Pursuit of intermittently illuminated moving targets in the human

1. Experiments have been conducted in order to establish the changes in oculomotor activity which take place when the human subject attempts to pursue an intermittently illuminated moving target. 2. In an initial experiment, target motion in the horizontal plane was composed of one or two sinusoids at frequencies between 0.11 and 0.2 Hz. The target was illuminated for varying durations (10-320 ms) at intervals between 40 and 960 ms. As pulse interval was increased or pulse duration was decreased there was a progressive increase in eye velocity gain for the smooth component of eye movement. Some smooth eye movement was generated even when the pulse interval was as large as 960 ms. 3. In a second experiment target motion consisted of a triangular waveform in which target presentation was timed to occur at regular intervals throughout each cycle. Overlaying and averaging the response from several cycles revealed a regular pattern of pulsatile activity associated with each target presentation. This response, which was particularly evident when the pulse interval was greater than 1 s, consisted of an initial build-up of smooth eye velocity followed by an exponential decay with a time constant of 0.5-2 s. When the pulse interval was less than 1 s there was a summation of the transient responses so that eye movement appeared quite smooth when pulse interval was reduced to 320 ms. 4. The pulsatile nature of the response was accentuated when the target was made to execute a staircase-ramp waveform in which the target was illuminated only during the ramp component. The elimination of position change between ramps and the ability to achieve higher target velocity led to clear evidence of the summation of transient oculomotor responses. 5. The summated effects, however, were not simply attributable to the addition of responses to individual target presentations as indicated by the timing of each response. The eye velocity pulse was frequently initiated 200-300 ms prior to target appearance, and well before the time (100 ms) at which visual feedback would be expected to become effective. 6. The effect of target step displacement alone was investigated by examination of the smooth eye movement initiated by varying numbers of steps in the waveform. This showed that the basic step response had a peak velocity of no more than 8-10 deg/s in most individuals.(ABSTRACT TRUNCATED AT 400 WORDS)

An unusual presentation of meningitis as a consequence of inadvertent dural puncture

Bacterial meningitis and aseptic meningitis have been reported as a consequence of spinal anaesthesia. The case we report is of an unusual presentation of meningitis, for which no cause was found, in a patient who received epidural anaesthesia, complicated by dural puncture.

The mechanism of prediction in human smooth pursuit eye movements

1. Experiments have been conducted on human subjects to determine the role of prediction in smooth eye movement control. Subjects were required to actively pursue a small target or stare passively at a larger display as it moved in the horizontal plane. 2. Target motion was basically periodic, but, after a random number of cycles an unexpected change was made in the amplitude, direction or frequency of target motion. Initially, the periodic stimulus took the form of a square waveform. In subsequent experiments, a triangular or sawtooth waveform was used, but in order to examine the timing of the response in relation to stimulus appearance, the target was tachistoscopically illuminated for 40-320 ms at the time that it passed through the mid-line position. 3. When subjects either actively pursued the target or stared passively at the larger display a characteristic pattern of steady-state eye movement was evoked composed of two phases, an initial build-up of eye velocity that reached a peak after 200 ms, followed by a decay phase with a time constant of 0.5-2 s. The build-up phase was initiated prior to target displacement for square-wave motion and before onset of target illumination for other waveforms. 4. The peak eye velocity evoked gradually increased over the first two to four cycles of repeated stimulation. Simultaneously, the response became more phase advanced, the reaction time between stimulus onset and the time at which peak velocity occurred decreasing from an average of 300 to 200 ms for triangular waveform stimuli. 5. When there was a sudden and unexpected change in amplitude and direction of the stimulus waveform, the eye movement induced had a peak velocity and direction that was inappropriate for the current visual stimulus, but which was highly correlated with the features of the preceding sequence in the stimulus. 6. When there was a sudden change in the frequency of the stimulus waveform the predictive eye movement was induced with a timing appropriate to the periodicity of the previous sequence but inappropriate to the new sequence. 7. The results indicate that prediction is carried out through the storage of information about both the magnitude and timing of eye velocity. The trajectory of the averaged eye velocity response was similar in form irrespective of the duration of target exposure or basic stimulus frequency, suggesting that the predictive estimate is released as a stereotyped volley of constant duration but varying magnitude under the control of a periodicity estimator.(ABSTRACT TRUNCATED AT 400 WORDS)

Vestibulo-ocular reflex suppression during high velocity head-free pursuit in normal subjects

Recordings of head and smooth pursuit eye movement were made during head-free pursuit of a pseudo-random target motion stimulus. The pseudo-random stimulus was composed of 2 high velocity sinusoids, of frequency 0.4 and 1.3 Hz, with the velocity of the higher frequency being varied as a ratio of the lower frequency velocity between 0 and 2. Slow-phase gaze velocity gain for the lower frequency component decreased significantly with an increase in velocity ratio, and with an increase in target velocity above 60 degrees/s. Gaze velocity gain was frequently less than head displacement gain which remained fairly constant, indicating that the eyes had been driven in the opposite direction to head movement as a result of inability of suppress the vestibulo-ocular reflex. Similar effects were seen when visual feedback was degraded by tachistoscopic illumination of a target composed of 2 low velocity sinusoids (0.11 and 0.13 Hz). These results indicate that visual feedback, rather than head displacement, is essential for suppression of slow-phase vestibular eye movement during head-free pursuit, even at high stimulus velocities.

The assessment of predictive effects in smooth eye movement control

Predictive mechanisms in pursuit were studied by requiring subjects to track a moving target which appeared for a brief period (40-320 ms) at regular intervals. With successive presentations the timing of the eye velocity trajectory became progressively more predictive of target appearance, whilst simultaneously increasing in peak velocity. Sudden changes in periodicity or velocity resulted in inappropriate eye movements correlated highly with previous stimulus cycles. The results suggest that prediction involves storage of velocity information and its release under the control of a periodicity estimator.

Head-free pursuit in the human of a visual target moving in a pseudo-random manner

1. Head and eye movements have been recorded in man during head-free pursuit of a target moving in a pseudo-random manner in the horizontal plane with a motion stimulus composed of the sum of four sinusoids. 2. In an initial experiment the three lowest frequencies remained constant at 0.11, 0.24 and 0.37 Hz, whilst the highest frequency (F4) took values of 0.39, 0.78, 1.56 and 2.08 Hz. Peak velocity of each component was 10 deg/s. When F4 was 0.39 Hz gaze displacement (i.e. the sum of head and eye displacement) was relatively smooth and had a mean velocity gain of 0.95. As F4 was increased gaze displacement contained more saccadic activity and gaze velocity gain for the three lower-frequency components was significantly (P less than 0.001) reduced to a minimum level of 0.66 when F4 was 1.56 Hz. 3. A similar reduction in gain of the lower-frequency components was obtained when the velocity of F4 was increased as a ratio of the velocity of the lower frequencies from 0 to 4. 4. When the frequency composition of the stimulus was varied so that the two highest frequencies were closely spaced, gaze velocity gain for the highest frequency was always significantly higher than that of the next lower frequency, indicating a true enhancement of the highest-frequency component. 5. Changing the lowest-frequency component of the stimulus resulted in a significant shift in the gaze velocity phase profile as a function of frequency, so that phase advance was always associated with the lowest frequency even when this was as high as 0.89 Hz. 6. These changes in gain and phase of gaze velocity with the frequency content of the stimulus were similar to those previously described for head-fixed pursuit and visual suppression of the vestibulo-ocular reflex (VOR) and implicate the frequency-dependent, non-linear visual feed-back mechanisms in gaze control. 7. A number of the non-linear characteristics of gaze velocity were also observed in a somewhat modified form in the head displacement gains and phases, implying that the drive to the neck muscles is also derived from the same non-linear visual feed-back source. 8. The role of the VOR in head-free pursuit was tested by exposing the subject to whole-body motion on a turntable which countered the volitional head movement generated by the subject.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect on motion sickness and oculomotor function of GR 38032F, a 5-HT3-receptor antagonist with anti-emetic properties

1. The 5-hydroxytryptamine (5-HT3) receptor antagonist, GR 38032F, which possesses potent anti-emetic properties in vomiting induced by cancer chemotherapeutic drugs, has been tested to determine its value in the prophylaxis of motion sickness induced by cross-coupled stimulation. The double-blind trial compared GR 38032F with both a placebo (lactose) and with hyoscine. In addition, studies of ocular pursuit and saccadic eye movements were carried out following the administration of each drug. 2. The prophylactic effect of GR 38032F on motion-induced nausea was indistinguishable from that of placebo, whereas following hyoscine subjects showed a highly significant (P less than 0.001) increase in tolerance to cross-coupled stimulation. Tests of oculomotor function showed no effect on saccadic eye movement from either drug. However, both drugs produced a significant (P less than 0.05) though small reduction in eye velocity gain during pursuit eye movement. 3. These findings suggest that the 5-HT3 receptor is not involved in the neural pathways that bring about motion sickness, but that it may have a role in the control of ocular pursuit. The absence of an anti-motion sickness effect from a drug that is effective in the treatment of vomiting induced by cancer chemotherapy serves to emphasize that different neural mechanisms are involved in the generation of motion sickness.

Head-free pursuit of pseudo-random target motion

Experiments have been conducted in which subjects were required to pursue a target moving in the horizontal plane with co-ordinated movements of the head and eyes. Target motion was pseudo-random in form, composed of four sinusoids. The three lower frequencies of the stimulus were maintained at 0.11, 0.24 and 0.37 Hz with a peak velocity of 10 degrees/s, whilst the frequency and velocity of the highest frequency component (F4) were varied. When all frequencies were below 0.4 Hz, eye movements were smooth and gaze velocity gain was high (0.95), but when F4 was increased up to 1.56 Hz or when the velocity of F4 was increased up to 40 degrees/s, gaze velocity gain decreased significantly. When voluntary head movements were countered by whole-body rotation to eliminate the input to the semicircular canals, gaze velocity gain increased because there was no longer any requirement to suppress the vestibulo-ocular response to head rotation. The results are in accord with those of previous experiments involving head-fixed pursuit and vestibulo-ocular suppression.

Eye movements induced by linear acceleration are modified by visualisation of imaginary targets

Lateral eye movement responses to linear acceleration in the lateromedial axis of the head have been examined in normal human subjects who were seated within a cabin, free to move on a horizontal linear track. The motion stimulus was either sinusoidal (0.2-0.8 Hz) or pseudo-random (0.11-1.25 Hz) in form, with a peak acceleration of 1.5 m.s-2. In darkness, while carrying out mental arithmetic, the mean ratio of slow-phase eye velocity to linear cart velocity increased from 2.8 degrees/m at 0.2 Hz to 10.5 degrees/m at 0.8 Hz during sinusoidal stimulation. When subjects were instructed to imagine a near head-fixed target in darkness, eye velocity decreased by 25%-48% during both sinusoidal and pseudo-random stimulation. When subjects were instructed to visualise an earth-fixed target during sinusoidal stimulation eye velocity was augmented by 47% when imagining a target 3 m distant and by 175% when visualising a target 0.6 m distant. Response augmentation was not as great during pseudo-random stimulation. The results indicate that the otolith-ocular response is highly modifiable by mental set.

Factors affecting the predictability of pseudo-random motion stimuli in the pursuit reflex of man

1. Experiments have been performed on human subjects to determine the principal mechanisms underlying the break-down in performance during ocular pursuit of pseudo-random target motion stimuli composed of a mixture of two, four or six sinusoids. As observed in a previous experiment there was a reduction in the ratio of eye velocity to target velocity (eye velocity gain) for lower-frequency components of the stimulus whenever the highest frequency exceeded 0.4 Hz, but the following effects were also observed. 2. Using a combination of four sinusoids in which the three lowest frequencies (0.11, 0.24 and 0.37 Hz) had a constant peak velocity (3 or 6 deg/s) it was shown that an increase in the velocity of the highest frequency (0.78 or 1.56 Hz) caused a progressive decline in gain of the low frequencies and a significant reduction in phase lag for the highest-frequency component. 3. Using a combination of two sinusoids (0.44 and 1.56 Hz), in which the peak velocity was varied over a wide range (4-32 deg/s), it was shown that the reduction in low-frequency gain was dependent on the velocity ratio between the frequency components rather than their absolute velocity. 4. Experiments using a combination of either four or six sinusoids in which the two highest frequencies were very close have revealed a true enhancement in the gain of the highest-frequency component in relation to other frequency components of the stimulus. 5. In the same experiments the phase relationships in the response were shown to vary according to the frequency range of the stimulus in such a way that phase advance was normally present at the lowest frequency even when this ranged up to 0.89 Hz. 6. When the oculomotor system was passively stimulated by allowing the subject to fixate a tachistoscopically illuminated stationary target, pseudo-random target motion induced a response which exhibited characteristics similar to those of active pursuit; that is, enhancement of the gain of the highest frequency and phase advance at the lowest frequency. 7. During passive stimulation the changes in gain of the low frequencies with increasing frequency of the highest-frequency component were not consistent with those of active pursuit. However, increasing the velocity of the highest-frequency component to simulate the retinal velocity error conditions of normal active pursuit caused a significant decrease in low-frequency gain and a subjective effect of high-frequency dominance similar to that observed during active pursuit.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of visual and non-visual mechanisms on the vestibulo-ocular reflex during pseudo-random head movements in man

1. The behaviour of the vestibulo-ocular reflex (VOR) in man was examined using pseudo-random and sinusoidal whole-body angular-motion stimuli applied about the yaw axis by a servo-controlled turn-table. 2. The VOR response was assessed in four conditions; during fixation on a head-fixed target (HFT); during attempted fixation in the dark of an imagined head-fixed (IHFT) or earth-fixed target (IEFT) and in darkness (DRK) whilst performing an auditory discrimination task. 3. When the pseudo-random stimulus was composed of four sinusoids, the three lowest frequencies (0.11, 0.24 and 0.37 Hz) were maintained constant whilst the highest frequency (F4) was varied from 0.39 to 2.08 Hz. In darkness (DRK condition) and when imagining a head-fixed target (IHFT condition) the gain of slow-phase eye velocity was not significantly affected by the frequency of the highest-frequency component, although there were significant changes in the phase for the IHFT condition. 4. During fixation of a real head-fixed target (HFT condition), both the gain and phase of eye velocity were significantly modified by the frequency (F4) of the highest-frequency component. When F4 was 0.39 Hz, all frequency components had a low gain (mean 0.05), but as F4 was increased there was a significant (P less than 0.001) increase in gain for all three low-frequency components which reached a maximum (mean 0.17) when F4 was 2.08 Hz. However, the gain for the highest frequency component always remained comparable to that obtained in response to a single discrete sinusoid of the same frequency. 5. When the stimulus was composed of only two sinusoids a similar increase in gain of the lower-frequency (0.22 Hz) component was observed in the head-fixed target condition as the frequency of the higher-frequency component was increased from 0.39 to 2.78 Hz. However, VOR gain was not significantly modified by the frequency of the higher-frequency component when subjects imagined a head-fixed or earth-fixed target in darkness. 6. The findings indicate that high levels of VOR suppression can be achieved in the head-fixed target condition with pseudo-random stimuli when all frequency components are below 0.4 Hz. But if the highest-frequency component rises above 0.8 Hz, optimum suppression is confined to the highest-frequency component, whilst suppression of the low-frequency components is significantly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

High-dose cisplatin and vinblastine infusion with or without radiation therapy in patients with advanced non-small-cell lung cancer

Non-small-cell lung cancer (NSCLC) patients with locally advanced or metastatic measurable disease were given a combination of cisplatin, 200 mg/m2 divided in five daily doses, and simultaneously, vinblastine, 7.5 mg/m2 as a continuous intravenous (IV) infusion over five days. Five courses of chemotherapy were planned. Afterwards or on progression, patients were randomized to receive maximally tolerated radiation to all sites of disease v observation only. Forty males and seven females were entered. Median age was 60 years (range, 37 to 74), median Karnofsky performance status was 70 (range, 30 to 90). Five patients had previous brain radiation therapy for metastatic disease, all others were previously untreated. Side effects in the 87 courses of chemotherapy administered included leukopenia (WBC less than 1,000/microL following nine courses) and thrombocytopenia (platelets less than 20,000/microL following four courses). Ten patients became septic, nine of them while leukopenic. Elevations of serum creatinine followed eight courses; in all cases the level was less than 3.0 mg/dL. Nausea and vomiting were mild to moderate. Five patients experienced mild hypoacusis and six had sensory polyneuropathy. The deaths of three patients were considered drug-related. The response rate was 28%. The median survival for the group was 22 weeks, 63.2 weeks for responders and 17.9 weeks for nonresponders. Twenty-six patients received radiation therapy, 16 randomized to this arm as planned, ten to palliate symptoms. Median survival of all irradiated patients was 24.8 weeks. Seven responders to chemotherapy were randomized to receive radiotherapy; their median survival was 25 weeks. In six responders randomized not to receive radiation, the median survival was 77.8 weeks (P greater than .3). Among nonresponding patients, the median survival of those radiated was 22.2 weeks, while that of nonradiated patients was 11 weeks. This regimen is cumbersome and toxic. It has offered no major survival benefits, or improvement in response rates, therefore, we do not recommend it for the standard treatment of NSCLC.

Predictive velocity estimation in the pursuit reflex response to pseudo-random and step displacement stimuli in man

1. Eye movements have been recorded in man in response to various forms of continuous and discontinuous target motions in the horizontal plane in an attempt to establish the mechanisms of prediction in the pursuit reflex. 2. In an initial experiment the target motion was composed of four sinusoids, each of peak velocity +/- 3.3 deg/s. The three lowest frequencies (0.11, 0.24 and 0.37 Hz) remained constant whereas the highest frequency (F4) was varied from 0.39 to 2.08 Hz. When F4 was 0.39 Hz, all frequency components had a high level of eye velocity gain (mean 0.92) but as F4 was increased there was a significant (P less than 0.001) decline in gain for all three low frequencies which reached a minimum (mean 0.53) when F4 was 1.56 Hz. However, the gain for F4 always remained at a high level, comparable to that evoked by a discrete frequency sinusoid of the same frequency. 3. When the highest-frequency sinusoid was replaced by a square wave of identical amplitude a similar decline in gain for the low frequencies was observed. Eye velocity exhibited a quasi-sinusoidal modulation at the frequency of the square wave even though the rapid steps did not constitute a suitable stimulus to the visual velocity feed-back mechanisms. 4. When only two sinusoids were mixed to form the pursuit stimulus a similar break-down in gain of the lower-frequency component was observed which reached a minimum (mean gain 0.58) when F2 was between 1 and 2 Hz. This implies that the predictability of stimulus motion is dependent, not on the complexity of the stimulus, but on its highest-frequency component. 5. Presentation of square-wave target displacements alone confirmed that smooth eye movements could be evoked by such a stimulus. Eye velocity was at a maximum between 1.0 and 1.5 Hz and was predictive of ensuing target displacement. Responses to staircase step sequences of varying duration indicated that prediction was based on an assessment of the duration of the preceding sequence. 6. Tachistoscopic presentation of targets during low-frequency sinusoidal oscillation indicated that illumination of the target for very short periods (10-320 ms) as few as two times per cycle during minimum velocity phases enhanced the perception of continuous movement. A predictive eye movement was evoked with a high level of peak velocity which then decayed until the subsequent exposure of the target.(ABSTRACT TRUNCATED AT 400 WORDS)

The interaction of conflicting retinal motion stimuli in oculomotor control

Oculomotor response has been assessed in humans during the presentation of conflicting retinal motion stimuli. In the majority of experiments a background stimulus was made to move with a constant velocity ramp in one direction followed by rapid resets at regular intervals. In the absence of an adequate fixation target this ramp-reset stimulus induced a nystagmus with a slow-phase velocity and saccadic frequency which remained almost constant as reset frequency was increased from 2 to 5 Hz. Moreover, the induced eye velocity could be considerably increased if the subject attempted 'active' matching of display velocity. During both 'active' and 'passive' responses eye velocity gain reached a peak when display velocity was between 2 degrees/s and 5 degrees/s. The presence of small stationary targets induced a suppression of the passive ramp-reset response which was modified by target eccentricity and by tachistoscopic target illumination. When subjects pursued a sinusoidally oscillating target against a stationary structured background, eye velocity gain was significantly less than for pursuit against a blank background. The degree of interaction between conflicting stimuli was found to be dependent on their relative size, peripheral location and velocity. However, it appears that the human observer is able selectively to enhance feedback gain from one particular source in order to dominate stimuli from other unwanted sources.

The effects of ethanol on visual-vestibular interaction during active and passive head movements

The effects on visual-vestibular interaction of a moderate dose of ethyl alcohol (blood alcohol 80 mg X 100 ml -1) have been investigated in two experiments. In the first, alcohol was shown to degrade both visual pursuit and suppression of the vestibulo-ocular reflex (VOR) in a similar manner when the vestibular response was induced by passive oscillation on a turntable at frequencies of 0.11-1.2 Hz. In the second experiment a similar degradation in VOR suppression was observed when subjects made volitional head movements at frequencies of 0.5-3 Hz. In addition, the effectiveness of vestibulo-ocular compensation was shown to be significantly reduced by alcohol when viewing an earth-fixed target during voluntary head movements. Although alcohol induced small changes in the vestibulo-ocular response recorded in darkness, the main effect on oculomotor performance was a reduced effectiveness in the visual feedback of retinal error information.

The influence of display characteristics on active pursuit and passively induced eye movements

A series of experiments has been conducted on human subjects to examine the effect of the movement of small targets located in the peripheral visual field on oculomotor response. Subjects were presented with either a single centrally positioned target or a pair of targets displaced at angles of +/- 5 degrees, +/- 10 degrees and +/- 20 degrees from centre. Target movement was in the horizontal plane, the paired targets always moving in unison. The stimulus waveform consisted of either a sinusoidal or random target motion encompassing a frequency range from 0.1 to 4 Hz with an angular displacement of +/- 3.5 degrees. Subjects made two types of response. First they were instructed to follow the single target or the centre point of the paired targets. In this 'active' pursuit condition the gain of slow-phase eye velocity progressively decreased as the moving targets were moved from the central position to the most peripheral location (+/- 20 degrees). Secondly, subjects were required passively to ignore the target movement by staring blankly ahead. During this 'passive' response nystagmic eye movements were induced for which the slow-phase eye velocity also decreased with increasing target eccentricity, but the gains were always less than those induced during 'active' pursuit. The frequency characteristics of the 'passive' response were very similar to those of the 'active' response, breaking down at frequencies beyond 1 Hz. The ability to suppress the 'passive' response was also investigated by the presentation of a tachistoscopically illuminated earth-fixed target. The response was found to decline as the interval between presentations of the fixation target was decreased from 3000 ms to 100 ms. It is suggested that the 'passive' response originates from a basic velocity drive to the oculomotor system resulting from image movement across the retina. This velocity drive may be cancelled with adequate fixation but must be enhanced to accomplish desired eye velocity during active pursuit.

Non-linear effects in visual suppression of vestibular nystagmus

Visual-vestibular interaction in the control of eye movement was investigated in six subjects during exposure to a low frequency (0.05 Hz) angular oscillation about the longitudinal axis of the body at four levels of peak head velocity: 30, 60, 90 and 120 degrees/s. Eye movements were recorded whilst the subject was presented with a head-fixed visual display consisting of either a single central target or a pair of targets placed at +/- 20 degrees in the periphery. For the lower stimulus levels (30 and 60 degrees/s) the degree of suppression was reasonably constant and the vestibular nystagmus was never completely suppressed. However, during oscillation at higher velocity levels (90 and 120 degrees/s) the relationship between eye velocity and head velocity became non-linear, the degree of suppression being much less during the high velocity periods of the waveform than when the head velocity was low. The changes in suppression may be interpreted as a decrease in gain of visual feedback as a function of increasing image velocity error on the retina.

The effects of strobe rate of head-fixed visual targets on suppression of vestibular nystagmus

The effects of degrading retinal image velocity information on suppression of the vestibulo-ocular reflex have been assessed through tachistoscopic presentation of target sources in man. Subjects were required to fixate a head-fixed display during exposure to a 0.5 Hz sinusoidal angular oscillation of the head at +/- 60 degrees/s. In the first experiment it was found that the degree of suppression was progressively degraded as the interval between successive target presentations was increased from 10 to 3,000 ms. In the second experiment no effect of changing the duration of the target pulse was observed over a range from 20 to 1,000 microseconds. The results appear consistent with a model of visual motion sensitivity in which relative velocity information is obtained by the temporal integration of responses from spatially separated retinal cells.

The effects of retinal target location on suppression of the vestibulo-ocular reflex

Experiments on human subjects exposed to angular oscillation whilst viewing a head-fixed display have indicated that the degree of suppression of the vestibulo-ocular reflex is dependent upon the peripheral location of the visual target. Suppression is greatest when fixating a central target and decreases in a graded manner for targets placed more peripherally. During central fixation a low-velocity nystagmus is still evident and there is no indication of any complete cancellation of the vestibulo-ocular reflex.

Non-linear characteristics of visual-vestibular interaction

Visual-vestibular interaction in the control of eye movement was investigated in six subjects during exposure to a low frequency (0.05 Hz) angular oscillation about the longitudinal axis of the body at four levels of peak head velocity: 30, 60, 90 and 120 degrees/s. For the lower stimulus levels (30 and 60 degrees/s) the degree of suppression of the vestibulo-ocular response was reasonably constant and the nystagmus was never completely suppressed. However, during oscillation at higher velocity levels (90 and 120 degrees/s) the degree of suppression became much less during high velocity periods of the waveform than when the head velocity was low. The changes in suppression may be interpreted as a decrease in gain of visual feedback as a function of increasing image velocity error on the retina.

The effects of ethyl alcohol on visual pursuit and suppression of the vestibulo-ocular reflex

A comparison of the frequency characteristics of the pursuit reflex and the suppression of vestibular nystagmus has been carried out in humans under carefully matched stimulus conditions, with and without the influence of ethyl alcohol (blood level approximately 77 mg/100 ml). In the pursuit task the ratio of eye displacement to target displacement was not significantly impaired by alcohol, but there was a significant decrease in the ratio of slow-phase eye velocity to target velocity. When subjects attempted to fixate a head-fixed target during whole-body angular oscillation the degree of suppression of the slow-phase velocity of the vestibulo-ocular response was also significantly impaired by alcohol and in both this and the control condition there was no significant difference between the amplitude ratio of suppression and the velocity error of the pursuit response.

Evaluation of the Vita-Stat automatic blood pressure recorder. A comparison with the Random-Zero sphygmomanometer

A total of 408 adult volunteers at Northwick Park Hospital, England, participated in an evaluation of the Vita-Stat automatic blood pressure (BP) recorder. During a five-day period in March 1981, readings of BP obtained by two Vita-Stat machines were compared with measurements obtained by two trained observers using Random-Zero sphygmomanometers. The order in which BP readers were obtained was randomized. A significant effect of order on systolic BP was removed from comparisons between techniques. For diastolic BP values, there was close agreement between the different measurement techniques and each was associated with a similar degree of repeatability between duplicate recordings. In contrast, both Vita-Stat machines seriously overestimated systolic BP and there was substantially more variability between duplicate readings of systolic BP obtained with the Vita-Stat machines than for Random-Zero values. The use of either Vita-Stat machine for population screening would have correctly identified almost all those with a high BP but would have led to considerable overdiagnosis of systolic hypertension. Despite their limitations, the accuracy of diastolic BP estimates and considerations of cost and convenience suggest that Vita-Stat machines might be useful as initial screening tools for detection of persons with high diastolic BP. However, the use of a particular Vita-Stat machine should be preceded by an evaluation of the machine against a standard method of BP estimation.

A procedure for the analysis of nystagmus and other eye movements

A simple procedure has been developed for the computer analysis of nystagmus and other eye movements using the superior pattern recognition capabilities of the human operator to distinguish between fast- and slow-phase eye movements. The basis of the procedure is that the operator can set up threshold limits around an expected slow-phase eye velocity waveform through the use of cursor facilities. Points lying outside the threshold limits are recognised as fast-phase eye movements and discarded. Various least-squares curve-fitting procedures are then used to establish the relationship between the oculomotor response and the stimulus waveform. Examples are given of analysis procedures for both periodic and transient responses of the vestibulo-ocular reflex. The method is particularly useful for the analysis of responses to voluntary head movement in which slow-phase velocity frequently approaches that of the fast-phase components.

The effects of visual discrimination of image movement across the stationary retina

The visual performance of human subjects has been investigated during presentation of a digital display moving at constant velocity with regard to the stationary eye. The display was presented for brief periods (10-80 ms) at different luminance levels (0.5-8 cd/m2), contrast levels (1-16), and display sizes (character height 12-24' of arc). The probability of correctly identifying the display decreased to 90% when the velocity reached 3-4 degrees/s. Performance was improved by increasing digit size, decreasing exposure time, or increasing display luminance. Theoretical arguments have been adduced to indicate the usefulness of the results in predicting performance during continuous display exposure.

Vestibulo-ocular function during co-ordinated head and eye movements to acquire visual targets

1. Experiments have been conducted on human subjects in an attempt to establish the role of the vestibulo-ocular reflex in the co-ordination of head and eye movements during visual target acquisition. 2. When the subject moved head and eyes to acquire visual targets in the horizontal plane, the eye movement consisted of an initial saccade in the direction of head movement followed by a slower return towards orbital centre which compensated for remaining head movement. 3. When the head was moved either voluntarily or passively in the dark the pattern of eye movement was very similar to that seen during target acquisition. 4. The mean latency between the start of head acceleration and the onset of the saccadic eye movement was greater in the dark (108 msec, S.D. 85 msec) than for the visually induced responses (14 msec, S.D. 59 msec), in which eye movement often preceded head movement when moving to small ( less than 45 degrees) target offset angles. 5. In all experimental conditions gaze displacement at the end of the initial saccade was normally related in a predictive manner to final head position, but when fixating visual targets offset by more than 60 degrees from the central position there were often large errors, 22% of responses undershooting the target by more than 15 degrees. 6. A highly significant (P less than 0.001) linear relationship was found between gaze displacement and head velocity under all experimental conditions. During target acquisition head velocity was normally positively correlated with amplitude of target offset. The large errors in gaze displacement in response to the larger target offsets occurred at levels of head velocity lower than normally associated with such target offsets. 7. The results have led to the suggestion of a dual mode of control for head-eye co-ordination. In one mode, normally associated with small target offsets (less than 45 degrees), control is mediated by retinal error information. In the other mode, associated with larger target offsets, gaze displacement is generated as an automatic response to head turning. 8. The observation of similar relationships between head and eye movement during passive head turning implicates the vestibulo-ocular reflex in the secondary mode of control, and provides support for the hypothesis that the role of the vestibular saccade is to induce a rapid offset of the eyes in the direction of head movement, thus facilitating rapid search and target location.

Cervical and vestibular afferent control of oculomotor response in man

Oculomotor response in the absence of vision has been compared in a group of 12 normal humans in two experimental conditions testing (a) the vestibulocular reflex by whole-body oscillation on a turntable, and (b) the cervico-ocular reflex by oscillation of the body with the head held stationary. The stimulus was a sinusoidal oscillation (peak angular velocity +/- 50 degrees/sec) at frequencies between 0.2 and 1.3 Hz. The slow-phase eye movements of the vestibulo-ocular response were compensatory for head movement and showed a mean gain of 0.54--0.90, increasing with frequency. The cervicoocular response was found to be very variable. The slow-phase eye movements were of low velocity (mean gain 0.05) and did not generally compensate for body movement. During neck torsion, some subjects exhibited large overall eye deviations composed of both slow and fast phase eye movements.

A comparison of the size distribution of collagen fibrils in connective tissues as a function of age and a possible relation between fibril size distribution and mechanical properties

Data on the distribution of collagen fibril diameters in various connective tissues have been collected and analysed for common features. The diameter distributions of the collagen fibrils at birth and in the foetal stages of development are unimodal, whereas at maturity the mass-average diameter of the collagen fibrils is generally larger than at birth and the distributions of fibril sizes may be either unimodal or bimodal depending on the tissue. At senescence, few data are available but in most instances both the mean and mass-average diameters of the collagen fibrils are smaller than those at maturity and the fibril distributions are mainly bimodal. The division between tissues showing unimodal or bimodal fibril distributions at maturity does not simply relate to the type I collagen/type II collagen classification, to the distinction between orientated and unorientated material or indeed directly to the levels of stress and strain encountered by the tissue. However, there may prove to be a relation between a bimodal fibril diameter distribution at maturity and the maintenance over long periods of time of either high stress in stretched tissues or low stress in compressed tissues. It has also been noted that the width of the collagen fibril diameter distribution at birth differs between altricious and precocious animals. The ultimate tensile strength of a connective tissue and the mass-average diameter of the constituent collagen fibrils have been shown to have a positive correlation. Further, the form of the collagen fibril diameter distribution can be directly related to the mechanical properties of the tissue. In particular, it is postulated that the size distribution of the collagen fibrils is largely determined by two factors. First, if the tissue is primarily designed to have high tensile strength, then an increase in the diameter of the collagen fibrils will parallel an increase in the potential density of intrafibrillar covalent crosslinks. Consequently large collagen fibrils are predicted to have a greater tensile strength than small fibrils. Secondly, if the tissue is designed to be elastic and hence withstand creep, then a reduction in the diameter of the collagen fibrils will effectively increase the surface area per unit mass of the fibrils thus enhancing the probability of interfibrillar non-covalent crosslinks between the collagen fibrils and the components of the matrix. The idealized description given may indicate how the mechanical properties of a tissue may be interpreted in terms of the collagen fibril diameter distribution.

Tendon and ligament from the horse: an ultrastructural study of collagen fibrils and elastic fibres as a function of age

A study has been made of the ultrastructural organization of the collagen fibrils and elastic fibres in tendons and ligaments from horses of ages ranging from 2 months premature to 19 years. Diameter distributions of the collagen fibrils in the common digital extensor tendon, the superficial flexor tendon and the suspensory ligament are unimodal in the foetal tissue and at birth, and at these stages of development the three collagenous tissues are virtually indistinguishable. However, at maturity, the ligament and flexor tendon have bimodal distributions similar to that found for rat-tail tendon. The fibril distribution for extensor tendon remains unimodal until the onset of maturity, beyond which the distribution becomes bimodal. Fibril diameter distributions for ligament, extensor and flexor tendon at old age are, as at birth, virtually identical. An estimate has been made of fibrillar collagen content in the three tissues as a function of age. As with rat-tail tendon, the collagen content increases steadily from birth to maturity, at which stage the content remains fairly constant though it does drop slowly with increasing age. The presence of well developed elastic tissue in foetal and immature tendon and ligament shows that the development of the elastic fibres does not parallel the development of the collagen fibrils. In diseased tissues from a 3 year suspensory ligament and an 8.5 year superficial flexor tendon only immature elastic fibres have been observed. Furthermore, since the collagen fibril diameter distributions in these specimens show a marked change from the norm, it would be expected that the mechanical properties of these tissues would be altered significantly.

Visual target acquisition and tracking performance using a helmet-mounted sight

Experiments have been conducted on human subjects to assess the efficiency with which a helmet-mounted sighting system can be used to locate and track target sources in the horizontal plane. In the first experimental condition, in which the sight was aligned with discrete stationary target sources, the latency between target presentation and final target location was in the 2-4 s range, dependent upon the amplitude of target offset and the rate of head movement. In the second condition, subjects were instructed to track a sinusoidally oscillating visual image with the sight. Tracking performance was found to be impaired when the frequency of oscillation was increased beyond 0.8-1.0 Hz. Recording of eye movement during both experimental conditions indicated that the impairment of performance could, in part, be attributed to involuntary eye movements consequent upon stimulation of the vestibulo-ocular reflex by the head turning movements.

Visual-vestibular interaction in the control of eye movement

Three experimental conditions have been used to investigate the extent to which inappropriate reflex eye movements of vestibular origin can be suppressed by visual feedback. First, the ability to read digits in a display which moved with the head was assessed during angular oscillation about the yaw and pitch axes of the body. Performance decrement was observed at frequencies above 0.2 Hz in pitch and 1.0 Hz in yaw, being greater at higher stimulus velocity levels. A second experiment revealed that the performance decrement was associated with eye movements relative to the head, which increased with the frequency of stimulation. Finally, the response of the pursuit reflex was investigated under similar experimental conditions. The results indicated that the breakdown in the pursuit reflex and in the suppression of the vestibulo-ocular reflex occurred over the same frequency band, implying the similarity of the mechanisms responsible for suppression and pursuit.

Vision during angular oscillation: the dynamic interaction of visual and vestibular mechanisms

A review is presented of the dynamic behaviour of two oculomotor control systems--the vestibulo-ocular and pursuit reflexes--responsible for the spatial and temporal stabilization of the image of an observed object on the fovea of the retina, and mathematical models adduced in which the contribution of physiological components of the systems can be identified. With angular oscillation of the head below 1-2 Hz, retinal information is used to maintain unity gain of the head/eye system, but at higher frequencies stabilization is determined primarily by vestibular inputs. When the observed target moves with the head, as in a head-mounted display, the suppression of inappropriate vestibulo-ocular responses is dependent upon the dynamic performance of the pursuit system. With such a display, impairment of visual acuity may be detected at frequencies of oscillation as low as 0.5 Hz.

Rostral displacement of the palatopharyngeal arch: a case report

A horse with rostral displacement of the palatopharyngeal arch was found to have a bilaterally symmetrical deformity of the laryngeal area. Both left and right cricopharyngeal muscles were absent. The shape of the thyroid cartilage was grossly abnormal and vestiges of the cricothyroid muscles were attached only to the cricoid cartilage. It was suggested that such an anomaly could have resulted from aberrant development of the fourth branchial arch.

Head movement induced by angular oscillation of the body in the pitch and roll axes

The transmission of angular acceleration to the head of the human subject has been investigated during sinusoidal angular oscillation of the body in either pitch or roll about an axis through the upper lumbar vertebrae. The results indicated that angular acceleration of the skull was induced in all three axes of the head by both pitch and by roll motion. At frequencies below 1-2 Hz the head moved with the body, but in the frequency range 2-8 Hz the amplitude of head acceleration was augmented indicating that oscillation about a centre of rotation low in the body may induce large angular movements in this frequency range because of the linear component of acceleration delivered at the cervical vertebrae. At higher frequencies, the acceleration at the head was attenuated with an associated increase in phase lag, probably due to the absorption of input acceleration by the upper torso.