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PubMed1984

Why the islands move.

Micronesian navigators routinely make voyages across large expanses of open ocean. To do this, a navigator must judge both the direction in which he is sailing and the distance he has travelled. The rising and setting points of the stars (and other cues) provide instantaneous information about direction, but distance can only be judged by integrating velocity-related information over time. Micronesian navigators judge distance in a way that seems odd. When they are out of sight of land, they imagine that the canoe is stationary and that the islands move back past them. For each voyage, they 'attend' to an island off to the side of the course which is out of sight over the horizon. As they sail, they imagine the island moving back along the horizon changing in bearing until it is imagined to be under the bearing it is known to have from the destination island. Then they know they are near their destination. There is good reason for using a frame of reference whose origin is defined by the boat. We show how it finesses a perceptual paradox--the rising and setting points of the stars do not exhibit motion parallax.

Cues↗
PubMed1985

Locomotor estimation of distance after visual scanning by children and adults.

Subjects (120 young adults and 120 children) were tested for their abilities to estimate visually the distance to a target 5 m away, then walk unaided by vision to that target as accurately as possible. Experimental groups were determined by visual scanning time (1, 5, or 10 s), delay between the end of visual scanning and the start of locomotion (0, 1, 3, or 5 s), and by age (9 year old children or young adult). Adults performed locomotor distance estimations based on visual scanning more accurately than children under all conditions. Increased scanning time resulted in more accurate performances by children but not by adults, and increased delays between the end of scanning and the start of locomotion caused decreases in accuracy for children only. These decrements were partially ameliorated by increased scanning time. The total time spent without vision after scanning the target (delay time plus walking time) was an important factor, with sharp increases in error for all delay conditions for children. The results are discussed in terms of trace decay effects and developmental aspects of visual guidance of locomotion.

Adult↗
PubMed1988

Accommodation and apparent distance.

The ability to judge egocentric distance was assessed in sixteen normal observers under conditions where monocular blur-driven accommodation provided the only source of information regarding changes in target dioptric distance. Accommodation and apparent distance were measured over an optical range of 2 to 6 D for each of three targets which differed in their effectiveness as accommodative stimuli (excellent, moderate, and poor). For the excellent and moderate accommodative targets, apparent distance decreased as accommodation increased, with both targets sharing a common linear function. Apparent distance, however, tended to exceed accommodative distance and was relatively compressed in scale. As expected, accommodative response and apparent distance diminished in range as target effectiveness was reduced. The poor accommodative target represented the limiting case wherein accommodation failed to deviate from the tonic level, and apparent distance remained constant. There were considerable intersubject differences in the ability to judge distances on the basis of accommodation. The results indicate that accommodation can indeed serve as a source of distance information, particularly for some individuals.

Accommodation, Ocular↗
PubMed1988

Visual information and skill level in time-to-collision estimation.

Previous studies on the visual origin of time-to-collision (Tc) information have demonstrated that Tc estimates can be based solely on the processing of target expansion rate (optic variable tau). But in the simulated situations used (film clips), there was little reliable information on speed (owing to reduced peripheral vision) and distance (owing to the absence of binocular distance cues) available. In order to determine whether these kinds of information are also taken into account, it is necessary to take an approach where the subject receives a more complete visual input. Thus, an experiment conducted on a circuit under actual driving conditions is reported. Experienced drivers and beginners, who were passengers in a car, had to indicate the moment they expected a collision with a stationary obstacle to take place. Subjects were blindfolded after a viewing time of 3 s. The conditions for speed evaluation (normal versus restricted visual field) and distance evaluation (binocular versus monocular vision) by subjects were varied. The approach speed (30 and 90 km h-1) and actual Tc (3 and 6 s) were also varied. The results show that accuracy of Tc estimation increased with (i) normal visual field, (ii) binocular vision, (iii) higher speeds, and (iv) driving experience. These findings have been interpreted as indicating that both speed and distance information are taken into account in Tc estimation. They suggest furthermore that these two kinds of information may be used differently depending on the skill level of the subject. The results are discussed in terms of the complementarity of the various potentially usable visual means of obtaining Tc information.

Acceleration↗
PubMed1988

Size constancy in children: a new interpretation.

Existing evidence indicates that there are differences between children and adults in size constancy when observation distances are large. Findings are reported which suggest that this phenomenon is based on a difference in the accessing of proximal stimulus information, which, in the case of size, refers to visual angle subtended. Age differences were found when a traditional size constancy task was used, but these differences disappeared when all the comparison objects subtended the same visual angle. Since this finding demonstrates that young children can make accurate size matches, it is suggested that the underconstancy previously reported is not necessarily the result of children's inability to use fully certain cues to distance. Rather, the findings suggest that children access proximal stimulus information more spontaneously than do adults.

Adolescent↗
PubMed1989

Inspection time and high-speed ball games.

It has been suggested that successful batsmen in cricket are not distinguished by their fast speed of visual information intake. A study is presented in which a season's batting averages for twenty regular cricketers, all members of the same local team, were correlated with the cricketers' visual inspection times. The correlation was -0.63 (p less than 0.005), suggesting that the successful batsmen were faster at picking up information from briefly presented visual displays. When the age factor was eliminated, the partial correlation between inspection time and batting average remained significant at -0.52 (p less than 0.01). This finding is discussed with respect to the sources of information available from a ball in flight.

Acceleration↗
PubMed1990

Visual localization and estimation of extent of target motion during ocular pursuit: a common mechanism?

The ability to localize a visual target and to estimate the distance through which it moves was studied during ocular pursuit. In the first experiment observers had to localize the position of a visually tracked moving target when they heard an acoustic signal. The signal was sounded near the beginning or near the end of the motion. The distance between the perceived positions was shorter than the distance between the corresponding physical positions of the target. The 'shortening' became more pronounced with higher tracking velocity. In another condition the observers estimated the length of the motion path between two successive sound signals, one presented near the beginning and one near the end of the motion. The length of path travelled was underestimated, the effect being stronger with higher tracking velocity. In the second experiment this effect of velocity on the underestimation of distance was shown to exist only during ocular pursuit and not during steady fixation. The hypothesis that localization and estimation of distance during ocular pursuit share a common mechanism is discussed.

Acceleration↗
PubMed1990

Low-frequency filtering and the processing of local-global stimuli.

The role of low-spatial-frequency information in the processing of global stimuli made up of local elements was examined. After selective removal of low spatial frequencies two major changes occurred in the pattern of results. First, response times to global stimuli were significantly slower and the usual speed advantage of global over local processing was lost. Second, when processing local features the usual decrease in response speed when the local and global letters are not the same (consistency effect) was not obtained. These effects could not be explained by changes in error rate, by contrast variation resulting from the process of filtering, or by loss of visual sensitivity due to greater eccentricity of global images.

Adult↗
PubMed1991

Predicting the future position of a moving target.

Three experiments were conducted to examine the processes underlying the prediction of the future position of a moving target. A target moved horizontally across a computer screen at constant velocity, disappearing partway across the screen, and subjects responded when they estimated the target would have passed a point on the far side of the screen, had it continued on its path. The first experiment demonstrated that visual tracking of the target is not necessary for successful position estimation. In the second experiment, the time over which the prediction was made rather than the interval for which the target was visible, the distance over which the prediction was made, or the velocity of the target, was found to affect performance. Finally, performance was not affected when markers were placed parallel to the trajectory of the target; the presence of gratings which masked portions of the target's path did not affect subjects' performance. The previous literature suggests that the spatial interval over which predictions are made is the important variable; we find that temporal factors are the major determinants of prediction.

Acceleration↗
PubMed1991

Thumb's rule tested: visual angle of thumb's width is about 2 deg.

Measures of the size of various hand parts and their viewing distance when held at arm's length were made on one hundred and eighteen undergraduate students. A simple rule of thumb can be confirmed: Visual angle of the width of the thumb held at arm's length is about 2 deg. The thumbnail subtends about 1.5 deg and the index fingernail about 1 deg in width, when both are held at arm's length. These figures are good approximations for males and females, although a significant, direct, linear relationship exists between hand-part size and the visual angle of the part at arm's length.

Adult↗
PubMed1991

A microgenetic study of the Müller-Lyer illusion.

In two experiments a decomposed Müller-Lyer pattern was used to measure the time course of the illusion. A partial report procedure was used to prevent the subjects from focusing only on parts of the pattern and to maximize visual processing. The Müller-Lyer figure was decomposed into two parts, its angles and its line. A configuration of three pairs of angles, each corresponding to a row in the usual partial report arrangement, was used. A line that did or did not fit the gap was shown with a variable delay (interstimulus interval, ISI). By this procedure the relevant row (line gap) was cued. The subject had to decide whether the line fitted or was too short/long. Two exposure times for the angles were used, 50 or 200 ms in one experiment and 50 or 500 ms in the other. The result of the first experiment, with outward-pointing fins, was a stable illusion for all values of ISI (25-400 ms) and all exposure times, with one significant exception: exposure for 50 ms with an ISI of 50 ms yielded an illusion peak. It was shown that this was not caused by a reduction in length-discrimination performance. In the second experiment, with inward-pointing fins, no such peak occurred. There was only a tendency for the illusion to vanish with zero ISI. The results are discussed with respect to 'global to local' theories of visual processing.

Distance Perception↗
PubMed1992

Visual judgements and misjudgements in cricket, and the art of flight.

To hit the ball with the centre of percussion of a bat so that the ball goes where he intends it to go, a batsman must estimate visually where the ball will be at a specific future time (when), and coordinate his swing accordingly. A number of visual cues are available to the batsman. Retinal image information provides an accurate indication of time to contact (ie when), even when the trajectory of the ball is inclined to the line of sight, and there is evidence that the human visual system is specifically sensitive to time-to-contact information. But only part of the necessary information about position (ie where) is available to the batsman. If the batsman's head is directly in the line of flight, the velocity ratio of the retinal images in the left and right eyes provides a precise cue to the trajectory of the ball in the horizontal plane. However, humans have only poor visual sensitivities to the absolute distance and to the line-of-sight velocity of a ball. Therefore, a batsman has inadequate retinal image information about the absolute vertical velocity of a ball. It is suggested in this paper that batsmen supplement inadequate retinal image information about where the ball will hit the ground with prior knowledge built up over the preceding few deliveries. Some slow bowlers can induce the batsman to misjudge where the ball will hit the ground. I suggest that these bowlers manipulate the flight of the ball so as to induce the batsman to supplement his inadequate retinal image information with inappropriate prior knowledge, and thus to misinterpret the vertical angular speed of the retinal image of the ball.

Animals↗
PubMed1992

The Poggendorff illusion and apparent interparallel extents.

An explanation of the Poggendorff misalignment effect in terms of apparent contraction of interparallel extent resulting from the Müller-Lyer illusion was tested in three experiments. Three of the eight stimulus figures had oblique transversals outside the parallels in the usual way, three had them inside, and two were controls consisting of the transversals only. Müller-Lyer forms were differently delineated between the parallels for the inside-transversal and outside-transversal figures, and were not delineated in the control figures. In the first experiment apparent misalignment occurred in four of the six parallel-line figures and in neither of the controls. In the second experiment oblique extent between the parallels was underestimated in six of the eight figures and right-angle extent was overestimated in all of them. The results of the third experiment showed that right-angle (horizontal) extent between the parallels without transversals is estimated without significant error. The data from the three experiments do not support the interparallel-extent explanation of apparent misalignment. Instead, the results are interpreted in terms of independent perceptual compromises, one involving alignment of the transversals and the other the distance between them.

Adult↗
PubMed1992

The reversed Müller-Lyer illusion and figure-ground organization theory.

When the shaft is shortened and reaches neither of the vertices of the two pairs of wings, a reversed Müller-Lyer illusion is observed: a shaft between inward-pointing wings appears to be longer than a shaft between the outward-pointing wings. In this paper it is examined whether this illusion can be explained in terms of figure-ground organization. A circle was used as the focal area, instead of a shaft or a pair of dots, so that the figure-ground character could be seen more definitely in this focal area. The apparent size of the focal circle was measured under different conditions with three variables (enclosure, wings direction, and depth). The focal circle appeared to be largest in the condition where the circle should appear most readily as a hole, ie in the single, wings-in, space condition. The circle appeared to be smallest in the condition where the circle should appear most readily as a disc, ie in the separate, wings-out, object condition. This is consistent with an explanation of the usual, as well as the reversed, Müller-Lyer illusion in terms of figure-ground organization theory.

Adult↗
PubMed1993

Illusion decrement and transfer of illusion decrement in Müller-Lyer figures.

In the work reported in the literature the reduction or decrement in the magnitude of the Müller-Lyer illusion with continued inspection has been typically investigated with the use of the composite illusion form. Three experiments are reported in which the illusion decrement was separately examined in the underestimated (wings-in) and the overestimated (wings-out) forms of the Müller-Lyer illusion, with particular attention paid to the transfer of illusion decrement between the two forms. Decrement occurred in both forms of the Müller-Lyer illusion, although there was considerable intersubject variability in decrement effects, and nonuniform rates of decrement across the inspection period. In none of the experiments did transfer of illusion decrement between the two forms occur. It is argued that the attentional/differentiation hypothesis of illusion decrement provides a plausible account of the present findings as well as of those found with the composite Müller-Lyer figure.

Adult↗
PubMed1993

Estimates of time to contact based on changing size and changing target vergence.

Apparent constant-velocity approaches of an outline circle were generated by changing the size and/or the lateral separation of dichoptically presented circles at a physically constant distance from subjects. The lateral separation of the circles defines target vergence which corresponds proximally to combinations of ocular vergence and (absolute) retinal disparity. In the first experiment, estimates of time to contact were found to depend both on changing size and on changing target vergence; in the case of conflicting information, changing size turned out to be the more powerful stimulus. In the second experiment, the size of the stimulus object was varied. The relatively stronger influence of changing size on estimates of time to contact was again found for the larger stimulus objects, but for the smaller stimulus objects changing target vergence became more powerful. The results of both experiments show that estimates of time to contact are not uniquely determined by changing size, as seems to be implied by some tests of the so-called "tau hypothesis', but that they are based on multiple sources of information that have situationally specific effects on the estimates.

Adult↗
PubMed1993

Apparent minification in an imaging display under reduced viewing conditions.

When extended outdoor scenes are imaged with magnification of 1 in optical, electronic, or computer-generated displays, scene features appear smaller and farther than in direct view. This has been shown to occur in various periscopic and camera-viewfinder displays outdoors in daylight. In four experiments it was found that apparent minification of the size of a planar object at a distance of 3-9 m indoors occurs in the viewfinder display of an SLR camera both in good light and in darkness with only the luminous object visible. The effect is robust and survives changes in the relationship between object luminance in the display and in direct view and occurs in the dark when subjects have no prior knowledge of room dimensions, object size or object distance. The results of a fifth experiment suggest that the effect is an instance of reduced visual size constancy consequent on elimination of cues for size, which include those for distance.

Adult↗
PubMed1993

Information processing by tree fellers: signal detection analysis.

Eighteen experienced tree fellers and eighteen forestry students watched video recordings of mature eucalypts being felled by a man using a chain saw, and then rated whether each tree had fallen normally or abnormally. Signal-detection analysis showed that the tree fellers were more accurate than the forestry students in predicting eventual outcome. Further, the tree fellers achieved peak accuracy in discrimination by the time the logger had completed cutting the scarf (typically several minutes before the tree hit the ground), whereas the forestry students predicted outcome most accurately only when a tree was falling (and about 1 s from hitting the ground). Study of the bases for information processing and decision making by tree fellers has implications for personnel selection and training, as well as for formulation of effective work practices.

Adult↗