Biomedical subjects

Anne Seidemann

Publications and source records attributed to Anne Seidemann.

4 recordsLinked to original sources

PubMed2004

Stimulating human accommodation without changes in focus.

PURPOSE: Inspired by the finding in chickens that preferential stimulation of the ON retinal system suppresses myopia induced by negative spectacle lens wear and that stimulation of the OFF system suppresses the hyperopia induced by positive lens wear, we sought to determine whether stimulation of the ON-OFF retinal systems could drive directional accommodation responses in humans. If emmetropisation and accommodation use similar image processing algorithms, more accommodation would be expected with OFF stimulation. METHODS: Accommodation responses were measured while viewing a computer-generated pattern designed to stimulate the ON-OFF systems. The stimulus comprised a rectangular field (12 x 9.5 cm) on a black background filled with 196 discs (diameters: 0.4-1.0 cm). These were presented on an LCD monitor in a dark room at a viewing distance of 55 cm (1.8 D). Thirteen subjects aged 21-37 years took part. The individual discs had saw-tooth shaped temporal luminance profiles with the same time period but with random phases with respect to each other, so that the mean brightness of the stimulus was constant. To eliminate accommodation responses based on other cues (i.e. proximity) a 0.5 mm artificial pupil was used to open the accommodation loop. Refraction in the vertical pupil meridian was continuously recorded with an infrared photorefractor (the PowerRefractor). To verify that computer-based stimuli presented within our experimental design were effective in driving accommodation, previously studied stimuli were also tested: changes in size (looming) and incremental low pass filtering. RESULTS: Preferential stimulation of the ON or OFF subsystems produced a convincing depth illusion in all subjects (which was psychophysically confirmed in four subjects). Although the stimulus appeared to move in depth it did not produce accommodation responses that were consistent with that, i.e. the accommodation system did not appear to fluctuate in rhythm with the temporal oscillations of the stimulus. As the target appeared to loom it induced a greater accommodation response then when it appeared to recede. The looming target produced changes in the accommodation response in nine of 13 subjects that were consistent with its perceived change in proximity (although the target did not actually move in depth). Incremental low pass filtering produced non-directional drifts of accommodation in all subjects. Combinations of the stimuli (i.e. looming and low pass filtering, ON/OFF and looming) were not more effective stimuli to accommodation. After removal of the artificial pupil (closed loop conditions), accommodation was no longer induced with any of these stimuli. CONCLUSIONS: Although the preferential ON or OFF stimulation produced a pronounced illusion of motion in depth despite constant average brightness, proximal accommodation was induced in only one subject. Therefore, the ON/OFF stimulation appeared to have only minor input into proximal accommodation. Potential inputs into reflex accommodation need to be defined in further studies.

Accommodation, Ocular↗

PubMed2003

An evaluation of the lag of accommodation using photorefraction.

OBJECTIVE: The lag of accommodation which occurs in most human subjects during reading has been proposed to explain the association between reading and myopia. However, the measured lags are variable among different published studies and current knowledge on its magnitude rests largely on measurements with the Canon R-1 autorefractor. Therefore, we have measured it with another technique, eccentric infrared photorefraction (the PowerRefractor), and studied how it can be modified. METHODS: Particular care was taken to ensure correct calibration of the instrument. Ten young adult subjects were refracted both in the fixation axis of the right eye and from the midline between both eyes, while they read text both monocularly and binocularly at 1.5, 2, 3, 4 and 5 D distance ("group 1"). A second group of 10 subjects ("group 2"), measured from the midline between both eyes, was studied to analyze the effects of binocular vs monocular vision, addition of +1 or +2 D lenses, and of letter size. Spherical equivalents (SE) were analyzed in all cases. RESULTS: The lag of accommodation was variable among subjects (standard deviations among groups and viewing distances ranging from 0.18 to 1.07 D) but was significant when the measurements were done in the fixation axis (0.35 D at 3 D target distance to 0.60 D at 5 D with binocular vision; p<0.01 or better all cases). Refracting from the midline between both eyes tended to underestimate the lag of accommodation although this was significant only at 5 D (ANOVA: p<0.0001, post hoc t-test: p<0.05). There was a small improvement in accommodation precision with binocular compared to monocular viewing but significance was reached only for the 5 D reading target (group 1--lags for a 3/4/5 D target: 0.35 vs 0.41 D/0.48 vs 0.47 D/0.60 vs 0.66 D, ANOVA: p<0.0001, post hoc t-test: p<0.05; group 2--0.29 vs 0.12 D, 0.33 vs 0.16 D, 0.23 vs -0.31 D, ANOVA: p<0.0001, post hoc t-test: p<0.05). Adjusting the letter height for constant angular subtense (0.2 deg) induced scarcely more accommodation than keeping letter size constantly at 3.5 mm (ANOVA: p<0.0001, post hoc t-test: n.s.). Positive trial lenses reduced the lag of accommodation under monocular viewing conditions and even reversed it with binocular vision. CONCLUSIONS: After consideration of possible sources of measurement error, the lag of accommodation measured with photorefraction at 3 D (0.41 D SE monocular and 0.35 D SE binocular) was in the range of published values from the Canon R-1 autorefractor. With the measured lag, simulations of the retinal images for a diffraction limited eye suggest surprisingly poor letter contrast on the retina.

Accommodation, Ocular↗

PubMed2002

Effects of longitudinal chromatic aberration on accommodation and emmetropization.

PURPOSE: Less accommodation was found when human subjects read in blue (peak at about 440 nm) than when they read in red light (above 600 nm; [Kroger & Binder, British Journal of Ophthalmology 84 (2000) 890]). On the other hand, emmetropization in chickens did not appear to compensate for the chromatic defocus (385 nm versus 665 nm; [Rohrer, Schaeffel & Zrenner, Journal of Physiology 449 (1992) 363]). The apparently contradictory result was studied in more detail in humans and chickens. METHODS: Accommodation was measured with an eccentric infrared photorefractor, the PowerRefractor, in human subjects reading under quasi-monochromatic illumination conditions. Chickens were refracted in quasi-monochromatic ambient illumination but with no particular fixation target. In a second experiment, they were also raised in monochromatic light for two days and subsequently refracted both in complete darkness, in monochromatic light, and in white light, both without and with cycloplegia. RESULTS: Consistent with the initial report by Kroger and Binder [British Journal of Ophthalmology 84 (2000) 890], accommodation in human subjects was found to shift in accordance with the chromatic aberration function. An immediate shift in accommodation tonus was also found in the chickens when they were refracted under red and in blue ambient illumination (average difference between refractions in both conditions: 1.26+/-0.54 D, p<0.001 paired t-test). This value is close to the chromatic focus difference between the two wavelengths (1.5 D [Mandelman & Sivak, Vision Research 23 (1983) 1555]). When chickens were raised in blue or red light for two days, and their refractions were subsequently measured in complete darkness, they showed also a difference in refractions (1.41+/-1.00 D; ANOVA: p<0.0012, post hoc t-test: at least p<0.05 among different groups). This difference was no longer significant when they were refracted in white light but became again significant when they were cyclopleged (0.57+/-0.58 D, p=0.039, unpaired t-test). The latter observation makes it unlikely that the difference resulted just from a shift in the resting tonus of accommodation. CONCLUSIONS: (1) Imposed chromatic defocus produces a shift in accommodation tonus in both humans and chickens which is, in the case of the chicken, followed by a shift in cycloplegic refractive state into the same direction, (2) the difference to the previous study by Rohrer, Schaeffel and Zrenner [Journal of Physiology 449 (1992) 363] can be explained from the fact that shorter wavelengths were used than in the present study, at which emmetropization was no longer functional and, (3) the small amplitude and the variability of the shifts in refraction do not allow clear statements about the role of the "lag of accommodation" in refractive development but they show that several cone types contribute to emmetropization.

Accommodation, Ocular↗

PubMed2002

Peripheral refractive errors in myopic, emmetropic, and hyperopic young subjects.

To gain more insight into the relationship between foveal and peripheral refractive errors in humans, spheres, cylinders, and their axes were binocularly measured across the visual field in myopic, emmetropic, and hyperopic groups of young subjects. Both automated infrared photorefraction (the "PowerRefractor"; www. plusoptix.de) and a double-pass technique were used because the PowerRefractor provided extensive data from the central 44 deg of the visual field in a very convenient and fast way. Two-dimensional maps for the average cross cylinders and spherical equivalents, as well as for the axes of the power meridians of the cylinders, were created. A small amount of lower-field myopia was detected with a significant vertical gradient in spherical equivalents. In the central visual field there was little difference among the three refractive groups. The established double-pass technique provided complementary data also from the far periphery. At 45 deg eccentricity the double-pass technique revealed relatively more hyperopic spherical equivalents in myopic subjects than in emmetropic subjects [+/-2.73 +/- 2.85 D relative to the fovea, p < 0.01 (+/- standard deviation)] and more myopic spherical equivalents in hyperopic subjects (-3.84 +/- 2.86 D relative to the fovea, p < 0.01). Owing to the pronounced peripheral astigmatism, spherical equivalents (refractions with respect to the plane of the circle of least confusion) became myopic relative to the fovea in all three groups. The finding of general peripheral myopia was unexpected. Its possible roles in foveal refractive development are discussed.

Adult↗