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Biomedical subjects

L S Mark

Publications and source records attributed to L S Mark.

16 recordsLinked to original sources
PubMed1999

The impact of visual exploration of judgments of whether a gap is crossable.

When observers look down into a gap in the ground plane, their judgments of the widest gap they can step across (gapmax) decrease as gap depth increases (Y. Jiang & L. S. Mark, 1994). This study investigated the possibility that Jiang and Mark's viewing conditions did not afford observers a sufficient opportunity to perform exploratory movements needed to detect information about gap width. Experiment 1 showed that the gap depth by gaze interaction disappeared only when restrictions were not imposed on observers' exploratory activities (eye, head, and body movements). Experiment 2 showed that observers tended to see the vertical surface as slanted away from them, which made the bottom of the surface appear farther away from them than the top. Only when observers were able to view the gap binocularly under conditions that did not restrict exploratory activity did their slant perception improve and their gapmax judgments no longer covary with gap depth. The data indicate that the exploratory movements of prospective actors are essential for the pickup of information about their action capabilities.

Adult↗
PubMed1998

Visual inertia of rotating 3-D objects.

Five experiments were designed to determine whether a rotating, transparent 3-D cloud of dots (simulated sphere) could influence the perceived direction of rotation of a subsequent sphere. Experiment 1 established conditions under which the direction of rotation of a virtual sphere was perceived unambiguously. When a near-far luminance difference and perspective depth cues were present, observers consistently saw the sphere rotate in the intended direction. In Experiment 2, a near-far luminance difference was used to create an unambiguous rotation sequence that was followed by a directionally ambiguous rotation sequence that lacked both the near-far luminance cue and the perspective cue. Observers consistently saw the second sequence as rotating in the same direction as the first, indicating the presence of 3-D visual inertia. Experiment 3 showed that 3-D visual inertia was sufficiently powerful to bias the perceived direction of a rotation sequence made unambiguous by a near-far luminance cue. Experiment 5 showed that 3-D visual inertia could be obtained using an occlusion depth cue to create an unambiguous inertia-inducing sequence. Finally, Experiments 2, 4, and 5 all revealed a fast-decay phase of inertia that lasted for approximately 800 msec, followed by an asymptotic phase that lasted for periods as long as 1,600 msec. The implications of these findings are examined with respect to motion mechanisms of 3-D visual inertia.

Contrast Sensitivity↗
PubMed1997

Postural dynamics and the preferred critical boundary for visually guided reaching.

How do people choose an action to satisfy a goal from among the actions that are afforded by the environment? In 3 experiments the action modes used by actors to reach for a block placed at various distances from them were observed. In each experiment, when actors were not restricted in how they could reach for the object, the transition from their reaching using only arm extension to a mode of reaching in which they used the upper torso to lean forward occurred at closer distances than each actor's absolute critical boundary, beyond which the former action was no longer afforded. In Experiments 2 and 3 actors' seated posture was varied so that the effect of postural dynamics on the distance at which actors actually chose to make the transition between action modes, the preferred critical boundary, could be examined. The results are consistent with the proposal that the preferred critical boundary reflects the relative comfort of available modes of reaching.

Adult↗
PubMed1994

The effect of gap depth on the perception of whether a gap is crossable.

Four experiments were performed in order to examine the effect of gap depth on human observers' perception of whether or not a gap is crossable. Experiments 1 and 2 showed that as the gap's depth increased, observers tended to increasingly underestimate the maximum width of a gap they could step across. Experiments 3 and 4 clarified this finding: The observed covariation of perceived gap crossability and gap depth depended on the observer's direction of gaze, rather than on the physical depth of the gap. The optical relations to which observers might be attending are discussed, as well as the possibility that cognitive-affective processes might have contributed to observers' underestimation of their actual capabilities.

Depth Perception↗
PubMed1987

Eyeheight-scaled information about affordances: a study of sitting and stair climbing.

Previous work has shown that both the perceived and actual critical (maximum) heights of surfaces that afford "sitting on " and "climbing on" can be expressed as constant proportions of each actor's leg length. The current study provides evidence that these judgments of critical action boundaries are based on an existing source of size and distance information that is already scaled with reference to the actor's eyeheight. In Experiment 1 changes in judgments of "perceived eyeheight" (an index of the intrinsic scalar) as a function of viewing distance were shown to be highly correlated with changes in the maximum height that was perceived to afford sitting on or climbing on. In Experiments 2 and 3 observers wore 10-cm blocks and made judgments about whether the heights of various surfaces afforded sitting or climbing. The use of eyeheight-scaled information as the basis for their estimates predicted the obtained pattern of errors in these judgments. With a modicum of experience wearing the blocks, however, observers were able to retune accurately their critical action boundary to a degree that would not have been predicted from their consistent overestimation of the height of the block on which they were standing. These results have implications for understanding how observers obtain information about their specific action boundary.

Body Height↗
PubMed1987

A biodynamic basis for perceived categories of action: a study of sitting and stair climbing.

Actors must determine whether the properties of the surface layout are sufficient to meet their specific requirements for performing an action. Warren's (1984) study of bipedal stair climbing has demonstrated the significance of intrinsic, body-scaled measures of environmental properties for defining perceptual categories relevant to action. Whereas the absolute measure of the perceptual boundary between "climbable" and "not climbable" varied according to the actor's size and mass, the perceived boundary was a constant proportion of each actor's leg length. Our current study examined the perceived maximum seat height (SHmax) for the act of sitting. Experiment 1 delineated the range of surface heights that were perceived to afford sitting on. When expressed as a function of each person's leg length (L), SHmax was remarkably stable across individuals. Unexpectedly, it was quite close to the maximum riser height determined by Warren. Experiment 2 examined whether this similarity reflected a common biodynamic requirement, since climbing and sitting require actors to lift their center of gravity above the surface of support. Perceived critical heights were obtained for both acts using the same methods and apparatus. The perceived maximum heights for each act were virtually identical. These findings are consistent with the possibility that the information used in determining critical action boundaries is already scaled with reference to some physical dimension of the actor.

Journal Article↗
PubMed1986

Structural support for the perception of growth.

Previous studies have shown that the effects of a particular class of geometric transformations, known as cardioidal strain, are perceived as growth when applied to a variety of animate and even inanimate objects. The current study demonstrates that the effects of these growth transformations are not completely independent of the object undergoing change, but depend critically on certain structural characteristics. When cardioidal strain is applied to a straight-line, right-angle, robotlike structure, there is no consistent effect on the age level of the figure. However, as the structural contours become more curved and less angular, the effects of this transformation are seen as increasingly more like growth. In contrast, the effects of a shear transformation are not perceived as growth on any of the profiles. These findings are examined in light of the critical physical properties that may be responsible for this notion of biological forms as well as their implications for our understanding of the information about events.

Age Factors↗
PubMed1981

Issues related to the prediction of craniofacial growth.

The prediction of craniofacial growth involves four central issues: (1) What frame of reference should be adopted for measuring change? (2) What type of coordinate system should be used? (3) How should the change be described? (4) How can the change be explained biologically? In an effort to address these issues within a common framework, we are presenting a mathematical model for predicting the course of craniofacial growth in any given individual. The model is derived from a few basic assumptions about the long-range effects gravitational pressure on the remodeling of bone and is expressed formally as a single geometric transformation. The validity of the model is examined empirically, using data for twenty individuals from the Denver Child Research Council's longitudinal growth study. The predictions of the model are found to be in close correspondence with the actual morphologic changes in each individual over periods ranging from 8 to 17 years. These findings suggest that a transformational approach to the study of human growth may provide clinicians with a valuable tool for long-range treatment planning.

Adolescent↗
PubMed1981

Perception of growth: a geometric analysis of how different styles of change are distinguished.

Although there have been many demonstrations that human observers can accurately recognize a variety of styles of change, such as rolling, walking, or growing, there are no existing theories capable of explaining how one style of change is distinguished from another. The present article offers a hypothesis that any recognizable style of change is uniquely specified by geometric invariants- the abstract properties of a visual display that are preserved by the change. In an effort to provide an empirical test of this hypothesis, several experiments involving the perception of growth were performed. Observers were required to make perceptual judgments of consequences of facial profiles, each of which was constructed by using a different mathematical transformation. The same pattern of results was obtained on both a free response task and a growth rating task: All transformations that were consistently identified as growth preserved the same geometric invariants.

Adolescent↗
PubMed1979

Perceptual information for the age level of faces as a higher order invariant of growth.

Previous work supports the hypothesis that cardioidal strain, a nonlinear topological transformation, offers a plausible mathematical model for the perceived global changes in human craniofacial morphology due to growth. Experiment 1 examined the generality of the effect of this growth transformation on relative age judgments by applying it to profiles of a dog, bird, and monkey. Experiment 2 investigated the abstractness of this transformation by looking at its effect on perceived age level of a Volkswagen "Beetle." In both experiments, cardioidal strain resulted in changes in the perceived age of the nonhuman profiles that were similar to those produced on human faces in earlier work. A second transformation, affine shear, failed to produce as significant an effect on perceived age as cardioidal strain when applied to the same structures. Because cardioidal strain produces changes in structures that do not share an isomorphism of rigid (Euclidian) local features or rigid feature configurations, this transformation seems both sufficiently general and abstract to specify what J.J. Gibson has called a "higher-order invariant of perceptual information.

Discrimination Learning↗