Spatial frequency selectivity of visual suppression during convergence eye movements.

Visual suppression of low-spatial frequency information during eye movements is believed to contribute to a stable perception of our visual environment. While visual perception has been studied extensively during saccades, vergence has been somewhat neglected. Here, we show that convergence eye movements reduce contrast sensitivity to low spatial frequency information around the onset of the eye movements, but do not affect sensitivity to higher spatial frequencies. This suggests that visual suppression elicited by convergence eye movements may have the same temporal and spatial characteristics as saccadic suppression.

Influence of central corneal thickness on measured intraocular pressure differentials: Nidek RKT-7700, Topcon CT-80 NCTs and Goldmann Tonometer.

PURPOSE: We sought to compare the intraocular pressure (IOP) measured by RKT-7700 and CT-80 noncontact tonometers (NCTs) with that measured by Goldmann applanation tonometry (GAT). We also examined the influence of central corneal thickness (CCT) on the agreement between both NCTs and GAT in a sub-population of healthy, young normals. METHODS: Triplicate IOP and CCT measurements were obtained twice from one randomized eye of 49 subjects (28 males and 21 females) aged 22.2 ± 1.3 (mean ± S.D.) years. Goldmann tonometry was performed subsequent to assessment with the RKT-7700 and CT-80, to negate the 'ocular massage effect', followed by ultrasound pachymetry. The results from each method were compared and assessed for repeatability and between-observer reproducibility. Analysis was performed to determine the correlation between the differences in IOP measurements and corneal thicknesses. RESULTS: The mean differences (±S.D.) in sessions 1 and 2 respectively between RKT-7700 and GAT (2.6 ± 2.0 and 2.7 ± 1.4 mmHg), between CT-80 and GAT (2.8 ± 2.0 and 3.2 ± 1.3 mmHg) were statistically significant (p < 0.0001). The repeatability coefficients in sessions 1 and 2 respectively were; ±1.2 and ±1.0 mmHg (GAT), ±1.3  and ±1.6 mmHg (CT-80), ±2.3  and ±1.7 mmHg (RKT-7700) and inter-observer reproducibility was; ±1.9 (RKT-7700), ±2.3 (GAT) and ±2.6 mmHg (CT-80). Between the corneal thickness and the differences in GAT-measured and NCT-measured IOPs, there was a significant negative correlation and a trend for a larger difference in thicker corneas. CONCLUSION: Neither the RKT-7700 nor the CT-80 can be used interchangeably with the Goldmann tonometer, although all three tonometers give repeatable measurements of IOP, in this sub-population. IOP differences between GAT and the NCTs significantly correlated with CCT, with the possibility of even greater differences in thicker corneas.

Investigating the mechanisms that may underlie the reduction in contrast sensitivity during dynamic accommodation.

Head and eye movements, together with ocular accommodation enable us to explore our visual environment. The stability of this environment is maintained during saccadic and vergence eye movements due to reduced contrast sensitivity to low spatial frequency information. Our recent work has revealed a new type of selective reduction of contrast sensitivity to high spatial frequency patterns during the fast phase of dynamic accommodation responses compared with steady-state accommodation. Here were report data which show a strong correlation between the effects of reduced contrast sensitivity during dynamic accommodation and velocity of accommodation responses, elicited by ramp changes in accommodative demand. The results were accounted for by a contrast gain control model of a cortical mechanism for contrast detection during dynamic ocular accommodation. Sensitivity, however, was not altered during attempted accommodation responses in the absence of crystalline-lens changes due to cycloplegia. These findings suggest that contrast sensitivity reduction during dynamic accommodation may be a consequence of cortical inhibition driven by proprioceptive-like signals originating within the ciliary muscle, rather than by corollary discharge signals elicited simultaneously with the motor command to the ciliary muscle.