Biomedical subjects
Publications and source records attributed to P Wenderoth.
Parks presented two example figures to illustrate that (i) familiarity can enhance illusory contour effects by completing an otherwise incomplete form and (ii) such contours are enhanced when their presence can explain an otherwise familiar but incomplete form. While familiarity probably does enhance illusory contours, additional perceptual factors may be involved in Parks's demonstrations.
Palmer and Hemenway (1978 Journal of Experimental Psychology: Human Perception and Performance 4 691-702) reported that shapes with multiple axes of symmetry are processed faster than those with single symmetry even when trials are blocked so that the subject knows that any symmetry axis will be vertical. Because their model of symmetry detection postulated a two-stage process in which all orientations are searched crudely at first, in no particular order, followed by second-stage scrutiny, the continued salience of multiple over single symmetry with blocking could not be explained. They claimed that stimuli with multiple axes of symmetry have an additional 'goodness'. Four experiments are reported in which it is demonstrated that both sensitivity (d') and response bias (beta) vary considerably in symmetry detection, not just as a function of the positive (symmetrical) stimuli used but also as a function of the negative or conjugate instances selected. Although stimuli with multiple axes of symmetry may well have extra salience due to pattern 'goodness', this factor may have been confounded with response bias in Palmer and Hemenway's experiments. It is suggested that several of their--as well as other researchers--results could be due to some combination of the effects of type of positive stimulus, type of negative stimulus, and response bias directed towards responding positively to highly symmetrical stimuli in a mix of less symmetrical stimuli. Palmer and Hemenway appear to have been correct in suggesting that subjects are more sensitive to quadruple than single symmetry, but the experiments indicate that subjects are also more willing to respond "symmetrical" to stimuli with quadruple symmetry when trials are not blocked, as in Palmer and Hemenway's experiment 1. However, it is demonstrated that the latter effect can be removed by blocking trials so that only one class of symmetrical pattern and one class of asymmetrical pattern occur in any block.
This report adds to existing evidence that a monocular, feature-sensitive motion mechanism is involved in two-dimensional (2-D) motion processing, and also accounts for an earlier, unexplained result [Alais et al.(1994) Vision Research, 34, 1823-1834]. The central finding is that the perceived direction of a monocularly viewed type II plaid changes over a period of continuous exposure such that post-adaptation direction judgements exhibit more of the component-direction bias known to occur with these stimuli than pre-adaptation judgements. These adaptation effects are confined to the adapted eye: when the adapting stimulus is presented to one eye, pre- and post-adaptation direction judgements made with the other, non-adapted eye are identical. These results strongly suggest the involvement of a monocular motion mechanism in two-dimensional motion processing, in addition to the more commonly presumed binocular mechanisms.
An analysis of previous studies of bilateral symmetry detection in dot patterns revealed what appeared to be an almost arbitrary choice of pattern parameters and constraints with no systemic examination of the effects of these parameters and constraints on observer performance. In Expt 1, 100-dot patterns either had no constraints on how they were plotted or had one or both of two constraints: either no dot was permitted to be plotted within a fixed distance of any other dot; or randomly selected dot radii were transformed to make the dot distribution more uniform. While a large vertical symmetry salience effect was obtained, both in number correct and reaction time, only marginal differences occurred between the various constraint conditions. However, when number of dots in the pattern was varied in Expt 2, increasing dot number from 10 to 80 had no effect at all on vertical symmetry detection but linearly decreased performance for other axis orientations. Experiments 3 and 4 together suggested that the critical variable producing the performance decrease was number of dots per se, not increasing dot density (which would tend to give all patterns a more circular outline) and not decreasing the distance between neighbouring dot. Thus, the relative salience of vertical over other symmetries is critically dependent on number of dots in the patterns and it is suggested that vertical symmetry is processed globally so that dot pairs are compared in parallel, whereas at other axis orientations symmetry is processed locally so that dot pairs are compared in serial fashion. Possible neurophysiological and cognitive factors are discussed which might account for the relative performances with different symmetry axis orientations.
Detection of vertical bilateral symmetry has previously been studied in patterns composed of black or white dots on a grey background under four conditions: (a) same contrast (black or white) for all dots (called BB or WW, for 'all black or all white'); (b) half of the dots black and half white with positive correspondence between symmetrical dot pairs (called MA for 'matched'); (c) half of the dots black and half white with negative correspondence between symmetrical dot pairs (called OPP for 'opposite'); and (d) black (white) dots on one side of the axis and white (black) dots on the other (called BW for 'one side black the other white'). It was found that performance was ordered BB (or WW) = MA > OPP = BW, where > indicates better performance. That experiment was repeated here in experiment 1 with symmetry axes not only at vertical but also at horizontal and the two diagonals. It was found overall that BB = MA > OPP, BW. However, OPP > BW when random trials were included in the analysis but when they were excluded BW > OPP. This was due to a very high false-alarm rate in condition BW which could be accounted for if grouping by colour occurs prior to symmetry detection. In experiment 2 it was shown that vertical-symmetry salience over other orientations remained about the same as OPP patterns progressively changed into BB patterns by varying the percentage same polarity between 0% and 100% in 12%-13% steps. Thus, dot-pair polarity affects performance without affecting relative axis salience, as was also found recently when dot pattern outlines were masked. All of the data indicate that although opposite dot polarity does reduce performance slightly, the symmetry-detection mechanism is remarkably resilient to such perturbation. The high false-alarm rate in the BW condition of experiment 1 may be accounted for by extremely salient global grouping of dots by luminance which effectively creates an integral stimulus which is perceptually difficult to break down into its component dot pairs, prohibiting the required point-by-point matching necessary to reject symmetry detection. The small detrimental effect of nonmatched polarity might be due to the polarity differences masking the grouping of dots into 'clumps' on either side of the axis, a process for which there is a great deal of independent evidence.
Neurones tuned for second-order stimuli--those which have edges defined by properties other than luminance and colour--have been frequently observed in prestriate cortex and in area V2 there are neurones which explicitly and unambiguously signal the orientation of purely subjective contours, i.e. contours with no Fourier components at the orientation of the perceived edge [von der Heydt, R. & Peterhans, (1989) Journal of Neuroscience, 9, 1731-1748]. No neurones in area V1 showed similar tuning characteristics. In addition, it has been demonstrated that like real contours, purely subjective test contours are subject to tilt aftereffects following prolonged viewing of an adapting stimulus. whether that stimulus is real or subjective [Paradiso, M. A., Shimojo, S. & Nakayama, K. (1989) Vision Research, 29, 1205-1213]. This result supports the assertion that the cortical processes responsible for real contour perception are also those giving rise to subjective contour perception. The data reported here further examined this hypothesis. Four experiments show that purely subjective contours exhibit both direct and indirect tilt aftereffects and tilt illusions like those observed with real contours. Further, they provide evidence that direct and indirect subjective contour effects, like direct and indirect real contour effects, arise via the operation of two mechanisms: a low level process, possibly lateral inhibition between orientation channels, and a second "higher-order" process. The data suggest that processing of orientation information beyond the striate cortex is similar to that which occurs in area V1 and the data are consistent with models of contour processing which assume that all perceived contours, both real and subjective, arise from a common mechanism.
Experiment 1 demonstrates that, while the outline shapes of bilaterally symmetrical dot patterns play a role in symmetry detection, the removal of the outline by a surrounding random-dot annulus merely reduces performance by a fixed amount. It does not affect the relative salience of different symmetry-axis orientations. The converse is also true: performance is almost as good when the symmetrical dot pattern is confined to the surrounding annulus but is disrupted similarly when the central area is filled with random dots (Experiment 2). In Experiment 3, any one or more of three areas--a central circular area and two surrounding annuli--could be filled with vertically symmetrical or random dots and symmetry was detected reliably only when the central circular area contained the symmetrical dots. A new paradigm was explored in Experiment 4: subjects judged the orientation--left or right of vertical--of 20-dot symmetrical patterns oriented 1-4 deg left or right of vertical and with or without surrounding random-outline masks. Surprisingly, the orientation judgments were extremely precise, with JNDs of the order of 1 deg or less, and outline masking dots made no difference. Experiment 5 showed that performance was even better with just two dots and it made no difference whether these were separated by just under 4 deg or just over 20 deg. It was concluded that while pattern outline plays some role in dot symmetry detection, the major mechanisms are located near the fovea. It is suggested that the relative salience of vertical and horizontal symmetry only in central vision is related to the preponderance of foveal cells tuned to those orientations (Mansfield, Science 186, 1133-1135, 1974; Mansfield and Ronner, Brain Res. 149, 229-234, 1978) but that attentional factors and scanning strategies also boost the salience of oblique symmetry and the salience of vertical over horizontal symmetry (Wenderoth, Perception 23, 221-236, 1994).
A number of recent studies have suggested that the "intersection of constraints" model of two dimensional motion perception, put forward by Adelson and Movshon [(1982) Nature, 300, 523-525], is incomplete. Evidence has been mounting that there is a second two-dimensional motion sensitive mechanism which is monocular and which appears to respond directly to the movement of the intersections (or "blobs") in a two-dimensional image. The current study extends these findings by demonstrating that the perceived coherence of a drifting plaid is largely under the control of a monocular mechanism. Prior exposure to a similarly drifting grating or plaid substantially raises the coherence threshold of a test plaid only if the same eye is adapted and tested. The threshold elevation is much more modest if the test plaid is presented to the unadapted eye, suggesting that coherence judgements are primarily based on the activity level of a monocular process--possibly the "blob tracking mechanism". The results of Expt 2 suggest the possibility that this monocular mechanism is inhibited by binocular exposure.
When motion aftereffects (MAEs) are measured by adapting to a drifting plaid (simultaneous adaptation) or by adapting to the plaid's component gratings in alternation (alternating adaptation), it has been shown that the velocity and duration of the MAE are smaller in the latter case [Wenderoth, P., Bray, R. & Johnstone, S. (1988) Perception, 17, 81-91; Burke, D. & Wenderoth, P. (1993) Vision Research, 33, 351-359]. However, Burke and Wenderoth additionally reported that the directions of MAEs induced by simultaneous and alternating adaptation were identical, an apparent inconsistency if the differences in duration and velocity were due to the presence of "blobs" at the component grating intersects in the simultaneous case. Presumably, the direction of the "blobs" should also affect perceived plaid direction during adaptation and, hence, the MAE direction. In five experiments, we have measured both perceived adapting plaid and MAE direction, tested with both alternating and simultaneous adaptation, measured interocular transfer of plaid-induced MAEs and obtained MAE and plaid direction judgments under monocular and binocular viewing conditions. All of the data indicate that there is a blob tracking mechanism which is preferentially stimulated by plaids whose component gratings have high spatial frequency, low temporal frequency and high contrast. Differences between simultaneous and alternating adaptation emerge only when more optimal blobs are used, thus accounting for Burke and Wenderoth's failure to find a difference. The data also support Burke and Wenderoth's claim that the blob tracking mechanism is monocular: alternating and simultaneous adaptation produce identical MAEs under interocular transfer conditions, even using plaids with more optimal blobs. We also report the unexpected finding that plaids with more- and less-optimal blobs appear to drift in directions 20 degrees apart yet their aftereffects differ in direction by only 3-5 degrees. That is, more optimal blob plaids--compared with less optimal blob plaids--change both perceived plaid direction during adaptation and subsequent perceived MAE direction but the latter change is much more modest. Possible explanations of this dissociation are considered.
It has long been accepted that amongst patterns which are bilaterally symmetrical, those which have their axis of symmetry vertical are more saliently symmetrical than patterns whose axis of symmetry is at some other orientation. The evidence regarding the relative salience of other orientations of axis of symmetry is somewhat more equivocal. In experiment 1, subjects were required to discriminate between symmetric or random-dot patterns when the axis of symmetry was at one of eighteen different orientations, spaced 10 degrees apart, both clockwise and counterclockwise of vertical to horizontal. The data indicated that vertical was most salient, then horizontal but that, unlike in the classical oblique effect for contrast sensitivity, performance for precisely diagonal axes was better than that for surrounding axis orientations. Additional data (from experiments 2 and 3) also showed that the salience of vertical and horizontal axes of symmetry can be manipulated extensively by varying the range of stimuli presented, presumably by manipulating the scanning or attentional strategy adopted by the observer. Many previous studies of symmetry perception may have confounded hard-wired salience for vertical symmetry with scanning or attentional strategies.
Motion aftereffects (MAEs) can be induced by adaptation to a pair of differently oriented drifting gratings whether the gratings are presented simultaneously, as a coherent plaid, or in alternation. The fact that the former MAEs were generally larger than the latter led to the suggestion that simultaneous adaptation involved higher-level extrastriate processes not involved in the alternating effects. In the past few years evidence has accumulated that the difference is in fact due to a low-level monocular process which can be termed the 'blob-tracking mechanism'. A review is presented of the evidence on MAEs induced by simultaneous and alternating adaptation, the evidence for the monocularity of the blob-tracking mechanism, the data which implicate the blob mechanism in the determination of MAE magnitude, perceived plaid drift direction, and in perceived plaid coherence.
Ferrera and Wilson [(1990) Vision Research, 30, 273-287] reported veridical perception of the direction of motion of Type I plaids, whose component gratings span the resultant direction, but marked misperception of the direction of motion of Type II plaids, whose component gratings both lie on one side of the resultant direction. Because they failed to find any effect of component direction (angular) separation on this misperception, Ferrera and Wilson concluded that the misperception was not due to perceptual repulsion of component directions. We report that component direction repulsion does occur, that plaid direction misperception is tuned to component separation, with larger repulsions for smaller angles. It is concluded that there is no fundamental difference in direction coding for Type I and Type II plaids, and that Ferrera and Wilson failed to find a direction separation effect because the range of separations they used was insufficiently broad to detect the slope of the angular function.
Wenderoth, Bray and Johnstone [(1988) Perception, 17, 81-91] measured motion aftereffects induced on stationary vertical sine-wave gratings by horizontally drifting two-dimensional patterns (plaids). The adapting plaid component gratings were simultaneously or alternately presented and were oriented left and right of vertical by 15, 45 or 75 degrees. It was found that aftereffects decreased linearly in the alternating conditions as the plaid component orientations changed but this was not the case in the simultaneous adaptation conditions, a finding taken to be consistent with the hypothesis that one-dimensional aftereffects have a low level site (possibly V1) whereas two-dimensional effects have a higher level site (possibly MT). In three experiments, we have examined in more detail the determinants of aftereffects induced by simultaneous and alternating plaid components. The data suggest that the mechanisms involved are more complex than those put forward by Wenderoth et al. and that plaid perception utilizes both higher and lower level processes which can be referred to, respectively, as an intersection of constraints algorithm and a moving "blob" detector.
Orientation illusions induced by two-dimensional stimuli, such as square outline frames or plaids, have been more or less adequately accounted for in terms of repulsion of the vertical test stimulus from the axis of symmetry nearest vertical of the inducing stimulus, whether that axis is real or virtual. Recently, data have been obtained which directly suggest a more complex mechanism: one in which the observed illusion is the sum of all effects--complementary and antagonistic--induced by all axes flanking vertical which are sufficiently close to vertical to exert a significant effect. Experiments are reported in which this latter hypothesis was directly tested by using nonorthogonal plaid component gratings and varying the real-axis orientations while a virtual plaid axis remained fixed in orientation at 10 degrees from vertical. The data indicate that the real component gratings modulate the virtual-axis effect.
A grey disc which exhibits a vertical gradient of shading usually appears convex if lighter above and concave if lighter below. This phenomenon was investigated by Howard et al (1990) who varied both the shading axis relative to gravity and the orientation of the head. Their results indicated that head-centric or retinocentric coordinates determined the depth effect rather than gravitational axes. However, several possible problems with their study were noted, not the least of which was the possible intrusion of response rather than perceptual factors in the task they used. Here, we attempted to use an indirect measure of the perception of depth from shading; rather than asking subjects whether discs looked convex or concave, we constructed ensembles of shaded discs which, in terms of depth from shading, were or were not bilaterally symmetrical about an horizontal axis. These stimulus displays were briefly flashed to prohibit the intrusion of conscious assumptions about direction of light sources. Subjects were never asked whether any discs looked concave or convex, merely whether the set of discs was or was not depth symmetrical. Results were generally consistent with those of Howard et al and supported the conclusion that depth from shading is largely a low-level and automatic mechanism.
Day (1990) and Day, Mitchell, and Stecher (1990) recently reported new data on the Bourdon illusion, showing that the effect occurs in novel variations of the classic figure--for example, with orthogonal and parallel test edges--and also that it occurs in the haptic modality. Day (1990) criticized theories of the Bourdon effect proposed by Wenderoth, Criss, and van der Zwan (1990), Wenderoth and O'Connor (1987a, 1987b), and Wenderoth, O'Connor, and Johnson (1986), and proposed his own "perceptual compromise" hypothesis. It is argued that Day has inaccurately portrayed the Wenderoth et al. theory, and that his own hypothesis lacks predictive power because it lacks any reference to neural mechanisms. Day's theory is thus unable to account for extant data, including his own. It is demonstrated how Day's novel experiments provide insights into the mechanisms of the Bourdon effect, and also how the Wenderoth et al. theory can account for the new data of Day (1990) and Day et al. (1990).
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.
Lucca, Dellantonio, and Riggio (1986) reported large distortions in a tactual analogue of the visual Poggendorff illusion. They also reported large effects in the direction opposite to the visual illusion, which they termed "inversions." However, their evidence for such effects is questionable; they used what we consider to be inappropriate measurement and analysis procedures. In attempting to replicate their experiment, and in conducting four additional experiments, we found no evidence at all for their alleged tactual analogue of the visual Poggendorff effect. Instead, we demonstrated that "inversions" are likely due to the use of a raised stimulus display that causes artifactual mistracking, which is totally unrelated to normal mechanisms of alignment judgment. We also discuss the possible role of intersensory factors in the generation of tactual illusions.