Biomedical subjects

Larry A Abel

Publications and source records attributed to Larry A Abel.

8 recordsLinked to original sources

PubMed2006

Effects of age on latency and error generation in internally mediated saccades.

Most studies on the effects of ageing on saccades have examined reflexive saccades; the only commonly studied volitional task has been the antisaccade task, with contradictory results. We examined in both young and elderly normal subjects the latency of anti-, memory-guided, and predictable saccades and the timing of self-paced saccades; we also evaluated errors made on the first two tasks. We expected errors to be correlated between tasks; we also expected antisaccade latencies and errors to be inversely correlated. We also expected antisaccade and memory-guided saccade latencies to be longer in individuals with a high self-paced rate. Except for predictable saccades, mean latencies were significantly higher in the elderly. However, their performance was more variable. Errors were also significantly more frequent on anti- and memory-guided saccade tasks. Most of the hypothesised correlations were not observed. Analysis of error latencies showed that whilst most antisaccade errors were reflexive, for memory-guided saccades both express errors and errors with latencies between 0.4 and 2.5 s were observed. The latter appeared to be a premature release of what would otherwise have been a properly planned response. Age thus impaired all but the predictable saccade task; nevertheless, there were few relationships between measures across tasks. This suggests that a range of processes mediate peoples' performance on these saccade paradigms.

Adolescent↗

PubMed2006

Psychosocial impact of amblyopia and its treatment: a multidisciplinary study.

BACKGROUND: To explore the meaning of amblyopia from both parents' and children's perspectives and to seek correlations between the experiential aspects of the condition and its treatment, the clinical characteristics of amblyopia and any apparent psychopathology. METHODS: Children with amblyopia and their parents were engaged in semistructured in-depth interviews. Children also underwent a vision assessment and, where applicable, parents and children completed a psychological inventory, the Behaviour Assessment System for Children. RESULTS: Dealing with stigma and the perceptions and responses of peers were found to be of central significance to the experience of amblyopia therapy. This had adverse consequences for some children's identity and psychosocial well-being. The clinical manifestations of amblyopia did not correlate with the social implications of the condition. However, children with strabismus were noted to have significantly greater conduct and externalizing problems. CONCLUSIONS: Given that amblyopia can affect children's psychosocial well-being, health outcomes need to integrate both vision and psychosocial implications of treatment. Although treatment should aim to reverse amblyopia and restore visual acuity, efforts to minimise any negative psychosocial consequences of treatment should be made. A way to balance managing amblyopia and ensuring children's psychosocial well-being should to be considered by clinicians and included in treatment guidelines.

Adolescent↗

PubMed2006

Infantile nystagmus: current concepts in diagnosis and management.

This review examines current approaches to the diagnosis and management of congenital forms of nystagmus. Emphasis is placed on diagnostic features that are amenable to clinical identification but those issues that can be addressed only with more detailed investigations, such as eye movement recording, are indicated. Non-surgical management, including prism spectacles, contact lenses and vision therapy, is discussed, as are surgical approaches. Because many aspects of congenital forms of nystagmus, particularly as experienced by patients with the condition in their normal lives, are poorly addressed in both the clinical and research literature, these limitations are also highlighted.

Electrooculography↗

PubMed2004

The effects of increased visual task demand on foveation in congenital nystagmus.

Commonly, when an individual with congenital nystagmus (CN) performs a visually demanding task their nystagmus intensifies and their visual acuity decreases, probably due to poorer foveation. However, the relationship between fixation attempt and nystagmus waveform has never been quantified. In this study 14 CN subjects viewed a Landolt C of varying orientation and size. They indicated its orientation via a button array whilst eye movements were recorded. Foveation was uncorrelated with optotype size. These results suggest that CN is not exacerbated by visual demand per se rather the need to do something visually demanding of importance to the individual.

Adolescent↗

PubMed2003

Latency and accuracy of saccades to somatosensory targets.

PURPOSE: To examine the accuracy and latency of reflexive saccades to vibratory stimulation of the fingertips made by normal human subjects and to compare the findings to those of visually guided saccades. METHODS: Eye movements were recorded using infrared oculography. Stimuli were presented via an array of audiometric bone vibrator transducers driven at 250 Hz and positioned at eye level in a darkened room. Target locations were at 0 and +/-5, 10, and 15 deg. Visual stimuli were green LEDs. Saccades were analysed interactively off-line and latency and amplitude measured for both types of saccade. RESULTS: Saccades to tactile stimuli had longer latencies and showed less accurate final eye positions than those to equivalent visual targets; they were unaffected by subject age. Error magnitude for the tactile saccades increased monotonically with increasing target eccentricity; because the fingers remained in a fixed position throughout the testing, this also meant that error was lowest for the thumb and increased with progression outwards towards the ring finger. Visually guided refixations were accurate and differed less than 0.2 deg across target locations. Subject age had no effect on performance. CONCLUSIONS: Human subjects may make relatively accurate refixations to tactile targets in the absence of visual cues, but only to certain locations/fingers; others were localised very poorly. Further studies are needed to determine whether finger selection or target location is the primary determinant of accuracy in this task.

Adult↗

PubMed2003

Effects of stimulus size and luminance on oscillopsia in congenital nystagmus.

Although the absence of oscillopsia is a common feature of congenital nystagmus (CN), it is occasionally noted by patients under poor viewing conditions and has been provoked in laboratory settings with stabilised images. In the present study, the effects of reductions in background stimulus size and luminance on perceptual stability in CN were examined. Sixteen CN subjects were first interviewed using a structured questionnaire about whether they ever experienced oscillopsia and, if so, under what circumstances and with what perceptions. They next fixated an LED centred in projected images of three sizes (21x14 degrees, 10x6 degrees and 7x4 degrees) and four luminance levels (115.5, 24.5, 2.7 and 0.1 cd/m2, with contrasts from 96 down to 20%). Eye movements were recorded with a limbal tracker. They were asked after viewing each image "whether anything happened to the image while they watched it." Occasional oscillopsia was reported by 12/16 of the CN subjects on the questionnaire. In the laboratory, 13/16 subjects experienced oscillopsia in some manner for at least one of the stimuli. 8/13 CN subjects experienced it for the dimmest and smallest slides. 11/13 perceived certain parts (either the LED or background) of the visual stimuli as moving, with the perception of LED movement most pronounced at low background luminance. Foveation did not differ when trials with and without reported oscillopsia were compared (independent samples t-test, p>0.05). Oscillopsia may occur in CN with normal viewing of bright fixation targets against dim backgrounds. Under these conditions, the oscillopsia may be spatially inhomogeneous. Luminance differences between the fixation point and surround may have caused transmission time differences as the image moved across the retina, therefore leading to the perception of motion in one portion of the scene and not the other.

Adolescent↗

PubMed2002

Ethanol effects on volitional versus reflexive saccades.

The effects of a moderate alcohol dose on reflexive and volitional saccades were compared. Healthy male subjects (n = 17) made horizontal reflexive saccades, antisaccades and self-paced saccades before and after the ingestion of a body-weight-related dose of alcohol. At a mean blood alcohol concentration of 0.044%, reflexive saccade latency significantly increased although accuracy remained unimpaired at each target amplitude. No significant ethanol effects were noted for antisaccade accuracy and latency. Antisaccade error rates decreased significantly, likely demonstrating a learning effect. The mean rate of self-paced saccades did not significantly change post-ethanol and their accuracy was also unimpaired. All saccades share a final common pathway for their execution. It thus seems reasonable to propose that ethanol modulated connections between the posterior parietal cortex and superior colliculus to increase reflexive saccade latency. Higher-order paradigms, which require inhibition and are mediated by frontal areas, might be impaired at greater blood alcohol concentrations.

Adolescent↗

PubMed2002

Effects of stimulus predictability and interstimulus gap on saccades in Alzheimer's disease.

OBJECTIVES: Studies of saccadic eye movement impairment in Alzheimer's disease (AD) have largely focused on simple reflexive eye movements and the antisaccade task. The effects of manipulating stimulus timing have been little studied. METHODS: Fourteen patients with mild to severe AD and 11 age-matched controls were studied on the antisaccade task, while latencies on simultaneous, gap and predictable tasks were recorded from 11 patients and 11 controls. Dementia severity was assessed with the Mini-Mental State Examination. RESULTS: As a group, patients' latencies were significantly higher and more variable on the simultaneous and gap tasks. Predictable task performance was similar in mean but significantly more variable. Grossly anticipatory responses by patients were common on the predictable, simultaneous and gap tasks. Exclusion of these from subject means revealed that AD patients, when making target-driven saccades, demonstrated a gap effect of similar magnitude to normal subjects. Patients made significantly fewer correct antisaccades and significantly more reflexive errors not followed by a corrective antisaccade than did controls. CONCLUSIONS: The frequent presence of grossly anticipatory saccades may reflect dysfunction of fixation mechanisms possibly involving projections from frontal lobe to superior colliculus. The less frequently seen, marked prolongation of latency may reflect changes in posterior parietal mechanisms mediating reflexive saccade generation. The presence of the gap effect demonstrates a continued ability to benefit from externally controlled stimulus disengagement. Patients' ability to make appropriately timed saccades to targets of known locations was particularly impaired, but the target sequence itself was at least grossly correctly learned. Larger studies may be able to identify clinically distinct populations of AD patients.

Aged↗