A high-intensity, goggle-mounted flash stimulator for short-latency visual evoked potentials.
The few studies that have been done on short-latency, subcortical visual evoked potentials (SVEPs) have all used stroboscopic flashes as the evoking stimulus. The dimensions of the stimulator, the acoustical artifacts and the photic spread to the examination room limited the use of SVEPs to research laboratories. With the advent of high-efficiency light-emitting diodes (LEDs), high-intensity flashes can now be generated from goggle-mounted LEDs. In this study, a goggle-mounted high-intensity stimulator was constructed and its flashes used to evoke SVEPs. The reproducibility of SVEPs across subjects and the ease of using the high-intensity LED flash stimulator make them a promising candidate for testing subcortical visual pathway function in the operating room.
Short latency visual evoked potentials in man.
Contrary to auditory and somatosensory evoked potentials, surface recorded visual evoked potentials which arise in subcortical neural elements have rarely been described. Considerable disagreement exists between the reports in the literature on such visual potentials. In this study, flash stimuli were used to evoke the potentials which were recorded from the skin overlying the infra-orbital ridge, outer canthus, middle of the forehead, vertex, mastoid ipsilateral to the stimulated eye and inion, using a non-cephalic reference. The potentials were amplified in a band which was chosen to omit slow retinal and cortical potentials, and to enhance activity which might include compound neural activity. Potentials were recorded from 9 subjects (13 eyes), and for each one the effects of eye position and stimulus intensity were studied. The results indicate that the series of components recorded within the first 100 msec following photic stimulation were volume-conducted activity generated by a subset of the visual system which is activated by luminosity changes. The generators of the first 4 or 5 components seem to be situated within the retina, the subsequent components seem to be generated in the optic nerve or tracts, and the later components may be thalamo-cortical in origin. These potentials may complement pattern evoked potentials in a more accurate definition of sites of lesions along the visual pathway.
OBSERVATIONS ON BLINK REFLEXES
A reflex wink, which resulted from a tap over the supraorbital region of the face, was described by Overend (1896). He observed that the reflex was the product of facial stimulation, for it was present in the blind. In the 25 years following this discovery it was realized that reflex winking (unilateral contraction of the homolateral orbicularis oculi) or blinking (bilateral contraction) could be elicited from many areas of the face, and some of these facial reflexes became incorporated into the routine examination of the nervous system (Weingrow, 1933). Wartenberg (1945) has admirably described the confusion that has resulted from the failure to recognize the essential identity of the facial reflexes, irrespective of the region of facial stimulation. He proposed that the term 'orbicularis oculi reflex' could well replace the long list of facial reflexes named after their illustrious discoverers, among them, McCarthy (1901), von Bechterew (1902), Weisenberg (1903), Guillan (1920), and Simchowicz (1922). The orbicularis oculi reflex was a simple myotatic reflex, Wartenberg believed, the result of brief stretch of facial musculature. This view was contrary to the generally accepted teleological opinion that a blink (in response to a facial stimulus) was a defence mechanism, designed to protect the eye from damage. However, Kugelberg (1952) conveniently reconciled both these views when he demonstrated, by electromyographic methods, that the blink reflex in response to a facial tap had two components, an initial proprioceptive (myotatic) reflex and a later nociceptive (defence) reflex. Kugelberg's detailed study confirmed Wartenberg's contention that the blink reflex was the same, no matter which area of the face was stimulated. Ekbom, Jernelius, and Kugelberg (1952) showed that the orbicularis oris reflex had many features in common with the blink reflex. The proprioceptive component of the facial reflexes raised again the question of the situation and nature of the sensory endings which were stimulated, and their afferent pathway. Whereas the afferent fibres of skeletal muscle form almost 50% of the muscular nerve (Adams, Denny-Brown, and Pearson, 1953), the facial nerve (of the cat) contains only 16% of sensory fibres and they are mostly of small diameter (Foley and DuBois, 1943). They have been described in human facial nerves by van Gehuchten (1906), by Edgeworth (1900), and by Wakeley and Edgeworth (1933). Bruesch (1944) demonstrated a few large afferents in the facial nerve of cats but found that they entered by the auricular nerve and were probably cutaneous in origin, and Van Buskirk (1945) confirmed this. Apart from taste afferents, the small afferent nerve fibres of the facial nerve