblyopic adults and 15 normal-sighted controls. Ricco's area was significantly larger in amblyopic eyes than in fellow non-amblyopic eyes. Compared to the size of Ricco's area in control eyes, Ricco's area measured significantly larger in amblyopic eyes. Additionally, Ricco's area in the fellow, non-amblyopic eye of amblyopic participants measured significantly smaller than in control eyes. Compared to controls, Ricco's area was larger in amblyopic eyes and smaller in fellow non-amblyopic eyes. Amblyopia type, binocularity, and inter-ocular difference in visual acuity were significantly associated with inter-ocular differences in Ricco's area in amblyopes. The physiological basis for Ricco's area is unlikely to be confined to the retina, but more likely representative of spatial summation at multiple sites along the visual pathway.

Grating visual acuity in infantile nystagmus in the absence of image motion.

PURPOSE: Infantile nystagmus (IN) consists of largely horizontal oscillations of the eyes that usually begin shortly after birth. The condition is almost always associated with lower-than-normal visual acuity (VA). This is assumed to be at least partially due to motion blur induced by the eye movements. Here, we investigated the effect of image motion on VA. METHODS: Grating stimuli were presented, illuminated by either multiple tachistoscopic flashes (0.76 ms) to circumvent retinal image motion, or under constant illumination, to subjects with horizontal idiopathic IN and controls. A staircase procedure was used to estimate VA (by judging direction of tilt) under each condition. Orientation-specific effects were investigated by testing gratings oriented about both the horizontal and vertical axes. RESULTS: Nystagmats had poorer VA than controls under both constant and tachistoscopic illumination. Neither group showed a significant difference in VA between illumination conditions. Nystagmats performed worse for vertically oriented gratings, even under tachistoscopic conditions (P < 0.01), but there was no significant effect of orientation in controls. CONCLUSIONS: The fact that VA was not significantly affected by either illumination condition strongly suggests that the eye movements themselves do not significantly degrade VA in adults with IN. Treatments and therapies that seek to modify and/or reduce eye movements may therefore be fundamentally limited in any improvement that can be achieved with respect to VA.

Stress and visual function in infantile nystagmus syndrome.

PURPOSE: Infantile nystagmus syndrome (INS) is an involuntary oscillation of the eyes that has been reported to impair vision and worsen under stress. This investigation aimed to measure visual function in terms of visual acuity (VA) and response time (RT), when INS subjects are placed under stress. METHODS: A total of 23 subjects with INS and 20 control subjects performed a 2-alternative forced choice (2AFC) staircase procedure identifying the gap in a Landolt C, under 4 experimental conditions: initial acclimatization (A); task demand (TD), during which subjects received a small electrical shock for every incorrect answer; anticipatory anxiety (AA), during which subjects received a small shock at random intervals; and relaxed (R). Arousal was monitored with galvanic skin conductance (SkC). In addition to VA and eye movements, RTs were recorded. RESULTS: The SkC was higher in the TD and AA periods and lower during A and R. Shock significantly increased nystagmus amplitude (P < 0.01) and intensity (P < 0.007), and reduced foveation periods (FPs, P < 0.022). In both groups, VA was not reduced, but showed a slight improvement. However, shock increased RT (P < 0.009), and INS subjects were slower than controls (P < 0.0005). CONCLUSIONS: Increased arousal ("stress") provoked more intense nystagmus eye movements. As seen in other studies, stress did not reduce VA despite the shorter FPs. Although VA and FP can correlate across subjects, there would appear to be little correlation, if any, within a subject. However, RTs did increase with stress and shorter FPs, which may have an adverse impact on the visual performance of those with INS.

Are high-pass resolution perimetry thresholds sampling limited or optically limited?

PURPOSE: It has been reported that high-pass resolution perimetry (HRP) provides a means of noninvasively determining retinal ganglion cell density. However, there is evidence to suggest that this may not be true. The purpose of the present study was to determine whether HRP thresholds are sampling limited, which is a necessary condition for being able to determine retinal ganglion cell density psychophysically. METHODS: This study measured resolution and detection performance for a range of grating-based stimuli under the testing conditions that HRP uses and compared these with performance of the ring stimulus. RESULTS: The results show that detection and resolution acuity under HRP test conditions were often equivalent, in accordance with previous investigations. However, the results also show that the thresholds underestimated the true level of resolution acuity in the periphery because increasing stimulus contrast increased performance. CONCLUSION: These findings suggest that HRP thresholds cannot be regarded as sampling limited, but rather they are optically limited. We therefore conclude that HRP thresholds cannot be regarded as a direct measure of the underlying ganglion cell density.