Modulation of saccadic intrusions by exogenous and endogenous attention.

Primary gaze fixation in healthy individuals is frequently interrupted by microsaccades and saccadic intrusions (SI). The neural systems responsible for the control of attention and eye movements are believed to overlap and in line with this, the behaviour of microsaccades appears to be affected by exogenous and endogenous attention shifts. In the current work we wished to establish whether SI would also be influenced by attention in order to provide evidence that SI and microsaccades exhibit similar behaviour and further investigate the extent of overlap between attention and eye movement systems. Twelve participants performed a cue-target task where they were cued exogenously or endogenously and had to respond to the appearance of a peripheral target with either a button press or saccade. Our results replicate earlier microsaccade research, indicating that SI are also influenced by exogenous and endogenous attention. In all conditions, SI frequency initially decreased following the cue, then rose to a maximum before falling to below baseline levels. Following the exogenous cue, SI were more frequently directed away from the cue as predicted by inhibition of return. Additionally, SI direction following the endogenous cue was biased towards the cue for the saccadic response mode only, suggesting that the degree to which the eye movement and attention systems overlap depends on whether an eye movement is required. In summary, our findings indicate that SI characteristics are modulated by exogenous and endogenous attention and in a similar way to microsaccades, suggesting that SI and microsaccades may lie on a continuum of fixational instabilities. Furthermore, as with microsaccades, SI are likely to provide additional insights into the relationship between attention and the oculomotor systems.

Paying attention to saccadic intrusions.

Fixation to a target in primary gaze is invariably interrupted by physiological conjugate saccadic intrusions (SI). These small idiosyncratic eye movements (usually <1 degrees in amplitude) take the form of an initial horizontal fast eye movement away from the desired eye position, followed after a variable duration by a return saccade or drift. As the aetiology of SI is still unclear, it was the aim of this study to investigate whether SI are related to exogenous or endogenous attentional processes. This was achieved by varying (a) the "bottom-up" target viewing conditions (target presence, servo control of the target, target background, target size) and (b) the 'top-down' attentional state (instruction change--'look' or 'hold eyes steady' and passive fixation versus active--'respond to change' fixation) in 13 subjects (the number of participants in each task varied between 7 and 11). We also manipulated the orientation of pure exogenous attention through a cue-target task, during which subjects were required to respond to a target, preceded by a non-informative cue by either pressing a button or making a saccade towards the target. SI amplitude, duration, frequency and direction were measured. SI amplitude was found to be significantly higher when the target was absent and SI frequency significantly lower during open loop conditions. Target size and background influenced SI behaviour in an idiosyncratic manner, although there was a trend for subjects to exhibit lower SI frequencies and amplitudes when a patterned background was present and larger SI amplitudes with larger target sizes. SI frequency decreased during the "hold eyes steady" passive command as well as during active fixation but SI direction was not influenced by the exogenous cue-target task. These results suggest that SI are related to endogenous rather than exogenous attention mechanisms. Our experiments lead us to propose that SI represent shifts in endogenous attention that reflect a baseline attention state present during laboratory fixation tasks and may prove to be a useful tool to explore higher cortical control of fixation.

Periodic alternating nystagmus in humans with albinism.

PURPOSE: To quantify the spatial and temporal nature of congenital periodic alternating nystagmus (PAN) and to test the hypothesis that PAN results from a temporal shift in the null zone. METHODS: Twenty-five subjects with oculocutaneous albinism (16 tyrosinase negative and 9 tyrosinase positive) and 7 with ocular albinism (5 x-linked and 2 autosomal recessive) participated in the study. Using infrared oculography, five features of the nystagmus were examined: amplitude, frequency, waveform, beat direction, and temporal nature of the cycle. RESULTS: Twelve subjects (37.5%) exhibited a PAN. The nystagmus waveforms encountered during the PAN active phases were either jerk-with-extended-foveation or pseudocycloid, whereas a variety of oscillations (including triangular and bidirectional) were evident during the quiet phases. For most of the 12 subjects, there was an asymmetric variation in nystagmus intensity during each PAN cycle. None of the 12 demonstrated a convergence null or an abnormal head posture. CONCLUSIONS: PAN is not an uncommon oscillation among humans with albinism. Changes in gaze position markedly influenced the periodicity of the ongoing nystagmus, thus supporting the hypothesis that PAN is the result of a temporal shift in the null zone.

Incremental light detection thresholds across the central visual field of human albinos.

PURPOSE: The authors investigated central retinal function in albinism by measuring incremental light detection thresholds in a group of oculocutaneous human albinos. METHODS: Eleven oculocutaneous human albinos (six tyrosinase negative and five oculocutaneous positive), six patients with idiopathic congenital nystagmus, and six normal control subjects participated in the study. Using a Goldmann bowl perimeter, incremental light detection thresholds were measured in the vertical meridian across the central +/- 30 degrees of the retina. Target presentation times were 1 sec for all subjects, and in the case of four albinos and one patient with idiopathic nystagmus, they were limited to the low-velocity period of each nystagmus cycle. RESULTS: For the normal control subjects, the maximum sensitivity was found to be -0.60 +/- 0.10 log units. By comparison, at 0 degrees, a range of sensitivities was obtained from the albino subjects (-0.9 to -2.1 log units) and from those with idiopathic nystagmus (-0.7 to -1.9 log units). The albinos had diverse retinal sensitivity profiles ranging from a near-normal peaked curve to a flat homogeneous profile. A variety of sensitivity profiles was also detected in those with idiopathic nystagmus, although, compared with the albino curves, a greater proportion were peaked. No sensitivity differences were found between the short and the longer target presentations. CONCLUSIONS: The variety of retinal sensitivity profiles obtained in this study suggests that, in albinism, considerable intersubject variability in the degree of foveal hypoplasia exists and that albino "foveal" function can reach near-normal levels, for at least some visual tasks.

Pursuit and optokinetic responses in latent/manifest latent nystagmus.

Abnormalities of foveal smooth pursuit and the monocular optokinetic response (OKR) have often been reported in subjects with latent nystagmus (LN) and manifest latent nystagmus (MLN). This abnormality typically takes the form of a monocular asymmetry with a deficit in the response to nasal-to-temporal (N-T) motion in the visual field. Previous studies have each presented different interpretations of this finding, depending on whether the characteristics of the spontaneous oscillation were considered when analyzing the measured eye movement response: one report has suggested that these asymmetries are in fact the cause of the spontaneous nystagmus. In this study, pursuit and OKRs were examined separately and, when working synergistically and antagonistically, to attempt to overcome this difficulty. Results suggest that pursuit and the OKR could be symmetric in LN and MLN for both binocular and monocular viewing, which leads to the conclusion that the asymmetric patterns of response often reported in LN/MLN result from either shifts in the zone of minimum-intensity oscillation or from non-stimulus-specific increases in the spontaneous nystagmus.

The distribution of macular pigment in human albinos.

This study investigated the variation in density of macular pigment across the central retina in normal and albino subjects. Luminance profiles were measured using a fundus camera and digital video techniques. The normal group had pigment spatial distributions consistent with previous studies. The albinos had no variation in absorbance across the central retina.

Waveform characteristics of manifest latent nystagmus.

PURPOSE: To examine the waveform characteristics of 37 subjects with manifest latent nystagmus (MLN) and determine the manner in which visual feedback influences the nature of the waveform. METHODS: Binocular recordings of the eye movements of all subjects were undertaken using an infrared tracking system. Subjects viewed the target binocularly and monocularly in primary gaze. The effect of visual feedback on the nature of the MLN waveform was examined by either removing the fixation target or by progressively stabilizing the target in relation to the retina. This progressive stabilization was achieved by feeding back the eye movement signal to move an otherwise stationary target. RESULTS: Four types of MLN were distinguished on the basis of the fixation characteristics seen during binocular and monocular viewing. First, under binocular viewing conditions, subjects could theoretically exhibit stable fixation (type 1 MLN). In addition, three other MLN types were recorded during binocular fixation: conjugate horizontal square-wave jerks (type 2 MLN), conjugate torsional nystagmus (type 3 MLN) and conjugate horizontal jerk MLN waveforms (type 4 MLN). Monocular viewing always gave rise to a conjugate horizontal jerk MLN waveform for each of the four types of MLN. More than 80% of the subjects exhibited either type 3 or type 4 MLN, both of which conform with previous classic descriptions of MLN. Much less common was type 2 MLN. Type 1 MLN (conventionally referred to as a latent nystagmus) appeared to be a rare occurrence. In addition to the two classic linear and decelerating MLN slow phases, four additional slow-phase shapes with either saccadic or pendular elements were recorded and described. Removing visual feedback generally reduced the mean slow-phase velocity and the number of fast phases. For each subject some variability of the slow-phase class was documented from session to session. CONCLUSIONS: Four types of MLN have been described. Their differences are based on their binocular oculomotor behavior, and it is proposed that type 1 MLN and type 4 MLN represent the absolute states and types 2 and 3 the intermediate levels of the MLN spectrum. All types of MLN appear to be strongly visually driven and are largely dependent on the attentional state of the subject and the target conditions. Six different classes of slow phase were found among the four MLN types. The introduction of visual feedback had an immediate effect on the subsequent slow phase or fast phase. It is likely that adaptation mechanisms are in play after a period of visual feedback.

Oscillopsia and tolerance to retinal image movement in congenital nystagmus.

PURPOSE: To determine the relationship between retinal image movement (RIM) and oscillopsia in subjects with idiopathic congenital nystagmus (CN). METHODS: Eye movements were recorded using an IRIS infrared system. The eye movement signal was fed back to move an otherwise stationary target on a screen and thereby modify the RIM experienced by each of the five CN subjects. The target was present with either no background (the absolute condition) or a textured background (the relative condition). Feedback gains were varied from -1.0 (i.e., 100% retinal image increase) to +1.0 (i.e., 100% retinal image decrease or complete stabilization), with 0 representing the zero feedback or stationary target condition. In the first experiment, RIM thresholds were determined for a range of feedback values. Using zero feedback, a second experiment measured the detection threshold for absolute and relative motions to a ramp-generated target movement for five CN and five control subjects. RESULTS: Under feedback control spatial constancy broke down for both increased and reduced RIM. The range of spatial constancy was greater for absolute (-0.56 to +0.44) compared with relative (-0.18 to +0.18) RIM. Motion detection thresholds for the CN group were 8 times less sensitive to the absolute and 17 times less sensitive to the relative motion of the target compared with the control group. CONCLUSIONS: These results suggest that in CN subjects perceptual stability is achieved primarily by extraretinal signals.

The nature of head postures in congenital nystagmus.

We investigated the factors that determine the nature and extent of abnormal head postures in patients with congenital nystagmus. The head positions and eye movements of 16 patients were monitored while they adopted a variety of gaze positions. Five patients displayed a single head posture and four displayed multiple head postures. Six of the nine head postures matched the minimum intensity zone. The extent of the head posture was also found to be dictated by the velocity distribution of the slow phase, the nystagmus beat direction, and the neutral zone. Our results suggest that the surgical management of a head posture should not always be based only on the relocation of the minimum intensity zone to the primary gaze position.

Monochromatic fundus photography of human albinos.

The clinical diagnosis of albinism can often be uncertain, particularly when one is confronted with blond patients with congenital nystagmus. Monochromatic fundus photography was carried out on 31 patients with congenital nystagmus to examine three retinal features: the macular pigment, foveal vasculature, and the retinal pigment epithelium. All subjects who were later classified as albinos did not exhibit any signs of macular pigmentation. Our results imply that monochromatic fundus photography can be a useful aid in the diagnosis of albinism.

Surgery and compensatory head postures in congenital nystagmus. A longitudinal study.

Null zones and head postures were studied in two patients with idiopathic congenital nystagmus who had been carefully selected from a larger group of patients who had nystagmus with compensatory head postures. Oculomotor and sensory features of the two patients indicated that they would have a reasonable chance of profiting from surgery. Neither patient had a periodic alternating or manifest latent nystagmus or any ocular disease of the fixating eye. Postoperative evaluation of the head posture and the nystagmus behavior of both patients was carried out over a period in excess of 6 years. Neither of the patients readopted a head turn, and visual acuity was found to be equivalent to that prior to surgery when using the null zone. In one of the patients the postoperative null zone was located in the primary position, while in the other the null zone was decreased from an eccentricity of 30 degrees to 15 degrees. These results suggest that the position of the postoperative null zone is not always an accurate predictor of head turn.

Visual resolution limits in human albinism.

The effects of the involuntary ocular oscillations on visual resolution was examined in 22 albinos and 11 idiopaths with congenital nystagmus. The idiopaths showed a linear relationship between the proportion of the slow phase spent at low velocities (less than or equal to deg/sec) and the log of the minimum angle of resolution; such that long dwell times were compatible with good resolution. For the albinos there appeared to be a critical duration of low retinal slip velocities above which there was no improvement in acuity. This supports the contention that factors other than the congenital nystagmus limit visual resolution in the albino.

Motor and sensory characteristics of infantile nystagmus.

BACKGROUND/AIMS: Past studies have explored some of the associations between particular motor and sensory characteristics and specific categories of non-neurological infantile nystagmus. The purpose of this case study is to extend this body of work significantly by describing the trends and associations found in a database of 224 subjects who have undergone extensive clinical and psychophysical evaluations. METHODS: The records of 224 subjects with infantile nystagmus were examined, where 62% were idiopaths, 28% albinos, and 10% exhibited ocular anomalies. Recorded variables included age, mode of inheritance, birth history, nystagmus presentation, direction of the nystagmus, waveform types, spatial and temporal null zones, head postures and nodding, convergence, foveation, ocular alignment, refractive error, visual acuity, stereoacuity, and oscillopsia. RESULTS: The age distribution of the 224 patients was between 1 month and 71 years, with the mean age and mode being 23 (SD 16) years and 16-20 years respectively. By far the most common pattern of inheritance was found to be autosomal dominant (n = 40), with the nystagmus being observed by the age of 6 months in 87% of the sample (n = 128). 139 (62%) of the 224 subjects were classified as idiopaths, 63 (28%) as albinos, and 22 (10%) exhibited ocular anomalies. Conjugate uniplanar horizontal oscillations were found in 174 (77.7%) of the sample. 32 (14.3%) had a torsional component to their nystagmus. 182 (81.2%) were classed as congenital nystagmus (CN), 32 (14.3%) as manifest latent nystagmus (MLN), and 10 (4.5%) as a CN/MLN hybrid. Neither CN nor MLN waveforms were related to any of the three subject groups (idiopaths, albinos, and ocular anomalies) MLN was found in idiopaths and albinos, but most frequently in the ocular anomaly group. The most common oscillation was a horizontal jerk with extended foveation (n = 49; 27%). The amplitudes and frequencies of the nystagmus ranged between 0.3-15.7 degrees and 0.5-8 Hz, respectively. Periodic alternating nystagmus is commonly found in albinos. Albino subjects did not show a statistically significantly higher nystagmus intensity when compared with the idiopaths (p>0.01). 105 of 143 subjects (73%) had spatial nulls within plus or minus 10 degrees of the primary position although 98 subjects (69%) employed a compensatory head posture. Subjects with spatial null zones at or beyond plus or minus 20 degrees always adopted constant head postures. Head nodding was found in 38 subjects (27% of the sample). Horizontal tropias were very common (133 out of 213; 62.4%) and all but one of the 32 subjects with MLN exhibited a squint. Adult visual acuity is strongly related to the duration and accuracy of the foveation period. Visual acuity and stereoacuity were significantly better (p<0.01) in the idiopaths compared to the albino and ocular anomaly groups. 66 subjects out of a sample of 168 (39%) indicated that they had experienced oscillopsia at some time. CONCLUSIONS: There are strong ocular motor and sensory patterns and associations that can help define an infantile nystagmus. These include the nystagmus being bilateral, conjugate, horizontal uniplanar, and having an accelerating slow phase (that is, CN). Decelerating slow phases (that is, MLN) are frequently associated with strabismus and early form deprivation. Waveform shape (CN or MLN) is not pathognomonic of any of the three subject groups (idiopaths, albinos, or ocular anomalies). There is no one single stand alone ocular motor characteristic that can differentiate a benign form of infantile nystagmus (CN, MLN) from a neurological one. Rather, the clinician must consider a host of clinical features.

Ocular motor behaviour of monozygotic twins with tyrosinase negative oculocutaneous albinism.

The involuntary nystagmus movements of 16-year-old monozygotic twins with tyrosinase negative oculocutaneous albinism were examined. On primary gaze both girls exhibited bilateral conjugate horizontal nystagmus, a jerk with extended foveation waveform, and similar frequencies (2.0 Hz:1.9 Hz), although the fast phases were in opposite directions. The mean amplitudes differed markedly (6.8 degrees:3.7 degrees), as did the position of the null zones (+20 degrees to +30 degrees:-25 degrees to -35 degrees) and the widths of the neutral zones (-25 degrees to +20 degrees:-25 degrees to -35 degrees). Since the twins have identical genotypes these differences must have arisen from other sources.

Contour interaction in the presence of congenital nystagmus.

Using a computer-generated acuity task, the resolution of vertical and horizontal Landolt Cs was assessed at three levels of contrast, each for three different crowding conditions. Eighteen adults participated; six had congenital idiopathic nystagmus, six were oculocutaneous albinos and six served as controls. Contour interaction was evident when bars were placed 1 gap-width from the Landolt C and was more marked as contrast increased. When scaled to the individual resolution limit, the magnitude of contour interaction displayed by the albinos was not significantly different from the controls, however the idiopaths did exhibit a greater crowding effect than the controls. These results imply that fixation instability is a contributory factor in contour interaction, but only when retinal image velocity is above a certain threshold.

Quick phase programming and saccadic re-orientation in congenital nystagmus.

Horizontal single-step stimuli were presented to 19 subjects with congenital nystagmus (CN). When the stimulus jump was in the same direction as the waveform beat, spatial and temporal measurements of the responses revealed that saccades occurring between 90 and 180 msec after the stimulus were directed to an average of the spatial locations of the initial and final target positions. Transition functions of response end position against delay resembled those collected from normal subjects responding to either double-step staircase or pulse-overshoot stimuli, according to the stimulus direction. An analogy is drawn between the response to a single-step stimulus in the presence of CN and responses made by normal subjects to double-step stimuli. These findings suggest that pathways involved in the computation of amplitude of visually guided saccades also participate in programming CN quick phases and that the time taken to programme CN quick phases is similar to that for voluntary saccades. Evidence of saccadic parallel processing was demonstrated in some, but not all, of the subjects. Responses to steps in the same direction as the nystagmus beat frequently overshot the target. The mean overshoot was proportional to the amplitude and intensity of the nystagmus. Responses to stimuli presented in a direction opposite to the nystagmus beat were as accurate as the responses of a normal control group.

The effects of simultaneous central and peripheral field motion on the optokinetic response.

The effects of central and peripheral retinal stimulation on the optokinetic response were examined using full-field, central field only, peripheral field only and simultaneous central and peripheral field motion. Stimulation of the central field in isolation produced similar responses to those obtained for full-field motion whereas peripheral field stimulation resulted in greatly reduced responses. Central field dominance was also evident during central and peripheral field motion in opposing directions. However with unidirectional, simultaneous central and peripheral stimulation the optokinetic response was determined not by the central stimulus but by whichever stimulus moved the slower.

Characteristics of saccadic intrusions.

Primary fixation is never perfectly stable, but is frequently interrupted by slow drifts, microsaccades and saccadic intrusions (SI). SI are involuntary, conjugate movements which take the form of an initial fast movement away from the desired eye position and followed after a short duration, by either a return secondary saccade or a drift. The purpose of this study was to examine the prevalence and metrics of SI in a population of 50 healthy subjects. Using both one and two dimensional recordings we find that all 50 members of the subject group exhibited SI. The SI were bilateral, conjugate and horizontal. No purely vertical SI were detected when examined in three subjects. SI amplitude mean and range was 0.6 degrees +/-0.5 degrees, 0.1 degrees -4.1 degrees; SI frequency mean and range was 18.0+/-14.3 per min, 1.0-54.8 per min; SI duration mean and range was 225+/-150, 20-870 ms. The mean SI amplitude and frequency when SI<0.5 degrees were removed was 0.97 degrees +/-0.56 degrees and 7.0+/-11.4 per min respectively. Age was positively correlated with SI amplitude (p<0.01), but there was no correlation between age and SI frequency. Three of four types of SI monophasic square wave intrusions (MSWI), biphasic square wave intrusions (BSWI) and double saccadic pulses (DSP) were found to be exclusively saccadic, whilst the fourth type, the single saccadic pulses (SSP), were confirmed to exhibit a slow secondary component. MSWI were the most frequently observed SI occurring in 47 out of 50 (94%) of the subjects with a mean amplitude, frequency and duration of 0.7 degrees +/-0.5 degrees, 11.5+/-11.6 per min, and 255+/-147 ms respectively. Mean amplitudes and frequencies for BSWI (n=20), SSP (n=11) and DSP (n=34) were found to be 0.50 degrees +/-0.2 degrees, 1.2+/-2.5 per min; 0.40 degrees +/-0.20 degrees, 0.4+/-1.0 per min and 0.3 degrees +/-0.4 degrees, 5.0+/-8.7 per min respectively. No differences in MSWI characteristics were found between binocular and monocular viewing. Possible explanations for SI occurrence include experimental viewing conditions, subject fatigue and covert shifts in attention.

Ocular oscillations on eccentric gaze.

Many normal individuals show ocular oscillations on eccentric gaze. This study was designed to investigate the effect of visual disengagement and visual feedback on the nature of these end point oscillations. Three test conditions were examined: target present, target absent and when the target position was determined by the subject's eye position via a variable feedback control system. Feedback gains (i.e. target velocity/eye velocity) ranged from 0, where the target position was decoupled from the subject's eye movements (i.e. the target is stationary on the screen), to +1.0 where the retinal image was stabilised (i.e. the target is driven by the subject's eye movements). Only subjects who exhibited sustained end-point oscillations with no latency were included in the study (n=6). Seven different oscillations including square-wave jerks were recorded in the abducting eye during eccentric gaze of a stationary target. The three most common oscillations were the jerk oscillations, with decelerating, linear or pendular slow phases. A number of additional previously unreported waveforms were also recorded. On removal of the target, the mean drift velocity of the slow phase was greatly reduced. The response to the introduction of a change in the visual feedback was specific to each subject, although in all cases, the end-point oscillations generally were of a lower velocity, and gaze was shifted by up to 8 deg in the direction of the slow phase within the first two seconds. The important role of slow eye movement control for maintaining gaze holding is discussed.

Monocular optokinetic nystagmus in humans with age-related maculopathy.

AIM: To investigate full field monocular optokinetic nystagmus (OKN) in patients with age-related maculopathy (ARM) and relative central scotoma. METHODS: Six patients aged 59-88 years with bilateral ARM and an aged-matched control group of six patients aged 54-83 years were examined. Visual fields were assessed with a Humphrey field analyser using the threshold 30-1 routine. Monocular full field horizontal optokinetic stimuli were presented on a hemicylindrical screen subtending 172 degrees horizontally and 50 degrees vertically. The stimulus was a projected random dot pattern and three stimulus velocities were used, 30, 50, and 70 degrees/s in both nasalward and temporalward directions. Each trial lasted between 30 and 40 seconds and eye movements were monitored using infrared oculography. RESULTS: The ARM patients had relative central scotomas with an average depth of 10 dB. Neither the ARM nor the age-matched groups displayed any directional preponderance or a buildup of the slow phase eye velocity with time. No statistically significant difference in the gain was found between the two groups (p > 0.05). CONCLUSIONS: Marked central field loss in ARM does not significantly impair OKN gain. This supports the view that complete central retinal integrity is by no means essential and that the peripheral retina provides an important input to the generation of OKN.