Vision standards for driving in Canada and the United States. A review for the Canadian Ophthalmological Society.

We review the vision requirements for driving with the goal of revising current Canadian Ophthalmological Society (COS) recommendations for vision standards for driving. The report comprises two sections. In the first section we report the results of a survey of transportation authorities in Canada and the United States conducted on behalf of the COS to determine the current standards and medical review procedures. The results suggest that although the standards in Canada are more consistent than those in the United States, few of the standards in either country are evidence-based. In the second section we review the recent literature on visual function and driving. We conclude from this review that adequate contrast sensitivity is as important as, if not more important than, good visual acuity for driving and that there is little evidence to support a monocular standard for acuity, contrast sensitivity or visual field. Although there is evidence that the extent of a visual field defect is related to the ability to perform driving tasks, there is little evidence to suggest a relation between the location of the visual field defect and fitness to drive.

Symptomatic and asymptomatic visual loss in patients taking vigabatrin.

PURPOSE: To investigate the clinical, perimetric, and electrophysiologic findings in patients with visual field loss on long-term treatment with the antiepileptic medication vigabatrin. DESIGN: Consecutive observational case series. PARTICIPANTS: Forty-one consecutive subjects taking vigabatrin referred for screening ophthalmologic assessment were studied. Twelve subjects with evidence of peripheral visual field constriction are presented. METHODS: Twelve subjects with evidence of peripheral visual field constriction on 60-4 perimetry underwent central 30-2 and blue-on-yellow (B/Y) perimetry, as well as electroretinography (ERG), electro-oculography (EOG), and visual-evoked potential (VEP) testing. MAIN OUTCOME MEASURES: Visual acuity; fundus abnormalities; visual field loss; and ERG, EOG, or VEP abnormalities were the main outcome measures. RESULTS: Eight of the 12 subjects with constricted visual fields were asymptomatic. The central 30-2 perimetry demonstrated bilateral visual field constriction in 9 of 12 patients and the B/Y perimetry in 8 of 9 patients tested. Of the ten patients tested electrophysiologically, four had abnormal ERGs, five had abnormal EOGs, and three had delayed VEPs. CONCLUSIONS: The incidence of visual field constriction in patients taking vigabatrin may be higher, and asymptomatic visual field loss more common, than reported previously. The authors postulate a possible Muller cell dysfunction in the peripheral retina. Patients taking vigabatrin should have regular peripheral visual field examinations.

The temporal order judgment paradigm: subcortical attentional contribution under exogenous and endogenous cueing conditions.

The role of subcortical attentional processing was investigated under exogenous and endogenous cueing conditions. As retinotectal projections arise predominantly from the nasal retina i.e., temporal hemifield, subcortical attention should be distributed asymmetrically under monocular viewing conditions with a temporal hemifield advantage. We compared the results of monocular and binocular viewing conditions using a temporal order judgment (TOJ) paradigm. Subjects fixated a centrally located cross and two stimuli were presented with a variable onset asynchrony. Three experiments were conducted: no cue, exogenous cue and endogenous cue. Subjects reported which stimulus seemed to appear first. An effect consistent with subcortical processing was found under exogenous cueing conditions. No such effect was found under endogenous cueing conditions. We believe that subcortical attentional processing in response to an exogenous cue facilitates rapid shifts in attention towards environmental stimuli. We found no evidence for subcortical processing in voluntary directed attention and believe this process to be cortical in nature.

Distinguishing subcortical and cortical influences in visual attention. Subcortical attentional processing.

PURPOSE: The purpose of the study was to investigate the role of subcortical processing in human visual attention. The midbrain contribution to visual attention is unclear. Although evidence exists for a subcortical attentional advantage in ocular motor tasks, such an advantage has not been shown in perceptual tasks. Because retinotectal projections arise predominantly from nasal retina (i.e., temporal hemifield), subcortical attention should be distributed asymmetrically for monocular viewing conditions with an advantage to the temporal hemifield. METHODS: To test for a subcortical attentional effect, the authors compared the results of binocular and monocular viewing conditions using the split priming motion induction paradigm. In this perceptual attention paradigm, priming cues are presented to the left and right of fixation followed by an instantaneously presented horizontal bar. As a result of attention to the priming cues, motion is perceived within the bar as it appears to draw in from the two lateral cues toward a central collision point. Asymmetrically distributed attention results in an asymmetry in the perception of motion within the bar, and thus the perceived collision point will be shifted away from the center. RESULTS: In two separate studies, one with and one without control of eye movements, the authors found significant differences between the results for monocular and binocular presentation. When the stimulus configuration is presented to the left eye, the perceived collision point is shifted toward the center consistent with a subcortical attentional effect. However, presentation of the stimulus configuration to the right eye yields the same results as those of binocular presentation. CONCLUSIONS: This pattern of results can be explained by a separate and additive interaction between cortical and subcortical attentional effects in the visual field. Dominance of the left visual field for cortical attention and dominance of the temporal visual field for subcortical attention act together when the initial priming cue occurs in the temporal (left) visual field of the left eye. However, these influences compete when the same stimulus configuration is presented to the right eye, where cortical attention predominates in the left visual field and subcortical attention predominates in the temporal (right) visual field.

Measuring the attentional speed-up in the motion induction effect.

Motion induction is the illusory motion within an elongated stimulus, such as a bar or a line, when it is preceded by a priming stimulus next to one of its ends. Motion is away from this primer. The presentation of two priming spots at both ends of a stimulus bar results in motion away from both spots with a collision in the center of the bar. With a sufficiently long delay between the spots, motion will be seen only as away from the second spot. Similarly, in a bar with a luminance gradient an illusory motion is perceived as away from the high-luminance end, presumably due to the known dependence of neural processing speed on luminance. In the present study, these two illusory motions were made to oppose each other. The particular luminance gradient which would just cancel the motion induction effect when motion is seen optimally as away from the second spot (cancellation gradient) was determined, resulting again in a collision near the center of the bar. Furthermore, the luminance dependence of the reaction time to stimulus detection was measured in a separate experiment. Thus for each observer, the processing time difference associated with the cancellation gradient was established. This delta t then gives the amount of time by which processing is speeded up in motion induction due to the priming spot. In a simple model of motion processing it can also be identified as the built-in delay delta t of a typical Reichardt-type motion detector. With the present conditions, it varied between 14 and 19 msec for different observers for a bar length of 5.3 deg. In this way, we show not only that the priming effect in motion induction can be understood as a speed-up of neural processing, but also provide a way of measuring the times involved. In additional experiments, we examined the effect of bar length and luminance profile. These results allow us to estimate the gradients of the attentional fields.