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PubMed2007

Travel distance estimation from visual motion by leaky path integration.

Visual motion can be a cue to travel distance when the motion signals are integrated. Distance estimates from visually simulated self-motion are imprecise, however. Previous work in our labs has given conflicting results on the imprecision: experiments by Frenz and Lappe had suggested a general underestimation of travel distance, while results from Redlick, Jenkin and Harris had shown an overestimation of travel distance. Here we describe a collaborative study that resolves the conflict by tracing it to differences in the tasks given to the subjects. With an identical set of subjects and identical visual motion simulation we show that underestimation of travel distance occurs when the task involves a judgment of distance from the starting position, and that overestimation of travel distance occurs when the task requires a judgment of the remaining distance to a particular target position. We present a leaky integrator model that explains both effects with a single mechanism. In this leaky integrator model we introduce the idea that, depending on the task, either the distance from start, or the distance to target is used as a state variable. The state variable is updated during the movement by integration over the space covered by the movement, rather than over time. In this model, travel distance mis-estimation occurs because the integration leaks and because the transformation of visual motion to travel distance involves a gain factor. Mis-estimates in both tasks can be explained with the same leak rate and gain in both conditions. Our results thus suggest that observers do not simply integrate traveled distance and then relate it to the task. Instead, the internally represented variable is either distance from the origin or distance to the goal, whichever is relevant.

Adult↗
PubMed1999

Influence of internal dynamics on accuracy of protein NMR structures: derivation of realistic model distance data from a long molecular dynamics trajectory.

In order to study the effect of internal dynamics on the accuracy of NMR structures in detail, we generated NOE distance data from a long molecular dynamics trajectory of BPTI. Cross-relaxation rates were calculated from the trajectory by analysis of the appropriate proton-proton vector autocorrelation functions. A criterion for the convergence of correlation functions was developed, and the analysis was restricted to those correlation functions that had converged within the simulation time. Effective distances were determined from the calculated cross-relaxation rates. Internal dynamics affected the derived distances in a realistic way, since they were subject both to radial averaging (which increases the cross-relaxation rate) and angular averaging (which decreases the cross-relaxation rate). The comparison of the effective distances with average distance between the protons during the trajectory showed that for most the effects of angular and distance averaging essentially cancel out. For these distances, the effective distance derived from an NOE is therefore a very good estimate of the average distance, or the distance in the average structure. However, for about 10% of the distances, the effective distance was more than 10% larger than the average distance, while for about 5%, it was more than 10% smaller, in some cases by more than 2 A. Little correlation is observed between the effects on cross-relaxation rates to different protons of the same residue. The results of this analysis have implications for the way structures are calculated from NOE distance data. For many distances, the assumption of a rigid structure is valid, and large error bounds would result in the loss of too much information content. On the other hand, the error bounds very often employed are not wide enough for some of the effects seen in our study.

Animals↗
PubMed1996

Mechanisms for distance reproduction in perceptual and motor tasks.

Two experiments were carried out: the control experiment and the doubling-distance experiment. In the control experiment subjects were presented with two visual stimuli whose distance was randomly varied. Subjects were required to reproduce the interstimulus remembered distance in two conditions. In one condition (reproduction by pointing) they pointed to a virtual position in space. In the other condition (visual reproduction) they matched the distance by using two other visual stimuli. In the doubling-distance experiment, distances between the two randomly presented stimuli were half of the distances used in the control experiment. Subjects were required to reproduce the double of the presented distance. As in the control experiment, reproduction was executed in two conditions: reproduction by pointing and visual reproduction. In both experiments variable and constant errors were measured. Pointing kinematics were also analysed. The results of the control experiment showed that subjects underestimated distance in reproduction by pointing, whereas they overestimated distance in visual reproduction. Variable errors increased with increasing distance, whereas they were not influenced by the type of reproduction. In the doubling-distance experiment, subjects generally overestimated distance by the same amount in both conditions. However, overestimation decreased with distance during reproduction by pointing. Pointing kinematics varied between the two experiments. The results of the control experiment confirm the hypothesis that perceptual judgement and visuo-motor transformation are two separate processes during which the same object attributes are independently analysed. However, the results of the doubling-distance experiment suggest that perceptual judgement and visuo-motor transformation use the same mechanisms when object attributes are deduced by mental elaboration.

Acceleration↗
PubMed2002

Comparison of the distances covered during 3 and 6 min walking test.

AIMS: To determine the reproducibility of the distance covered in 3 min and its correlation with the 6 min walking test, as well as compare the distances covered at different time intervals. Secondly, to evaluate the relationship between the distances covered during these time periods and the maximum oxygen intake obtained during a bicycle ergometer test. METHODS: Forty-five Chronic Obstructive Pulmonary Disesase patients were included in the study. Subjects who were either physically limited or familiar with the test, or those with acute exacerbation in the month prior to the study, were excluded. Three walking tests were carried out each day. In 30 patients for three consecutive days, and the distances covered in periods of 3 and 6 min were measured with 20 min rest between each walk. No incentive was given and the patients knew that the distances covered in 3 and 6 min would be quantified. Oxygen saturation, heart rate and degree of breathlessness (modified Borg scale) were registered at baseline. After 3 min, the distance covered and degree of breathlessness were also measured. After 6 min, oxygen saturation, heart rate, degree of breathlessness and distance covered in meters were registered. Spirometry was performed daily on each patient, and those with an FEV1 variation of less than 10% were considered clinically and functionally stable. An exercise test using bicycle ergometer was carried out to determine maximum oxygen intake. A 3 min walking test was performed in 15 patients, independently on the same day, which was followed after 20 min rest with a 6 min walking test. RESULTS: A significant increase was observed in the distance covered over 3 and 6 min in the first 5 walks, with the greatest increase seen in the first 3 walks. The correlation between the distance covered in 3 and 6 min was 0.98. The correlation between the distance covered in 3 min and oxygen intake was 0.64. No significant differences were observed between the distances covered in the 0-3 and 3 to 6 min periods. During the walking test, breathlessness was measured using the modified Borg scale, which was 1.8 after 3 min, and 3.2 after six min and 8.6 at the end of bicycle ergometer test. No significant differences were observed between the distance covered during the 3 minute test and the distance in the first 3 min of the 6 min walking test. CONCLUSIONS: A learning effect was observed when the walking test is carried out repeatedly over short time periods, with a significant increase in the first 5 walks. Correlation between the distances covered in 3 and 6 min is very good, and acceptable when the distance covered over these periods is compared with oxygen intake and walking speed is constant.

Aged↗
PubMed2006

Optimum view distance for laparoscopic surgery.

BACKGROUND: Proper visualization of the surgical field without fatigue is essential in laparoscopic surgery and reduces the risk of iatrogenic injuries. One of the important factors influencing visualization is the viewing distance between the surgeon and the monitor. This was the subject of the current investigation. METHODS: For this study, 14 surgeons participated in experiments designed to determine two working distances from a standard 34-cm (14 in. diagonal) cathode ray tube (CRT) monitor: (a) the maximum view distance permitting small prints of a near vision chart to be identified clearly by sight, (b) and the minimum view distance (of a standard resolution chart) just short of flicker, image degradation, or both. The range of the monitor optimal working distance for laparoscopic surgery was extrapolated from these data sets. RESULTS: The maximum view distance allowing identification of detail averaged 221 cm (range, 166-302 cm). The mean minimal view distance short of flicker/image degradation was 136 cm (range, 102-168 cm). The coefficient of variation for the two view distances was almost identical (18% vs 17%, respectively), and a frequency histogram confirmed the normality of the two data sets. Thus, for most surgeons, the extrapolated monitor view distances for laparoscopic surgery using a 14-in. diagonal (34-cm) monitor range from 139 to 303 cm (57-121 in.) for maximal distance viewing and from 90 to 182 cm (36-73 in.) for close-up viewing (i.e., a monitor optimal working distance ranging from 90 to 303 cm (36-121 in.). CONCLUSIONS: For most surgeons operating from a 14-in. diagonal CRT monitor, both the maximal and minimal (close-up) view distances are individually variable, but the surgeon should never be farther than 3 m (10 ft) or less than 0.9 m (3 ft) from the monitor. However, within limits, the maximal view distance increases with increasing monitor size. The limit for close-up distance is 0.9 m, irrespective of monitor size.

Clinical Competence↗
PubMed2004

Factors that motivate and deter rehabilitation educators from participating in distance education.

The major purpose of the study was to conduct exploratory research on the motivational levels of rehabilitation educators whose programs have Comprehensive Service Personnel Development (CSPD; Department of Education grant) grants targeted toward distance education. Additionally, the study attempted to identify whether significant factors existed that would inhibit faculty participation in distance education. There were three research questions to examine: (a) Do distance educators and non-distance educators differ significantly in intrinsic motivational factors? (b) Do distance educators and non-distance educators differ significantly in extrinsic motivational factors? and (c) Do distance educators and non-distance educators differ significantly in inhibiting factors? The results showed that rehabilitation faculty with CSPD grants who are distance educators are more extrinsically motivated (such as increase in salary, monetary support for participation, job security, working conditions, technical support, and requirement by department) than non-distance educators. There were no significant differences in levels between distance educators and non-distance educators that are intrinsically motivated (scholarly pursuit, personal research tool, and job satisfaction). There was no significant difference between distance educators and non-distance educators in inhibiting factors.

Adult↗
PubMed2006

Assessment of a new Distance Randot stereoacuity test.

PURPOSE: We sought to develop a new "Distance Randot" test, establish a normative data set for children and adults, and compare with established measures of stereoacuity. METHODS: Distance Randot, distance Frisby-Davis 2 (FD2), and near Preschool Randot stereoacuity (Stereo Optical Co., Inc., Chicago, IL) were assessed in 23 normal children (ages 4-14 years), 21 normal adults (ages 20-36 years), and 131 patients with a variety of strabismic conditions (ages 4-85 years). For each test, stereoacuity was defined as the smallest disparity in which 2 targets were correctly identified. The simultaneous prism and cover test (SPCT) and the alternate prism and cover test (APCT) were used to assess misalignment. RESULTS: For the new Distance Randot test, normative results were similar to published data obtained with established near and distance stereoacuity tests. In the patient cohort, comparing stereoacuity data obtained from Distance Randot stereoacuity test and the Near Preschool Randot stereoacuity test, most of the patients with discordant scores had poorer distance than near stereoacuity. Comparing the Distance Randot stereoacuity test and the Distance FD2, all of the patients with discordant scores had poorer Distance Randot than FD2 scores. CONCLUSIONS: Distance Randot test is more likely to detect abnormalities in distance stereopsis and may provide a useful tool in measuring distance stereoacuity in patients with and without strabismus. However, further studies are needed to define the efficacy of the Distance Randot test in monitoring progression specific conditions such as intermittent exotropia, where it may prove useful as a guide to the timing of intervention.

Adolescent↗
PubMed2002

Bioenergetic constraints on tactical decision making in middle distance running.

BACKGROUND: The highest velocity that a runner can sustain during middle distance races is defined by the intersection of the runner's individual velocity-time curve and the distance-time curve. The velocity-time curve is presumably fixed at the onset of a race; however, whereas the race distance is ostensibly fixed, the actual distance-time curve is not. That is, it is possible for a runner to run further than the race distance if he or she runs wide on bends in track races. In this instance, the point of intersection of the individual velocity-time curve and the distance-time curve will move downwards and to the right, reducing the best average velocity that can be sustained for the distance. METHODS: To illustrate this point, the race tactics used by the gold and silver medallists at 800 m and 5000 m in the Sydney Olympics were analysed. The paths taken by the runners were carefully tracked and the total distance they covered during the races and the average velocity they sustained over the distances they actually covered were calculated. RESULTS: In both the Olympic 800 m and 5000 m finals, for example, the winner was not the runner who ran at the highest average velocity in the race. Rather, the winners of these races were able to husband their metabolic resources to better effect by running closer to the actual race distance. CONCLUSIONS: Race results in middle distance running events are dependent not just on the energetic potential of the runners at the start of the race and their strategy for pace allocation, but also on the effect of their tactical approach to positioning on the total distance covered in the race. Middle distance runners should be conscious of minimising the distance covered in races if they wish to optimise their performance.

Decision Making↗
PubMed2001

The effects of production and presentation level on the auditory distance perception of speech.

Although both perceived vocal effort and intensity are known to influence the perceived distance of speech, little is known about the processes listeners use to integrate these two parameters into a single estimate of talker distance. In this series of experiments, listeners judged the distances of prerecorded speech samples presented over headphones in a large open field. In the first experiment, virtual synthesis techniques were used to simulate speech signals produced by a live talker at distances ranging from 0.25 to 64 m. In the second experiment, listeners judged the apparent distances of speech stimuli produced over a 60-dB range of different vocal effort levels (production levels) and presented over a 34-dB range of different intensities (presentation levels). In the third experiment, the listeners judged the distances of time-reversed speech samples. The results indicate that production level and presentation level influence distance perception differently for each of three distinct categories of speech. When the stimulus was high-level voiced speech (produced above 66 dB SPL 1 m from the talker's mouth), the distance judgments doubled with each 8-dB increase in production level and each 12-dB decrease in presentation level. When the stimulus was low-level voiced speech (produced at or below 66 dB SPL at 1 m), the distance judgments doubled with each 15-dB increase in production level but were relatively insensitive to changes in presentation level at all but the highest intensity levels tested. When the stimulus was whispered speech, the distance judgments were unaffected by changes in production level and only decreased with increasing presentation level when the intensity of the stimulus exceeded 66 dB SPL. The distance judgments obtained in these experiments were consistent across a range of different talkers, listeners, and utterances, suggesting that voice-based distance cueing could provide a robust way to control the apparent distances of speech sounds in virtual audio displays.

Adult↗
PubMed1999

Effects of viewing distance on the responses of horizontal canal-related secondary vestibular neurons during angular head rotation.

Effects of viewing distance on the responses of horizontal canal-related secondary vestibular neurons during angular head rotation. The eye movements generated by the horizontal canal-related angular vestibuloocular reflex (AVOR) depend on the distance of the image from the head and the axis of head rotation. The effects of viewing distance on the responses of 105 horizontal canal-related central vestibular neurons were examined in two squirrel monkeys that were trained to fixate small, earth-stationary targets at different distances (10 and 150 cm) from their eyes. The majority of these cells (77/105) were identified as secondary vestibular neurons by synaptic activation following electrical stimulation of the vestibular nerve. All of the viewing distance-sensitive units were also sensitive to eye movements in the absence of head movements. Some classes of eye movement-related vestibular units were more sensitive to viewing distance than others. For example, the average increase in rotational gain (discharge rate/head velocity) of position-vestibular-pause units was 20%, whereas the gain increase of eye-head-velocity units was 44%. The concomitant change in gain of the AVOR was 11%. Near viewing responses of units phase lagged the responses they generated during far target viewing by 6-25 degrees. A similar phase lag was not observed in either the near AVOR eye movements or in the firing behavior of burst-position units in the vestibular nuclei whose firing behavior was only related to eye movements. The viewing distance-related increase in the evoked eye movements and in the rotational gain of all unit classes declined progressively as stimulus frequency increased from 0.7 to 4.0 Hz. When monkeys canceled their VOR by fixating head-stationary targets, the responses recorded during near and far target viewing were comparable. However, the viewing distance-related response changes exhibited by central units were not directly attributable to the eye movement signals they generated. Subtraction of static eye position signals reduced, but did not abolish viewing distance gain changes in most units. Smooth pursuit eye velocity sensitivity and viewing distance sensitivity were not well correlated. We conclude that the central premotor pathways that mediate the AVOR also mediate viewing distance-related changes in the reflex. Because irregular vestibular nerve afferents are necessary for viewing distance-related gain changes in the AVOR, we suggest that a central estimate of viewing distance is used to parametrically modify vestibular afferent inputs to secondary vestibuloocular reflex pathways.

Animals↗
PubMed1988

Effects of English admixture and geographic distance on anthropometric variation and genetic structure in 19th-century Ireland.

The analysis of anthropometric data often allows investigation of patterns of genetic structure in historical populations. This paper focuses on interpopulational anthropometric variation in seven populations in Ireland using data collected in the 1890s. The seven populations were located within a 120-km range along the west coast of Ireland and include islands and mainland isolates. Two of the populations (the Aran Islands and Inishbofin) have a known history of English admixture in earlier centuries. Ten anthropometric measures (head length, breadth, and height; nose length and breadth; bizygomatic and bigonial breadth; stature; hand length; and forearm length) on 259 adult Irish males were analyzed following age adjustment. Discriminant and canonical variates analysis were used to determine the degree and pattern of among-group variation. Mahalanobis' distance measure, D2, was computed between each pair of populations and compared to distance measures based on geographic distance and English admixture (a binary measure indicating whether either of a pair of populations had historical indications of admixture). In addition, surname frequencies were used to construct distance measures based on random isonymy. Correlations were computed between distance measures, and their probabilities were derived using the Mantel matrix permutation method. English admixture has the greatest effect on anthropometric variation among these populations, followed by geographic distance. The correlation between anthropometric distance and geographic distance is not significant (r = -0.081, P = .590), but the correlation of admixture and anthropometric distance is significant (r = 0.829, P = .047). When the two admixed populations are removed from the analysis the correlation between geographic and anthropometric distance becomes significant (r = 0.718, P = .025). Isonymy distance shows a significant correlation with geographic distance (r = 0.425, P = .046) but not with admixture distance (r = -0.052, P = .524). The fact that anthropometrics show past patterns of gene flow and surnames do not reflects the greater impact of stochastic processes on surnames, along with the continued extinction of surnames. This study shows that 1) anthropometrics can be extremely useful in assessing population structure and history, 2) differential gene flow into populations can have a major impact on local genetic structure, and 3) microevolutionary processes can have different effects on biological characters and surnames.

Adult↗
PubMed1999

Coordination of spinal locomotor activity in the lamprey: long-distance coupling of spinal oscillators.

The extent and strength of long-distance coupling between locomotor networks in the rostral and caudal spinal cord of larval lamprey were examined with in vitro brain/spinal cord preparations, in which spinal locomotor activity was initiated by chemical microstimulation in the brain, as well as with computer modeling. When locomotor activity and short-distance coupling were blocked in the middle spinal cord for at least 40 segments, burst activity in the rostral and caudal spinal cord was still coupled 1:1, indicating that long-distance coupling is extensive. However, in the absence of short-distance coupling, intersegmental phase lags were not constant but decreased significantly with increasing cycle times, suggesting that long-distance coupling maintains a relatively constant delay rather than a constant phase lag between rostral and caudal bursts. In addition, under these conditions, intersegmental phase lags, measured between rostral and caudal burst activity, were significantly less than normal, and the decrease was greater for longer distances between rostral and caudal locomotor networks. The above result could be mimicked by a computer model consisting of pairs of oscillators in the rostral, middle, and caudal spinal cord that were connected by short- and long-distance coupling. With short-distance coupling blocked in the middle spinal cord, strychnine was applied to either the rostral or caudal spinal cord to convert the pattern locally from right-left alternation to synchronous burst activity. Synchronous burst activity in the rostral spinal cord resulted in a reduction in right-left phase values for burst activity in the caudal cord. These results also could be mimicked by the computer model. Strychnine-induced synchronous burst activity in the caudal spinal cord did not appear to alter the right-left phase values of rostral burst activity. Taken together, the experimental and modeling results suggest that the descending and ascending components of long-distance coupling, although producing qualitatively different effects, are comparatively weak. In particular, the descending component of long-distance coupling appears to become progressively weaker with increasing distance between two given regions of spinal cord. Therefore, short-distance coupling probably contributes substantially to normal rostrocaudal phase lags for locomotor activity along the spinal cord. However, short-distance coupling may be more extensive than "nearest neighbor coupling."

Animals↗
PubMed2006

Visual and refractive status at different focal distances after implantation of the ReSTOR multifocal intraocular lens.

PURPOSE: To evaluate the visual acuity and determine the refraction in emmetropic pseudophakic eyes at different focal distances after implantation of the AcrySof SA 60D3 ReSTOR multifocal intraocular lens (IOL) (Alcon Laboratories). SETTING: Private clinical practice. METHODS: Thirty-seven eyes of 20 patients received the ReSTOR IOL. Visual acuity was measured at several distances, and best distance corrected, best intermediate corrected (60 cm), and best near corrected (33 cm) visual acuities were determined 3 months after surgery. Subjective outcomes were assessed by a questionnaire. RESULTS: Postoperatively, 36 of 37 eyes (97.3%) were within +/-1.00 diopter (D) of the manifest refraction spherical equivalent and cylinder. Monocularly, the mean best distance corrected, best distance corrected intermediate, and best distance corrected near visual acuities were 20/20, 20/46, and 20/23, respectively. The best distance corrected acuities at 40, 50, 60, and 70 cm were significantly worse than the best distance corrected and best distance corrected near acuities (P<.05). On the questionnaire, all patients noted intermediate blur, but 75% had little or no difficulty seeing and were bothered by the blur occasionally or never. The mean additional add to best distance correction for best intermediate corrected visual acuity was +1.20 diopters (D) +/- 0.27 D or -1.68 +/- 0.24 D and for best near corrected visual acuity, -0.26 +/- 0.24 D. CONCLUSIONS: Implantation of the ReSTOR IOL offered excellent visual acuity at distance and near distance and functional visual acuity in the intermediate range. For patients who need excellent vision at intermediate range, leaving 1 eye with distance myopia or hyperopia to compensate for intermediate vision may provide consistent good binocular vision over the full range in cases of bilateral implantation.

Acrylic Resins↗
PubMedDate not supplied

Distance alternate-letter suppression test for objective assessment of sensorial status in intermittent exotropia.

PURPOSE: To investigate the value of distance alternate-letter suppression testing in the objective assessment of fusional control of intermittent exotropia. METHODS: A total of 71 consecutive patients with intermittent exotropia (study group) and 112 normal subjects (control group) underwent a series of measurements including distance alternate-letter suppression testing, Worth 4 Dot test at distance and near, and distance and near stereo acuity tests to see whether sensorial behavior differed in the study and control groups. The distance alternate-letter suppression test and distance stereo acuity tests were performed using the Mentor B-VAT II-SG Video Acuity Tester and Binocular Vision Testing System. The TNO test was used to measure near stereo acuity. RESULTS: Seventeen patients with intermittent exotropia (24%) presented suppression with the distance alternate-letter suppression test, but none of the normal subjects (p<0.001, chi2 test). Only, three patients (4%) showed suppression on the Worth 4 Dot test at distance, and all had fusion on the Worth 4 Dot test near. Patients with intermittent exotropia had significantly diminished distance stereo acuity compared to normal subjects (p<0.001). Normal subjects and patients had good near stereo acuity (p>0.05). Patients with intermittent exotropia who had fusion with the distance alternate-letter suppression test had significantly better distance stereo acuity than patients who had suppression (p<0.001). CONCLUSIONS: Distance alternate-letter suppression testing and distance stereo acuity tests may be useful as objective measures for assessing sensory loss in patients with intermittent exotropia.

Adolescent↗
PubMed2001

Distance learning in academic health education.

Distance learning is an apparent alternative to traditional methods in education of health care professionals. Non-interactive distance learning, interactive courses and virtual learning environments exist as three different generations in distance learning, each with unique methodologies, strengths and potential. Different methodologies have been recommended for distance learning, varying from a didactic approach to a problem-based learning procedure. Accreditation, teamwork and personal contact between the tutors and the students during a course provided by distance learning are recommended as motivating factors in order to enhance the effectiveness of the learning. Numerous assessment methods for distance learning courses have been proposed. However, few studies report adequate tests for the effectiveness of the distance-learning environment. Available information indicates that distance learning may significantly decrease the cost of academic health education at all levels. Furthermore, such courses can provide education to students and professionals not accessible by traditional methods. Distance learning applications still lack the support of a solid theoretical framework and are only evaluated to a limited extent. Cases reported so far tend to present enthusiastic results, while more carefully-controlled studies suggest a cautious attitude towards distance learning. There is a vital need for research evidence to identify the factors of importance and variables involved in distance learning. The effectiveness of distance learning courses, especially in relation to traditional teaching methods, must therefore be further investigated.

Accreditation↗
PubMed2005

Dissociation between visual perception of allocentric distance and visually directed walking of its extent.

Walking without vision to previously viewed targets was compared with visual perception of allocentric distance in two experiments. Experimental evidence had shown that physically equal distances in a sagittal plane on the ground were perceptually underestimated as compared with those in a frontoparallel plane, even under full-cue conditions. In spite of this perceptual anisotropy of space, Loomis et al (1992 Journal of Experimental Psychology. Human Perception and Performance 18 906-921) found that subjects could match both types of distances in a blind-walking task. In experiment 1 of the present study, subjects were required to reproduce the extent of allocentric distance between two targets by either walking towards the targets, or by walking in a direction incompatible with the locations of the targets. The latter condition required subjects to derive an accurate allocentric distance from information based on the perceived locations of the two targets. The walked distance in the two conditions was almost identical whether the two targets were presented in depth (depth-presentation condition) or in the frontoparallel plane (width-presentation condition). The results of a perceptual-matching task showed that the depth distances had to be much greater than the width distances in order to be judged to be equal in length (depth compression). In experiment 2, subjects were required to reproduce the extent of allocentric distance from the viewing point by blindly walking in a direction other than toward the targets. The walked distance in the depth-presentation condition was shorter than that in the width-presentation condition. This anisotropy in motor responses, however, was mainly caused by apparent overestimation of length oriented in width, not by depth compression. In addition, the walked distances were much better scaled than those in experiment 1. These results suggest that the perceptual and motor systems share a common representation of the location of targets, whereas a dissociation in allocentric distance exists between the two systems in full-cue conditions.

Adult↗
PubMed1998

Human horizontal vestibulo-ocular reflex initiation: effects of acceleration, target distance, and unilateral deafferentation.

The vestibulo-ocular reflex (VOR) generates compensatory eye movements in response to angular and linear acceleration sensed by semicircular canals and otoliths respectively. Gaze stabilization demands that responses to linear acceleration be adjusted for viewing distance. This study in humans determined the transient dynamics of VOR initiation during angular and linear acceleration, modification of the VOR by viewing distance, and the effect of unilateral deafferentation. Combinations of unpredictable transient angular and linear head rotation were created by whole body yaw rotation about eccentric axes: 10 cm anterior to eyes, centered between eyes, centered between otoliths, and 20 cm posterior to eyes. Subjects viewed a target 500, 30, or 15 cm away that was extinguished immediately before rotation. There were four stimulus intensities up to a maximum peak acceleration of 2,800 degrees/s2. The normal initial VOR response began 7-10 ms after onset of head rotation. Response gain (eye velocity/head velocity) for near as compared with distant targets was increased as early as 1-11 ms after onset of eye movement; this initial effect was independent of linear acceleration. An otolith mediated effect modified VOR gain depending on both linear acceleration and target distance beginning 25-90 ms after onset of head rotation. For rotational axes anterior to the otoliths, VOR gain for the nearest target was initially higher but later became less than that for the far target. There was no gain correction for the physical separation between the eyes and otoliths. With lower acceleration, there was a nonlinear reduction in the early gain increase with close targets although later otolith-mediated effects were not affected. In subjects with unilateral vestibular deafferentation, the initial VOR was quantitatively normal for rotation toward the intact side. When rotating toward the deafferented side, VOR gain remained less than half of normal for at least the initial 55 ms when head acceleration was highest and was not modulated by target distance. After this initial high acceleration period, gain increased to a degree depending on target distance and axis eccentricity. This behavior suggests that the commissural VOR pathways are not modulated by target distance. These results suggest that the VOR is initially driven by short latency ipsilateral target distance dependent and bilateral target-distance independent canal pathways. After 25 ms, otolith inputs contribute to the target distance dependent pathway. The otolith input later grows to eventually dominate the target distance mediated effect. When otolith input is unavailable the target distance mediated canal component persists. Modulation of canal mediated responses by target distance is a nonlinear effect, most evident for high head accelerations.

Acceleration↗
PubMed2006

Perceptual distance and the constancy of size and stereoscopic depth.

The relationship between distance and size perception is unclear because of conflicting results of tests investigating the size-distance invariance hypothesis (SDIH), according to which perceived size is proportional to perceived distance. We propose that response bias with regard to measures of perceived distance is at the root of the conflict. Rather than employ the usual method of magnitude estimation, the bias-free two-alternative forced choice (2AFC) method was used to determine the precision (1/sigma) of discriminating depth at different distances. The results led us to define perceptual distance as a bias free power function of physical distance, with an exponent of approximately 0.5. Similar measures involving size differences among stimuli of equal angular size yield the same power function of distance. In addition, size discrimination is noisier than depth discrimination, suggesting that distance information is processed prior to angular size. Size constancy implies that the perceived size is proportional to perceptual distance. Moreover, given a constant relative disparity, depth constancy implies that perceived depth is proportional to the square of perceptual distance. However, the function relating the uncertainties of depth and of size discrimination to distance is the same. Hence, depth and size constancy may be accounted for by the same underlying law.

Adult↗