Documenta Ophthalmologica 44,1 : 203-206, 1977 NYSTAGMUS INDUCED BY INTERMITTENT PHOTIC STIMULATION ( F L A S H INDUCED NYSTAGMUS ( F I N ) ) J.T.W. VAN DALEN
(Amsterdam /Nijrnegen ) Keywords: Flash-induced nystagmus, Latent nystagmus, Nystagmus.
ABSTRACT In man, as has been shown previously in rabbits and monkeys, a nystagmus can be elicited by monocular intermittent photic stimulation. The direction of nystagmus (FIN) is always towards the stimulated eye. This nystagmus strongly resembles latent nystagmus. Here the fast phase is always directed towards the 'non-occluded' eye. The FIN may, therefore, be used as a model for the study of mechanisms underlying these pathological conditions. For further elucidation of FIN, studies were made in rabbits. A previously unknown type of nystagmus induced by intermittent photic stimulation was described in rabbits by Costin, Chaimowitz & Bergmann in 1965 and by Itin in 1969. Some years later Pasik & Pasik (1970) demonstrated that this nystagmus could be evoked in monkeys by monocular flashing, while the contralateral eye, as in rabbit, was shielded from light. Since then this nystagmus has been called: Flash induced Nystagmus (FIN). The direction of flash nystagmus is always towards the stimulated eye. This is regarded as the most characteristic feature of this type of nystagrnus. The oculomotor pattern of response to monocular photic stimulation is strikingly similar to the condition known in man as latent nystagmus. In latent nystagmus, at least the classical cases, nystagmus becomes apparent only on occlusion of one eye, while the direction of the fast phase is also to the side of the uncovered eye. The cause of latent nystagmus is still a matter of dispute. Since the fast phase of flash nystagmus is always directed towards the stimulated eye, the phenomenon of FIN may be used as a model for the study of mechanisms underlying these pathological conditions. The presence of this visuo-oculomotor reflex in animals with such dissimilar visual systems suggests, that analogous responses might be obtained in man, whose visual anatomy is very similar to that of the Rhesus monkey. Nevertheless, Keane (1972), to his surprise, could not evoke a flash induced nystagmus in man under similar testing conditions as in monkeys. He ex-
203
plained the absence of such nystagmus as being possibly due to the decreased extrageniculostriate visual function in human beings. Our investigations have been concerned with the presence of FIN in man and rabbits. EXPERIMENTS IN MAN In man stimulation was monocular, the contralateral eye being shielded from light. In order to provide stimulation in a homogeneous field ('GanzFeld'), a semitransparent occluder, made of half a ping-pong ball, was placed before the stimulated eye. This occluder was found to be essential: stimulation without this device did not give the nystagmic response. The contralateral eye could be illuminated by a special device. The experiments were carried out in a darkened room; it seemed that background illumination abolished the response only when it approached or exceeded the luminance of the flash. Results for one subject are given here as an example. Subject A.B.: a 10year-old boy with amblyopia of the left eye and an intermittent divergent squint. Stimulation of the right eye elicited a nystagmus with the fast phase towards the stimulated eye. The effective stimulation ranged from 2 - 5 5 flashes/sec. The optimal stimulation frequency was 5 - 1 0 ftashes/sec, with a maximum response of 124 beats/60 see. (Fig. 1). 1 sec I I
I.
--7
----L
.
I , --
I
OD
I, ,,,
I.
,.,J
= .d,.......
...... "
A.B. o~ , tc I , HF 1 5 ,
,e,t right,
I
"
50 tlash I s e c , 1 c m = 2 0 0 ~JV, 0.1 c a n d e l a / s e c , OS = o c c l u d e d , O D = s t i m u l a t e d , 10"=160 AJV.
Fig. 1. Subject A.B.: Stimulation of the right eyE, the left eye being occluded, elicited a nystagmus with the fast phase to the right. Stirhulation at a frequency of 50 flashes/ sec was still effective.
204
EXPERIMENTS IN RABBITS For further elucidation of the FIN, studies were also made in rabbits. The rabbits were tested monocularly; the contralateral eye was blindfolded. Background illumination abolished the response only when it approached or exceeded the luminance of the flash. All animals showed the p h e n o m e n o n without premedication, the fast phase of the nystagmus being always directed towards the stimulated eye. The optimal stimulation frequency was 30 flashes/sec, in all rabbits, with a maximum response of 170 beats/60 sec. After a short tatency there was a build up of the nystagmus until a steady state was reached (Fig. 2). Usually an after-nystagrnus was seen with a duration of approximately 3 0 - 6 0 sec. The direction of this after-nystagmus was towards the stimulated eye. Occasionally, an after-after-nystagmus was seen. Background illumination did not abolish this after response. As it is known that the vestibular organ plays an important role in FIN, several animals were subjected to unilateral or bilateral labyrinthectomy. After bilateral labyrinthectomy there was no FIN at all. After unilateral labyrinthectomy (left side), there was a diminished response: stimulation of the left eye gave 16 beats/40 sec; stimulation of the right eye gave 25 beats/40 sec at 30 flashes/sec, compared to a normal response of 70 beats per 40 sec in normal animals.
i
lsec
I
OS [ ~t~t~r162162162162162162
rabbit 1636, tc 1, H F"15, left right , 25 tlashes/sec, 11 cm=650/JV, intensity OA Joule/flash, 10 crn distance, OS = stimulated, OD = occluded, 10"= 930 ~V.
Fig. 2. Rabbit 1635: After starting the stimulation of the left eye, we observed a latency, followed by a gradual build-up of the nystagmus. The direction of the nystagmus was always towards the stimulated eye.
205
DISCUSSION AND CONCLUSION In man, as we could demonstrate, a nystagmus, conjugate for the two eyes, can be induced by intermittent photic stimulation. The direction of this 'flash-nystagmus' was towards the stimulated eye. The contralateral eye should be occluded. Binocular stroboscopic stimulation did not evoke a flash-induced nystagmus (FIN). Since in latent nystagmus the fast phase of the nystagmus is also directed towards the 'non-occluded' eye, there may be a correlation between the two types of nystagmus. In rabbit research an interesting feature was that FIN was n o t present at all after bilateral labyrinthectomy; this in contradiction to optokinetic nystagmus. REFERENCES Costin, A., M. Chalmowitz & F. Bergman. Nystagmus evoked by intermittent photic stimulation of the rabbit eye. Experientia 21:167-168 (1965). Itin, W. Le nystagmus photique provoqud par stimulation lumineuse intermittente. Rev. Laryng. (Bordeaux) 90:520-528 (1969). Keane, J.R. Flash-evoked nystagmus: absence in man. Neurology 22:551-553 (1972). Pasik, P., T. Pasik & J. Valciukas. Nystagmus induced by repetitive light flashes in monkeys. Brain Res. 19:313-317 (1970).
Author's addresses: Electrophysiology Unit Department of Ophthalmology Wilhelmina Gasthuis University of Amsterdam le Helmerstraat 104 Amsterdam The Netherlands Department of General Neurophysiology Catholic University of Nijmegen Nijmegen The Netherlands
206
(Amsterdam /Nijrnegen ) Keywords: Flash-induced nystagmus, Latent nystagmus, Nystagmus.
ABSTRACT In man, as has been shown previously in rabbits and monkeys, a nystagmus can be elicited by monocular intermittent photic stimulation. The direction of nystagmus (FIN) is always towards the stimulated eye. This nystagmus strongly resembles latent nystagmus. Here the fast phase is always directed towards the 'non-occluded' eye. The FIN may, therefore, be used as a model for the study of mechanisms underlying these pathological conditions. For further elucidation of FIN, studies were made in rabbits. A previously unknown type of nystagmus induced by intermittent photic stimulation was described in rabbits by Costin, Chaimowitz & Bergmann in 1965 and by Itin in 1969. Some years later Pasik & Pasik (1970) demonstrated that this nystagmus could be evoked in monkeys by monocular flashing, while the contralateral eye, as in rabbit, was shielded from light. Since then this nystagmus has been called: Flash induced Nystagmus (FIN). The direction of flash nystagmus is always towards the stimulated eye. This is regarded as the most characteristic feature of this type of nystagrnus. The oculomotor pattern of response to monocular photic stimulation is strikingly similar to the condition known in man as latent nystagmus. In latent nystagmus, at least the classical cases, nystagmus becomes apparent only on occlusion of one eye, while the direction of the fast phase is also to the side of the uncovered eye. The cause of latent nystagmus is still a matter of dispute. Since the fast phase of flash nystagmus is always directed towards the stimulated eye, the phenomenon of FIN may be used as a model for the study of mechanisms underlying these pathological conditions. The presence of this visuo-oculomotor reflex in animals with such dissimilar visual systems suggests, that analogous responses might be obtained in man, whose visual anatomy is very similar to that of the Rhesus monkey. Nevertheless, Keane (1972), to his surprise, could not evoke a flash induced nystagmus in man under similar testing conditions as in monkeys. He ex-
203
plained the absence of such nystagmus as being possibly due to the decreased extrageniculostriate visual function in human beings. Our investigations have been concerned with the presence of FIN in man and rabbits. EXPERIMENTS IN MAN In man stimulation was monocular, the contralateral eye being shielded from light. In order to provide stimulation in a homogeneous field ('GanzFeld'), a semitransparent occluder, made of half a ping-pong ball, was placed before the stimulated eye. This occluder was found to be essential: stimulation without this device did not give the nystagmic response. The contralateral eye could be illuminated by a special device. The experiments were carried out in a darkened room; it seemed that background illumination abolished the response only when it approached or exceeded the luminance of the flash. Results for one subject are given here as an example. Subject A.B.: a 10year-old boy with amblyopia of the left eye and an intermittent divergent squint. Stimulation of the right eye elicited a nystagmus with the fast phase towards the stimulated eye. The effective stimulation ranged from 2 - 5 5 flashes/sec. The optimal stimulation frequency was 5 - 1 0 ftashes/sec, with a maximum response of 124 beats/60 see. (Fig. 1). 1 sec I I
I.
--7
----L
.
I , --
I
OD
I, ,,,
I.
,.,J
= .d,.......
...... "
A.B. o~ , tc I , HF 1 5 ,
,e,t right,
I
"
50 tlash I s e c , 1 c m = 2 0 0 ~JV, 0.1 c a n d e l a / s e c , OS = o c c l u d e d , O D = s t i m u l a t e d , 10"=160 AJV.
Fig. 1. Subject A.B.: Stimulation of the right eyE, the left eye being occluded, elicited a nystagmus with the fast phase to the right. Stirhulation at a frequency of 50 flashes/ sec was still effective.
204
EXPERIMENTS IN RABBITS For further elucidation of the FIN, studies were also made in rabbits. The rabbits were tested monocularly; the contralateral eye was blindfolded. Background illumination abolished the response only when it approached or exceeded the luminance of the flash. All animals showed the p h e n o m e n o n without premedication, the fast phase of the nystagmus being always directed towards the stimulated eye. The optimal stimulation frequency was 30 flashes/sec, in all rabbits, with a maximum response of 170 beats/60 sec. After a short tatency there was a build up of the nystagmus until a steady state was reached (Fig. 2). Usually an after-nystagrnus was seen with a duration of approximately 3 0 - 6 0 sec. The direction of this after-nystagmus was towards the stimulated eye. Occasionally, an after-after-nystagmus was seen. Background illumination did not abolish this after response. As it is known that the vestibular organ plays an important role in FIN, several animals were subjected to unilateral or bilateral labyrinthectomy. After bilateral labyrinthectomy there was no FIN at all. After unilateral labyrinthectomy (left side), there was a diminished response: stimulation of the left eye gave 16 beats/40 sec; stimulation of the right eye gave 25 beats/40 sec at 30 flashes/sec, compared to a normal response of 70 beats per 40 sec in normal animals.
i
lsec
I
OS [ ~t~t~r162162162162162162
rabbit 1636, tc 1, H F"15, left right , 25 tlashes/sec, 11 cm=650/JV, intensity OA Joule/flash, 10 crn distance, OS = stimulated, OD = occluded, 10"= 930 ~V.
Fig. 2. Rabbit 1635: After starting the stimulation of the left eye, we observed a latency, followed by a gradual build-up of the nystagmus. The direction of the nystagmus was always towards the stimulated eye.
205
DISCUSSION AND CONCLUSION In man, as we could demonstrate, a nystagmus, conjugate for the two eyes, can be induced by intermittent photic stimulation. The direction of this 'flash-nystagmus' was towards the stimulated eye. The contralateral eye should be occluded. Binocular stroboscopic stimulation did not evoke a flash-induced nystagmus (FIN). Since in latent nystagmus the fast phase of the nystagmus is also directed towards the 'non-occluded' eye, there may be a correlation between the two types of nystagmus. In rabbit research an interesting feature was that FIN was n o t present at all after bilateral labyrinthectomy; this in contradiction to optokinetic nystagmus. REFERENCES Costin, A., M. Chalmowitz & F. Bergman. Nystagmus evoked by intermittent photic stimulation of the rabbit eye. Experientia 21:167-168 (1965). Itin, W. Le nystagmus photique provoqud par stimulation lumineuse intermittente. Rev. Laryng. (Bordeaux) 90:520-528 (1969). Keane, J.R. Flash-evoked nystagmus: absence in man. Neurology 22:551-553 (1972). Pasik, P., T. Pasik & J. Valciukas. Nystagmus induced by repetitive light flashes in monkeys. Brain Res. 19:313-317 (1970).
Author's addresses: Electrophysiology Unit Department of Ophthalmology Wilhelmina Gasthuis University of Amsterdam le Helmerstraat 104 Amsterdam The Netherlands Department of General Neurophysiology Catholic University of Nijmegen Nijmegen The Netherlands
206
