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At least 1,171 records · Page 65Linked to original sources
PubMed2000

Neural consequences of acting in near versus far space: a physiological basis for clinical dissociations.

We used PET to determine which brain regions are implicated when normal volunteers bisect horizontal lines and point to dots in near (peripersonal) or far (extrapersonal) space. Studies of line bisection in patients with right hemisphere lesions have shown that bisection performance can be severely impaired in either near or far space while remaining within normal limits in the other spatial domain. Likewise, clinical dissociations between pointing to objects in near and far space have been reported. The normal functional anatomy of these dissociations has not been demonstrated convincingly. Regional cerebral blood flow measurements using PET were carried out in 12 healthy right-handed male volunteers who bisected lines or pointed to dots in near or far space, using a laser pen. Subjects performing either task in near space showed neural activity in the left dorsal occipital cortex, left intraparietal cortex, left ventral premotor cortex and left thalamus. In far space, subjects performing either task showed activation of the ventral occipital cortex bilaterally and the right medial temporal cortex. These data provide physiological support for the clinically observed dissociations demonstrating that attending to and acting in near space differentially employs dorsal visuomotor processing areas, whereas attending to and acting in far space differentially draws on ventral visuoperceptual processing areas, even when the motor components of the tasks are identical when performed in the two spaces.

Adult↗
PubMed2005

Causes of cross-over in unilateral neglect: between-group comparisons, within-patient dissociations and eye movements.

Patients with left unilateral neglect bisect long horizontal lines to the right of the true centre. However, when given short lines, many of the same patients mark the midpoint to the left of the true centre, towards the otherwise neglected space. This paradoxical phenomenon has been termed 'cross-over' and is difficult to explain based on current accounts of the neglect syndrome. To explore the causes of cross-over, in a first study we evaluated bisection of 20, 100 and 200 mm horizontal lines in groups of unilateral brain-damaged patients with neglect and hemianopia, with neglect and no hemianopia, with hemianopia and no neglect and without neglect or hemianopia. Cross-over of 20 mm lines was found only in neglect patients with hemianopia. To ascertain further the influence of visual field defects on cross-over, in a second study we compared the performance of two right-brain-damaged patients with contralesional neglect and inferior quadrantanopia with that of a patient with inferior quadrantanopia and no neglect. Patients bisected lines oriented so as to cross or uncross the blind quadrant of the visual field. When short 20 mm lines crossed the blind quadrant, neglect patients showed cross-over; when the same lines crossed the seeing quadrants cross-over was absent. These findings were confirmed by the examination of a neglect patient with sparing of the central 5 degrees of the contralesional left visual hemifield in the right eye and no sparing in the left eye. In monocular viewing, cross-over was present when 20 mm lines were bisected with the left eye and absent when bisected with the right eye. Recording of eye movements showed that at the moment of bisection left eye fixations shifted towards the contralesional line endpoint whereas right eye fixations remained anchored to the centre of the line. With long lines, both eyes deviated ipsilesionally. These results show that in neglect patients ipsilesional deviation in the bisection of long lines turns into apparently paradoxical contralesional bisection of short ones only when these cross a retinotopically blind sector of the neglected space. Cross-over seems to depend on the small spatial effects produced by reflexive contralesional gaze shifts allowing eccentric fixations with the seeing hemifield. During the bisection of long lines, these effects are cancelled out by the strong attentional deviation induced by the marked extension of the ipsilesional line segment. This explanation establishes coherence between cross-over and current accounts of the neglect syndrome.

Adult↗
PubMed2002

Aging and the representation of spatial situations in narrative understanding.

Age differences in the construction of the situation model during text understanding were investigated. Situation model processing was measured in terms of the distance effect, the tendency for readers to process information about objects in a narrative more quickly when the objects are spatially closer to the protagonist than when they are farther away. To examine readers' ability to construct the model directly from the text, the distance effect was contrasted for objects that were either presented in a layout of the narrative setting prior to reading (learned objects) or introduced in the narrative itself (new objects). Both younger and older readers showed strong distance effects for learned objects. When objects had not been learned but were only introduced in the text, however, younger adults did not show a reliable distance effect. Older adults with high levels of comprehension did, lending support to the position that older readers differentially rely on the situation model for effective narrative understanding.

Adolescent↗
PubMed1980

The epidemiology of myopia.

Many people have reduced unaided vision because of myopia, a spherical error of refraction. The biological theory of myopia views myopia as the result of genetically determined characteristics of eye tissues, whereas the use-abuse theory views myopia as the result of habitual use of the eye at a near focal length, near-work. The use-abuse theory implies that myopia is preventable whereas the biological theory does not. Myopia varies over age, gender, race, ethnicity, level of education, social class and degree of urbanization. The explanation of the epidemiology of myopia in the use-abuse theory is that some types of people do more near-work than others. Using data from the Health Examination Survey of 12 to 17-year-olds conducted by the US Public Health Service from 1966--1970, this paper finds that the use-abuse theory can explain at least some of the variance of myopia and much of the socially patterned variance. This finding raises the possibility that at least some of the myopia extant in a population is preventable.

Adolescent↗
PubMed1999

Is neuronal encoding of subject-object distance dependent on learning?

Recordings were made in the intermediate and medial part of the hyperstriatum ventrale of behaving domestic chicks which had been imprinted (trained) by being exposed to a training stimulus. Neurons were tested for responsiveness to the training stimulus and to an alternative stimulus at each of three distances (d = 0.5 m, 1 m, 2 m) from the chick. For responses to the training stimulus 24/78 (31%) responsive neurons were d-sensitive, i.e. responses changed with distance. For responses to the alternative stimulus, a similar proportion of neurons was d-sensitive (16/57, 28%). Six d-invariant neurons responded similarly at each distance: four to the training and two to the alternative stimulus. Thus no effect of learning on d-sensitive or d-invariant neuronal responsiveness was found.

Animals↗
PubMed2002

Visuospatial updating of reaching targets in near and far space.

The brain constructs multiple representations of near and far space but it is unclear which spatial mechanism guides reaching across eye movements in near space. Retinocentric reaching representations are known to exist in parietal cortex, but must be updated during eye movements, in order to remain accurate. In contrast, non-retinal (e.g. muscle-centered) reaching plans in motor cortex do not require updating, and so may provide a more stable encoding mechanism. To test between these, we employed a behavioral test. Subjects briefly foveated a target (located at various depths in near and far space) looked peripherally, then reached toward its remembered location. Surprisingly, subjects did not use the stable non-retinal reaching plan (compared to controls without eye movements). Instead, the intervening eye movements induced a systematic pattern of reaching errors for targets at all depths consistent with updating in a retinal frame. We conclude that a common eye-centered updating mechanism prevails in programming arm movements in both near and far space.

Adult↗
PubMed1995

The temporal-distance effect in subjects with dementia of the Alzheimer type.

The efficacy of temporal-distance processing for spatial locations was evaluated in college students, elderly controls, and subjects with mild and moderate dementia of the Alzheimer type (DAT). The elderly controls showed a slightly lower level of performance than college students, but both elderly controls and college students displayed a temporal-distance effect. The mild DAT subjects performed the task accurately only at the greatest temporal distances. The moderate DAT subjects performed at chance levels for all distances. Because similar changes in the temporal-distance function have been described for humans and animals with small hippocampal or prefrontal cortex damage, there might be important hippocampal or prefrontal cortex involvement or both in early stages of Alzheimer disease.

Adult↗
PubMed2000

Measurement of daily walking distance-questionnaire versus pedometer.

PURPOSE: This study compared measurements of daily walking distance on the College Alumnus questionnaire (CAQ) and a pedometer. METHODS: A total of 96 men and women (25-70 yr of age) with a wide range of physical activity habits were studied. Physical activity index was computed from the College Alumnus questionnaire (PAI-CAQ) as the sum of the energy expended in stair climbing, walking, and sports and recreational physical activity. Data on walking distance were compared with values obtained from the Yamax electronic pedometer (DW-500B). Participants wore the pedometer for 7 consecutive days, except when sleeping, showering, or performing sports and recreational activities. RESULTS: Subjects underestimated their daily walking distance on the CAQ, compared with the pedometer (1.43 +/- 1.01 vs 4.17 +/- 1.61 km x d(-1)). The energy expended in walking was correspondingly lower on the CAQ, compared with the pedometer-derived values (555 +/- 405 versus 1608 +/- 640 kcal x wk(-1)). CONCLUSIONS: These findings suggest that electronic pedometers are useful for examining questions about walking distance on physical activity questionnaires.

Adult↗
PubMed1978

Accommodation dynamics I. Range nonlinearity.

We show the presence of a new nonlinearity in accommodation dynamics. Positive accommodation is slower when the eye is operating in the near range; conversely, relaxation of accommodation is apparently slower during work in the far range. Latency of accommodation is unaffected by working range. This new dynamic range nonlinearity has been defined in connection with a continuing research study of accommodation responses in normal subjects and in clinic patients. Instrumentation and analysis procedures are described.

Accommodation, Ocular↗
PubMed1978

Population study of fixation disparity, heterophoria, and vergence.

Data on phorias, vergences, and fixation disparities were gathered for distant and near viewing conditions from a nonclinical, young adult population. Normative values were computed from the data for analysis and comparison. Our study revealed slightly less exophoria at far and near than Morgan's study, together with large base-out vergence readings. We found proportionately more esophores with exo fixation disparity who require base-in prism to neutralize the fixation disparity than Ogle's studies found.

Adult↗
PubMed1980

Best retinal focus for various fication distances in phakic and aphakic eyes.

When an eye focuses on a distant target, it is often found that the long wavelength rays are focused on the receptors, but when the same eye looks at a target close to the eye, the green or blue rays are in focus in the retinal receptors. The authors investigated this phenomenon by means of a laser optometer to find out whether the shift in focus represented an attempt to reduce accommodative effort, or was used to aid distance judgement, or was simply a by-product of the control mechanisms. Six phakic individuals showed a shift of 50 nm over an accommodation range of 2.75 D. The single aphakic observer preferred a retinal focus within a very narrow wavelength range. The authors favor the view that the wavelength shift is an inability of the accommodative control mechanism to make a precise response to a given stimulus.

Accommodation, Ocular↗