Development of the response AC/A ratio over the first year of life.

This study investigated the development of the link from accommodation to vergence in infants by occluding one eye thus removing binocular cues. Occluded adults continue to converge partially demonstrating that the accommodative drive to vergence (the AC/A link) and proximal cues are sufficient to drive vergence. For infants of all ages, AC/A ratios were found to be in the normal adult range. We conclude that infants can use monocular cues to drive vergence and that this occurs before the age when there is a substantial increase in the accuracy of oculomotor processes. There is flexibility in the developing visual system which is able to produce early vergence responses by relying upon alternative cues.

Estimating the size of hard-to-reach populations: a novel method using HIV testing data compared to other methods.

OBJECTIVE: To estimate population size of hard-to-reach groups such as injecting drug users and men who have sex with men. DESIGN: Several different methods were used to estimate the size of these populations in Canada's three largest cities (Toronto, Montreal and Vancouver). METHODS: A novel method (referred to as the indirect method) was developed for use in Toronto and Vancouver that combines HIV serodiagnostic information with data on HIV testing behavior. Population size estimates were obtained by dividing the number of injecting drug users or men who have sex with men recorded in HIV serodiagnostic databases in a given year by the proportion of the corresponding group that reported being tested in a 1-year period. Results of this method were compared with four other methods: (1) population surveys; (2) capture-recapture (for injecting drug users only); (3) a modified Delphi technique; and (4) a method based on the proportion of never-married men aged 45 and over (for men who have sex with men only). Only these other methods were used in Montreal. RESULTS: The survey method gave the lowest estimates which are best viewed as minimum estimates given the relative inability of surveys to access these populations and the reluctance of participants to admit to sensitive behaviors. The indirect method produced results more closely comparable with those obtained by other methods, but they are probably slight overestimates, at least for injecting drug users, due to possible underestimation of the proportion tested for HIV. Point estimates using the indirect method were 17,700 and 17,500 for injecting drug users in Toronto and Vancouver, respectively, and 39,100 and 15,900 for men who have sex with men. In Toronto, results for the other methods ranged from 12,300-13,360 for injecting drug users and 18,800-35,000 for men who have sex with men. For Vancouver, these ranges were 6400-11,670 and 7000-26,500, respectively. In Montreal, ranges were 4300-12,500 for injecting drug users and 18,500-40,000 for men who have sex with men. CONCLUSIONS: This novel method provides estimates of population size of hard-to-reach groups such as injecting drug users and men who have sex with men that are comparable with results derived by other methods. These estimates may be useful for the purposes of planning, implementing and evaluating prevention and care services, especially when they are combined with the results of other estimation methods to improve the degree of confidence in the resulting estimates.

Variations in accommodation and convergence responses in a minimally controlled photorefractive setting.

PURPOSE: A remote haploscopic photorefractor, designed for assessment of accommodation and convergence in infants and clinical groups, was used to determine heterophoria accommodative convergence/accommodation (AC/A) ratios in normal naïve adults. These were compared with conventional clinical measures. METHODS: Twenty-one naïve subjects were used to compare occluded and unoccluded prism cover test responses with the remote haploscopic photorefractor using a text and picture target. RESULTS: Although luminance was generally low for both targets, binocular vergences were appropriate for target demand in both studies. Binocular accommodation showed greater lag for the highest target accommodative demand and the less demanding target. Occlusion not only reduced vergence response, but also frequently caused a marked reduction in accommodation, especially to the picture target. Normal mean AC/A values were found, but with wide variations between individual subjects. CONCLUSIONS: Although mean accommodation, vergence, and AC/A values were comparable with published data, we suggest that in these conditions using naïve subjects, accommodation is frequently inaccurate, especially on occlusion, without concomitant loss of vergence, at least at low light levels. Accommodative convergence may play a less important part in, and other cues contribute more to, the near reflex than has been previously suggested.

The response to prism deviations in human infants.

Previous research has suggested that infants are unable to make a corrective eye movement in response to a small base-out prism placed in front of one eye before 14-16 weeks [1]. Three hypotheses have been proposed to explain this early inability, and each of these makes different predictions for the time of onset of a response to a larger prism. The first proposes that infants have a 'degraded sensory capacity' and so require a larger retinal disparity (difference in the position of the image on the retina of each eye) to stimulate disparity detectors [2]. This predicts that infants might respond at an earlier age than previously reported [1] when tested using a larger prism. The second hypothesis proposes that infants learn to respond to larger retinal disparities through practice with small disparities [3]. According to this theory, using a larger prism will not result in developmentally earlier responses, and may even delay the response. The third hypothesis proposes that the ability to respond to prismatic deviation depends on maturational factors indicated by the onset of stereopsis (the ability to detect depth in an image on the basis of retinal disparity cues only) [4] [5], predicting that the size of the prism is irrelevant. To differentiate between these hypotheses, we tested 192 infants ranging from 2 to 52 weeks of age using a larger prism. Results showed that 63% of infants of 5-8 weeks of age produced a corrective eye movement in response to placement of a prism in front of the eye when in the dark. Both the percentage of infants who produced a response, and the speed of the response, increased with age. These results suggest that infants can make corrective eye movements in response to large prismatic deviations before 14-16 weeks of age. This, in combination with other recent results [6], discounts previous hypotheses.