I. J. Murray

Road traffic casualties: understanding the night-time death toll

A disproportionate number of fatal injuries occur after dark. The paper presents some statistics of road traffic injuries in a novel way which suggests that low luminance plays a major role in this effect. A sound physiological explanation for this is advanced based on the poor temporal characteristics of rod photoreceptors. It is argued that processing information based on low luminance, low contrast targets is much slower than that for high contrast bright targets. To test the idea, simple visual reaction times were measured under typical low visibility conditions encountered on non-lit roads and were found to be substantially longer than under optimal conditions. It is shown that longer reaction times translate into significantly increased stopping distances. This important point has received insufficient attention in the road safety literature, by the Highways Agency, the police, injury prevention officials, and the UK Highway Code.

The Ocular Stress Monitor; a New Device for Measuring Discomfort Glare.

Discomfort glare is always accompanied by a strong contraction or spasm in the muscles surrounding the eye. A portable device for measuring the electrical activity generated by this muscle spasm is described. The device samples the signal from electrodes placed around the eye. It is composed of a narrow-band amplifier/transmitter, a receiver/frequency converter and a tone generator. The signal amplitude is proportional to the vertical illuminance at the eye and can therefore be used as an objective index of the discomfort induced. The results compare favourably with subjective assessment.

The dazzle reflex: electrophysiological signals from ocular muscles reveal strong binocular summation effects

Under dark adapted or dim conditions the mammalian visual system is carefully programmed to respond rapidly to the sudden onset of bright lights. This response, called the dazzle reflex, is controlled from sub‐cortical structures of the brain. It is known anecdotally that exposure to a bright light when dark adapted induces an instinctive closure of one eye to reduce the pain associated with dazzle. This binocular summation of the dazzle response has not previously been reported. The dazzle reflex can be measured in human subjects by recording the electrical activity from surface electrodes located near the muscles around the eye. In this paper we report an investigation of the apparent binocular summation of the dazzle reflex using this technique. The data reveal a clear difference between monocular and binocular stimulation, with the binocular response being much larger than the monocular response. Furthermore this monocular/binocular difference arises only if the stimulus duration is longer than approximately 1 s. These observations are interpreted in terms of the known physiology of blink mechanisms.