Biomedical subjects

T S Collett

Publications and source records attributed to T S Collett.

9 recordsLinked to original sources

PubMed1991

Does vertical disparity scale the perception of stereoscopic depth?

It has been suggested that a measure of the gradients of vertical disparity over a surface may scale the mapping between horizontal disparity and perceived depth. We have investigated this possibility by obtaining estimates of the depth within stereograms that simulated two apposed fronto-parallel planes placed at different distances from an observer. The gradients of vertical disparity in a stereogram were set to simulate those appropriate to a viewing distance of 12.5 cm, 25 cm, 50 cm or 100 cm, whereas the distance specified by vergence and accommodative cues was always fixed at 50 cm. Judgements of the perceived depth between the two planes were uninfluenced by changes in the gradients of vertical disparity. It thus seems that the human visual system does not employ vertical disparity as a scaling parameter in stereoscopic depth judgements.

Depth Perception↗

PubMed1991

The interaction of oculomotor cues and stimulus size in stereoscopic death constancy.

In the natural world, observers perceive an object to have a relatively fixed size and depth over a wide range of distances. Retinal image size and binocular disparity are to some extent scaled with distance to give observers a measure of size constancy. The angle of convergence of the two eyes and their accommodative states are one source of scaling information, but even at close range this must be supplemented by other cues. We have investigated how angular size and oculomotor state interact in the perception of size and depth at different distances. Computer-generated images of planar and stereoscopically simulated 3-D surfaces covered with an irregular blobby texture were viewed on a computer monitor. The monitor rested on a movable sled running on rails within a darkened tunnel. An observer looking into the tunnel could see nothing but the simulated surface so that oculomotor signals provided the major potential cues to the distance of the image. Observers estimated the height of the surface, their distance from it, or the stereoscopically simulated depth within it over viewing distances which ranged from 45 cm to 130 cm. The angular width of the images lay between 2 deg and 10 deg. Estimates of the magnitude of a constant simulated depth dropped with increasing viewing distance when surfaces were of constant angular size. But with surfaces of constant physical size, estimates were more nearly independent of viewing distance. At any one distance, depths appeared to be greater, the smaller the angular size of the image. With most observers, the influence of angular size on perceived depth grew with increasing viewing distance. These findings suggest that there are two components to scaling. One is independent of angular size and related to viewing distance. The second component is related to angular size, and the weighting accorded to it grows with viewing distance. Control experiments indicate that in the tunnel, oculomotor state provides the principal cue to viewing distance. Thus, the contribution of oculomotor signals to depth scaling is gradually supplanted by other cues as viewing distance grows. Binocular estimates of the heights and distances of planar surfaces of different sizes revealed that angular size and viewing distance interact in a similar way to determine perceived size and perceived distance.

Adult↗

PubMed1989

The appearance of surfaces specified by motion parallax and binocular disparity.

The experiments reported in this paper were designed to investigate how depth information from binocular disparity and motion parallax cues is integrated in the human visual system. Observers viewed simulated 3-D corrugated surfaces that translated to and fro across their line of sight. The depth of the corrugations was specified by either motion parallax, or binocular disparities, or some combination of the two. The amount of perceived depth in the corrugations was measured using a matching technique. A monocularly viewed surface specified by parallax alone was seen as a rigid, corrugated surface translating along a fronto-parallel path. The perceived depth of the corrugations increased monotonically with the amount of parallax motion, just as if observers were viewing an equivalent real surface that produced the same parallax transformation. With binocular viewing and zero disparities between the images seen by the two eyes, the perceived depth was only about half of that predicted by the monocular cue. In addition, this binocularly viewed surface appeared to rotate about a vertical axis as it translated to and fro. With other combinations of motion parallax and binocular disparity, parallax only affected the perceived depth when the disparity gradients of the corrugations were shallow. The discrepancy between the parallax and disparity signals was typically resolved by an apparent rotation of the surface as it translated to and fro. The results are consistent with the idea that the visual system attempts to minimize the discrepancies between (1) the depth signalled by disparity and that required by a particular interpretation of the parallax transformation and (2) the amount of rotation required by that interpretation and the amount of rotation signalled by other cues in the display.

Attention↗

PubMed1988

Frogs use retinal elevation as a cue to distance.

We have investigated the role that retinal elevation plays in a frog's (Rana pipiens) estimate of prey distance. We dissociated retinal elevation from other depth cues by artificially increasing the height of the frogs' eyes above the ground. Frogs then snapped short of their prey in their ventral visual field as if their estimate of distance were determined primarily by the retinal elevation of the image of the prey. The data suggest that the frog assumes its eyes to be about 3 cm above the ground. Other cues modify depth judgements when targets are close to this assumed ground-plane.

Animals↗

PubMed1988

The retrieval of visuo-spatial memories by honeybees.

In order to explore how honeybees manage to retrieve the right landmark-memory in the right place, we trained bees along a short foraging route which consisted of two identical huts 33 m apart. Bees entered each hut to collect a drop of sucrose on the floor. The location of the drop was defined by the same arrangement of four blue and yellow cylindrical landmarks. However, in one hut the drop was between two yellow cylinders and in two other it was to the east of the blue cylinders. On tests with the sucrose missing, bees tended to search in the appropriate area in each hut (Fig. 1), thus showing that they used cues other than the sight of the local landmarks to select the appropriate memory. In a second experiment, the position of the sucrose was specified by yellow cylinders in one hut and by blue triangles in the other. When the arrays were swapped between huts, bees searched in the position specified by the array they encountered (Fig. 2). Thus, memories can be triggered by visual features of local landmarks. Bees were also trained outside to collect food from two platforms 40 m apart. The location of sucrose on one platform was defined by yellow cylinders, and on the other it was defined by blue triangles. When these arrays were exchanged between platforms, bees searched on each platform as though the landmarks had not been swapped. It seems that the more distant surroundings, which fill most of the visual field, may be more potent than the local landmarks in deciding which memory should be retrieved.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

A possible mechanism for binocular depth judgements in anurans.

Two experiments were performed to analyze how anurans (Bufo marinus) use binocular cues to gauge the distance of their prey. In the first, bilateral lesions of the nucleus isthmi eliminated the major source of input from the ipsilateral eye to the tectum. These lesions did not disrupt the animals' ability to use binocular cues to judge distance, suggesting that frogs and toads may not employ binocular disparity-selective cells to assess prey distance. They may instead use a scheme more overtly akin to triangulation, with each tectum providing an output signal encoding the angular position of the prey with respect to the contralateral eye and with distance extracted from the difference between these tectal outputs. In the second experiment, prisms imposed large (13.5 degrees) vertical disparities between the two eyes' images. The toads continued to use binocular cues. The added vertical disparities, like added horizontal ones, caused toads to undershoot their prey. Thus the binocular system must tolerate such vertical disparities and fail to distinguish them from horizontal ones.

Animals↗

PubMed1987

The use of visual landmarks by gerbils: reaching a goal when landmarks are displaced.

Gerbils (Meriones unguiculatus) can specify the location of a goal by means of visual landmarks and will return to such a goal from different starting positions in the vicinity of the landmarks. To discover whether landmark-cues are used continuously during an approach to the goal, gerbils were trained to forage for sunflower seeds close to a single illuminated light-bulb on the floor of an arena. As they approached the bulb, it was switched off and another bulb in a variable position with respect to the first turned on. On 52 out of 71 trials the gerbils changed their trajectory (latency ca. 240 ms) to aim for the newly lit bulb (Fig. 1 A, B). On the remaining trials, gerbils maintained their original course towards the first bulb as though it were still lit and then paused after a longer delay before eventually changing direction (Fig. 1 C). Thus, an approach to a beacon is usually under continuous visual control. This ensures that the gerbil will reach its goal correctly despite any inaccuracies in its initial computation of its approach. When switches were made between more complex arrays of landmarks, the gerbils' behaviour was less clear-cut. Possible reasons for this difference are suggested.

Animals↗

PubMed1986

Landmark learning and visuo-spatial memories in gerbils.

The aim of this study is to understand what a rodent (Meriones unguiculatus) learns about the geometrical relations between a goal and nearby visual landmarks and how it uses this information to reach a goal. Gerbils were trained to find sunflower seeds on the floor of a light-tight, black painted room illuminated by a single light bulb hung from the ceiling. The position of the seed on the floor was specified by an array of one or more landmarks. Once training was complete, we recorded where the gerbils searched when landmarks were present but the seed was absent. In such tests, gerbils were confronted either with the array of landmarks to which they were accustomed or with a transformation of this array. Animals searched in the appropriate spot when trained to find seeds placed in a constant direction and at a constant distance from a single cylindrical landmark. Since gerbils look in one spot and not in a circle centred on the landmark, the direction between landmark and goal must be supplied by cues external to the landmark array. Distance, on the other hand, must be measured with respect to the landmark. Tests in which the size of the landmark was altered from that used in training suggest that distance is not learned solely in terms of the apparent size of the landmark as seen from the goal. Gerbils can still reach a goal defined by an array of landmarks when the room light is extinguished during their approach. This ability implies that they have already planned a trajectory to the goal before the room is darkened. In order to compute such a trajectory, their internal representation of landmarks and goal needs to contain information about the distances and bearings between landmarks and goal. For planning trajectories, each landmark of an array can be used separately from the others. Gerbils trained to a goal specified by an array of several landmarks were tested with one or more of the landmarks removed or with the array expanded. They then searched as though they had computed an independent trajectory for each landmark. For instance, gerbils trained with an array of two landmarks were tested with the distance between two landmarks doubled. The animals then searched for seeds in two positions, which were at the correct distance and in the right direction from each landmark.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1985

Extrapolating and interpolating surfaces in depth.

Random-dot stereograms were generated with a blank area placed in part of the right-hand image so making a patchwork of monocular and binocular areas. The perceived depth and shape of the monocular region, where depth was not explicitly marked, depended in part on the depth and surface orientation of adjacent binocular areas. Thus a monocular rectangle flanked by two binocular rectangles which were placed in different fronto-parallel planes was seen as a sloping surface spanning the depth between the binocular regions, and, under some conditions, the gradient of a sloping binocular plane extended into a neighbouring monocular area. Division of the monocular region into two by textural discontinuities or discontinuities of motion sometimes altered the shape of the extrapolated surface. Often, though, the shape was unchanged by such discontinuities implying that both two- and three-dimensional features are used to segment a scene into separate surfaces. Pictorial cues also contribute to the shape and apparent depth of the monocular surface. For instance, when subjects viewed a display consisting of portions of a cube of which two ends were shown stereoscopically and one side monocularly, the monocular side was seen in three dimensions filling the gap between the ends. When stereo cues were pitted against pictorial cues, sometimes pictorial cues and sometimes stereo cues dominated, and sometimes the surface contained sharp discontinuities enabling both to be accommodated.

Computers↗