Assessing the effect of colored filters on glare sensitivity of post refractive surgery patients.

PURPOSE: The use of colored filters is one recommended way for patients to reduce glare. This study assessed the effect of colored filters for reducing glare symptoms in post refractive surgery patients. METHOD: Log MAR visual acuity was determined in 70 participants (140 eyes) who had undergone PRK refractive surgery using three colored filters (Yellow, Green, and Red) in different light conditions. RESULTS: Measured visual acuity was significantly reduced with red and green filters in glare and nonglare conditions when compared to its measurement without a filter. An asymmetrical light condition also revealed a significant difference in the visual acuity of the right and left eye when compared to each other. However, no significant differences were observed for the yellow filter in either the measured visual acuity of each eye in both light conditions or of the right and left eyes compared to each other. CONCLUSION: Red and green filters are poor choices for reducing glare in post refractive surgery patients but a yellow filter could be used to decrease glare and improve acuity.

Effect of lens-induced myopia on visual cortex activity: a functional MR imaging study.

BACKGROUND AND PURPOSE: Myopia is a type of refractive error that blurs retinal image and in turn can change neural signals transferred from retina to visual cortex. The purpose of this study was to evaluate the effect of induced myopia on occipital visual cortex activity by fMRI results. MATERIALS AND METHODS: BOLD fMRI was performed in 13 emmetropic volunteers (refractive error, <±0.50D) with normal visual acuity, good binocular vision, and no history of neurologic illness. Visual stimulus was counterphasing vertical luminance sinusoidal grating with spatiotemporal frequency of 1.84 cycles per degree/8 Hz and contrast of 60%. The functional images were acquired in block design, during normal refractive state and induced myopia produced by convex noncoating plastic lenses of +1D, +3D, +5D, by using an EPI gradient-echo sequence in a 1.5T MR imaging scanner. fMRI data were processed by using FSL software. RESULTS: fMRI responses to visual stimuli demonstrated that percentage of BOLD signal intensity change and number of activated voxels within occipital visual cortex were reduced remarkably in induced myopic states of 1D, 3D, and 5D in comparison with normal refractive state; the results did not show a significant and regular decreasing trend in number of activated voxels and BOLD signal intensity change in these 3 different values of induced myopia. CONCLUSIONS: The findings suggest that induced myopia has a considerable effect on visual cortex activity, because myopia induced by lens of +1D is sufficient to change fMRI results significantly. Accordingly, it is essential to correct myopia before visual fMRI studies, even if it is at low levels.

Spatial frequency modulates visual cortical response to temporal frequency variation of visual stimuli: an fMRI study.

The brain response to temporal frequency (TF) variation has already been reported, but with no study for different TF with respect to various spatial frequencies (SF). Functional magnetic resonance imaging (fMRI) was performed with a 1.5 Tesla General Electric system in 14 volunteers during square-wave reversal checkerboard visual stimulation with different temporal frequencies of 4, 6, 8 and 10 Hz in two states of low SF of 0.4 and high SF of 8 cpd (cycles/degree). The activation map was created using the data obtained from the block-designed fMRI study. Voxels whose Z value was above a threshold of 3.0, at a significance level P = 0.05, were considered activated. The results demonstrated that the percentage BOLD signal change in response to different TFs was the maximum value at 6 Hz for a high SF of 8 cpd, whereas it was the maximum at TF of 8 Hz for a low SF of 0.4 cpd. The results of this study agree with the results of animal invasive neurophysiological studies showing spatial and temporal frequency selectivity of neurons in visual cortical areas. These results can be useful for vision therapy (such as the treatment of amblyopia) and selecting a visual task in fMRI studies.

Spatial frequency modulates the human visual cortical response to temporal frequency variation: an fMRI study.

The brain response to temporal frequencies (TF) has been already reported, but with no study for different TFs with respect to various spatial frequencies (SF). Functional Magnetic Resonance Imaging (fMRI) experiments were performed by 1.5 Tesla General Electric-system in 14 volunteers (9 males and 5 females, range 19-26 years) during square-wave reversal checkerboard visual stimulation with different temporal frequencies of 4, 6, 8 and 10 Hz in two states of low SF of 0.5 and high SF of 8 cpd (cycles/degree). The activation map was created using the data obtained from the block designed fMRI study. Pixels whose correlation coefficient value was above a threshold of 0.33, in significant level P<0.01 were considered activated. The average percentage BOLD (blood oxygenation level dependent) signal change for all activated pixels within the occipital lobe, multiplied by the total number of activated pixels within the occipital lobe, was used as the criterion for the strength of the fMRI signal at each state of TF&SF. The results demonstrated that the strength of the fMRI signal in response to different TFs was maximum in 6Hz for high SF of 8 cpd, while it was maximum at TF of 8Hz for low SF of 0.5 cpd. The results of this study agree with the results of animal invasive neurophysiological studies showing spatial and temporal frequency selectivity of neurons in visual cortical areas. These results can be useful for vision therapy (such as the treatment of Amblyopia) and selecting visual task in fMRI studies.

Spatial frequency dependence of the human visual cortical response to temporal frequency modulation studied by fMRI.

The brain response to temporal frequencies (TF) has been already reported, but with no study for different TF in respect to various spatial frequencies (SF). fMRI was performed by 1.5T GE-system in 14 volunteers during checkerboard visual stimulation, with TFs of 4, 6, 8 and 10Hz in low and high SFs of 0.5 and 8cpd. The averaged percentage BOLD signal changes demonstrated the amplitude of the fMRI response to different TFs which was maximally at 6Hz for high SF of 8cpd, and at 8Hz for low SF of 0.5cpd. The results are useful for vision therapy (such as the treatment of Amblyopia) and for fMRI applications incorporating visual tasks.