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PubMed1994

Invariance from the Euclidean geometer's perspective.

It is remarkable how well the human visual system can cope with changing viewpoints when it comes to recognising shapes. The state of the art in machine vision is still quite remote from solving such tasks. Nevertheless, a surge in invariance-based research has led to the development of methods for solving recognition problems still considered hard until recently. A nonmathematical account explains the basic philosophy and trade-offs underlying this strand of research. The principles are explained for the relatively simple case of planar-object recognition under arbitrary viewpoints. Well-known Euclidean concepts form the basis of invariance in this case. Introducing constraints in addition to that of planarity may further simplify the invariants. On the other hand, there are problems for which no invariants exist.

Attention↗
PubMed1994

Ambiguous pictorial depth cues and perceptions of nonrigid motion in the three-loop figure.

The three-loop figure is a two-dimensional (2-D) pattern that generates (mis)perceptions of nonrigid three-dimensional (3-D) structure when rotated about its centre. Such observations have been described as counterexamples to the principle whereby a moving object is presumed to be rigid, provided that a rigid interpretation is possible (ie the 'rigidity constraint'). In the present investigation we demonstrated that stationary three-loop figures exhibit many of the classic properties of multistable/ambiguous figures, with any one of several possible 3-D configurations being reported at any one instant. Further investigation revealed that perceived nonrigidity during rotation was markedly reduced (and rigidity enhanced) when the figure was modified with static pictorial depth cues (eg shading, interposition). These cues had no effect on the overall proportion of time that observers reported 3-D organisations in stationary versions of the figure, but significantly reduced the frequency of perceptual reorganisation, and increased the duration for reporting a particular organisation. Since each of the perceived 3-D structures in a stationary ambiguous 2-D figure has a unique kinetic counterpart (ie rigid transformation), we attribute the nonrigid structure perceived when the figure rotates to the integration of these otherwise inconsistent kinetic components; and have further illustrated this with modified versions of a Penrose impossible triangle. Under kinetic versions of the classical size/distance invariance hypothesis, the rigidity constraint may be considered to represent a special instance of size/shape constancy, in which case counterexamples involving (mis)perceptions of nonrigid structure are comparable to other well-known exceptions to such principles of minimum object change (eg classical illusions).

Cues↗
PubMed1994

A model for identifying the perceptual centre of polygons.

An attempt is made to develop a general model of perceptual-centre determination. The model describes the possible 'perceptual operations' involved in determining the perceptual centre of flat figures. The specific steps in the model are: (i) determining the orientation and size of the figure; (ii) determining the 'base' of the figure and its 'top-bottom axis'. It is suggested that anisotropy along the main axes of the figures determines a displacement of the perceptual centre toward the phenomenal top of the shape. A quantitative measure for the displacement of the perceptual centre from the area barycentre of the figure is introduced.

Form Perception↗
PubMed1995

Perceiving heading in the presence of moving objects.

In most models of heading from optic flow a rigid environment is assumed, yet humans often navigate in the presence of independently moving objects. Simple spatial pooling of the flow field would yield systematic heading errors. Alternatively, moving objects could be segmented on the basis of relative motion, dynamic occlusion, or inconsistency with the global flow, and heading determined from the background flow. Displays simulated observer translation toward a frontal random-dot plane, with a 10 deg square moving independently in depth. The path of motion of the object was varied to create a secondary focus of expansion (FOE') 6 deg to the right or left of the actual heading point (FOE), which could bias the perceived heading. There was no effect when the FOE was visible, but when the object moved in front of it, perceived heading was biased toward the FOE' by approximately 1.9 degrees with a transparent object, and approximately 3.4 degrees with an opaque object. The results indicate that scene segmentation does not occur prior to heading estimation, which is consistent with spatial pooling weighted near the FOE. A simple template model based on large-field, center-weighted expansion units accounts for the data. This may actually represent an adaptive solution for navigation with respect to obstacles on the path ahead.

Analysis of Variance↗
PubMed1995

Auditory isochrony: time shrinking and temporal patterns.

It has previously been reported that the duration of short time intervals is conspicuously underestimated if they are preceded by shorter neighbouring time intervals. This illusion was called 'time shrinking' and it was argued that it strongly affects the perception of auditory rhythms. In the present study this supposition has been pursued in three experiments. In the first, temporal patterns consisting of two, three, and four intervals had to be judged for anisochrony, which was invoked by offsetting the last sound from its isochronous position. By a constant method, it was determined that the last sound of fast sequences (50 ms base interval) had to be delayed by about 30 ms in order for isochronous rhythms to be perceived. Another interesting finding was that for sound sequences with base intervals up to 200 ms it was the difference limen, rather than Weber's ratio, that was constant for anisochrony detection. In the second experiment, the temporal patterns comprised two intervals, presented serially or separately. The deviation of isochrony could be on either the first or the second interval. The data, gathered by an adaptive method, showed time shrinking to be effective even up to a base interval of 200 ms. The third experiment involved a constant method and anisochrony was implemented on the first interval of two interval patterns. Time shrinking affected perceived isochrony in sequences with base intervals of 50, 100, and 200 ms. It is argued that the paradoxical results of anisochrony detection can be explained in terms of time shrinking. Some anomalies of rhythm perception and production that are the result of time shrinking are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

The perceptual flattening of three-dimensional scenes enclosed by a frame.

The effects of a visible frame around a three-dimensional scene on perceived depth within the scene was investigated in three experiments. In experiment 1 subjects judged the slant of an object that had been rotated about a vertical axis. Judged slant was reduced when the frame was illuminated. In experiments 2 and 3 subjects judged the shape (width-to-height ratio) of the object. The object was judged to be narrower when the frame was illuminated (experiment 2) or when a frame was added to the scene in an illuminated room (experiment 3). These results demonstrate that the presence of a frame around a three-dimensional scene serves as a flatness cue, reducing perceived depth within the scene.

Analysis of Variance↗
PubMed1996

Qualitative cues in the discrimination of affine-transformed minimal patterns.

An important factor in judging whether two retinal images arise from the same object viewed from different positions may be the presence of certain properties or cues that are 'qualitative invariants' with respect to the natural transformations, particularly affine transformations, associated with changes in viewpoint. To test whether observers use certain affine qualitative cues such as concavity, convexity, collinearity, and parallelism of the image elements, a 'same-different' discrimination experiment was carried out with planar patterns that were defined by four points either connected by straight line segments (line patterns) or marked by dots (dot patterns). The first three points of each pattern were generated randomly; the fourth point fell on their diagonal bisector. According to the position of that point, the patterns were concave, triangular (three points being collinear), convex, or parallel sided. In a 'same' trial, an affine transformation was applied to one of two identical patterns; in a 'different' trial, the affine transformation was applied after the point lying on the diagonal bisector was perturbed a short, fixed distance along the bisector, inwards for one pattern and outwards for the other. Observers' ability to discriminate 'same' from 'different' pairs of patterns depended strongly on the position of the fourth, displaced, point: performance varied rapidly when the position of the displaced point was such that the patterns were nearly triangular or nearly parallel sided, consistent with observers using the hypothesised qualitative cues. The experimental data were fitted with a simple probabilistic model of discrimination performance that used a combination of these qualitative cues and a single quantitative cue.

Adult↗
PubMed1996

Segregation of motion structure from random visual noise.

A potential source of information about spatial layout, surface slant, and self-motion is provided by transformations of the optic flow field. Theoretical analyses have demonstrated that such flow fields can be decomposed into translation, rotation, expansion, and deformation components. The objective in this study was to investigate the effectiveness of these motions as visual primitives for preattentive scene segregation. The stimuli consisted of two-frame patterns containing 998 dots. A variable number of these dots (5 to 50) were defined as target dots, representing a specific motion structure, with the residual dots defined as a noise field. The observers' task was to identify the location and type of motion structure. On the basis of the number of target dots needed to detect and correctly identify the target structure, it was determined that translation was the most easily detected structure, followed by rotation, expansion, and compression. Deformation was found to have a detection threshold as high as compression. The results are related to other studies of these motion components.

Adult↗
PubMed1996

Spatial context in recognition.

In recognizing objects and scenes, partial recognition of objects or their parts can be used to guide the recognition of other objects. Here, the role of individual objects in the recognition of complete figures and the influence of contextual information on the identification of ambiguous objects were investigated. Configurations of objects that were placed in either proper or improper spatial relations were used, and response times and error rates in a recognition task were measured. Two main results were obtained. First, proper spatial relations among the objects of a scene decrease response times and error rates in the recognition of individual objects. Second, the presence of objects that have a unique interpretation improves the identification of ambiguous objects in the scene. Ambiguous objects were recognized faster and with fewer errors in the presence of clearly recognized objects compared with the same objects in isolation or in improper spatial relations. The implications of these findings for the organization of recognition memory are discussed.

Adult↗
PubMed1996

There is no evidence that Kanizsa-type subjective contours can be detected in parallel.

Davis and Driver presented evidence suggesting that Kanizsa-type subjective contours could be detected in a visual search task in a time that is independent of the number of nonsubjective contour distractors. A linking connection was made between these psychophysical data and the physiological data of Peterhans and von der Heydt which showed that cells in primate area V2 respond to subjective contours in the same way that they respond to luminance-defined contours. Here in three experiments it is shown that there was sufficient information in the displays used by Davis and Driver to support parallel search independently of whether subjective contours were present or not. When confounding properties of the stimuli were eliminated search became slow whether or not subjective contours were present in the display. One of the slowest search conditions involved stimuli that were virtually identical to those used in the physiological studies of Peterhans and von der Heydt to which Davis and Driver wish to link their data. It is concluded that while subjective contours may be represented in the responses of very early visual mechanisms (eg in V2) access to these representations is impaired by high-contrast contours used to induce the subjective contours and nonsubjective figure distractors. This persistent control problem continues to confound attempts to show that Kanizsa-type subjective contours can be detected in parallel.

Cues↗
PubMed1997

Investigation into the origin of the haptic aftereffect of curved surfaces.

In haptics, the perceived (phenomenal) flatness of a surface is strongly influenced by a previous surface which has been statically touched. The mechanisms underlying this haptic aftereffect of curved surfaces are investigated. It is shown that the representation of curvature abstracted from the sense of touch, ie a high-level representation, is not affected during the aftereffect. This is concluded because: (1) the aftereffect does not exhibit intermanual transfer; (2) the way in which two successive surfaces are touched can influence the magnitude of the aftereffect; and (3) it is not necessary to touch a surface-active muscular contraction can also result in a shift of the phenomenal flatness. Furthermore, it is suggested that the physiological process involved in the aftereffect is a central process, ie it is located in the brain but it is distinct for each hemisphere. This is supported by the findings that: (1) the decay rate of the aftereffect is not influenced by the degree of peripheral stimulation during the decay; and (2) the aftereffect does not transfer from the adapted hand to the unadapted hand.

Humans↗
PubMed1997

The Thatcher illusion as a test of configural disruption.

Bartlett and Searcy's recent account for the Thatcher illusion suggests that inversion impairs holistic facial information. This illusion is used to compare the effects of inverting and negating faces. Subjects made a speeded response to whether just the mouth and the eyes of a face have been inverted. Performance was found to be slower when faces were inverted rather than upright. Presenting faces in photographic negative also hindered performance implying that this transformation also disrupts holistic facial information.

Analysis of Variance↗
PubMed1997

Blur and contrast as pictorial depth cues.

Studies have shown that blur can act as a pictorial cue to depth perception. But blurring a stimulus reduces its contrast, and studies have also shown that contrast can act as a pictorial cue to depth perception. To determine whether blur and contrast have separate influences on depth perception, each variable was independently manipulated in two experiments. Observers reported depth alternations in a simple reversible figure. Both contrast and blur were found to influence depth perception, but blur had its greatest effect at moderate contrasts. When blurred and sharp stimuli were equated on either Michelson or RMS contrast, blur continued to affect depth perception. Hence blur can act as a depth cue independently of contrast. It is speculated that blur is effective as a pictorial cue because of its usual association with other depth cues, particularly in pictures and photographs.

Adult↗
PubMed1997

Fractal timing of verbal transforms.

The paper deals with the timing pattern of perceptual change elicited by multiple repetitions of a syllable (the verbal-transformation effect). We show that the distribution of the dwell time, the time spent perceiving a given phonemic form before switching to another form, obeys a power law with an exponent valued between 1 and 2. This result is robust, occurring for meaningless syllables and for English words of different initial phonemic salience. Experiment 2 demonstrates that subjects are aware of the temporal dynamics of perceptual change. On the basis of these results, it is argued that within this paradigm the notion of a mean dwell time is ill defined and there is apparently no characteristic time scale for perceptual change.

Adolescent↗
PubMed1999

Evidence for holistic processing of faces viewed as photographic negatives.

Inversion and photographic negation both impair face recognition. Inversion seems to disrupt processing of the spatial relationship between facial features ('relational' processing) which normally occurs with upright faces and which facilitates their recognition. It remains unclear why negation affects recognition. To find out if negation impairs relational processing, we investigated whether negative faces are subject to the 'chimeric-face effect'. Recognition of the top half of a composite face (constructed from top and bottom halves of different faces) is difficult when the face is upright, but not when it is inverted. To perform this task successfully, the bottom half of the face has to be disregarded, but the relational processing which normally occurs with upright faces makes this difficult. Inversion reduces relational processing and thus facilitates performance on this particular task. In our experiments, subjects saw pairs of chimeric faces and had to decide whether or not the top halves were identical. On half the trials the two chimeras had identical tops; on the remaining trials the top halves were different. (The bottom halves were always different.) All permutations of orientation (upright or inverted) and luminance (normal or negative) were used. In experiment 1, each pair of 'identical' top halves were the same in all respects. Experiment 2 used differently oriented views of the same person, to preclude matches being based on incidental features of the images rather than the faces displayed within them. In both experiments, similar chimeric-face effects were obtained with both positive and negative faces, implying that negative faces evoke some form of relational processing. It is argued that there may be more than one kind of relational processing involved in face recognition: the 'chimeric-face effect' may reflect an initial 'holistic' processing which binds facial features into a 'Gestalt', rather than being a demonstration of the configurational processing involved in individual recognition.

Adult↗
PubMed1998

Increasing-loudness aftereffect following decreasing-intensity adaptation: spectral dependence in interotic and monotic testing.

Listening to decreasing intensity leads to illusory increasing loudness afterwards. Evidence suggests that this increasing-loudness aftereffect may have a sensory component concerned with dynamic localisation. This was tested by comparing the spectral dependence of monotic aftereffect (adapting and testing one ear) with the spectral dependence of interotic aftereffect (adapting one ear and testing the other ear). Existence of the proposed component implies that monotic aftereffect should be more spectrally dependent than interotic aftereffect. Three listeners were exposed to a 1 kHz adapting stimulus. From responses of "growing softer" or "growing louder" to test stimuli changing in intensity, nulls were calculated; test carrier frequencies ranged from 0.5 kHz to 2 kHz. Confirming the hypothesis, monotic aftereffect was about three times as strong as interotic aftereffect for the 1 kHz test carrier frequency, while monotic and interotic aftereffects were comparable in magnitude for test carrier frequencies below about 0.8 kHz and above about 1.2 kHz. The latter residual aftereffects are attributed to cognitive processing, perhaps concerning response bias. Sensitivity did not vary systematically across conditions; this is consistent with evidence that changing intensity entails mainly direct processing. The results cannot be attributed to the loudness adaptation elicited by steady stimuli.

Adaptation, Psychological↗