Evaluating the decay gradient for collinearity bias with lateral displacement from the axis of induction

The misperception of alignment which is found in many geometric illusions can be quantified using relatively simple stimulus configurations. Perceived collinearity of one segment (designated as the test segment) is biased by a second segment (designated as the induction segment), with the size of effect being a function of the relative angle between the two segments. The process can be described as angular induction. The strength of bias is greatest when the induction segment is centered at the tip of the test segment. Tong and Weintraub have reported that lateral displacement from the tip, i.e., at right angles to the axis of the induction segment, produces a sharp drop in the strength of effect. This decline is described as a decay gradient for the angular induction. One experiment replicates and provides better quantification of this decay gradient. Two other experiments examine the decay gradient using a pair of induction segments, one on each side of the tip of the test segment. Displacement of the segments (either in the same direction or in opposite directions) produces substantially the same gradient of effect. Therefore, previous evidence of tandem boosting of effect for segment pairs does not depend on collinearity among the stimulus components. Finally, a fourth experiment finds that an induction segment which is at a fixed position and orientation differentially affects the influence of a variable induction segment. At some angles the influence of the variable segment is augmented, and at others it is suppressed. These findings are discussed in a neuroreductionist context, and a simple model for angular induction is presented.     

Angle discrimination in raised-line drawings

We investigated the angular resolution subserving the haptic perception of raised-line drawings by measuring how accurately observers could discriminate between two angle sizes under various conditions. We found that, for acute angles, discrimination performance is highly dependent on exploration strategy: mean thresholds of 2.9 degrees and 6.0 degrees were found for two different exploration strategies. For one of the strategies we found that discriminability is not dependent on the bisector orientation of the angle. Furthermore, we found that thresholds almost double when the angular extent is increased from 20 degrees to 135 degrees. We also found that local apex information has a significant influence on discrimination for acute as well as obtuse angles. In the last experiment we investigated the influence of depiction mode but did not find any effect. Overall, the results tell us that the acuity with which angles in raised-line drawings are perceived is determined by the exploration strategy, local apex information, and global angular extent.     

Processing orientation and emotion recognition

There is evidence that some emotional expressions are characterized by diagnostic cues from individual face features. For example, an upturned mouth is indicative of happiness, whereas a furrowed brow is associated with anger. The current investigation explored whether motivating people to perceive stimuli in a local (i.e., feature-based) rather than global (i.e., holistic) processing orientation was advantageous for recognizing emotional facial expressions. Participants classified emotional faces while primed with local and global processing orientations, via a Navon letter task. Contrary to previous findings for identity recognition, the current findings are indicative of a modest advantage for face emotion recognition under conditions of local processing orientation. When primed with a local processing orientation, participants performed both significantly faster and more accurately on an emotion recognition task than when they were primed with a global processing orientation. The impacts of this finding for theories of emotion recognition and face processing are considered.     

Large errors in the perception of vertically are generated by luminance borders (integrated across space) not by subjective borders

The rod-and-frame illusion shows large errors in the judgment of visual vertical in the dark if the frame is large and there are no other visible cues (Witkin and Asch, 1948 Journal of Experimental Psychology 38 762-782). Three experiments were performed to investigate other characteristics of the frame critical for generating these large errors. In the first experiment, the illusion produced by an 11 degrees tilted frame made by luminance borders (standard condition) was considerably larger than that produced by a subjective-contour frame. In the second experiment, with a 33 degrees frame tilt, the illusion was in the direction of frame tilt with a luminance-border frame but in the opposite direction in the subjective-contour condition. In the third experiment, to contrast the role of local and global orientation, the sides of the frame were made of short separate luminous segments. The segments could be oriented in the same direction as the frame sides, in the opposite direction, or could be vertical. The orientation of the global frame dominated the illusion while local orientation produced much smaller effects. Overall, to generate a large rod-and-frame illusion in the dark, the tilted frame must have luminance, not subjective, contours. Luminance borders do not need to be continuous: a frame made of sparse segments is also effective. The mechanism responsible for the large orientation illusion is driven by integrators of orientation across large areas, not by figural operators extracting shape orientation in the absence of oriented contours.     

Individual differences in collinearity judgment as a function of angular position

Previous research indicates that perceived orientation and/or alignment of segments and points can vary as a function of the angular position of the stimulus elements. Several studies show that the variability of the responses is least and accuracy of judgment is greatest where segments and dots are aligned with a cardinal axis. Additionally, some report assimilation of judgments toward the nearest cardinal axis--that is, the segments (or dots) are seen as being closer to the horizontal or vertical than is true. The present research confirms that judgments of collinearity are least variable and most accurate when the segment being judged is aligned with a cardinal axis. However, we do not find any consistent tendency for cardinal axis assimilation. Plotting the collinearity error (delta) as a function of angular position (phi), we find a distinctive profile of oscillation for each subject. Furthermore, subjects who were evaluated in two sessions showed very similar profiles of delta oscillation from Day 1 to Day 2. Harmonic analysis indicated a wide-ranging pattern of significant components. The components at the 4th harmonic and below were more likely to be significant, but each subject showed differential loadings in terms of which of the components were significant, as well as in the sign and amplitude of significant components. These results may reflect idiosyncratic fixation tendencies, or individual differences in the design of neural mechanisms that encode the angular positions of stimuli.     

Recognition of vector patterns under transformations: Local and global determinants

The extent to which combined local position and orientation information contributes to the recognition of patterns under transformation was investigated. Vector patterns, which consist of arrays of line segments composed according to specific rules, were presented in pairs. Discrimination performance was measured both as a function of the degree of local perturbation in one member of an otherwise identical pair, and in terms of a global rotation of one pattern with respect to the other. Differences were found between vector-pattern types on this task that point to a two-component process for pattern recognition under transformation. One component involves the comparison of local orientation/position information in the original pattern with that in its transform. The second component is global and is related to the degree to which the vector pattern is invariant under certain whole-field (1-parameter) transformations.     

Angular induction is modulated by the orientation of the test segment but not its length

Angular induction is the basic process by which the orientation of line segments can affect the perceived orientation of other segments as well as their alignment. In this laboratory, we have found that the effect on alignment follows approximately linear rules, with the inducing segment having its greatest influence when its orientation is near to that of the segment being judged. Other laboratories, however, have reported peak effects when the relative angle between the two is at 45°, and with the inducing segment being aligned with one of the cardinal axes of the observer. It has been said that the length of the test segment being judged is a critical factor, but the first experiment of the present study shows a similar linear decline of induction strength irrespective of test segment length. The second experiment indicates that the orientation of the test segment relative to the observer modulates the induction to determine the location of peak effects. A two-factor linear model predicts the observed pattern of results.     

The effect of discordant sensory information in graphic production: two distinct subject groups

The present study investigated the underlying processes used to cope with discordant sensory information induced in a mirror-drawing task. Two experiments were carried out in which adults copied simple geometrical figures made up of either horizontal and vertical segments or oblique segments meeting at a right angle in both a normal and a mirror condition. Experiment 1 identified individual differences in relation to preferred graphic movement directions; some subjects preserved the visual directions that occurred in normal drawing by reversing the direction of drawing movements (perceived-direction group), while others preserved normal drawing directions that produced reversed visual directions (performed-direction group). Experiment 2 was performed to elucidate whether these two distinct behaviors resulted from different strategies used to cope with visuo-proprioceptive discordances. The main results showed that preference for the perceived directions led to longer pauses, slower movement velocity, greater movement dysfluency, and greater spatial orientation accuracy. By contrast, longer reaction time and greater angular accuracy characterized performance in the performed-direction group. These results were interpreted as indicating that two distinct information-processing strategies can be used when resolving sensory discordance in graphic production.     

Local and global orientation in visual search

Six experiments investigated the role of global (shape) and local (contour) orientation in visual search for an orientation target. Experiment 1 demonstrated thatsearch for a conjunction of local contours with a distinct global orientation was less efficient than search for a target featurally distinct in terms of both global and local contour orientation. However, Experiments 2 and 4 demonstrated that the presence of a unique line contour was neither sufficient nor necessary to allow efficient search. Experiment 5 found thatsearch for a local orientation difference was strongly impeded by irrelevant variation in global orientation, arguing for a preeminent role for global orientation. Finally, Experiment 6 demonstrated that the orientation search asymmetry holds for the global orientation of stimuli. Taken together, the results are consistent with visual search processes guided predominately by a representation of global orientation.     

Estimating local shape from shading in the presence of global shading

In theory, global shading may help with the estimation of local surface structurefrom shading (e.g., in specifying the illuminant direction). Empirically, we do not know whether human observers combine the information given by the local and global shading to estimate local shape. Observers had to indicate the orientation of a local elongated perturbation with or without global shading information provided by a background surface. Our psychophysical results show the following:

1. Observers do not estimate the orientation of the local perturbation more accuratelywith global shading information than they do in the absence of such information.
2. Responses depend dramatically on the inclination between the illuminant direction and the viewing direction. For an inclination of 20°, observers indicate more or less the orientation of the local ridge; however, for an inclination of 40°, they indicate either the direction of the illuminant or an orientation close to the shadow edge of the perturbation. Most subjects show some combination of these behaviors. This behavior is not altered by global shading information.

We conclude that in our paradigm, global shading information does not aid the estimation of local shape.     

Local form–motion interactions influence global form perception

Object motion perception depends on the integration of form and motion information into a unified neural representation. Historically, form and motion perception are thought to be independent processes; however, research has demonstrated that these processes interact in numerous and complex ways. For example, an object??s orientation relative to its direction of motion will influence its perceived speed (Georges, Seriès, Frégnac, & Lorenceau, Vision Research 42:2757?C2772, 2002). Here, we investigated whether this local form?Cmotion interaction influences global form processing. In Experiment 1, we replicated the effect of orientation-dependent modulation of speed. In Experiment 2, we investigated whether the perceived speed of local elements could influence the perceived shape of a global object constructed from grouping of those elements. The results indicated that the orientation of local elements indeed influenced the perceived shape of a global object. We propose that inputs from local form?Cmotion processes are one of perhaps many neural mechanisms underlying global form integration.     

Demonstration of a mental analog of an external rotation

Subjects imagined a designated two-dimensional shape rotating within a blank, circular field at a self-determined rate. At some point during the mental rotation, a test shape was presented at one of 12 picture-plane orientations, and the subject was required to determine as rapidly as possible whether the test shape was the same as the originally designated shape or was its mirror image. When the test shape was presented in the expected orientation (the orientation assumed to correspond to the current orientation of the rotating internal representation), reaction time was short and constant, regardless of the angular departure of that orientation from a previously trained position. This was true even when the test shape was presented in an orientation which had not previously been tested. When the test shape was presented at some other, unexpected orientation, reaction time increased linearly with the angular difference between the expected orientation and the orientation of the test shape. It is argued that these results provide a demonstration of the \ldanalog\rd nature of mental rotation.     

Classical geometric illusion effects with nonclassical stimuli: Angular induction from decomposing lines into point arrays

Angular induction is the process by which one line segment can bias judgment of orientation and/or collinearity of another segment, and it has been established that the magnitude of error is a determinate function of the relative angle between the two. We examined how these known relationships are affected by decomposing the induction segment into an array of scattered points. The bias that was produced by such arrays was found to be consistent with a formal model of angular induction, with the strength of the effect decreasing as the scatter among the points was increased. This decline in strength was almost linear with a logarithmic transform of the dimensions of the stimulus array. We also evaluated the hypothesis that the induction stimulus is detected by one or more channels—for example, neurons—for which the sensitivity profiles are modeled as Gabor wavelets. The change in induction strength with increasing point scatter was not predicted by a single width of channel. However, the combined activity of an ensemble of channels that differed in width did match the perceptual effects if one also stipulated that each channel would respond maximally to a fine-line stimulus.     

Representation of anatomical constraints in motor imagery: mental rotation of a body segment

Classically, the mental rotation paradigm has shown that when subjects are asked to judge whether objects that differ in orientation are spatially congruent, reaction times increase with angular discrepancy, although some reports have shown that this is not always the case. Would similar results be obtained with realistic figures of body segments? In this work, the mental rotation of a hand attached to its forearm and arm in anatomically possible and impossible starting positions is compared with the mental rotation of a hammer. The main results show that reaction times increase monotonically with the angle of discrepancy for both stimuli and that the speed of rotation is higher for anatomically possible orientations in the case of the hand. Thus, mental rotation of body segments follows the same empirical rules as objects of another nature, and biomechanical constraints imposed to the motility of these segments can be considered as attributes of the mental representation.     

Culture variation in the average identity extraction: The role of global vs. local processing orientation

ABSTRACT

Research has shown that observers often spontaneously extract a mean representation from multiple faces/objects in a scene even when this is not required by the task. This phenomenon, now known as ensemble coding, has so far mainly been based on data from Western populations. This study compared East Asian and Western participants in an implicit ensemble-coding task, where the explicit task was to judge whether a test face was present in a briefly exposed set of faces. Although both groups showed a tendency to mistake an average of the presented faces as target, thus confirming the universality of ensemble coding, East Asian participants displayed a higher averaging tendency relative to the Westerners. To further examine how a cultural default can be adapted to global or local processing demand, our second experiment tested the effects of priming global or local processing orientation on ensemble coding via a Navon task procedure. Results revealed a reduced tendency for ensemble coding following the priming of local processing orientation. Together, these results suggest that culture can influence the proneness to ensemble coding, and the default cultural mode is malleable to a temporary processing demand.     

The rod-and-frame effect: the whole is less than the sum of its parts

Since the discovery of the influence of the tilted frame on the visual perception of the orientation perceived as vertical (VPV), the frame has been treated as a unitary object-a Gestalt. We evaluated the effect of 1-line, 2-line, 3-line, and 4-line (square frame) stimuli of two different sizes, and asked whether the influence of the square frame on VPV is any greater than the additive combination of separate influences produced by the individual lines constituting the frame. We found that, for each size, the square frame is considerably less influential than the additive combination of the influences of the individual lines. The results conform to a mass action rule, in which the lengths and orientations of the individual line components are what matters and the organization of the lines into a square does not-no higher-level Gestalt property is involved in the induction effect on VPV.     

Angular induction as a function of contact and target orientation

The Poggendorff effect is seen as misalignment of two obliques, or misprojection of one, when the obliques are placed outside a set of parallel lines. To understand better the mechanisms behind this effect, the orientation of the lines which are normally parallel was systematically manipulated. The results indicate that projection bias is affected by the orientation of either line, is at a minimum where the line is orthogonal to the oblique, and is maximal at small angles. This is in line with classic theories which attribute the illusion to misperception of angular size. However, such explanations presuppose that in order to be effective the induction line must be proximal to the oblique so that an angle can be formed. Results are reported which show that the angle formed by the oblique and a line placed at a distance from the oblique, serving as the target of the projection, follows an angular rule of effectiveness similar to what is seen when the line is placed directly in contact with the oblique. The underlying process is described as 'angular induction'.     

Local and global mechanisms of one- and two-dimensional orientation illusions

One-dimensional (1-D) orientation illusions induced on a test grating by a tilted and-surrounding 1-D inducing grating have a well-known angular function that exhibits both repulsion and attraction effects. Two-dimensional (2-D) orientation illusions are those induced on a test grating by 2-D image modulation, such as a pair of superimposed inducing gratings at different orientations, usually orthogonal (a plaid). Given the known angular functions induced by the plaid component gratings, two hypotheses were developed that predicted different plaid-induced illusion functions. Hypothesis 1 states that the 1-D component-induced effects simply add linearly; Hypothesis 2 states that there is an additional mechanism that responds to the virtual axes of mirror symmetry of the plaid and adds to the effect. The data of two experiments were consistent with the predictions from the second hypothesis but not the first. Possible neural substrates of mechanisms that extract axes of symmetry are discussed; it is suggested that such global symmetry axes may underlie the perceived orientation of complex shapes.