Search PubMed⌕ Search

Biomedical subjects

G R Barnes

Publications and source records attributed to G R Barnes.

At least 73 records · Page 4Linked to original sources
PubMed1987

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)

Eye Movements↗
PubMed1987

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.

Adult↗
PubMed1985

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.

Electronystagmography↗
PubMed1985

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.

Dark Adaptation↗
PubMed1984

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.

Eye Movements↗
PubMed1984

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.

Ethanol↗
PubMed1984

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.

Eye Movements↗
PubMed1983

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.

Eye Movements↗
PubMed1983

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.

Eye Movements↗
PubMed1983

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.

Animals↗
PubMed1983

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.

Adolescent↗
PubMed1982

Innervation of the equine intrinsic laryngeal muscles.

The nerve supply to the intrinsic laryngeal muscles of the horse was studied by gross dissection and by electromyography which was carried out before, during and after section of various intralaryngeal nerve branches. The anatomical relationships and passage of the laryngeal nerves throughout the larynx were defined. Unlike the dog and man there was no evidence of the passage of motor nerve fibres from one side of the larynx to the other.

Journal Article↗
PubMed1982

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.

Computers↗
PubMed1981

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.

Form Perception↗