Biomedical subjects

J L Barbur

Publications and source records attributed to J L Barbur.

At least 19 recordsLinked to original sources

PubMed1997

Pattern of neuronal activity associated with conscious and unconscious processing of visual signals.

Following striate cortex damage in monkeys and humans there can be residual function mediated by parallel visual pathways. In humans this can sometimes be associated with a "feeling" that something has happened, especially with rapid movement or abrupt onset. For less transient events, discriminative performance may still be well above chance even when the subject reports no conscious awareness of the stimulus. In a previous study we examined parameters that yield good residual visual performance in the "blind" hemifield of a subject with unilateral damage to the primary visual cortex. With appropriate parameters we demonstrated good discriminative performance, both with and without conscious awareness of a visual event. These observations raise the possibility of imaging the brain activity generated in the "aware" and the "unaware" modes, with matched levels of discrimination performance, and hence of revealing patterns of brain activation associated with visual awareness. The intact hemifield also allows a comparison with normal vision. Here we report the results of a functional magnetic resonance imaging study on the same subject carried out under aware and unaware stimulus conditions. The results point to a shift in the pattern of activity from neocortex in the aware mode, to subcortical structures in the unaware mode. In the aware mode prestriate and dorsolateral prefrontal cortices (area 46) are active. In the unaware mode the superior colliculus is active, together with medial and orbital prefrontal cortical sites.

Brain↗

PubMed1996

Motion discrimination of single targets: comparison of preliminary findings in normal subjects and patients with glaucoma.

BACKGROUND: Luminance, global motion and flicker sensitivities are affected in patients with primary open-angle glaucoma. Although no theoretical model has been put forward to explain the observed reduction in sensitivity in this patient group, these findings have often been attributed to diffuse and selective loss of large optic nerve fibres. METHODS: Movement processing was investigated using an optical projection system that generates smooth, continuous motion with control of speed, displacement and motion direction. Motion-displacement and direction-discrimination thresholds were measured in eight normal subjects and in three patients with diagnosed glaucoma. At each speed tested, targets were presented for a range of displacements and thresholds were extracted after probit analysis. The measurements were carried out both foveally and at 19 degrees in the periphery and provided the data necessary to develop and optimise a model of motion perception based on multiple time delays for the correlation of signals that map progressively more distant parts of the visual field. RESULTS: Our preliminary findings show that direction discrimination can be at chance level even for large displacements when motion is detected 80% of the time. Model simulations show that specific changes in the spatial sampling interval and the speed of transmission of the motion signals involved can cause the observed reduction in motion sensitivity and direction discrimination in patients with glaucoma. CONCLUSIONS: A model for motion detection and direction discrimination of single targets has been proposed to account for the measured functional relationship between motion displacement thresholds and target speed in normal subjects. Tested patients with glaucoma show reduced motion sensitivity and poor discrimination of motion direction. The type of degraded performance observed experimentally in glaucoma patients is also predicted by the model. Such predictions require specific changes in model parameters that may be indicative of changes in the retina caused by the disease.

Adult↗

PubMed1995

Parameters affecting conscious versus unconscious visual discrimination with damage to the visual cortex (V1).

When the visual (striate) cortex (V1) is damaged in human subjects, cortical blindness results in the contralateral visual half field. Nevertheless, under some experimental conditions, subjects demonstrate a capacity to make visual discriminations in the blind hemifield (blindsight), even though they have no phenomenal experience of seeing. This capacity must, therefore, be mediated by parallel projections to other brain areas. It is also the case that some subjects have conscious residual vision in response to fast moving stimuli or sudden changes in light flux level presented to the blind hemifield, characterized by a contentless kind of awareness, a feeling of something happening, albeit not normal seeing. The relationship between these two modes of discrimination has never been studied systematically. We examine, in the same experiment, both the unconscious discrimination and the conscious visual awareness of moving stimuli in a subject with unilateral damage to V1. The results demonstrate an excellent capacity to discriminate motion direction and orientation in the absence of acknowledged perceptual awareness. Discrimination of the stimulus parameters for acknowledged awareness apparently follows a different functional relationship with respect to stimulus speed, displacement, and stimulus contrast. As performance in the two modes can be quantitatively matched, the findings suggest that it should be possible to image brain activity and to identify the active areas involved in the same subject performing the same discrimination task, both with and without conscious awareness, and hence to determine whether any structures contribute uniquely to conscious perception.

Adult↗

PubMed1994

Insights into the different exploits of colour in the visual cortex.

A new method that allows controlled masking of luminance contrast has been developed to study the use of chromatic signals in human vision. The method also makes it possible to examine the different uses of chromatic signals (e.g. the generation of perceived colour, or the construction and representation of object structure and form). By using this technique, we studied the threshold detection of chromatic signals in normal trichromats. The results show that chromatic signals are virtually unaffected by ongoing, randomly varying, luminance contrast changes. These findings suggest that chromatic signals are either processed independently or can be separated completely from any confounding luminance contrast components in the stimulus. Thresholds for detection of colour changes only, and for extraction of stimulus structure from chromatic signals in normal trichromats, in subjects with single cone receptor deficiency (i.e. dichromats) and in three subjects with abnormal colour vision caused by bilateral damage to ventromedial, extra-striate visual cortex (i.e. subjects with cerebral achromatopsia) have also been measured. No significant difference in thresholds for the two conditions was observed either in normal trichromats or in dichromats. Subjects with cerebral achromatopsia, however, reveal markedly different thresholds. The results suggest that chromatic signals are processed independently to generate perceived object colour or to construct spatially structured objects, and that these functions involve different neural substrates. The results help to explain, at least in part, why cerebral achromatopsia is a heterogeneous disorder, and why there can be significant differences in the effective use of chromatic signals in subjects described as cerebral achromatopsics.

Color Perception↗

PubMed1994

Spatial and temporal response properties of residual vision in a case of hemianopia.

Residual vision in subjects with damage of the primary visual cortex (striate cortex) has been demonstrated in many previous studies and is taken to reflect the properties of known subcortical and extrastriate visual pathways. In this report we describe psychophysical experiments carried out on a subject clinically blind in half of his visual field (i.e. homonymous hemianopia) caused by striate cortex damage. They reveal the existence of two distinct channels mediating such vision. One channel responds to spatial structure and the other to light flux changes. The spatially tuned channel has a peak response at about 1.2 cycles per degree and shows rapid loss of sensitivity at both high and low spatial frequencies. This channel does not respond to diffuse illumination. The light flux channel, however, responds only to sudden increments in light flux levels on the retina and shows extensive spatial summation. Both channels require transient inputs, with a peak sensitivity at about 10 cycles per second and show virtually complete attenuation at temporal frequencies below 2 cycles per second. The spatiotemporal characteristics of these two channels account for much of the reported limits of visual performance attributed to subcortical or extrastriate pathways in some patients, and especially for their relatively good sensitivity for the detection of abrupt, transient stimuli or fast-moving targets. A new method is also applied to the measurement of the amount of light scatter in the eye. The measurements show that light scatter into the sighted hemifield could not account for the results obtained with the stimuli used to characterized the residual vision of this subject.

Accidents, Traffic↗

PubMed1994

Pupillary function in human amblyopia.

Quantitative measurements of pupillary function (response amplitude and latency) were made for normal eyes and for normal and fellow amblyopic eyes of groups of strabismic and anisometropic amblyopes. Stimuli consisted of luminance modulation of a large, evenly lit area (pupil light reflex) as well as contrast modulation of sinusoidal gratings (pupil grating response) of fixed, space-averaged luminance. Measurements were made of the direct and the consensual reflex under monocular stimulation. A comparison of the amplitude of the pupil light reflex as a function of luminance modulation showed no significant differences between normal and fellow amblyopic eyes for both the strabismic and anisometropic groups of amblyopes studied. A similar comparison of the associated response latencies showed significant difference between normal and fellow amblyopic eyes for both groups. In general, reductions in response amplitude and latency of the pupil grating response were found in individuals from each group when comparing the good and the affected eyes, although the observed group differences were only significant in the strabismic group. Interestingly, statistically significant reductions in both amplitude and latency for both the pupil light reflex and the pupil grating response were found between the eyes of normal observers and the so-called normal eyes of amblyopes in both groups studied. These results suggest that the type of pupillary deficit in amblyopia is a complicated one, depending not only on the type of amblyopia (strabismic or anisometropic) and the type of stimulus employed (light or pattern), but also on the parameter assessed (amplitude or latency) and whether the amblyopic result is referenced to its fellow normal eye or to the normal eye of a non-amblyopic observer. Since the pupil response to light flux changes is not mediated exclusively via the retinal projection to the midbrain and may also involve the activity of central visual pathways, the results obtained in this study cannot be used to provide definitive evidence for the site of abnormality in amblyopia.

Adolescent↗

PubMed1993

Conscious visual perception without V1.

We used the technique of PET to determine whether visual signals reach visual area V5, specialized for visual motion, when a human patient, blinded by a lesion in area V1, discriminates the direction of motion of visual stimuli and shows, through his verbal reports, that he is consciously aware of both the nature of the visual stimulus and its direction of motion. The results showed that area V5 was active without a parallel activation of area V1, implying that the visual input can reach V5 without passing first through V1 and that such an input is sufficient for both the discrimination and the conscious awareness of the visual stimulus.

Adult↗

PubMed1992

New method based on random luminance masking for measuring isochromatic zones using high resolution colour displays.

A new method of measuring normal hue discrimination ellipses and dichromatic zones using a high resolution colour monitor is described. The test involves the detection of chromatic bars on a grey background (x = 0.305, y = 0.323) having a luminance of 34 cd m-2. Elements of the background matrix of square checks are varied randomly in luminance in space and time to provide random luminance masking (RLM) which compensates for differences in the relative luminous efficiency of different observers. The measurement technique provides a rapid and comprehensive colour vision test. Typical results are presented for normal trichromats, protanopes and deuteranopes without RLM and with the RLM set of 25%. The size of the discrimination ellipse in normal observers is the same in both viewing conditions, but the use of the RLM technique reveals the extent of the isochromatic zones in colour deficient observers.

Color Perception Tests↗

PubMed1992

Pupillary responses to stimulus structure, colour and movement.

Pupillary responses to stimuli which favour the preferential stimulation of neural mechanisms involved in the detection of visual attributes such as colour, spatial structure, movement and light flux changes on the retina have been measured and compared. Pupil responses to a decrement in stimulus luminance (i.e., a flash of darkness), suggest that at least three components are involved in this response, their relative contribution being determined largely by stimulus size, contrast and presentation time. A comparison of pupil responses to gratings of equal and lower space-averaged luminance shows that the amplitude of pupillary constriction at grating onset for the equal luminance condition is about twice that measured with similar gratings in the lower luminance condition. Pupillary responses to chromatic isoluminant gratings are in general of longer latency when compared to responses of similar amplitude elicited by achromatic gratings. Small pupillary constrictions elicited by the onset of coherent movement in dynamic, random dot patterns are also demonstrated under stimulus conditions which eliminate pupillary responses to sudden light flux changes on the retina. The results support an earlier hypothesis which suggests that the onset of sudden changes in neural activity in the visual cortex when a visual stimulus is presented to the eye causes an overall perturbation which weakens transiently the regulatory inhibitory input to the pupillomotor nucleus. This, in turn, results in a transient increase in the efferent parasympathetic innervation of the iris sphincter muscle and hence the observed constriction of the pupil. The characteristics of the pupillary response reflect the properties of the mechanisms and the number of neurones which participate in the detection of each stimulus attribute.

Color Perception↗

PubMed1991

Factors affecting visual sensitivity in a hemianopic subject.

A well-studied subject with visual cortex damage (G.Y.) was tested in his hemianopic field with temporally modulated sinusoidal and square-wave gratings. The purpose was to use an extended range of parameters to obtain a detailed spatiotemporal specification of his residual vision and to try to resolve the discrepancy between negative findings of Hess and Pointer (1989) and previous positive claims. Both the spatial and temporal parameters could be Gaussian-weighted. Detection as a function of spatial frequency, contrast, temporal modulation frequency, stimulus size, and slope of the temporal and spatial Gaussian functions was investigated using a two-alternative forced-choice procedure. The most important parameters for this subject were found to be the slope of the temporal Gaussian function and the size and contrast of gratings. With optimum parameters he could reliably achieve a score of 95-100% correct in his 'blind' field. The results are consistent with earlier studies of this subject, especially his ability to respond to moving stimuli, and also may account for why negative results had been reported for him when particular fixed parameters were used.

Contrast Sensitivity↗

PubMed1988

Human saccadic eye movements in the absence of the geniculocalcarine projection.

This paper describes the results of an investigation into the spatial and temporal properties of the saccadic eye-movement system in a subject with a 'blind' hemifield caused by accidental damage to the left geniculostriate projection. The results show that blind hemifield stimulation can elicit goal-directed saccades of short latency provided the contrast of the visual stimulus is above the threshold required for saccade generation at the corresponding stimulus eccentricity. Signal processing associated with blind hemifield pathways shows reduced sensitivity by comparison with the normal hemifield and greater dependence on stimulus size, eccentricity and mode of presentation. Higher contrast levels are always needed for blind field stimulation and larger variance in mean saccadic amplitudes and latencies is normally observed. Large stimulus eccentricities which would normally require corrective saccades exhibit a different response pattern to that observed in normal vision. The results show that the centre of gravity effect and the generation of rapid, 'express-saccades' observed in normal vision are not inevitable outcomes of midbrain processing of visual information. The subject's performance reveals strategic patterns of eye-movement responses which maximize the use of his normal hemifield. Visual performance in the control of saccadic eye-movement responses appears to be optimum when the detection of the visual stimulus is associated with both the geniculostriate and the midbrain pathways. Experiments designed to test for the effects of light scatter into the normal hemifield show that the subject's saccadic eye-movement performance following visual stimulation of the blind hemifield cannot be attributed either to the amount or the pattern of light scatter into the normal hemifield. The implications of present findings on the results of other studies of eye-movements in subjects with scotomas associated with cortical lesions are also examined.

Adult↗

PubMed1987

Pupil response as an objective measure of visual acuity.

Pupillary responses to brief presentations of sinusoidal grating patterns were measured. The results show that the amplitude of the pupillary responses varies systematically with grating spatial frequency. Gratings of spatial frequencies in the range 1-5 c deg-1 elicited the maximum percentage change in pupil area, with higher and lower spatial frequency gratings producing smaller responses. The bandpass nature of the pupil response function was compared to contrast sensitivity functions obtained under the same viewing conditions. The effects of defocus and eccentric fixation on both contrast sensitivity and pupil response functions were qualitatively similar. Grating acuities calculated from contrast sensitivity data correlated well with similar data extracted from pupil response functions. The results suggest that automated pupillometry can provide an attractive means for objective measurement of visual acuity.

Fixation, Ocular↗

PubMed1986

The perception of moving comets at high retinal illuminance levels: a rod-cone interaction effect.

A small circular target of high retinal illuminance level can have a comet-like appearance when presented moving continuously with a speed as low as 0.2 deg/s. This perceived lengthening of the circular target increases with the speed of movement and is only observed for target presentations outside the foveal region. Data on the parametric properties of the "comet effect" are presented together with related results on the time-course of recovery of retinal sensitivity following brief exposure to intense stimuli. Measurement of target spectral irradiance levels which are just sufficient to yield the comet effect suggests that the lengthening of the circular target reflects a rod-cone interaction and therefore it may be due to unsuppressed, saturated rod responses at high retinal illuminance levels. The restriction of the comet effect to areas outside the foveal region is used to produce spatial maps of what appears to be the rod-free area of the retina. A model simulation by means of a computational approach shows that the predicted appearance of the moving target matches very closely the experimental observations on the comet effect. Model predictions based on psychophysical estimates of comet length for the stimulus conditions of these experiments yield an overall response time for the rod system of some 600 ms.

Humans↗

PubMed1986

A new photographic-based system for the measurement of contrast sensitivity.

A new photographic-based system has been developed for the measurement of contrast sensitivity in human vision. The system has several advantages over other photographic methods and is relatively easy and inexpensive to produce. This paper describes the design of the new system and the methods used in the production of the photographic prints. Preliminary tests show that full contrast sensitivity curves can be obtained with a small number of photographic prints and that these curves are very similar to those obtained using a carefully calibrated electronic pattern generator.

Form Perception↗

PubMed1985

Speed discrimination and its relation to involuntary eye movements in human vision.

In addition to a selective response to a narrow range of motion directions, a neural mechanism specialized for motion detection must also be able to discriminate between different speeds of target movement. Many psychophysical and electrophysiological investigations of motion perception have largely been concerned with identifying possible schemes or mechanisms capable of discriminating motion direction, but the ability to discriminate faster or slower movement in the same direction has so far received comparatively little attention. Two schemes capable of motion detection and speed discrimination are reported here, together with experimental data which show that the visual system employs both schemes, one for the slow speed range (i.e. less than 3 degrees/s) and the other for larger speeds of target movement. It is also shown that the use of both schemes ensures that retinal image displacements due to involuntary eye movements (i.e. slow drifts and microsaccades) are not detected as target movement.

Discrimination, Psychological↗

PubMed1985

Displacement thresholds for motion detection under conditions of chromatic adaptation.

The smallest, continuous target displacement which is sufficient to cause a sensation of movement was measured under conditions of chromatic adaptation. The experiments were carried out for test target and background field wavelengths which selectively isolate the activity of the colour mechanisms of the human eye. The results show that, when target detection is largely mediated by the red or green colour mechanism, a target displacement of 2-3' in the foveal region is sufficient to cause a reliable sensation of movement. These displacement thresholds show only a small change as a function of target contrast or background retinal illuminance level. Similar results were obtained when the colour of the test target was the same as that of the background field (e.g. green, yellow or red), when no attempt was made to isolate a single colour mechanism. Test target and background wavelengths which isolate the activity of the blue colour mechanism yield displacement thresholds in the range 5-7' and show a stronger dependence on target contrast and background retinal illuminance level. The experiments were repeated at several eccentricities and the results show that, for a test target and background field of the same colour and under conditions which isolate the green or red spectral response mechanism, the minimum target displacement required for motion detection appears to increase in a manner similar to the centre-width receptive-field size of broad-band, centre--surround transient neurons in the rhesus monkey retina [De Monasterio and Gouras (1975) J. Physiol., Lond. 251, 167-195].(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Ocular↗

PubMed1982

Reaction-time determination of the latency between visual signals generated by rods and cones.

This paper reports the results of a series of reaction-time measurements in response to light flashes which stimulate preferentially either the rod of the cone receptor mechanisms in human vision. The average response latency between rod- and cone-generated signals as determined from these measurements was found to be 80 ms. Similar response latency values were also obtained from measurements involving only visual responses to real- and apparent-motion stimuli. The results show that the rod-cone response latency, measured by reaction-time or visual methods is relatively constant for target presentation locations between 4 and 9 degrees of visual angle from the fovea along the horizontal.

Adult↗

PubMed1981

The spatial and temporal organisation of motion perception units in human vision.

Measurements of threshold illumination levels for detection of retinally non-localised moving targets show that detection of a moving target is influenced by both the spatial and the temporal modulation of the background field. The temporal response characteristics obtained from these measurements are similar to those obtained from experiments on detection of temporal flicker. Experiments with spatially modulated background fields reveal visual mechanisms with spatial properties which are essentially independent of many stimulus parameters. The response amplitude of the spatial filter which characterises these mechanisms increases linearly as a function of background contrast and is independent of the relative orientation between the background structure and the direction of target movement. These properties are used to compute the two-dimensional spatial characteristics of mechanisms involved in the detection of moving targets.

Adult↗