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PubMed2004

Perceiving distance: a role of effort and intent.

Perceiving egocentric distance is not only a function of the optical variables to which it relates, but also a function of people's current physiological potential to perform intended actions. In a set of experiments, we showed that, as the effort associated with walking increases, perceived distance increases if the perceiver intends to walk the extent, but not if the perceiver intends to throw. Conversely, as the effort associated with throwing increases, perceived distance increases if people intend to throw to the target, but not if they intend to walk. Perceiving distance combines the geometry of the world with our behavior goals and the potential of our body to achieve these goals.

Adult↗
PubMed2005

Limits of visual acuity in the frontal field of the rock pigeon (Columba livia).

The eye of the rock pigeon is typical of a granivorous lateral-eyed bird, in that it has both a laterally projecting central fovea and a second high-density cellular area in peripheral retina (area dorsalis) which projects to the binocular frontal field below the beak. Such a dual system is faced with potentially different optical restraints arising from central and peripheral vision. We asked whether the frontal axis can support high resolution vision from a refractive resting position (predicted to be 25-33 cm; Fitzke et al, 1985 Journal of Physiology 369 33-44) to some near point of accommodation. We measured the visual acuity on the frontal axis in five pigeons using an operant discrimination of high-contrast square-wave gratings at a series of distances from 7 to 80 cm from the eye. The peak average acuity was 11.04 cycles deg(-1), which occurred 10 cm from the eye. The average of the maximum acuity of each bird at 10 cm was 12.8 +/- 1.1 cycles deg(-1), a value equal to the Nyquist frequency calculated from the peak ganglion cell density of the area dorsalis. However, this maximum acuity was restricted to a narrow depth in space, located around 10 cm from the eye, and at greater distances fell exponentially such that acuity was 50% of its maximum at 35 cm and less than 1 cycle deg(-1) at 100 cm. We propose that the range of high-acuity vision is limited in the frontal field by either increased refractive power and/or inaccuracy in frontal accommodation, and is optimized for a preferred far point located 10 cm from the eye.

Accommodation, Ocular↗
PubMed2004

Judgments of exocentric direction in large-scale space.

Judgments of exocentric direction are quite common, especially when judging where others are looking or pointing. To investigate these judgments in large-scale space, observers were shown two targets in a large open field and were asked to judge the exocentric direction specified by the targets. The targets ranged in egocentric distance from 5 to 20 m with target-to-target angular separations of 45 degrees, 90 degrees, and 135 degrees. Observers judged exocentric direction using two methods: (i) by judging which point on a distant fence appeared collinear with the two targets, and (ii) by orienting their body in a direction parallel with the perceived line segment. In the collinearity task, observers had to imagine the line connecting the targets and then extrapolate this imagined line out to the fence. Observers indicated the perceived point of collinearity on a handheld 360 degrees panoramic cylinder representing their vista. The two judgment methods gave similar results except for a constant bias associated with the body-pointing response. Aside from this bias, the results of these two methods agree with other existing research indicating an effect of relative egocentric distance to the targets on judgment error--line segments are perceived as being rotated in depth. Additionally, verbal estimates of egocentric and exocentric distance suggest that perceived distance is not the cause for the systematic errors in judging exocentric direction.

Adult↗
PubMed2005

Global versus local image expansion in estimating time-to-contact from complex optic flow.

Previous work (Harris and Giachritsis 2000, Vision Research 40 601-611) has shown that, when global and local image expansion are placed in conflict, estimates of time-to-contact (TTC) are based almost exclusively upon global expansion. Here we extend this finding by demonstrating that global image expansion continues to predominate even under conditions that seem more favourable to a local analysis. We added a global rotation to the stimulus so that the global pattern of expansion was distorted while leaving the local expansion unaffected. Even under relatively high rotation rates (30 degrees s(-1)), local expansion continued to have little systematic effect upon estimates of TTC.

Distance Perception↗
PubMed2005

The roles of inducer size and distance in the Ebbinghaus illusion (Titchener circles).

Although the Ebbinghaus illusion is commonly used as an example of a simple size-contrast effect, previous studies have emphasised its complexity by identifying many factors that potentially influence the magnitude of the illusion. Here, in a series of three experiments, we attempt to simplify this complexity. In each trial, subjects saw a display comprising, on one side, a target stimulus surrounded by inducers and, on the other, an isolated probe stimulus. Their task was to indicate whether the probe appeared larger or smaller than the target. Probe size was adjusted with a one-up, one-down staircase procedure to find the point of subjective equality between probe and target. From these experiments, we argue that the apparent effects of inducer size are often confounded by the relative completeness of the inducing surround and that factors such as the similarity of the inducers and target are secondary. We suggest a simple model that can explain most of the data in terms of just two primary and independent factors: the relative size of the inducers and target, and the distance between the inducers and the target. The balance between these two factors determines whether the size of the target is underestimated or overestimated.

Analysis of Variance↗
PubMed2006

The role of part structure in the perceptual localization of a shape.

The process of object localization may be accomplished with respect to a particular reference location, such as the center of gravity, COG (eg Vishwanath and Kowler, 2003 Vision Research 43 1637-1653). Here, we investigated how part structure affects an object's reference location. The reference location was evaluated with a measure of the illusory displacement of an internal target element embedded within a larger object (Morgan et al, 1990 Vision Research 30 1793-1810). To examine whether the reference location is different for shapes with part structure, two shapes were tested: circle (small and large; no part structure) and bell (shape with two parts, one larger than the other). Results were examined with respect to two predictions: either the location of an object is based on its shape as a whole, disregarding part structure (ie a single, overall COG), or the parts are processed separately (different COGs). With the circles, the results showed a systematic illusory displacement of the internal target toward the COG. With the bell, the illusion was significantly weaker than with both circles--even though the main part of the bell had the same size as the small circle, and its horizontal axis had the same extent as the large circle. Moreover, the distance judgments for the bell were consistent with a (weaker) reference point being located at the COG of the larger part, rather than at the COG of the entire bell. These results show that the part structure of a shape plays a role in the representation of its location, and that for complex shapes the perceived location of an embedded element depends more on the parts within which it is embedded, rather than on the whole shape.

Distance Perception↗
PubMed1989

Two motion perception mechanisms revealed through distance-driven reversal of apparent motion.

We demonstrate two kinds of visual stimuli that exhibit motion in one direction when viewed from near and in the opposite direction from afar. These striking reversals occur because each kind of stimulus is constructed to simultaneously activate two different mechanisms: a short-range mechanism that computes motion from space-time correspondences in stimulus luminance and a long-range mechanism in which motion computations are performed, instead, on stimulus contrast that has been full-wave rectified (e.g., on the absolute value of contrast).

Journal Article↗
PubMed2003

Cortical modulation of whole body movements in brain-damaged patients.

The aim of the present study was to assess whether the consistent bias in reproducing distances and lengths on visual tasks that characterizes hemispatial neglect is also present when whole body displacements have to be calculated and reproduced. Two different experiments were proposed to participants with right brain lesions and neglect syndrome (RN+), right (RN-) and left brain lesions (LN-) without neglect and to participants without brain damage (C). In Experiment 1, participants actively reproduced passive linear displacements in the same or in a different direction. This task could be performed using only vestibular and somatosensory input, since no relevant visual input was available. In Experiment 2, relevant visual information had to be integrated with vestibular and somatosensory information in order to make the active reproduction. In Experiment 1, all brain-damaged groups reproduced the horizontal displacements similarly to the control group and without any spatial asymmetry. In Experiment 2, when vestibular, somatosensory and visually remembered information was required to produce an integrated mental representation, RN+ processed contralesional displacements differently from ipsilesional ones. Rectilinear displacements of the whole body in space were not affected by focal left and right brain lesions, suggesting that the computation of nonvisual information can be accomplished by brain structures different from those involved in spatial visual processes. However, when body displacements in space required a mental representation based on visual and nonvisual sensory information, a significant asymmetry appeared only in patients with hemispatial neglect. Some attempts are made to identify the neural substrates involved in this integration.

Aged↗
PubMed2005

Dynamic visual acuity using "far" and "near" targets.

CONCLUSIONS: DVA may be useful for assessing the functional consequences of an impaired gaze stabilization mechanism or for testing the effectiveness of a rehabilitation paradigm. Because target distance influences the relative contributions of canal and otolith inputs, the ability to measure DVA at near and far viewing distances may also lead to tests that will independently assess canal and otolith function. OBJECTIVE: To present and test a methodology that uses dynamic visual acuity (DVA) to assess the efficacy of compensatory gaze mechanisms during a functionally relevant activity that differentially measures canal and otolith function. MATERIAL AND METHODS: The effect of treadmill walking at a velocity of 1.79 m/s on subjects' visual acuity was assessed at each of two viewing distances. A custom-written threshold determination program was used to display Landolt C optotypes on a laptop computer screen during a "far" (4 m) target condition and on a micro-display for a "near" (50 cm) target condition. The walking acuity scores for each target distance were normalized by subtracting a corresponding acuity measure obtained while standing still on the treadmill belt. RESULTS: As predicted by subjective reports of relative target motion, the decrease in visual acuity was significantly greater (p < 0.00001) for the near compared to the far condition.

Adult↗
PubMed1995

Preferred viewing distances for handheld and structurally fixed displays.

This research was conducted to define viewing distance characteristics of individuals engaged in ordinary reading tasks. Specific attention was directed to assessing the potential relationship between observed distance and individual resting point accommodation. Consistent, statistically significant differences were observed across variable handheld and structurally-fixed hardcopy display configuration conditions. Relationships between observed viewing distance and resting point accommodation were not apparent. These findings suggest that simple eye-to-display viewing distance is fundamentally different for handheld text presentations and for the same text presented in a configuration similar to that of a video display terminal.

Accommodation, Ocular↗
PubMed1996

Relations between individual differences in oculomotor resting states and visual inspection performance.

Individual differences in the oculomotor resting states (dark vergence and dark focus) have previously been linked to subjective and visual consequences of near visual work. The present experiment investigated whether these resting states are related also to performance on a near visual inspection task. Dark vergence and dark focus were measured in 38 students before and after they spent 40 min searching for a target letter among distractor letters on a video display terminal at a distance of 20 cm. Subjects with relatively near dark vergence positions performed the inspection task significantly more quickly than subjects with relatively far dark vergence positions. Also, subjects who showed a relatively large inward shift in dark vergence tended to perform quickly. Inspection performance was not related to individual differences in dark focus. These results extend existing oculomotor theory and suggest that the performance of visual inspectors is maximized when the mismatch between the task distance and their dark vergence posture is minimized.

Accommodation, Ocular↗
PubMed2002

Correlation between hand preference and distance of focusing points of two eyes in the horizontal plane.

Relationships among hand preference, ocular dominance, and the degree of ocular shifting were studied in 78 right-handed and 16 left-handed subjects. Ocular dominance was assessed with the Miles test. The shifting degree of eye was assessed using a modified Miles test. The shifting distance of the right-eye was marginally greater, although significant, in the left-handers as compared to the right-handers. The shifting distance of the left-eye was greater in the right-handers than in the left-handers. The distance of focusing points of two eyes in the horizontal plane was greater in the right-handers than in the left-handers. In the total sample, there was a significant negative Pearson correlation between hand-preference and the shifting distance of the right-eye, and there was a positive correlation between hand-preference and the shifting distance of the left-eye. In addition, there was a significant positive correlation between hand preference and the distance of focusing points of two eyes. These results suggest that hand preference may be related to the degree of ocular asymmetry.

Adolescent↗