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Biomedical subjects

A Sahraie

Publications and source records attributed to A Sahraie.

12 recordsLinked to original sources
PubMed2002

Psychophysical and pupillometric study of spatial channels of visual processing in blindsight.

To date no systematic method has been used for characterising the residual capacity of blindsight subjects that would allow comparison and generalisation across all subjects. The detection of isoluminant gratings of varying spatial and temporal frequencies commends itself for detailed between-subject comparison, and for mapping results onto physiological properties in relation to neuronal circuitry. We report the ability of a blindsight subject (CS) to detect suprathreshold sine-wave gratings over a range of spatial and temporal frequencies using psychophysical techniques. A band-pass spatial channel with an upper cutoff below 3.5 cycles/deg is specified. The data also have been analysed to compare differences between two types of blindsight performances, type I and type II. Spatial gratings were also used to elicit a pupillary grating response, offering an objective method that is free of verbal nuances and response bias, and the resulting band-pass channel can be used both for clinical screening and for prediction and comparisons with psychophysical profiles. Finally, we have compared our results with those reported in studies of a well-known subject, GY, which demonstrate remarkable similarities. Implications are discussed in relation to blindsight research.

Adult↗
PubMed2002

New Farnsworth-Munsell 100 hue test norms of normal observers for each year of age 5-22 and for age decades 30-70.

AIMS: To provide normative data for chromatic discrimination on the Farnsworth-Munsell 100 hue test particularly for observers under 23 years of age. METHODS: Normal observers were screened for congenital colour vision deficiencies using the Ishihara test leaving 382 observers. RESULTS: New total error score (TES) norms (means and 95th percentiles) are presented for each year of age from 5-22 and for 10 year age groups from the 30s to the 70s. These norms are presented as actual values (TES) and also as square root values ( radical TES). Other data include partial error scores for red-green and blue-yellow axes discrimination. CONCLUSION: This study provides the most detailed set of normative data to date. The data are also in agreement with other reports of chromatic discrimination, showing that the performance in this task varies as a U-shape function with age, the best being achieved at 19 years of age.

Adolescent↗
PubMed2001

Attention induced motion blindness.

Recent studies have shown evidence for modulation of cortical activity by attention in visual areas involved in motion processing. Behavioural effects of this modulation have only been reported for high-order, but not for luminance-based motion. We show that attentional load can even affect the perception of a first-order motion inducing a short-termed motion blindness. The detection of transient coherent motion embedded in a rapid serial visual presentation was severely impaired if colour features were to be processed simultaneously. The findings reported here show attentional requirements can affect motion perception. This effect can not be explained by motion adaptation or priming and may instead arise from the suppression of irrelevant stimuli.

Analysis of Variance↗
PubMed1998

Awareness and confidence ratings in motion perception without geniculo-striate projection.

We have previously reported that under certain experimental conditions, a subject with damaged primary visual cortex performed significantly above chance in discriminating motion direction and orientation either with or without awareness of the stimulus presentation in his blind hemifield. The subject's performance varied as a function of stimulus speed, excursion length, and background luminance. Present observations confirm the previous findings of above chance discrimination scores both with and without awareness, but nevertheless indicate an overall increase in sensitivity over the past 2 years. In addition to discriminating the direction of motion, the subject was asked on every trial to report either awareness or confidence or both, on a six-point scale. The results show that the introduction of a six-point scale for the reported level of awareness yielded similar results to those when the subject was given a binary choice to indicate the presence or absence of conscious awareness. The area under the receiver operating characteristic (ROC) curves obtained from the rating data was used to compare results across different experimental conditions. It was found that although the subject's reported confidence and awareness varied monotonically as a function of stimulus speed, they were not equivalent measures.

Adult↗
PubMed1998

Residual processing of chromatic signals in the absence of a geniculostriate projection.

We have investigated the residual processing of chromatic signals in a subject with unilateral damage to the primary visual cortex using psychophysical, pupillometric and functional magnetic resonance imaging (fMRI) methods. Of particular interest was to establish the correlation between the subject's ability to make use of chromatic signals in the blind hemifield to discriminate between different coloured targets, the corresponding residual pupil colour responses and the level and location of cortical activation generated by the same stimuli as revealed by fMRI. The results obtained using the three different experimental approaches are consistent and suggest that retrograde degeneration of thalamic and retinal chromatic processing mechanisms caused by damage to the primary visual cortex in man does not abolish completely the ability to process chromatic signals particularly when large, long-wavelength stimuli are employed.

Journal Article↗
PubMed1998

Residual processing of chromatic signals in the absence of a geniculostriate projection.

We have investigated the residual processing of chromatic signals in a subject with unilateral damage to the primary visual cortex using psychophysical, pupillometric and functional magnetic resonance imaging (fMRI) methods. Of particular interest was to establish the correlation between the subject's ability to make use of chromatic signals in the blind hemifield to discriminate between different coloured targets, the corresponding residual pupil colour responses and the level and location of cortical activation generated by the same stimuli as revealed by fMRI. The results obtained using the three different experimental approaches are consistent and suggest that retrograde degeneration of thalamic and retinal chromatic processing mechanisms caused by damage to the primary visual cortex in man does not abolish completely the ability to process chromatic signals particularly when large, long-wavelength stimuli are employed.

Color Perception↗
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↗
PubMed1997

Pupil response triggered by the onset of coherent motion.

BACKGROUND: Recent studies have shown that transient constrictions of the pupil can be elicited by visual stimuli that do not cause an increment in light flux level on the retina. Such stimuli include achromatic gratings and isoluminant chromatic patterns. METHOD: We investigated pupillary responses to the onset of coherent movement generated in a pattern of dots in random motion. Measurements were carried out in normal observers and in a subject with hemianopia caused by damaged primary visual cortex. RESULTS: The experimental findings show that the onset of coherent motion triggers systematic constrictions of the pupil that cannot be accounted for in terms of a pupil light reflex response. We labelled these constrictions Pupil motion responses (PMRs). Results show that PMRs have large response latencies and on average are of small response amplitudes. The dependence of PMRs on changes in motion parameters such as stimulus speed and direction of motion has been investigated. CONCLUSIONS: The existence of PMRs to the onset of the coherent motion in human vision has been demonstrated. These new findings are discussed in relation to the psychophysical and physiological data on motion perception and the possible pathways involved in the control of the pupil response.

Hemianopsia↗
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↗
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↗