Kevin T. Willeford

Amacrine Cell Subtypes Differ in Their Intrinsic Neurite Growth Capacity

PURPOSE Amacrine cell neurite patterning has been extensively studied in vivo, and more than 30 subpopulations with varied morphologies have been identified in the mammalian retina. It is not known, however, whether the complex amacrine cell morphology is determined intrinsically, is signaled by extrinsic cues, or both. METHODS Here we purified rat amacrine cell subpopulations away from their retinal neighbors and glial-derived factors to ask questions about their intrinsic neurite growth ability. In defined medium strongly trophic for amacrine cells in vitro, we characterized survival and neurite growth of amacrine cell subpopulations defined by expression of specific markers. RESULTS We found that a series of amacrine cell subtype markers are developmentally regulated, turning on through early postnatal development. Subtype marker expression was observed in similar fractions of cultured amacrine cells as was observed in vivo, and was maintained with time in culture. Overall, amacrine cell neurite growth followed principles very similar to those in postnatal retinal ganglion cells, but embryonic retinal ganglion cells demonstrated different features, relating to their rapid axon growth. Surprisingly, the three subpopulations of amacrine cells studied in vitro recapitulated quantitatively and qualitatively the varied morphologies they have in vivo. CONCLUSIONS Our data suggest that cultured amacrine cells maintain intrinsic fidelity to their identified in vivo subtypes, and furthermore, that cell-autonomous, intrinsic factors contribute to the regulation of neurite patterning.

Effect of chromatic filters on visual performance in individuals with mild traumatic brain injury (mTBI): A pilot study

Purpose Spectral filters have been used clinically in patients with mild traumatic brain injury (mTBI). However, they have not been formally assessed using objective techniques in this population. Thus, the aim of the present pilot study was to determine the effect of spectral filters on reading performance and visuo-cortical responsivity in adults with mTBI. Methods 12 adults with mTBI/concussion were tested. All reported photosensitivity and reading problems. They were compared to 12 visually-normal, asymptomatic adults. There were several test conditions: three luminance-matched control filters (gray neutral density, blue, and red), the patient-selected ‘precision tint lens’ that provided the most comfort and clarity of text using the Intuitive Colorimeter System, and baseline without any filters. The Visagraph was used to assess reading eye movements and reading speed objectively with each filter. In addition, both the amplitude and latency of the visual-evoked potential (VEP) were assessed with the same filters. Results There were few significant group differences in either the reading-related parameters or VEP latency for any of the test filter conditions. Subjective improvements were noted in most with mTBI (11/12). Conclusions The majority of patients with mTBI chose a tinted filter that resulted in increased visual comfort. While significant findings based on the objective testing were found for some conditions, the subjective results suggest that precision tints should be considered as an adjunctive treatment in patients with mTBI and photosensitivity.

Smoking and age-related macular degeneration: biochemical mechanisms and patient support.

Abstract A small percentage of the population associates smoking with ocular disease. Most optometrists do not stress the importance of smoking cessation to their patients, and the centrality of smoking regarding the risk for ocular disease is not emphasized in optometric education. Age-related macular degeneration has strong epidemiological associations with smoking, and so serves as an appropriate model for the adverse effects of cigarette smoke on the eye. This article aims to provide basic scientific information to optometrists and optometry students so that they can better understand the pathogenesis of age-related macular degeneration and provide education and support to their patients wishing to stop smoking.

The effect of spectral filters on VEP and alpha-wave responses

Purpose Spectral filters are used to treat light sensitivity in individuals with traumatic brain injury (TBI); however, the effect of these filters on normal visual function has not been elucidated. Thus, the current study aimed to determine the effect of spectral filters on objectively-measured visual-evoked potential (VEP) and alpha-wave responses in the visually-normal population. Methods The full-field (15°H × 17°V), pattern-reversal VEP (20′ check size, mean luminance 52 cd/m2) was administered to 20 visually-normal individuals. They were tested with four Intuitive-Colorimeter-derived, broad-band, spectral filters (i.e., gray/neutral density, blue, yellow, and red), which produced similar luminance values for the test stimulus. The VEP N75 and P100 latencies, and VEP amplitude, were recorded. Power spectrum analysis was used to derive the respective powers at each frequency, and peak frequency, for the selected 9–11 Hz components of the alpha band. Results Both N75 and P100 latencies increased with the addition of each filter when compared to baseline. Additionally, each filter numerically reduced intra-session amplitude variability relative to baseline. There were no significant effects on either the mean VEP amplitude or alpha wave parameters. Conclusions The Intuitive Colorimeter filters significantly increased both N75 and P100 latencies, an effect which is primarily attributable (∼75%) to luminance, and in some cases, specific spectral effects (e.g., blue and red). VEP amplitude and alpha power were not significantly affected. These findings provide an important reference to which either amplitude or power changes in light-sensitive, younger clinical groups can be compared.

Objective Assessment of Eye Dominance Using the VEP: A Pilot Study.

Objectives: To develop a clinical protocol for the determination of eye dominance using an objective method (i.e., the visual evoked potential [VEP]), and furthermore to determine the correlation of these objective findings with clinical subjective tests of eye dominance to provide guidance in clinical monovision refractive correction. Methods: The Diopsys NOVA-TR system was used to record the VEP amplitude and latency of 10 visually-normal, presbyopic, adult subjects aged 50 to 70 years ( =60, SEM=0.17 years). First, eye dominance was assessed in two ways: a sensory-based “sensitivity to blur task” and a motor-based “sighting task.” Next, while monocularly defocused, subjects binocularly viewed a black-and-white checkerboard (20-min arc size), pattern-reversal stimulus under 7 different test conditions: (1) baseline, (2) dominant (DE) eye blurred +1 diopter (D), (3) nondominant (NDE) eye blurred +1 D, (4) DE +2 D, (5) NDE +2 D, (6) DE +3 D, and (7) NDE +3 D. Results: Under nearly all conditions (22/24=92%), the amplitude and latency were significantly different from baseline with any amount of defocus (P<0.05). Monocular defocus decreased, and increased, the mean binocular VEP amplitude and latency, respectively. There were no significant differences in either the binocular VEP amplitude or latency values when comparing defocused dominant vs. nondominant eyes. Conclusions: Eye dominance type (i.e., sensory or motor) and status (i.e., DE or NDE) did not provide a differential effect with respect to the VEP findings. Thus, the VEP technique does not show promise as an objective adjunct in the fitting of monovision correction.