Drugs online research references
Perception. 1999;28(2):143-53.
The effects of eccentricity and vergence angle upon the relative tilt of corresponding vertical and horizontal meridia revealed using the minimum motion paradigm.
Ledgeway T, Rogers BJ.
Department of Experimental Psychology, University of Oxford, UK.
When the corresponding horizontal meridia of the two eyes are aligned, the corresponding vertical meridia are tilted outwards in a temporal direction, a phenomenon first described by Helmholtz. However, it is not known if this effect is confined to the principal meridia or whether the same relationship exists between corresponding horizontal and corresponding vertical meridia at eccentric retinal locations. We sought to address this issue by exploiting the technique of Nakayama (1977 Proceedings of the Society of Photo-Optical Instrument Engineers 120 2-9) in which the positions of alternating dichoptic images that produce minimal apparent motion were used to measure the relative tilt of corresponding meridia at a range of eccentricities up to +/- 16 deg away from the fovea. Stimuli were composed of dichoptic images, one containing a blank field and the other a pair of dots, which alternated at a rate of 0.63 Hz and the relative tilt (binocular orientation difference) between the pairs of dots presented to the two eyes was varied between +/- 11 degrees. Nonius lines were used to maintain vergence angle, which was varied between 28 cm and infinity. Subjects judged which pair of alternating images produced the smallest amount of apparent motion (position change). It was found that at all eccentricities examined the corresponding horizontal meridia were generally aligned but the corresponding vertical meridia were consistently offset (extorted) by about +/- 2 degrees. The tilts of corresponding principal meridia were typically unaffected when vergence angle was varied, indicating that little or no cyclovergence accompanied changes in horizontal vergence. The results suggest that the binocular correspondence system appears to be mapped by a horizontal shear distortion that extends to retinal locations at least as far as 16 deg away from the foveae. The invariant extortion of corresponding principal vertical meridia with vergence state is consistent with previous suggestions that the empirical vertical horopter becomes progressively inclined with respect to the vertical as viewing distance increases.
online pharmacy ref source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10615456&dopt=Abstract
ccit.arizona.edu
PURPOSE: To provide normative data on the effect of a central competing stimulus on monocular visual field extent in 3.5- to 9-month-olds and adults, and binocular visual field extent in 11- to 30-month-olds. METHODS: Visual field extent along diagonal meridia was measured in 180 infants and children (N = 30 at 3.5, 7, 9, 11, 17, and 30 months) and 20 adults, using static perimetry. Stimuli were 3-deg, 10-Hz flickering, yellow light-emitting diodes (LEDs). Each subject was tested with 12 trials in which the central stimulus remained on and 12 trials in which the central stimulus was turned off during presentation of the peripheral stimulus. RESULTS: A competing stimulus produced a decreased frequency of eye movements at 17 months and evidence suggestive of decreased measured field extent at 17 and 30 months. CONCLUSIONS: A continuously present central stimulus does not produce underestimation of visual field extent in 3.5- to 9-month-old infants with normal vision and adults tested monocularly and 11-month-old infants tested binocularly. However, visual field extent may be underestimated in 1- and 2-year-olds if a competing central stimulus is used.
online pharmacy ref source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9380375&dopt=Abstract
eyes.arizona.edu
Visual field extent along the four diagonal meridia was measured cross-sectionally in 180 normal children (infants and toddlers), and 22 adults. Infants were tested monocularly at 3.5, 7, or 9 months, and toddlers were tested binocularly at 11, 17, or 30 months. Adult control data were obtained under monocular viewing. Three testing methods were investigated: static and hybrid static-kinetic perimetry, using LED arrays under computer control, and kinetic perimetry, using white styrofoam spheres manipulated by hand. Data analysis included corrections for false positives in the method of constant stimuli and for errors of anticipation in the ascending method of limits. Across all data sets from children, kinetic perimetry yielded larger, more adult-like fields, which approached adult levels around 17 months, whereas static and hybrid static-kinetic perimetry yielded smaller visual fields, approaching adult levels only at 30 months.
online pharmacy ref source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9775323&dopt=Abstract
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