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At least 1,027 records · Page 57Linked to original sources
PubMed1998

Terrain influences the accurate judgement of distance.

Mathematically, three-dimensional space can be represented differently by the cartesian, polar, and other coordinate systems. However, in physical sciences, the choice of representation system is restricted by the need to simplify a machine's computation while enhancing its efficiency. Does the brain, for the same reasons, 'select' the most cost-efficient way to represent the three-dimensional location of objects? As we frequently interact with objects on the common ground surface, it might be beneficial for the visual system to code an object's location using a ground-surface-based reference frame. More precisely, the brain could use a quasi-two-dimensional coordinate system (x(s), y(s)) with respect to the ground surface (s), rather than a strictly three-dimensional coordinate system (x, y, z), thus reducing coding redundancy and simplifying computations. Here we provide support for this view by studying human psychophysical performance in perceiving absolute distance and in visually directed action tasks. For example, when an object was seen on a continuous, homogeneous texture ground surface, the observer judged the distance to the object accurately. However, when similar surface information was unavailable, for example, when the object was seen across a gap in the ground, or across distinct texture regions, distance judgement was impaired.

Brain↗
PubMed2001

Ant odometry in the third dimension.

Desert ants (Cataglyphis) are renowned for their ability to perform large-scale foraging excursions and then return to the nest by path integration. They do so by integrating courses steered and the distances travelled into a continually updated home vector. Whereas the angular orientation is based on skylight cues, how the ants gauge the distances travelled has remained largely unclear. Furthermore, almost all studies on path integration in Cataglyphis, as well as in spiders, rodents, and humans, have aimed at understanding how the animals compute homebound courses in the horizontal plane. Here, we investigate for the first time how an animal's odometer operates when a path integration task has to be accomplished that includes a vertical component. We trained Cataglyphis ants within arrays of uphill and downhill channels, and later tested them on flat terrain, or vice versa. In all these cases, the ants indicated homing distances that corresponded not to the distances actually travelled but to the ground distances; that is, to the sum of the horizontal projections of the uphill and downhill segments of the ants' paths.

Africa, Northern↗
PubMed2001

Distance determined by the angular declination below the horizon.

A biological system is often more efficient when it takes advantage of the regularities in its environment. Like other terrestrial creatures, our spatial sense relies on the regularities associated with the ground surface. A simple, but important, ecological fact is that the field of view of the ground surface extends upwards from near (feet) to infinity (horizon). It forms the basis of a trigonometric relationship wherein the further an object on the ground is, the higher in the field of view it looks, with an object at infinity being seen at the horizon. Here, we provide support for the hypothesis that the visual system uses the angular declination below the horizon for distance judgement. Using a visually directed action task, we found that when the angular declination was increased by binocularly viewing through base-up prisms, the observer underestimated distance. After adapting to the same prisms, however, the observer overestimated distance on prism removal. Most significantly, we show that the distance overestimation as an after-effect of prism adaptation was due to a lowered perceived eye level, which reduced the object's angular declination below the horizon.

Accommodation, Ocular↗
PubMed1995

A negatively powered lens in the chameleon.

Chameleons are arboral lizards that spot their prey visually and catch it by highly precise shots with their long sticky tongue. They scan their environment by large-amplitude independent saccadic eye movements; once an insect is detected, the head axis is aligned towards the target ('head tracking', both eyes come forward to fixate the insect and, in a phase called 'initial protrusion', the sticky tongue is loaded with tension by a special hyoid apparatus and subsequently shot out of the mouth with great precision. Lenses placed in front of the eyes produce predictable errors in distance estimation, suggesting that chameleons rely on accommodation cues when measuring the distance to their prey, but focusing has never been measured directly. Using a new technique to measure accommodation, we now show that accommodation is precise enough to serve as the major distance cue. Because accurate focusing requires large retinal images, we have tested image magnification and find that it is higher than in any other vertebrate eye scaled to the same size. This is a result of a unique optical design: unlike other vertebrate eyes, the crystalline lens of the chameleon has negative refractive power. Although there is a trend among vertebrates to increase corneal power and to decrease lens power with higher visual acuity, only in the chameleon eye has this tendency led to a reversal of the sign of the power of the lens.

Accommodation, Ocular↗
PubMed1999

Impact of cataract surgery on visual acuity and subjective functional outcomes: a population-based study in Sweden.

PURPOSE: First, to determine the effects of cataract surgery on subjectively experienced visual function and visual acuity in a defined population, at a specific frequency of surgery. Secondly, to validate questionnaire data regarding the visual function of cataract patients. METHODS: A prospective population-based investigation of the subjective visual functional and visual acuity outcomes of cataract surgery over a 1 year time interval at one institution was conducted. All operated cases (n = 459) were grouped into three levels of visual impairment, according to the preoperative visual acuities of their better eyes. Subjective reading, TV watching, distance estimation and ability to orientate in unfamiliar surroundings, before and after surgery, were assessed using self-administered questionnaires. The subjective outcomes were related to the subjects' post-operative visual acuities. The statistical evaluations comprised analyses of variance, Yates'-corrected chi-squared tests, weighted kappa and correlation statistics. RESULTS: The pre-operative subjective visual disabilities of the patients were significantly correlated with the pre-operative visual acuities of the patients' better eyes. There was an improvement in subjective reading ability, distance estimation and ability to orientate in unfamiliar surroundings for most patients at all three pre-operative visual acuity levels. After surgery there was a stronger correlation between the subjective functional improvement and the increase in visual acuity for the operated eye than for the better eye. CONCLUSIONS: An incidence of cataract surgery of 3.3 per 1000 population for the year the present study was conducted seems not to be an over-utilisation of resources. Irrespective of the visual acuity level before cataract surgery, the vast majority of patients gain better subjective visual function and better acuity after surgery. It is possible to gain valid information from cataract surgery patients using a short questionnaire.

Activities of Daily Living↗
PubMed2004

Perceiving distance accurately by a directional process of integrating ground information.

By itself, the absolute distance of an object cannot be accurately judged beyond 2-3 m (refs 1-3). Yet, when it is viewed with reference to a flat terrain, humans accurately judge the absolute distance of the object up to 20 m, an ability that is important for various actions. Here we provide evidence that this is accomplished by integrating local patches of ground information into a global surface reference frame. We first show that restricting an observer's visual field of view to the local ground area around the target leads to distance underestimation, indicating that a relatively wide expanse of the ground surface is required for accurate distance judgement. Second, as proof of surface integration, we show that even with the restricted view, the observer can accurately judge absolute distance by scanning local patches of the ground surface, bit by bit, from near to far, but not in the reverse direction. This finding also reveals that the surface integration process uses the near-ground-surface information as a foundation for surface representation, and extrapolation to the far ground surface around the target for accurate absolute distance computation.

Darkness↗
PubMed1983

Magnitude estimation of oral cavity distances.

Adult subjects were asked to estimate relative distances between points marked on artificial palates fitted to their own palatal vaults. The artificial palates were less than 1 mm thick and extended posteriorly along the hard palate from the central incisors to approximately the second molars. Distances of 4, 8, 12, 16, and 20 mm were marked on four planes on the palates, one corresponding to the alveolar ridge, one anterior and two posterior to the ridge. The subjects felt raised points on the palates with their tongues and assigned number referenced to a standard distance on the alveolar plane. The eight adults performed the task very reliably, and the growth of scores corresponded closely to the increases in distance among the points judged. Accuracy did not vary with the particular plane, and performance was as accurate on the first trial as it was on the fifth. These subjects displayed very precise proprioceptive-kinesthetic knowledge of the oral cavity.

Adult↗
PubMed1995

Effects of distance on vocal intensity.

The vocal response of speakers to change of distance from a listener is in dispute. Warren (1968) found that speakers obeyed the inverse square law when compensating for distance changes; that is, they decreased their vocal intensity by 6 dB when distance was halved. However, speakers in a study of Johnson, Pick, Siegel, Cicciarelli, and Garber (1981) changed their vocal intensity by much less than 6 dB. This study was an attempt to reconcile the conflicting results and to gain better understanding of what people know implicitly about the effects of distance on intensity. Speakers in the present study significantly changed their vocal intensity to compensate for changes in distance, but by a maximum of 2.46 dB. Possible reasons for the different results are discussed.

Adult↗