PubMed · 14830753
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R J FLETCHER. 1951. Astigmatic accommodation.. https://pubmed.ncbi.nlm.nih.gov/14830753/
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To stabilize objects of interest on the fovea during translation, vestibular-driven compensatory eye movements [translational vestibulo-ocular reflex (TVOR)] must scale with both target distance and eccentricity. To identify the neural correlates of these properties, we recorded from different groups of eye movement-sensitive neurons in the prepositus hypoglossi and vestibular nuclei of macaque monkeys during lateral and fore-aft displacements. All neuron types exhibited some increase in modulation amplitude as a function of target distance during high-frequency (4 Hz) lateral motion in darkness, with slopes that were correlated with the cell's pursuit gain, but not eye position sensitivity. Vergence angle dependence was largest for burst-tonic (BT) and contralateral eye-head (EH) neurons and smallest for ipsilateral EH and position-vestibular-pause (PVP) cells. On the other hand, the EH and PVP neurons with ipsilateral eye movement preferences exhibited the largest vergence-independent responses, which would be inappropriate to drive the TVOR. In addition to target distance, the TVOR also scales with target eccentricity, as evidenced during fore-aft motion, where eye velocity amplitude exhibits a "V-shaped " dependence and phase shifts 180 degrees for right versus left eye positions. Both the modulation amplitude and phase of BT and contralateral EH cells scaled with eye position, similar to the evoked eye movements during fore-aft motion. In contrast, the response modulation of ipsilateral EH and PVP cells during fore-aft motion was characterized by neither the V-shaped scaling nor the phase reversal. These results show that distinct premotor cell types carry neural signals that are appropriately scaled by vergence angle and eye position to generate the geometrically appropriate compensatory eye movements in the translational vestibulo-ocular reflex.
Presbyopia, the inability to accommodate, affects almost everyone at middle age. Recently, it has been shown that there is a massive increase in the stiffness(1) of the lens with age and, since the shape of the lens must change during accommodation, this could provide an explanation for presbyopia. In this review, we propose that presbyopia may be the earliest observable symptom of age-related nuclear (ARN) cataract. ARN cataract is a major cause of world blindness. The genesis of ARN cataract can be traced to the onset of a barrier within the lens at middle age. This barrier restricts the ability of small molecules, such as antioxidants, to penetrate into the centre of the lens leaving the proteins in this region susceptible to oxidation and post-translational modification. Major protein oxidation and colouration are the hallmarks of ARN cataract. We postulate that the onset of the barrier, and the hardening of the nucleus, are intimately linked. Specifically, we propose that progressive age-dependent hardening of the lens nucleus may be responsible for both presbyopia and ARN cataract.
PURPOSE: To quantify and characterize higher order aberrations in children and to investigate the influence of refractive error and cycloplegia. DESIGN: Observational cross-sectional study. METHODS: setting: Clinical practice. patients: One hundred sixty-two eyes of 82 children were examined. The mean age of the children was 6.7 years (range, 4 to 14 years), and the mean manifest refractive spherical equivalent was 2.39 +/- 3.35 diopters (range, -8.98 to +8.45 diopters). Aberrometry was performed with a wavefront analyzer (after cycloplegia and mydriasis with cyclopentolate 1%). Aberrometry was also conducted on a subgroup of 52 eyes of 27 children, both before and 30 minutes after the instillation of cyclopentolate 1% to investigate the effect of paralysis of accommodation. RESULTS: Mean root mean square values of total-, third-, fourth-, and fifth-order aberrations were 0.37 +/- 0.13 microm, 0.23 +/- 0.12 microm, 0.16 +/- 0.11 microm, and 0.08 +/- 0.12 microm, respectively. Mean root mean square values of total coma (Z(3)(-1), Z(3)(1), Z(5)(-1), Z(5)(1)) and total trefoil (Z(3)(-3), Z(3)(3), Z(5)(-3), Z(5)(3)) were 0.27 +/- 0.31 microm and 0.21 +/- 0.29 microm, respectively. Myopes had statistically significant greater levels of total (P = .005) and fourth order (P = .002) aberrations and Zernicke terms (Z(3)(-3), Z(3)(-1), Z(3)(3), Z(4)(-4), and Z(4)(2)) compared with hyperopes. Cycloplegia had minimal influence on higher-order aberrations. CONCLUSION: Significant levels of higher-order aberrations were found in these children. They were influenced by refractive error, because myopes had significantly greater levels compared with hyperopes. Paralysis of accommodation with cyclopentolate appeared to have little effect on higher-order aberrations.