Orientation specificity in biological motion perception

We addressed the issue of how display orientation affects the perception of biological motion. In Experiment 1, spontaneous recognition of a point-light walker improved abruptly with image-plane display rotation from inverted to upright orientation. Within a range of orientations from 180 degrees to 90 degrees, it was dramatically impeded. Using ROC analysis, we showed (Experiments 2 and 3) that despite prior familiarization with a point-light figure at all orientations, its detectability within a mask decreased with a change in orientation from upright to a range of 90 degrees-180 degrees. In Experiment 4, a priming effect in biological motion was observed only if a prime corresponded to a range of deviations from upright orientation within which the display was spontaneously recognizable. The findings indicate that display orientation nonmonotonically affects the perception of biological motion. Moreover, top-down influence on the perception of biological motion is limited by display orientation.     

Category effects on the processing of plane-rotated objects

We examined the effects of plane rotations on the identification of exemplars of three semantic categories. In the first two experiments line drawings belonging to three categories (animals, inanimate objects, and vegetables) were presented at four orientations (0 degree, 60 degrees, 120 degrees, and 180 degrees of clockwise rotation). The response time was found to depend on stimulus category. In particular, whereas rotation effects were shown for animals, no effect at all was found for vegetables and only partial effects were found for inanimate objects. The unclear pattern found for inanimate objects was further examined in experiment 3 where the orientation effects on the identification of two subsets of the inanimate category were studied. The hypothesis of view-observation frequency was confirmed. In experiment 4, line drawings of objects at different orientations were presented in physically degraded versions. The minimum amount of visual information necessary to identify rotated stimuli was found to vary as a function of stimulus category as well. Results are discussed, combining current research on both viewpoint-dependence/independence and neural systems involved in category processing.     

Mental rotation in perspective problems

The present paper demonstrates that mental rotation as used in the processing of disoriented objects (Cooper and Shepard 1973) can also be used as an explanatory concept for the processing of perspective problems in which the task is to imagine how an environment will appear from another vantage point. In a cognitive map, subjects imagined an initial line of vision and subsequently processed a reorientation stimulus, requesting them to imagine a turn over 0, 45, 90, 135, or 180 degrees. Time for a reorientation increased linearly with the size of the imaginary turn up to 135 degrees and decreased for turns of 180 degrees; apparently, about-faces were relatively easy to imagine. The increment of reorientation time between 0 and 135 degrees was larger for maps presented in unfamiliar orientations such as South-West up. Both the increment and the interaction with familiarity are consistent with an explanation in terms of mental rotation.     

Influence of body roll on visually induced sensations of self-tilt and rotation

Illusory self-tilt and illusory self-motion (vection) produced by rotation of a 360 degrees visual scene about the subject's roll axis was measured as a function of the presence or absence of actual rotation of the subject during acceleration of the visual scene. Rotation of the subject to a tilt of 15 degrees was at two levels of acceleration (onset) and with or without a delay between initial rotation and subsequent return (washout) to the vertical position. In one set of conditions, visual motion and subject motion were in opposite directions (concordant) and in another set they were in the same direction (discordant). In two control conditions, the subject was rotated while the visual scene remained stationary. For concordant motion the main effect of body rotation was to reduce the time taken by the subject to indicate self-tilt as compared with the response time to visual motion alone. The magnitude of estimated self-tilt was increased by actual body tilt as could be expected from addition of the perceived actual body tilt and the illusory body tilt induced by visual rotation. This effect of augmented body tilt did not persist after the body was returned to the vertical. The magnitude of vection was not markedly influenced by body rotation and washout. For discordant motion of body and the visual scene, subjects were confused and their responses were very variable, suggesting a nonlinear visual--vestibular interaction.     

Are the Orientations of the Head and Arm Related During Pointing Movements?

The head, eye, and shoulder are each free to rotate around three mutually orthogonal axes. These three degrees of freedom allow a given gaze or pointing direction of the eye, head, or arm to be obtained in many different possible orientations. Unlike translations in three dimensions, three-dimensional (3-D) rotations are noncommutative. Therefore, the orientation of a rigid body following sequential rotations about two different axes depends on the order of the rotations. In this article, we demonstrate that only two degrees of freedom are used during orienting movements of the head and pointing movements of the arm. This provides a unique orientation of head and arm for each gaze or pointing direction despite the noncommutativity of three-dimensional rotations. This observation is in itself not new. We found, however, that (a) the two-dimensional lquot;rotation surface,rquot; which describes the orientation of the head for all gaze directions, is curved, unlike the analogous flat plane for the eye. (b) The rotation surface for the head is curved differently than that for the arm. This result argues against the hypothesis that the orientations of head and arm are directly coupled during pointing. It also implies that the orientation of the eye in space during gaze shifts of the eye and head is not uniquely determined for a given direction of gaze. This finding argues against a perceptual basis for the reduction of rotational degrees of freedom.     

Rotating objects to determine orientation, not identity: evidence from a backward-masking/dual-task procedure

The effects of picture-plane rotations on times taken to name familiar objects (RTs) may reflect a process of mental rotation to stored viewpoint-specific representations: the rotate-to-recognize hypothesis. Alternatively, mental rotation might be used after stored object representations are activated by a misoriented stimulus in order to verify a weak or distorted shape percept: the double-checking hypothesis. We tested these two accounts of rotation effects in object recognition by having subjects verify the orientations (to within 90 degrees) and basic-level names of 14-msec, backward-masked depictions of common objects. The stimulus-mask interval (SOA) varied from 14 to 41 msec, permitting interpolation of the SOA required for 75% accuracy (SOAc). Whereas the SOAc to verify orientation increased with rotation up to 180 degrees, the SOAc to verify identity was briefer and asymptoted at approximately 60 degrees. We therefore reject the rotate-to-recognize hypothesis, which implies that SOAc should increase steadily with rotation in both tasks. Instead, we suggest that upright and near-upright stimuli are matched by a fast direct process and that misoriented stimuli are matched at a featural level by a slightly slower view-independent process. We also suggest that rotation effects on RTs reflect a postrecognition stage of orientation verification: the rotate-to-orient hypothesis, a version of double-checking that also explains the well-known reduction in orientation effects on RTs when naming repeated objects.     

Patterns of perceived similarity cannot be generalized from long to short exposure durations and vice versa

Subjects rated the perceived similarity between patterns and their 180-deg rotational variants. The stimuli, adapted from Halpern, Fishbein, and Warm (1979), were randomly generated dot patterns and line drawings and polygons generated from the dot patterns. Inspection time was varied in a between-subject design. As in the Halpern et al. (1979) study, it was found that the variables of form type (dots, lines, polygons) and axis of rotation (x, y, z) had no effect on perceived similarity in the long exposure conditions. However, when inspection time was limited to 150 msec, both variables had a substantial impact. This finding undermines the frequent practice of selecting stimuli for use in reaction time or other experiments with limited inspection time on the basis of similarity ratings gathered under typical free inspection conditions.     

Perceptual constraints on implicit learning of spatial context

If perspective views of an object in two orientations are displayed in alternation, observers will experience the object rotating back and forth in three-dimensional space. Rotational motion is perceived even though only two views are displayed and each view is two-dimensional. The results of 5 experiments converge on the conclusion that the perception of apparent rotational motion produces representations in visual memory corresponding to the spatial structure of the object along its arc of rotation. These representations are view-dependent, preserving information about spatial structure from particular perspectives, but do not preserve low-level perceptual details of the stimulus.     

Discrimination learning of rotated faces: a Markov analysis of coding and association processes]

30 subjects participated in a discrimination experiment learning face-letter associations under four rotation conditions (45 degrees, 90 degrees, 135 degrees, 180 degrees). Under each condition two thirds of the faces were presented twice, upright and rotated away from the vertical; the remaining faces were presented once, upright or rotated. Learning is described by a joint Markov model: For faces that are presented twice it assumes a separate association and encoding process (two-stage-model), for faces that are presented once it assumes an association process (all-or-none-model). The Markov model fits the data for all four rotation conditions. The angle of rotation does not affect learning for faces that are presented once. For faces that are presented twice it influences both the association and the encoding process. For the angles employed, the effect of rotation can be approximated linearly. The results suggest that the encoding of a rotated face differs increasingly from an upright face as a function of these angles of rotation. This confirms analogous conclusions from mental rotation experiments.     

Perceived bending motions from a quadrangle changing form

In an earlier study by Jansson and Johansson it was found that rotation of a rigid object is perceptually preferred over bending motion, and that bending motion in turn is preferred over two-dimensional stretching. The aim of the present experiment was to study if the same preference order is retained also when the proximal stimuli are changing in a physically more complex way. The stimuli were quadrangular outline figures with two stationary and two moving corners; the figures differed in degree of phase lag between the motions of the two corners. The result was that the preference order found earlier was retained. It was also found that the relative frequency of two subcategories of bending motion, bending proper and folding, varied with phase lag. The relation of the result to a principle of minimum object change was discussed.     

Accuracy and precision of the PEAK Performance Technologies Motion Measurement System

This study evaluated the accuracy and precision of the PEAK Performance Technologies, lnc.'s motion analysis system for three-dimensional angle reconstruction. Pendular motion of a bar, on which 18 retroreflective markers were mounted, was videotaped at three different orientations (parallel, and rotated 30 degrees right and left) to a plane at which two standard video cameras were aimed. The videotaped motion was digitized off-line, and 32 angles between the 18 markers were calculated. intraclass correlation coefficients (ICCs) were calculated between trials within each pendulum orientation and across orientations to determine system precision, and between randomly selected trials and actual angles to determine accuracy. lCCs were in all cases greater than.99. Within-trial standard deviations ranged between 0.05-0.8 degrees for the different angles. Deviations from the actual angle averaged 0.0-0.8 degrees across all angles and orientations. The results indicate that accurate and reliable angular measurements can be made with this motion analysis system.     

Sex differences in mental rotation with polygons of different complexity: Do men utilize holistic processes whereas women prefer piecemeal ones?

Sex differences in mental rotation were investigated as a function of stimulus complexity with a sample size of N = 72. Replicating earlier findings with polygons, mental rotation was faster for males than for females, and reaction time increased with more complex polygons. Additionally, sex differences increased for complex polygons. Most importantly, however, mental rotation speed decreased with increasing complexity for women but did not change for men. Thus, the sex effects reflect a difference in strategy, with women mentally rotating the polygons in an analytic, piecemeal fashion and men using a holistic mode of mental rotation.     

Sex differences in mental rotation with polygons of different complexity: Do men utilize holistic processes whereas women prefer piecemeal ones?

Sex differences in mental rotation were investigated as a function of stimulus complexity with a sample size of N = 72. Replicating earlier findings with polygons, mental rotation was faster for males than for females, and reaction time increased with more complex polygons. Additionally, sex differences increased for complex polygons. Most importantly, however, mental rotation speed decreased with increasing complexity for women but did not change for men. Thus, the sex effects reflect a difference in strategy, with women mentally rotating the polygons in an analytic, piecemeal fashion and men using a holistic mode of mental rotation.     

The rubber pencil illusion

When a straight, rigid line segment is put into certain types of motion, it appears to an observer to lose its rigidity and become rubbery, as in the well-known “rubber pencil illusion.“ The factors controlling this illusion were examined, including the nature of the motion employed (harmonic or linear oscillation), the amplitudes of the translational and rotational components of the motion, and the phase relationship between these two components. The effect is shown to be due to visual persistence. The status of the illusion as a potential counterexample to the rigidity principle (that moving, two-dimensional arrays will be perceived as rigid) is discussed.     

The integration of orientation information in the motion correspondence problem

We examine how differently oriented components contribute to the discrimination of motion direction along a horizontal axis. Stimuli were two-frame random-dot kinematograms that were narrowband filtered in spatial frequency. On each trial, subjects had to state whether motion was to the left or the right. For each stimulus condition, Dmax (the largest displacement supporting 80% correct direction discrimination performance) was measured. In experiment 1, Dmax was measured for orientationally narrowband stimuli as a function of their mean orientation. Dmax was found to increase as the orientation of the stimuli became closer to the axis of motion. Experiment 2 used isotropic stimuli in which some orientation bands contained a coherent motion signal, and some contained only noise. When the noise band started at vertical orientations and increased until only horizontal orientations contained a coherent motion signal, Dmax increased slightly. This suggests that near-vertical orientations interfere with motion perception at large displacements when they contain a coherent motion signal. When the noise band started at horizontal and increased until only vertical orientations contained the motion signal, Dmax decreased steadily. This implies that Dmax depends at least partly on the most horizontal motion signal in the stimulus. These results were contrasted with two models. In the first, the visual system utilises the most informative orientations (nearest horizontal). In the second, all available orientations are used equally. Results supported an intermediate interpretation, in which all orientations are used but more informative ones are weighted more heavily.     

Mental rotation, physical rotation, and surface media.

Subjects made mirror-normal discriminations on alphanumeric characters shown in different orientations in the picture plane. Either the characters or the background rotated during stimulus presentation in Experiments 1-3. Character rotation in the direction of mental rotation facilitated mental rotation, whereas rotation in the opposite direction inhibited it. In Experiment 4, characters were presented in different surface media so as to stimulate only one representation at a time. Mental rotation performance was similar whether the stimuli were defined by luminance, color, texture, relative motion, or binocular disparity, suggesting that mental rotation occurs at a level beyond that of the independent analyses of these different media. These results support those of Experiments 1-3 in excluding the participation of low-level motion analysis centers in the mental rotation processes.     

The perception of identity by 6 1/2-month-old infants.

In Study 1, sixteen 6 1/2-month-olds were habituated to a Reversible stimulus (an upright face that could be perceived as an entirely different upright face when it was rotated 180 degrees) and to a Nonreversible stimulus (a face that could be perceived as upright in only one orientation). Following habituation for each type of stimulus, test trials paired the habituated face with a novel stimulus (an inversion of the same face). For both Reversible and Nonreversible stimuli, the physical difference between the old and new test stimuli was the same (a 180 degrees rotation); however, infants devoted more visual attention to the 180 degrees rotation only when it was a Reversible face, suggesting that the identity change was detected. Experiment 2 ruled out the explanation that infants might have failed to dishabituate to the inversion of the Nonreversible stimulus because they could not remember it. Results are interpreted as evidence that 6 1/2-month-old infants are not limited to face recognition based on similarity in pattern arrangement alone, but are capable of processing faces at a representational level.     

Flipping and spinning: Spatial transformation procedures in the identification of rotated natural objects

The proposal that identification of inverted objects is accomplished by either a relatively slow rotation in the picture plane or a faster rotation in the depth plane about the horizontal axis was tested. In Experiment 1, subjects decided whether objects at 0° or 180° corresponded to previously learned normal views of the upright objects, or were mirror images. Instructions to mentally flip an inverted object in the depth plane to the upright produced faster decision times than did instructions to mentally spin the object in the picture plane. In Experiment 2, the effects of orientation were compared across an object-naming task and a normal-mirror task for six orientations from 0° to 300°. In the normal-mirror task, objects at 180° were cued for rotation in the picture plane or in the depth plane in equal numbers. The naming function for one group of subjects did not differ from the normalmirror function where inverted objects had been mentally rotated to the upright. For both functions, response time (RT) increased linearly from 0° to 180° and the slopes did not differ. The naming function for a second group of subjects did not differ from the normal-mirror function where inverted objects had been mentally flipped to the upright. For both functions, RT increased linearly at a similar rate from 0° to 120°, but decreased from 120° to 180°. The results are discussed in terms of theories of orientation-specific identification.     

Effect of field size, head motion, and rotational velocity on roll vection and illusory self-tilt in a tumbling room

The effect of field size, velocity, and visual fixation upon the perception of self-body rotation and tilt was examined in a rotating furnished room. Subjects sat in a stationary chair in the furnished room which could be rotated about the body roll axis. For full-field conditions, complete 360 degrees body rotation (tumbling) was the most common sensation (felt by 80% of subjects). Constant tilt or partial tumbling (less than 360 degrees rotation) occurred more frequently with a small field of view (20 deg). The number of subjects who experienced complete tumbling increased with increases in field of view and room velocity (for velocities between 15 and 30 degrees s-1). The speed of perceived self-rotation relative to room rotation also increased with increasing field of view.