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PubMedDate not supplied

The characteristics of static visual fields in children with psychogenic visual disturbances.

PURPOSE: It is well-known that patients with psychogenic visual disturbances (PVD) exhibit characteristic kinetic visual fields. Even when the kinetic fields are normalized, the static fields of PVD children frequently remain abnormal. To verify this finding, we performed static perimetry on those children whose kinetic fields were initially normal or which normalized during the follow-up period, and compared the results with those of children with psychosomatic disorders (PSD) and normal children. METHODS: We examined 9 PVD children (17 eyes), 16 PSD children (32 eyes), and 16 normal children (16 eyes). Program 30-2 or 24-2 of the Humphrey Field Analyzer was used in the examinations on all subjects. RESULTS: The average mean deviation (MD) of the PVD group was significantly lower than that of the other groups (P <. 01). False negative errors and short-term fluctuations were significantly higher in the PVD group than in the other groups (P <. 05). CONCLUSION: Although PVD and PSD children possess a similar underlying psychological dysfunction, their performances in visual field testing proved to be quite different. In the PVD group, even when kinetic fields were normal, functional visual field loss in the static fields was common and had characteristic response properties.

Child↗

PubMed1997

Comparing the visual deficits of a motion blind patient with the visual deficits of monkeys with area MT removed.

The performance of a 'motion blind' patient on a series of tasks in which the perception of motion played an essential or no role was compared with that of a human subject with normal vision and with that of macaque monkeys in which cortical visual area MT had been removed and adjacent areas damaged. The patient experienced difficulties on those tasks in which the perception of motion was essential, but was unimpaired on those tasks that did not require it. Similarly, the tasks which the 'motion blind' patient found impossible or difficult were precisely those tasks on which monkeys lacking area MT performed poorly. Similarly, the tasks on which the patient performed well also presented no difficulties for the animals lacking cortical area MT. The close correlation between the pattern of visual perceptual impairments in the patient and monkeys indicates that the patient's inability to perceive most forms of visual movement is attributable to total loss of, or extensive damage to, a cortical visual area that is the human equivalent of area MT and perhaps its adjacent areas.

Adult↗

PubMed1999

Does visual sensitivity improve between 5 and 8 years? A study of automated visual field examination.

In 74 normal subjects (62 children aged 5-8 years and 12 adults), we tested the widely-held belief that visual sensitivity improves substantially during childhood. Maturation of the retino-striate pathways is generally invoked to account for age-related changes in visual sensitivity. We evaluated the extent to which attentional factors unduly emphasized the effect of age on the purely physiological mechanisms. After a specially-designed familiarization procedure, sensitivity was fully evaluated at two locations in the superior temporal field using a bracketing technique (Octopus 2000R). False-positive (FP) and false-negative (FN) catch-trials were interspersed with the sequence of stimuli. Analyses demonstrated that: (1) age affected sensitivity; and (2) the general level of attentiveness varied not only with age, but also among subjects in the same age group. We then estimated the extent to which improved visual sensitivity may reflect a concomitant evolution of vigilance. Firstly, controlled variance analyses indicated that factors for evaluating attentiveness (rate of FN responses, slope of the psychometric function at the median, and goodness of fit) were indeed much better predictors than age of the sensitivity measured. Secondly and more significantly, the grouping of subjects into homogeneous subgroups, on the basis of their attentional performance, showed that children as young as 5 years may have a visual sensitivity that is only marginally lower than that of adults.

Adult↗

PubMed2004

A historical review of the representation of the visual field in primary visual cortex with special reference to the neural mechanisms underlying macular sparing.

This article comprises a historical review of the literature pertaining to the representation of the visual field in human primary visual cortex. A brief survey of the anatomy of the visual system is followed by a critical evaluation of the key studies that have informed both the issue of the disproportionate representation of central vision within primary visual cortex, and the anatomical basis underlying the phenomena of macular sparing and macular splitting hemianopia.

Hemianopsia↗

PubMed2003

Visual and electrical evoked response recorded from subdural electrodes implanted above the visual cortex in normal dogs under two methods of anesthesia.

Sensitive methods are required to record electrical evoked potentials over the visual cortex to evaluate the efficacy and safety of a retinal prosthesis before it can be implanted on the retinal surface of patients afflicted by outer retinal diseases. This study was designed to examine subdural electrodes as a mean to evaluate cortical evoked potentials in response to light and electrical stimulation of the retina in three dogs under two methods of anesthesia-halothane and propofol. Results showed that subdural electrodes could be stabilized over the visual cortex for several (3-5) months, and that they were 6.95 times more sensitive than subdermal electrodes in recording cortical visual evoked potentials (VEPs) and 4.31 times more sensitive in recording cortical electrical evoked potentials under both methods of anesthesia. The waveforms' shape changed for each electrode in the subdural array during 6/6 (100%) and 20/38 (52%) multi-channel recording sessions under halothane and propofol, respectively. This change could point to a cortical retinotopic organization versus hierarchical organization of different cortical areas for a given retinal stimulus. In summary, subdural electrodes show promising results for recording visual and electrical evoked responses (EERs) and thus for evaluation of the retinal prosthesis.

Anesthesia↗

PubMed1999

Alpha-band oscillations in visual cortex: part of the neural correlate of visual awareness?

It was originally proposed that synchronized gamma-band oscillations in neurons of the visual cortex constitute the neural correlate of visual awareness. Two hypotheses are presented in this article: (a) both alpha-band and gamma-band oscillatory activity correspond to visual awareness; and (b) alpha-band activity occurs in the parvocellular processing pathway and gamma-band activity occurs in the magnocellular pathway, and the two modes of oscillation are correlated. It is further proposed that the frequency of the gamma-band oscillation in a group of neurons representing a particular visual scene segment is three times that of the alpha-band oscillation in the neurons representing the same segment. These hypotheses are discussed in relation to recent neurophysiological and psychophysiological experiments.

Alpha Rhythm↗

PubMedDate not supplied

The effects of an NDMA receptor antagonist on delayed visual differentiation in monkeys and rearrangements of neuron spike activity in the visual and prefrontal areas of the cortex.

The effects of perfusion of field 17 with the glutamate receptor antagonist 2-amino-5-phosphonovaleric acid (APV) on the characteristics of visual recognition and short-term memory were studied, along with the effects of APV on the responses of neurons in the visual and prefrontal areas of the cortex in rhesus macaques. In the test for delayed visual differentiation of stimuli of different colors, behavioral data were recorded simultaneously with multichannel recordings of the spike activity of single cells in cortical field 17 (directly within the microdialysis zone) and field 8. Multifactor dispersion analysis (ANOVA) showed that APV significantly worsened the behavioral characteristics in monkeys, with significant reductions in the duration of short-term storage of information (by factors of 2-4) and significant increases in the motor response times. These changes in cognitive characteristics induced by APV were accompanied by changes in the spike activity of neurons in the visual and prefrontal areas of the cortex during the sensory analysis and delay stages; changes in spike activity consisted of significant desynchronization. These results show that cognitive dysfunctions consisting of worsening of short-term remembering of information and increases in the duration of motor responses during exposure to APV may be caused by desynchronization of neuron activity in various areas of the cortex, these being involved in neuron ensembles responsible for the mechanisms of short-term memory, in which glutamatergic structures play an important role.

2-Amino-5-phosphonovalerate↗

PubMed2000

Activity in primary visual cortex predicts performance in a visual detection task.

Visual attention can affect both neural activity and behavior in humans. To quantify possible links between the two, we measured activity in early visual cortex (V1, V2 and V3) during a challenging pattern-detection task. Activity was dominated by a large response that was independent of the presence or absence of the stimulus pattern. The measured activity quantitatively predicted the subject's pattern-detection performance: when activity was greater, the subject was more likely to correctly discern the presence or absence of the pattern. This stimulus-independent activity had several characteristics of visual attention, suggesting that attentional mechanisms modulate activity in early visual cortex, and that this attention-related activity strongly influences performance.

Action Potentials↗

PubMed1993

Visual-tactual and tactual-visual transfer between objects and pictures in 2-month-old infants.

Previous studies have provided evidence for transfer of perception of object shape from touch to vision, but not from vision to touch, in young infants. Previous studies also indicate that intermodal recognition can produce a preference either for a matching or for a nonmatching object. We investigated the causes of asymmetries in intermodal transfer and of conclusions: (i) Transfer from vision to touch is possible under certain conditions and is facilitated by the use of two-dimensional (2-D) visual representations rather than three-dimensional (3-D) visual objects. (ii) The direction of preferences in a transfer task depends on the degree of dissimilarity between the haptically and visually presented objects. Familiarity preferences increase with increasing difference between the object to be recognised and the familiar object. (iii) Infants are able to perceive the 3-D shape of an object both visually and haptically, and they are sensitive both to commonalities and to discrepancies between the shapes of 3-D objects and of their 2-D representations. Hierarchical levels of perceptual processing are proposed to account for these findings.

Female↗

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↗

PubMed2000

Visual hallucinatory syndromes and the anatomy of the visual brain.

We have set out to identify phenomenological correlates of cerebral functional architecture within Charles Bonnet syndrome (CBS) hallucinations by looking for associations between specific hallucination categories. Thirty-four CBS patients were examined with a structured interview/questionnaire to establish the presence of 28 different pathological visual experiences. Associations between categories of pathological experience were investigated by an exploratory factor analysis. Twelve of the pathological experiences partitioned into three segregated syndromic clusters. The first cluster consisted of hallucinations of extended landscape scenes and small figures in costumes with hats; the second, hallucinations of grotesque, disembodied and distorted faces with prominent eyes and teeth; and the third, visual perseveration and delayed palinopsia. The three visual psycho-syndromes mirror the segregation of hierarchical visual pathways into streams and suggest a novel theoretical framework for future research into the pathophysiology of neuropsychiatric syndromes.

Aged↗

PubMed2000

The multiple roles of visual cortical areas MT/MST in remembering the direction of visual motion.

Although the role of cortical areas MT and MST (MT/MST) in the processing of directional motion information is well established, little is known about the way these areas contribute to the execution of complex behavioral tasks requiring the use of such information. We tested monkeys with unilateral lesions of these areas on a visual working memory task in which motion signals not only had to be encoded, but also stored for brief periods of time and then retrieved. The monkeys compared the directions of motion of two random-dot stimuli, sample and test, separated by a temporal delay. By increasing the temporal delay and spatially separating the two stimuli, placing one in the affected visual field and the other in the intact visual field, we were able to assess the contribution of MT/MST to specific components of the task: encoding (sample), retention (delay) and encoding/retrieval/comparison (test). We found that the effects of MT/MST lesions on specific components depended upon the demands of the task and the nature of the visual motion stimuli. Whenever stimuli consisted of random dots moving in a broad range of directions, MT/MST lesions appeared to affect encoding. Furthermore, when the lesions affected encoding of the sample, retention of the direction of stimulus motion was also affected. However, when the stimulus was coherent and the emphasis of the task was on the comparison of small direction differences, the absence of MT/MST had major impact on the retrieval/comparison component of the task and not on encoding or storage.

Animals↗

PubMed2005

The effects of severe visual challenges on steering performance in visually healthy young drivers.

PURPOSE: Two experiments explored the extent to which induced blur, reduced luminance, and reduced visual fields affect drivers' steering performance in a driving simulator. METHODS: In experiment 1, ten young participants (M = 21.2 years) drove at approximately 89 km/h (55 mph) along a curvy roadway while being exposed to blur (0 to + 10 D), luminance (0.003 to 16.7 cd/m), and visual field (1.7 and 150 degrees) manipulations. In experiment 2, a new group of ten young participants (M = 18.5 years) drove while exposed to seven visual field sizes (1.7 to 150 degrees). RESULTS: Steering was sensitive to a reduced field size but not to the blur and luminance challenges. Acuity, on the other hand, was sensitive to the blur and luminance challenges but not to reduced field size. DISCUSSION: In healthy young drivers, steering performance is remarkably robust to severe blur and to extremely low luminances. These results support a key element of the selective degradation hypothesis advanced by Leibowitz and colleagues--that steering abilities are preserved at night even when the ability to recognize objects and hazards is not. Additional research should address the other element of this hypothesis--that drivers fail to appreciate the extent to which their visual abilities are degraded at night.

Adult↗

PubMed1991

Visual evoked potentials and visual acuity after transurethral resection of the prostate.

Changes in visual evoked potentials, visual acuity, blood ammonia levels and serum electrolytes (Na+ and K+) after transurethral resection of the prostate using glycine as an irrigating fluid performed under subarachnoid block were studied in 12 patients, in the pre-operative and immediate postoperative periods. Visual evoked potentials (p100 latency), recorded by shift of a checkerboard pattern, increased significantly from a pre-operative value of mean (SEM) 101.18 (1.63) msec in the right eye, and 102.5 (1.47) msec in the left eye to 108.91 (1.8) msec (p less than 0.01) and 108.08 (2.53) msec (p less than 0.01) respectively in the postoperative phase. There were no changes in visual acuity as assessed by a Snellen's chart, blood ammonia levels and serum electrolyte concentrations. The amount of glycine used intra-operatively for irrigation ranged from 3 to 31 litres.

Aged↗

PubMed1998

Electrode position and the multi-focal visual-evoked potential: role in objective visual field assessment.

PURPOSE/METHODS: To improve the performance of visual-evoked potentials (VEP) in the assessment of the human visual field, the multi-focal cortically scaled pattern VEP was recorded up to 250 of eccentricity in normal subjects. Monopolar and varying bipolar electrode positions were used. RESULTS: The monopolar response was strongly biased towards the lower hemifield. Bipolar leads straddling the inion (2 cm above and below) achieved approximately equal signals from the upper and lower visual field. Division into sectors of similar wave-form augments the analysis compared with summed full-field responses. CONCLUSION: With this technique, the multi-focal VEP can be used to objectively assess the visual field.

Cerebral Cortex↗

PubMed1998

Increased oxygen consumption in human visual cortex: response to visual stimulation.

To test whether a sufficiently complex visual stimulus causes the consumption of oxygen to rise in the human visual cortex, we used positron emission tomography (PET) to measure the cerebral metabolic rate of oxygen (CMRO2) during visual stimulation in 6 healthy normal volunteers. A yellow-blue checkerboard, reversing its contrast at a frequency of 8 Hz, was presented for a period of 7 min, beginning 4 min before the onset of a 3-min scan. In the baseline condition, subjects fixated a cross-hair from 30 s before until the end of the 3-min scan. The CMRO2 was calculated with the two-compartment weighted integration method (1). The checkerboard minus baseline subtraction yielded statistically significant increases in CMRO2 in the primary (V1) and higher order visual cortices (V4 and V5). The significant CMRO2 increases were detected in these regions in both the group average and in each individual subject.

Adult↗

PubMed2001

Fast backprojections from the motion to the primary visual area necessary for visual awareness.

Much is known about the pathways from photoreceptors to higher visual areas in the brain. However, how we become aware of what we see or of having seen at all is a problem that has eluded neuroscience. Recordings from macaque V1 during deactivation of MT+/V5 and psychophysical studies of perceptual integration suggest that feedback from secondary visual areas to V1 is necessary for visual awareness. We used transcranial magnetic stimulation to probe the timing and function of feedback from human area MT+/V5 to V1 and found its action to be early and critical for awareness of visual motion.

Adult↗

PubMed2004

Visual acuity and the ability of the visually impaired to read medication instructions.

BACKGROUND/AIMS: Although medical information is often communicated in writing, little attention is given as to whether patients can read it--for example, the miniscule writing on the side of the medication bottles (equivalent to 6/6 reduced Snellen). The authors wished to determine a distance visual acuity threshold, using routine assessments of visual acuity (VA), beyond which patients are unable to read their instructions. METHODS: 180 patients, subgrouped according to the best line of Snellen acuity (from 6/9 to 6/60) achieved in either eye were recruited. Subjects were asked to read the printed manufacturer's instructions on the side of the drops box without magnification. RESULTS: Those subjects with best corrected VA of lower than or equal to 6/24 showed a significantly diminished ability to read the instructions on their eye drops bottles (p<0.001 for each comparison). When this group selected a font size of their choosing, the mode for preferred Arial font sizes were 16 for the 6/24 group, 18 for 6/36, and 22 for 6/60. CONCLUSION: This study documents the inability of patients with visual impairment to read the instructions on their bottle of eye drops. There is evidence of a distance VA threshold effect such that, although the majority of patients with 6/18 VA are able to read their instructions, patients whose VA is worse cannot. This latter group would benefit from a larger font size, a size of Arial 22 being sufficient. We recommend that all printed information for patients with visual acuities between 6/24 and 6/60 be printed in font Arial 22 or equivalent.

Adult↗