Browsing by Subject "Visual acuity"

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Quantifying the impact of thermal lensing on visual function in ocular media
(2012-08) Weber, Erica L.; Dunn, Andrew Kenneth, 1970-
Several studies have been conducted in the past which determined that some near-infrared (NIR) sources are capable of inducing a thermal lens within ocular media of rhesus and, potentially, human subjects. Typically, the role of thermal lensing in the eye was explored in terms of its influence on damage thresholds for these NIR lasers entering the eye. However, the effect of a thermal lens on visible wavefronts entering the eye has yet to be explored. In recent years military and law enforcement agencies in the United States and elsewhere have devoted considerable resources to the area of "non-lethal weapons." Devices such as tasers, spike strips and ocular interruption (OI) devices provide the user with an escalation of force while minimizing casualties and collateral damage. One particular form of OI device, the laser dazzler, employs a visible laser capable of saturating retinal receptors causing a temporary flash blindness effect. While these visible devices have proven safe and effective in the field, an inherent risk exists when any light source is used to saturate retinal tissue. By adding the use of a thermal lens, these OI devices would create significant distortions in the visible wavefront to alter vision and/or increase the diameter of a focused visible dazzler at the retina to both improve safety and effectiveness of the visible device. This dissertation describes experiments involving artificial eye, human subject, and computational modeling which were conducted to quantify the impact of thermal lensing on visual acuity.
Visual acuity in a cathemeral lemur (Eulemur macaco flavifrons) : implications for strepsirrhine visual ecology
(2006-12) Veilleux, Carrie Cecilia; Kirk, E. Christopher, 1974-
For many nocturnal, cathemeral, and diurnal strepsirrhines, acute vision is important in foraging for food, detecting predators, and communicating with conspecifics. However, comparable visual acuity measurements are currently only available from four strepsirrhines: Lemur catta, Galago senegalensis, Otolemur crassicaudatus, and Microcebus murinus. In light of the role of increased visual acuity in early primate evolution and the controversies surrounding the selective pressures influencing primate origins, it is important to understand how visual acuity varies with ecological variables such as diet or activity pattern. In this study, the first behavioral assessment of visual acuity in a cathemeral strepsirrhine was conducted. Acuity in two blue-eyed black lemurs (Eulemur macaco flavifrons), a 3-year old male and a 16-year old female, was measured using a two-alternative forced choice discrimination task. Visual stimuli consisted of high contrast square wave gratings of seven spatial frequencies. Acuity threshold was determined using a 70% correct response criterion. Results indicate a maximum visual acuity of 5.1 cycles per degree (c/deg) for the female (1718 trials) and 3.8 c/deg for the male (718 trials). Acuity was also assessed anatomically from retinal ganglion cell densities for Microcebus murinus (2.8 c/deg) and Tarsius syrichta (8.9 c/deg) reported in the literature. Preliminary analyses of ecological effects on prosimian visual acuity were conducted. Although the results are not statistically significant, acuity appears to vary with activity pattern, diet, and prey mobility. In particular, functional, behavioral, and anatomical evidence suggest that cathemeral strepsirrhines are balancing the visual acuity necessary for diurnal activity with the visual sensitivity necessary for nocturnal activity. Additionally, visual predation may be an important selective factor favoring high acuity.