The efficiency of the central and peripheral retina in driving human optokinetic nystagmus

Previous experiments to decide whether the gain of optokinetic nystagmus (OKN) is increased or decreased by occlusion of the central retina involved the use of stationary edges on the occluder and unmatched contrasts. With these factors controlled, it was confirmed that OKN gain is severely reduced by occlusion of the central retina but only at stimulus velocities above about 30°/sec. The gain of horizontal OKN was found not to increase with increasing width of the display if the lateral edges are blurred. The high gain of centrally driven OKN may be related to the ability of higher mammals to stabilize the images of objects at a given distance in a complex parallactic visual field.     

Human optokinetic nystagmus in response to moving binocularly disparate stimuli

Physiological and behavioral evidence shows that the directionally preponderant subcortical control of optokinetic nystagmus (OKN) in lower mammals is supplemented in higher mammals by bidirectional cortical control. It is hypothesized that this cortical control allows higher mammals to cope with the parallactic movement of the scene produced by linear motion of the body. In particular, it is hypothesized that a coupling between OKN and stereopsis allows higher mammals to stabilize the images of objects within the plane of fixation while ignoring motion signals from objects at other distances. According to this hypothesis the gain of the slow phase of OKN should be highest for binocularly fused moving stimuli and attenuated for binocularly disparate displays. The results of Experiment 1 confirmed this prediction although the effects of accommodation were not ruled out completely. In Experiment 2 a display moving in one direction was presented across the central retina at the same time as one moving in the opposite direction was presented in the upper and lower periphery. It was found that subjects do not show OKN in the direction of the peripheral display unless it is binocularly fused and the central display is disparate. In Experiment 3 a stationary display of dots was superimposed on a moving display. It was found that OKN is not inhibited by the stationary display when it has a horizontal disparity and the moving display is fused. Experiment 4 found that horizontal OKN is disrupted by the sudden introduction of a vertical disparity in the stimulus. Since accommodative state was kept constant in the last three experiments, the data show that binocular disparities can help a person to stabilize selectively the image of one moving display while ignoring conflicting motion signals from another display.     

Optokinetic nystagmus in patients with central scotomas in age related macular degeneration

BACKGROUND: Reports on the impact of a loss in the central field of vision on optokinetic nystagmus (OKN) are varied. A study was therefore undertaken to reassess the role of the central retina in the generation of OKN in a large group of patients with age related macular degeneration. METHODS: Four groups of 20 patients were examined: a control group without scotoma and three groups with absolute central scotomas measuring 1 degrees--10 degrees, 11 degrees--20 degrees, and 21 degrees--30 degrees. OKN was elicited with black and white stripes moving nasally to temporally or temporally to nasally on a screen subtending 54 degrees x 41 degrees at four velocities (15, 30, 45, and 60 degrees /s). OKN gain was measured using infrared oculography. RESULTS: There was no significant difference in OKN gain between the control group and those with scotomas of 1 degrees--10 degrees and 11 degrees--20 degrees. A significant difference in OKN gain was found between the group with scotomas of 21 degrees--30 degrees and all other groups at stimulus velocities of 30, 45, and 60 degrees/s (p<0.05). OKN gain significantly diminished with increasing stimulus velocity (p<0.05). No statistically significant difference was found in OKN gain between stimuli moving temporally to nasally and nasally to temporally. CONCLUSION: Abnormalities of OKN gain were noted only in patients with large scotomas. An intact macula is therefore not necessary for the generation of OKN.     

Role of the stimulus size in the generation of optokinetic nystagmus in normals and in patients with retinitis pigmentosa

BACKGROUND: To find out the smallest stimulus size still able to elicit optokinetic nystagmus (OKN). To discuss the differences in the generation of OKN between normals and patients with tunnel vision. METHODS AND PATIENTS: OKN was elicited monocularly with black and white stripes of 2 degrees moving on a screen temporally-to-nasally or nasally-to-temporally at velocities of 15, 30, 45, and 60 degrees /s. In healthy subjects (n = 10) the size of the stimulus was gradually increased from minimal 8 degrees x 0.5 degrees to maximal 48 degrees x 10 degrees. OKN was elicited in retinitis pigmentosa (RP) patients (n = 2) with visual field reduced to the central 10 degrees. OKN gain was measured using infrared oculography. OKN response was considered as normal if it was within 2 standard deviations of the mean OKN of age-matched control groups (n = 29). RESULTS: In healthy subjects the size of the stimulus necessary to elicit normal OKN gain was smaller at low velocity of 15 degrees /s (16 degrees x 1 degrees ) than at higher velocities of 30, 45, and 60 degrees /s (16 degrees - 24 degrees x 3 degrees ). Normal OKN gain was measured in the RP patients only at the low velocity of 15 degrees /s. CONCLUSIONS: Small stimuli are sufficient in normals to elicit good OKN answers. Therefore, poor OKN gain in RP patients may not be explained by their tunnel vision only. We postulate that in these patients the reduction of gain is due to a multifactorial decrease of their visual perception related to disorders of the visual field, the visual acuity, and the contrast sensitivity.     

Age related change of optokinetic nystagmus in healthy subjects: a study from infancy to senescence

BACKGROUND: Optokinetic nystagmus (OKN) gain is asymmetrical between temporal to nasal (TN) and nasal to temporal (NT) stimulation in infancy and decreases at older ages. The age at which OKN gain becomes symmetrical and decreases is debated. The aim was to investigate OKN over the whole lifespan in a large sample of healthy subjects. METHODS: In a prospective, cross sectional study OKN was tested monocularly using TN and NT small field stimulation. Stimulation velocity was 15 degrees /s and 30 degrees /s for children aged under 1 year (n = 97), and 15 degrees /s, 30 degrees /s, 45 degrees /s, and 60 degrees /s for older subjects (1-9 years, n = 66; 10-89 years, n = 86). Gain was measured using infrared oculography. RESULTS: Significant OKN gain asymmetry in favour of TN versus NT stimulation was found during the first 5 months of life (p<0.05). Only at 11 months of age was OKN symmetrical in 100% of the subjects. The percentage of children with symmetrical OKN decreased with increasing stimulus velocity. OKN gain increased in the second and third years (p<0.05 for 15 degrees /s), remained stable until 50 years of age, and showed a small but significant decrease afterwards for the tested velocities (between 6% and 18%, p<0.05). CONCLUSIONS: Infrared oculography is an accurate method to assess OKN, especially in children. Knowledge about change of OKN in healthy subjects could be helpful to interpret OKN in patients with abnormal binocular vision or lesions of the central nervous system.     

Visual completion of partly occluded grating in infants under 1 month of age

Four groups of eight infants (3 weeks of age on average) were each habituated to one of four displays consisting of a grating of either low (0.4 cpd) or high (1.2 cpd) spatial frequency, whose central portion was covered up with a horizontal occluder which was either narrow (1.33 degrees) or broad (4.17 degrees). These habituation displays are referred to as LN (low spatial frequency grating and narrow occluder), LB (low and broad), HN (high and narrow), and HB (high and broad) displays. Posthabituation-test displays consisted of a complete grating (CG) of the same frequency as the habituated grating along with a separate grating (SG) whose central portion was replaced with a black gap of the same height as the occluder in the habituation displays. Infants habituated to the LN display looked significantly longer at the SG than the CG display during posthabituation-test trials. Infants habituated to the LB and HN displays looked at the CG and SG displays, almost equally. In contrast, infants habituated to the HB display looked longer at the CG than the SG display. These results show that infants under 1 month of age can perceive the continuation of the grating behind the occluder, and that their visual completion on habituation displays can be evoked according to the interaction between the spatial frequency of the grating and the occluder height.     

Modulation of spatial attention with unidirectional field motion: an implication for the shift of the OKN beating field

During optokinetic nystagmus (OKN) the mean eye position of gaze (the beating field) shifts in the direction of the fast phases. The function of this shift may be to re-orient the eyes in the direction of self-motion which optic flow implies (in-coming field). This idea leads to the hypothesis that visual attention may be directed toward the In-coming field. In Experiment 1, subjects detected a visual flash presented against unidirectional field motion. The OKN beating field was shifted toward the In-coming field, and manual reaction times were shorter when the target appeared in the In-coming field. Experiment 2 revealed that this In-coming field advantage occurred even when OKN (and thus the mean eye-position shift) was suppressed. Subsequent experiments showed that the In-coming field advantage is not due to a local motion interaction (Experiment 3), survives subject's voluntary allocation of attention (Experiment 4), and develops over less than 320 ms after the onset of the motion field (Experiment 5). These results suggest that unidirectional field motion tends to automatically shift visual attention toward the In-coming field.     

Torsional optokinetic nystagmus: normal response characteristics

BACKGROUND/AIMS: Few studies have investigated normal response characteristics of torsional optokinetic nystagmus (tOKN). The authors have investigated the effect of stimulus velocity and central/peripheral stimulation on tOKN. METHODS: Torsional OKN was elicited using a sinusoidal grating rotating at velocities of 3 degrees /s to 1000 degrees /s in clockwise and anticlockwise directions. To investigate the effect of central stimulation, stimulus size was varied from 2.86 degrees to 50.8 degrees. An artificial scotoma placed over a 50.8 degrees stimulus was varied from 2.86 degrees to 43.2 degrees to investigate peripheral stimulation. Eight subjects participated in each experiment and torsional eye movements were recorded using video-oculography. The mean slow phase velocity (MSPV) and gain were calculated. RESULTS: The maximum gain occurred in response to 8 degrees /s stimulation. The MSPV increased up to a stimulus velocity of 200 degrees /s achieving a maximum of 3 degrees /s in both directions. MSPV was linearly correlated with the log of stimulus velocity. The smallest field size, rotating at 40 degrees /s, evoked 10% of the gain elicited by the largest display. When the most peripheral stimulus was used, the gain was maintained at 50% of the gain evoked when the full display was used. CONCLUSIONS: A wide range of stimulus velocities can elicit tOKN and peripheral field stimulation contributes significantly to its response.     

Comparison of latent nystagmus and nasotemporal asymmetries of optokinetic nystagmus in adult humans and macaque monkeys who have infantile strabismus

To determine whether macaque monkeys with infantile strabismus have latent nystagmus and directional asymmetries of horizontal optokinetic nystagmus (OKN) similar to those of humans with infantile strabismus, the authors recorded eye movements under conditions of monocular viewing. The presence of latent fixation nystagmus was tested by requiring the subjects to steadily fixate a stationary target subtending less than I deg of visual arc. OKN was tested using high-contrast, vertically-oriented moving stripes that filled 80 deg of the visual field. A macaque monkey who had infantile strabismus induced by alternating occlusion from birth showed latent nystagmus highly similar to that recorded in an adult human subject with infantile strabismus. The strabismic monkey also had asymmetric OKN similar to that of the strabismic human, favoring nasally-directed stimulus motion when viewing with either eye. Neither nystagmus nor an OKN asymmetry was observed in a normal macaque or in humans who had normal binocular vision. The findings of latent nystagmus and OKN asymmetries in the strabismic monkey support the notion that monkeys who have infantile-onset strabismus are an appropriate ocular motor model of human infantile strabismus.     

Human horizontal optokinetic nystagmus elicited by the upper versus the lower visual fields

A 30-deg-high horizontally rotating random-dot display was presented to the central field, and with its more central edge at vertical eccentricities of 0, 2.5, 5, and 10 deg above or below the horizon. Stimulus velocities of 25-100 deg/s and two directions of motion were presented. The mean gain of the slow phases of optokinetic nystagmus (OKN) for five subjects was significantly higher when the stimulus was presented to the lower visual field than when the stimulus was presented to the upper field. This difference was most pronounced when the display was displaced 5 deg from the fovea and moving below 100 deg/s. Our results are consistent with existing psychophysical and physiological evidence for the superiority of the upper retina. In addition, four of the five observors showed significant directional asymmetries.     

Asymmetry of the optokinetic nystagmus in lesion of the central nervous system

BACKGROUND: Unilateral lesions of the central nervous system (CNS) may be associated with a reduction of the optokinetic nystagmus (OKN) slow component in the direction of the lesion. The aim of our study was to assess the role played in these cases by the direct injury of the OKN pathways and/or by a possible associated visual field defect. METHODS AND PATIENTS: Monocular OKN was elicited with black and white stripes moving temporally-to-nasally (TN) or nasally-to-temporally (NT) at velocities of 15, 30, 45 and 60 degrees /s. Patients with cortical or chiasmal lesions associated with visual field defects were investigated. OKN was considered asymmetrical if the gain difference between TN and NT stimulation was not within 2 standard deviations of an age-matched control group (n = 86). RESULTS: We examined 12 patients with cortical lesions and 4 patients with chiasmal lesions. Asymmetric OKN gain was measured in 7 patients with cortical lesions associated with a visual field defect, and in 2 patients with chiasmal compression and bitemporal hemianopia. In 2 patients with isolated occipital lesions, OKN asymmetry was explained by the associated visual field defect. CONCLUSION: The interpretation of OKN asymmetry in patients with CNS lesions should not only consider a direct lesion of the OKN pathways but also a sensory deficit due to a visual field defect.     

AN ASYMMETRY IN THE OPTOKINETIC NYSTAGMUS RESPONSE IN NORMAL INDIVIDUALS

Optokinetic nystagmus (OKN) was induced by using a modified projector system to provide a moving pattern of irregular spots of light. A systematic electro-oculographic study of horizontal and vertical OKN for a series of stimulus velocities from 10 to 100 degrees s-1 was conducted on 10 healthy young subjects. It was found that the vertical responses, in particular to upward-moving stimuli, were less well sustained at velocities above 30 degrees s-1 than the horizontal responses. In view of these observations, care should be taken in the interpretation of OKN results clinically, and only when the response is absent in one direction or an extreme asymmetry is present should clinical significance be attached. This applies particularly for the vertical OKN responses. Clearly, the use of stimulus velocities below 40 degrees s-1 is advised and the standard suggested by Holm -Jensen and Peitersen [Acta oto-lar. 88, 110-116 (1968)] of a target moving at 20 degrees s-1 seems a reasonable choice.     

Optokinetic nystagmus elicited by filling-in in adults with central scotoma

PURPOSE: Filling-in is the perceptual completion of physiological, pathologic, or artificial scotomas. Three patients are described, in whom optokinetic nystagmus (OKN) was present during filling-in. METHODS: Three patients with age-related macular degeneration with large central scotomas were included in the study. OKN was elicited with black and white stripes moving nasally to temporally or temporally to nasally at four velocities. OKN gain was measured using infrared oculography. RESULTS: While looking at the OKN stimuli, the patients either did not see the stimulus (without perceiving a positive scotoma) or filled in the scotoma and perceived the stripes. Simultaneously with filling in the scotoma, OKN eye movements were elicited in all three patients. The filling-in phenomenon was present for all stimulus directions and velocities, appeared within seconds, and was followed immediately by eye movements corresponding to OKN. OKN gains during filling-in were similar to those of age-matched control subjects without scotomas. No asymmetry was noted between temporal to nasal or nasal to temporal stimulation. CONCLUSIONS: Motion-sensitive areas of the visual cortex may be activated and trigger the generation of OKN, supporting an active process. Alternatively, OKN is suppressed when subjects are not filling-in, while the subjects are fixating the central scotoma or edges of the scotoma. That the subjects did not perceive positive scotomas suggests that an active process is more likely.     

7th International Congress of Neuro-Ophthalmology and Neurogenetics of the World Federation of Neurology and 5th Meeting of the International Neuro-Ophthalmology Society

Since the localizing value of optokinetic nystagmus (OKN) in cerebral hemispheric lesions has been questioned by some investigators, 96 patients with hemispheric lesions of various origin demonstrated on CT scan were studied by EOG (optokinetic stimulation at 30, 60 and 120 deg/ sec), EEG, perimetry and neurologic examination.

A severe OKN diminution was found in 17 cases, a moderate diminution in ninecasesand a slight diminution only in seven cases (contralateral deficit in OKN). M onocular stimulation revealed further OKN deficits in two additional cases. Both severe and moderate OKN diminution could be located significantly in the deep white matter of the dorsoparietal and dorsotemporal lobe (CT 2A, 2B, 3A). In contrast to this specific OKN deficit a slight unilateral diminution determined only the involved hemisphere in general but did not correlate with any anatomic area within one hemisphere.     

Reflexive optokinetic nystagmus in younger and older observers under photopic and mesopic viewing conditions

PURPOSE: To investigate the effect of age on optokinetic nystagmus (OKN) in response to stimuli designed to preferentially stimulate the M-pathway. METHOD: OKN was recorded in 10 younger (32.3 +/- 5.98 years) and 10 older (65.6 +/- 6.53) subjects with normal vision. Vertical gratings of 0.43 or 1.08 cpd drifting at 5 degrees /s or 20 degrees /s and presented at either 8% or 80% contrast were displayed on a large screen as full-field stimulation, central stimulation within a central Gaussian-blurred window of 15 degrees diameter, or peripheral stimulation outside this window. All conditions apart from the high-contrast condition were presented in a random order at two light levels, mesopic (1.8 cdm(-2)) and photopic (71.5 cdm(-2)). RESULTS: Partial-field data indicated that central stimulation, mesopic light levels, and lower temporal frequency each significantly increased slow-phase velocity (SPV). Although there was no overall difference between groups for partial-field stimulation, full-field stimulation, or low-contrast stimulation, a change in illumination revealed a significant interaction with age: there was a larger decrease in SPV going from photopic to mesopic conditions for the older group than the younger group, especially for higher temporal frequency stimulation. CONCLUSIONS: OKN becomes reflexive in conditions conducive to M-pathway stimulation, and this rOKN response is significantly diminished in older healthy adults than in younger healthy adults, indicative of decreased M-pathway sensitivity.     

Look and stare optokinetic nystagmus in healthy subjects and in patients with no measurable binocularity. A prospective study

BACKGROUND: The aim of our study was to assess the difference between look and stare optokinetic nystagmus (OKN) in healthy subjects and in patients with early onset strabismus and no measurable binocularity. METHODS AND PATIENTS: OKN was elicited monocularly with black and white stripes moving horizontally or vertically at 15, 30, 45 and 60 degrees /s. Subjects were instructed either to follow individual stripes across the screen (look OKN) or to fixate stripes in the centre of the screen (stare OKN). We examined 20 healthy subjects (mean age 29 years; range 21 - 39), and 10 patients with no measurable binocularity (mean age 12.7 years; range 5 - 37). OKN gain was measured with photo-oculography. RESULTS: In both groups and at stimulation with the higher velocities gains for look OKN were significantly better than for stare OKN, and gains with horizontal stimuli were significantly better than with vertical stimuli (p < 0.05). In the group with no measurable binocularity horizontal look and stare OKN answers were significantly better for temporal-to-nasal stimulation than for nasal-to-temporal stimulation (p < 0.05). CONCLUSIONS: The performance of look or stare OKN influences the gain in healthy subjects and in patients with no measurable binocularity. Data about both modes of OKN stimulation are helpful to better interpret OKN responses especially in subjects with poor cooperation.     

Optokinetic nystagmus in strabismus: are asymmetries related to binocularity?

PURPOSE: Strabismus may be associated with an asymmetry of monocular horizontal optokinetic nystagmus (OKN). It is unclear whether OKN asymmetries are associated with deficiency in binocular and/or stereovision. In the current study, patients with different degrees of binocularity were investigated. METHODS: OKN was examined in the dominant and nondominant eyes of four groups of patients: (1) no measurable binocularity (NB), (2) poor binocularity (PB)-that is, showing binocularity on the Bagolini Test and/or rudimentary stereovision, (3) good binocularity (GB) with good stereoacuity after squint surgery, and (4) a control group. Monocular OKN was elicited with black-and-white stripes moving temporally to nasally (TN) or nasally to temporally (NT) at velocities of 15, 30, 45, and 60 deg/s. Eye movements were recorded with infrared oculography. RESULTS: Only subjects in the NB group showed a significant OKN asymmetry, with preference for TN stimulation in dominant and nondominant eyes. Subjects with PB did not have significant OKN asymmetries but reduced OKN gains in both stimulus directions. Subjects with GB had normal mean OKN gains without asymmetry. Larger OKN asymmetries were correlated with younger age at detection of strabismus if NB and GB were grouped together, but not if each group was analyzed separately. CONCLUSIONS: For the first time, a large groups of patients classified by level of binocular vision has been investigated. The results show that OKN gain and asymmetry are associated with the development of binocular vision. OKN investigation may be helpful to identify patients with binocularity or binocular potential in strabismus.     

Optimal occluder luminance for seeing stationary visual phantoms

If the central portion of a vertical grating is covered up by an opaque horizontal occluder, a phantom grating is perceived to continue across the occluded region. The phantoms can be seen even if the inducing grating is stationary, but are not seen when the occluder luminance is near to the space-average luminance of the grating. Optimal occluder luminance for the perception of phantoms was measured, while occluder height and the inducing grating contrast varied. Phantom visibility was maximal when the occluder luminance was near to either the minimum or to the maximum luminance of the grating. When the occluder luminance was set between the two levels, opposite-phase illusory gratings were dominantly observed. The reciprocal relationship between the visual phantoms and the grating induction effect of McCourt (1982) is discussed.     

The visual-vestibular interaction in normal subjects

Full-field optokinetic nystagmus (OKN) at constant speed of 40 0/s and 60 0/s, vestibular ocular reflex (VOR) (sinusoidal 0.2 Hz, 60 0/s peak velocity) with different background (light and dark) and VOR-fix were observed in 72 normal subjects (144 normal eyes). The gain, FCV and DP of OKN, VVOR, VOR were also analysed. The gain of OKN is reduced accompanied with stimulation increasing, while the FCV is increased. The gain of VVOR is 1.08 +/- 0.09 (X +/- SD), while the gain of VOR is 0.64 +/- 0.09. The FCV of VOR is much reduced than that of VVOR. The FCV in the groups under 30 years of age are much faster than the groups above 30 years of age. The VOR--fix gain is 0.05 +/- 0.04. The DP of OKN, VVOR, and VOR are 0.05 +/- 0.04. The results showed an co-operation between visual and vestibular systems and the results also suggested that the series methods of visual-vestibular interaction (OKN, VVOR, VOR, VOR-fix) might be useful in eye movement examination.     

Asymmetrical vertical phorias in normal subjects: the influence of unbalanced illumination

PURPOSE: One goal of this investigation was to determine whether asymmetrical vertical hetereophorias, which are qualitatively similar to a dissociated vertical deviation (DVD), occur commonly in normal people. A second goal was to establish whether the magnitude or direction of vertical phoria depends on the illumination of the occluded eye, as would be expected if vertical eye position were influenced by a dorsal light reflex. METHODS: Vertical phorias were estimated in 30 normal subjects using a Maddox rod in conjunction with a dark and an illuminated occluder. RESULTS: On average, subjects exhibited a hypophoria of less than 1 min arc with both the dark and the illuminated occluder. Asymmetrical vertical phorias that were consistent with previously published criteria for a minute DVD were found in eight subjects when a dark occluder was used and in four subjects when an illuminated occluder was used. The proportions of subjects whose vertical phorias were consistent with a DVD did not differ significantly under the two conditions of occlusion. CONCLUSIONS: Asymmetrical vertical phorias that mimic a minute DVD do not occur commonly in normal people. Our finding that vertical phorias do not change systematically when an illuminated instead of a dark occluder is used suggests that normal subjects exhibit little or no vestige of the dorsal light reflex.