Human optokinetic afternystagmus. Charging characteristics and stimulus exposure time dependence in the two-component model

The dependence of human optokinetic afternystagmus (OKAN) velocity storage (charging) and optokinetic nystagmus (OKN) characteristics on optokinetic (OK) stimulus exposure time was investigated, using the two-component double exponential model for OKAN decay. Results are compatible with our previously proposed concept of two velocity storage integrators, one responsible for the short time constant decay (pursuit-mediated) and the other for the long time constant decay (OK system-mediated). The dependence of the long time constant integrator of OKAN on stimulus exposure time was clearly demonstrated. The short time constant integrator appeared to be independent of stimulus exposure time within the range studied. We conclude that the charging time-course of each component is distinct from that of the other. The time constants of each component decay were found to be invariant. A left-right asymmetry observed in both OKN and OKAN responses suggests that the integrators are direction sensitive.     

Transfer of optokinetic activity to vestibular nystagmus

Transfer of activity generated by prior optokinetic (OK) stimuli of one minute's duration to nystagmus induced in darkness by a subsequent vestibular stimulus consisting of step velocities to and from 40 degrees/s-1 was studied in 10 normal subjects. Four types of OK stimuli were used: (a) full field 'passive'; (b) full field 'active'; (c) full field in the presence of optic fixation, and (d) small OK drum stimulation. Transfer (T) was evident under all conditions and resulted in an enhancement of the vestibulo-ocular (VO) response when activity from the two stimuli were in the same direction (S) and a suppression when in the opposite (O). Expressed by the equation: Formula See Text. the respective transfer values obtained for the above conditions were (a) 66%, (b) 58%, (c) 22%, and (d) 54%. In all tests, rightward OK drum movement was more effective than leftward. In respect of passive OKN the resultant response can be well represented as the algebraic summation of the expected optokinetic after-nystagmus (OKAN) and the VO response, though opposing OKAN is more effective than enhancing. Passive OKN is more effective than active and this can be accounted for by the small contribution made by retinal slip in the former (the indirect path). Surprisingly, the small drum proved almost as effective as active OKN in terms of transfer. Fixation in the presence of full-field OK stimuli induces a non-directionally specific depression of the subsequent VO response, implying that retinal slip could contribute to the mechanism of VO response suppression.     

Human optokinetic afternystagmus. Slow-phase characteristics and analysis of the decay of slow-phase velocity

Events following the extinction of lights after 1-minute exposures of naive, normal subjects to an optokinetic stimulus at 40 deg/sec have been closely examined and quantified. Mean eye displacement in each slow phase decreased from 10.12 +/- 1.61 deg during optokinetic nystagmus (OKN) to 3.36 +/- 2.32 deg during optokinetic afternystagmus (OKAN). Slow-phase duration increased from 0.26 +/- 0.03 sec during OKN to 0.45 +/- 0.195 sec during OKAN. Eye displacement per slow phase remained fairly constant during OKAN, suggesting a spatial reference for the resetting of gaze. OKAN decay is a two-component process which can be closely approximated by a sum of two exponentials, one with a short time constant of 1.15 sec and the other with a long time constant of 48.8 sec. OKAN decay commenced at a time after lights out which depended upon the presence and timing of an intervening fast phase. When a fast phase intervened, OKAN decay commenced about 230 msec after it, and about 460 msec after lights out. When lights out occurred during the fast phase, OKAN decay commenced about 340 msec later.     

Human optokinetic after-nystagmus: variability in serial test results.

Test-retest variability of values for directional asymmetry in primary and secondary horizontal optokinetic after-nystagmus (OKAN I and OKAN II, respectively) was studied in 16 apparently healthy subjects. OKAN was induced by 60 s of whole-field optokinetic stimulation at speeds of 60 degrees/s and 90 degrees/s in either direction (left and right), each subject being tested on the same respective weekday once a week for 4 consecutive weeks. Values for directional asymmetry were calculated as the relative side-difference between response to drum rotation toward the right and toward the left. The subjects manifested considerable variation in values for directional asymmetry in OKAN I. This suggests prediction of a given individual's true value for directional asymmetry in OKAN I to require several measurements. On the other hand, 15/16 subjects manifested no asymmetry in OKAN II (the 16th subject was a further investigation found to have significant asymmetric caloric responses). As both OKAN I and OKAN II are known to reflect asymmetric vestibular function it is suggested that studying OKAN II may require fewer measurements of directional asymmetry, compared with studying OKAN I, when assessing the course of peripheral vestibular asymmetry.     

Is "ambient vision" distributed in the brain? Effects of wide-field-view visual yaw motion on PET activation

Ambient vision comprises the visual functions that are associated with the maintenance of spatial orientation and that depend on peripheral, preconscious visual inputs. Although a limited number of brain areas appear to be activated by coherent wide-field-of-view (WFOV) motion in more than one axis, a diffuse pattern of lateralized brain activity occurs in response to clockwise or counterclockwise ambient visual roll motion. In the present study involving positron emission tomography (PET), a similar finding was shown for rightward versus leftward yaw stimulation. A total of 18 PET scans were obtained from six subjects in response to either leftward or rightward WFOV motion in a collimated display subtending > 100 degrees horizontally. Rightward stimulation elicited mainly activation throughout the right hemisphere, whereas leftward stimulation elicited mainly activation throughout the left hemisphere. These findings provide further evidence that the ambient vision signal is either processed or transmitted throughout the entire brain, as befits a visual function that is fundamental to all other perceptual systems.     

Directional preponderance in pitch circular vection

We used optokinetic stimulation (OKS) in eighteen normal adults aged 18-30 years to investigate vertical self-motion perception. In order to induce self-rotation, either a stripe pattern or a random dot pattern was projected onto the inner wall of a hemispherical dome with a diameter of 150 cm. The pattern was rotated either about the subject's vertical axis (yaw) or about the subject's interaural axis (pitch) for 80 s at a constant acceleration of 1 deg/s2. Stimuli were randomly repeated three to four times in each direction. The latency of onset as well as the perceived intensity of circular vection (CV) was measured for each stimulus presentation. CV latencies for upward rotational stimulation were significantly longer than those for downward rotational stimulation under both types of stimulus conditions. There was no significant difference in CV latency between rightward and leftward rotational stimulation. For most subjects, the magnitudes of the perceived CV for rightward rotational stimulation were equal to those for leftward rotational stimulation, whereas the magnitudes of the perceived CV for vertical stimulation showed large intersubject variability. These results provide additional evidence that fundamental differences exist between different types of self-motion. Possible explanations for the directional asymmetry in vertical perception of self-motion will also be discussed.     

Suppression of optokinetic velocity storage in humans by static tilt in pitch.

The effects of static tilts about the pitch axis on human horizontal optokinetic after-nystagmus OKAN (HOKAN) were examined. Static tilts in pitch produced tilt-dependent HOKAN suppression. The slow decay (indirect pathway) component (coefficient C and long time constant 1/D) of the two-component model for OKAN was significantly reduced, while the short decay (direct pathway) component (coefficient A and short time constant 1/B) remained invariant as angle of tilt was increased. These results provide further evidence that otolith organ activity can couple to horizontal velocity storage in humans, in accordance with models proposed in the literature.     

Slow cumulative eye position to quantify optokinetic afternystagmus.

In 30 normal subjects we computed the slow cumulative eye position (SCEP) of optokinetic afternystagmus (OKAN) that followed 60 seconds of full-field optokinetic stimulation at 60 degrees/s. The mean SCEP was 112.8 degrees +/- 65.0 degrees. The lower and upper fifth percentile limits for directional preponderance of the SCEP were -38.8% and 44.3%, respectively. The time constant, which we calculated by dividing the SCEP by the initial velocity, was 12.0 +/- 7.4 seconds. This value is nearly identical to the time constant obtained from semilogarithmic regression of the decay of OKAN slow-phase velocity versus time. We conclude that the SCEP is a good measure of OKAN and that it reflects the substantial amount of variability and directional asymmetry observed in the optokinetic responses of normal subjects.     

Human optokinetic afternystagmus. Stimulus velocity dependence of the two-component decay model and involvement of pursuit

The dependence of human OKAN characteristics on optokinetic (OK) stimulus velocity was examined using the two-component double exponential model for OKAN decay. Drum velocities studied were between 10 degrees and 70 degrees deg/sec over a constant exposure period of 60 sec. Results reveal two distinct types of response: a 'low'-level response at lower drum velocities (10 degrees, 20 degrees, 30 degrees/sec) and a 'high'-level response at higher drum velocities (40 degrees, 60 degrees, 70 degrees /sec). These findings support our previous proposal that OKAN decay is a two-component process, and extend it by demonstrating that these two components have differing stimulus velocity sensitivities, as would be predicted if it were assumed that they represented direct (pursuit) and indirect (non-pursuit) pathways respectively.     

Optokinetic afternystagmus in humans: normal values of amplitude, time constant, and asymmetry

It has been suggested that the appearance of directional asymmetry and/or a reduced time constant of optokinetic afternystagmus (OKAN) might be a clinical index of vestibular imbalance. However, we do not know the limits for OKAN parameters in normal humans. Accordingly, we studied OKAN in 30 normal subjects using a "sampling" method, in which a number of values of OKAN are obtained by turning out the lights periodically during optokinetic stimulation. We found that the initial velocity of OKAN has a large intrasubject variability. Accordingly, if precision is desired so as to obtain 95% confidence that the measured mean of the initial velocity of OKAN is within 25% of the true mean in an individual subject, at least eight measurements of the initial OKAN velocity must be taken. When 12 measurements are made, all subjects had a minimum value of 5 degrees/s initial OKAN, and there was little directional asymmetry (mean of -0.47 degree/s +/- 3.13 degrees/s). The intrasubject variability of the time constant of OKAN was similar to the variability of initial OKAN velocity. However, because it is not possible to obtain repeated measures of the time constant in a short period of time, the time constant of OKAN is less likely to be useful in clinical testing.     

Age-Related Hearing Loss in C57BL/6J Mice has both Frequency-Specific and Non-Frequency-Specific Components that Produce a Hyperacusis-Like Exaggeration of the Acoustic Startle Reflex

Auditory brainstem-evoked response (ABR) thresholds were obtained in a longitudinal study of C57BL/6J mice between 10 and 53 weeks old, with repeated testing every 2 weeks. On alternate weeks, acoustic startle reflex (ASR) amplitudes were measured, elicited by tone pips with stimulus frequencies of 3, 6, 12, and 24 kHz, and intensities from subthreshold up to 110 dB sound pressure level. The increase in ABR thresholds for 3 and 6 kHz test stimuli followed a linear time course with increasing age from 10 to 53 weeks, with a slope of about 0.7 dB/week, and for 48 kHz a second linear time course, but beginning at 10 weeks with a slope of about 2.3 dB/week. ABR thresholds for 12, 24, and 32 kHz increased after one linear segment with a 0.7 dB slope, then after a variable delay related to the test frequency, shifted to a second segment having slopes of 3–5 dB/week. Hearing loss initially reduced the ASR for all eliciting stimuli, but at about 6 months of age, the response elicited by intense 3 and 6 kHz stimuli began to increase to reach values about three times above normal, and previously subthreshold stimuli came to elicit vigorous responses seen at first only for the intense stimuli. This hyperacusis-like effect appeared in all mice but was especially pronounced in mice with more serious hearing loss. These ABR data, together with a review of histopathological data in the C57BL/6 literature, suggest that the non-frequency-specific slow time course of hearing loss results from pathology in the lateral wall of the cochlea, whereas the stimulus-specific hearing loss with a rapid time course results from hair cell loss. Delayed exaggeration of the ASR with hearing loss reveals a deficit in centrifugal inhibitory control over the afferent reflex pathways after central neural reorganization, suggesting that this mouse may provide a useful model of age-related tinnitus and associated hyperacusis.     

Reverse optokinetic after-nystagmus generated by gaze fixation during optokinetic stimulation

Gaze fixation during optokinetic stimulation generates an after-nystagmus with a slow component towards the reverse direction of the optokinetic stimulation. The duration and maximum slow component velocity (SCV) of this "reverse OKAN" were observed by changing the duration, velocity and direction of the optokinetic stimulation in nine normal volunteers. The duration of reverse OKAN increased with increasing stimulation time but was unaffected by changes in the stimulation velocity. The maximum SCV of reverse OKAN decreased with an increase in the stimulation velocity but was not significantly affected by changes in the optokinetic stimulation time. There was no directional difference among the horizontal, upwards and downwards reverse OKANs. The reverse OKAN was thought to be generated by a mechanism different from the velocity storage mechanism which produced optokinetic nystagmus and the first phase of OKAN. Retinal slip during the optokinetic stimulation was considered to be an input to the mechanism which generated the reverse OKAN. We hypothesize that the mechanism causing the reverse OKAN may be a generator of the second phase of OKAN, which was also intimately connected with self-motion sensation during the optokinetic stimulation.     

Reverse Optokinetic After-nystagmus Generated by Gaze Fixation During Optokinetic Stimulation

Gaze fixation during optokinetic stimulation generates an after-nystagmus with a slow component towards the reverse direction of the optokinetic stimulation. The duration and maximum slow component velocity (SCV) of this "reverse OKAN" were observed by changing the duration, velocity and direction of the optokinetic stimulation in nine normal volunteers. The duration of reverse OKAN increased with increasing stimulation time but was unaffected by changes in the stimulation velocity. The maximum SCV of reverse OKAN decreased with an increase in the stimulation velocity but was not significantly affected by changes in the optokinetic stimulation time. There was no directional difference among the horizontal, upwards and downwards reverse OKANs. The reverse OKAN was thought to be generated by a mechanism different from the velocity storage mechanism which produced optokinetic nystagmus and the first phase of OKAN. Retinal slip during the optokinetic stimulation was considered to be an input to the mechanism which generated the reverse OKAN. We hypothesize that the mechanism causing the reverse OKAN may be a generator of the second phase of OKAN, which was also intimately connected with self-motion sensation during the optokinetic stimulation.     

Optokinetic nystagmus and after-nystagmus during a 6 hour bedrest study

CONCLUSIONS: A lengthy alteration of gravity direction produced different effects on the intrinsic horizontal and vertical optokinetic oculomotor systems. OBJECTIVE: To examine both optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) in a 6 h 6 degrees head-down bedrest study, in which the subjects were kept lying under simulated micro-gravity conditions. SUBJECTS AND METHODS: In six normal healthy adults, we repeatedly (five times) and comparatively studied OKN and OKAN evoked by horizontal and vertical stimuli. Stage 1 was an upright sitting position. During the 6 h bedrest condition, we studied OKN and OKAN in 90 degrees recumbent lateral positions (stages 2, 3, and 4). In stage 5 the subject returned to an upright position. RESULTS: We confirmed that the change in gravity direction had various effects on the condition of OKN and OKAN. Also, we found that it took more than 3 h to reach a desirable level of systemic adaptive modification to the unique environmental condition. We considered that the early change was basically due to the changes in sensory inputs through the otolith organs, and the latter changes represented the adaptive process of the spatial orientation system. During the tilt, the occurrence rates of both horizontal and vertical OKANs were decreased; however, the conditions of these changes were different.     

The effects of intense click sounds on velocity storage in optokinetic after-nystagmus

Vestibular evoked myogenic potentials (VEMP) occurring after click stimulation in cervical muscles are thought to be a polysynaptic response of otolith-vestibular nerve origin. In optokinetic after-nystagmus (OKAN) the direction of after-nystagmus changes and slow-phase velocity decreases with head tilt. This phenomenon may be an otolith response to the direction of gravity. We assumed that intense clicks might have some influence on OKAN via the otolith-vestibular nerve. Twelve normal subjects who showed VEMP at 75 dB normal hearing level (nHL) clicks were examined. The OKAN was recorded under four conditions: right monaural, left monaural and binaural stimulation by 75 dB nHL clicks, and absence of click stimulation. Horizontal optokinetic stimulation was applied using stepwise increasing speeds from 30 deg/s to 90 deg/s. Two seconds before the stimulus ended, clicks were sounded. The slow-phase velocity of the recorded electro-nystagmography was manually measured. There was no effect on OKAN with unilateral stimulation but binaural stimulation suppressed it. These results suggest that a velocity storage integrator is influenced by intense clicks via the otolithic area. Received: 17 November 1999 / Accepted: 30 May 2000     

Asymmetric optokinetic afterresponse in patients with vestibular neuritis.

The symmetry of primary and secondary optokinetic afternystagmus (OKAN I and OKAN II, respectively) was studied in 14 patients with vestibular neuritis, as well as in 50 normals. The patients were examined at onset of symptoms and at follow-up 3 and 12 months later. At onset, OKAN was found mainly to reflect the spontaneous nystagmus. Although the spontaneous nystagmus disappeared in all patients within 3 months, both OKAN I and OKAN II was asymmetric at the 3- and 12-month check-ups. OKAN beating toward the lesioned ear was weaker than the OKAN beating toward the healthy ear. Thus, the asymmetric vestibular function was reflected not only in the OKAN I, but also by an asymmetry in OKAN II. Between the 3- and 12-month check-ups, asymmetry in OKAN declined, even among those patients who showed no improvement in caloric response during that time. The decreasing asymmetry in OKAN with time after lesion was, however, related to the disappearance of a positional nystagmus. Hence, the results may be interpreted as suggesting OKAN not only to be affected by vestibular side-difference, but also to be modified by the process responsible for vestibular compensation following a peripheral vestibular lesion.     

Periodic alternating nystagmus during caloric stimulation

Periodic alternating nystagmus (PAN) is a form of horizontal jerk nystagmus characterized by periodic reversals in direction. We report a case who exhibited transient PAN induced by caloric stimulation. The patient was a 75-year-old male. He had experienced floating sensation in January 2010. Eight months later, he was referred to our university hospital. Gaze nystagmus and positional tests revealed no nystagmus. Only weak right-beating horizontal nystagmus was observed during left Dix–Hallpike maneuver. Electronystagmography showed normal saccadic and smooth pursuit eye movements. The optokinetic nystagmus pattern test was also bilaterally normal. However, during the caloric stimulation to the right ear, at 166 s from the start of irrigation, the direction of nystagmus alternated from leftward to rightward, and thereafter this reversal of direction repeated 15 times. Magnetic resonance imaging showed no significant lesion except for chronic ischemia in the brain. The patient probably had some kind of latent lesion of impaired velocity storage and exhibited transient PAN induced by caloric stimulation. Caloric stimulation is useful and simple examination to disclose latent eye movement disorders of which velocity storage mechanism is impaired.     

Neuronal responses in chinchilla auditory cortex after postnatal exposure to frequency-modulated tones

Early postnatal exposure to an abnormal acoustic environment has been shown to significantly influence the behaviour of neurons in the auditory cortex. In the present study, we ask if sustained neonatal exposure to an FM sweep affects the development of responses to tonal and FM stimuli in chinchilla auditory cortex. Newborn chinchilla pups were exposed continuously to an upward linear FM sweep (0.1-20 kHz) at 0.05 kHz/ms for 4 weeks. Neuronal responses to pure tones and bidirectional linear FM sweeps (range: 0.1-20 kHz; speeds: 0.05-0.82 kHz/ms) were assessed in anesthetized animals following the exposure period as well as in age-matched controls (P28). We hypothesized that constant FM exposure would increase the response selectivity of cortical neurons to the environmental FM sweep. However, our results show that while tonal response latencies increased after the exposure period (p < 0.0001, one-way ANOVA), the exposure stimulus had minimal effect on neuronal direction sensitivity and decreased neuronal selectivity for any of the presented FM sweep speeds (p < 0.05, one-way ANOVA). We therefore suggest that the development of FM direction sensitivity is experience-independent while normal acoustic experience may be required to maintain FM speed tuning.     

Optokinetic nystagmus and after-nystagmus during a 6 hour bedrest study

Conclusions. A lengthy alteration of gravity direction produced different effects on the intrinsic horizontal and vertical optokinetic oculomotor systems. Objective. To examine both optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) in a 6 h 6° head-down bedrest study, in which the subjects were kept lying under simulated micro-gravity conditions. Subjects and methods. In six normal healthy adults, we repeatedly (five times) and comparatively studied OKN and OKAN evoked by horizontal and vertical stimuli. Stage 1 was an upright sitting position. During the 6 h bedrest condition, we studied OKN and OKAN in 90° recumbent lateral positions (stages 2, 3, and 4). In stage 5 the subject returned to an upright position. Results. We confirmed that the change in gravity direction had various effects on the condition of OKN and OKAN. Also, we found that it took more than 3 h to reach a desirable level of systemic adaptive modification to the unique environmental condition. We considered that the early change was basically due to the changes in sensory inputs through the otolith organs, and the latter changes represented the adaptive process of the spatial orientation system. During the tilt, the occurrence rates of both horizontal and vertical OKANs were decreased; however, the conditions of these changes were different.     

Cross-coupling between horizontal and vertical eye movements during optokinetic nystagmus and optokinetic afternystagmus elicited in microgravity

The occurrence of horizontal jerks with larger amplitudes than on Earth was observed during vertical optokinetic nystagmus in astronauts tested throughout a 7-day spaceflight. During early exposure to microgravity, a horizontal spontaneous-like nystagmus was recorded in darkness following both vertical and horizontal optokinetic stimulation. In addition, the time constant of vertical OKAN with slow phase up was larger than on the ground. These effects disappeared on flight day 2. Then, the horizontal and vertical OKAN time constants decreased, and gradually returned to the preflight values, as previously observed with the gain of the vestibulo-ocular reflex. The early changes in microgravity are in agreement with those obtained on Earth in monkeys and humans during static tilt relative to gravity. Our findings suggest that the absence of otolithic input in microgravity may have an effect on the optokinetic system which could be mediated by the velocity storage mechanism.