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PubMed1984

The effects of illumination level and retinal size on the depth stratification of subjective contour figures.

The apparent stratification in depth of subjective contour figures over their backgrounds was investigated as a function of illumination level, figure size, and viewing distance. Magnitude estimation, with a real contour figure serving as the modulus, was used to measure the stratification in depth of a subjective contour figure over its background. Illumination level and retinal size both had significant effects on the depth stratification of the subjective contour figures. The greatest apparent depth differences were obtained for figures of small retinal size under low levels of illumination. These results paralleled previous findings for judgments of subjective contour strength. Consequently, both contour clarity and depth stratification of subjective contour figures are affected in similar ways by illumination level, figure size, and viewing distance. The implications of this response coupling are discussed in terms of current theories of subjective contours.

Adult↗
PubMed1985

Haploscopic colour mixtures with and without contours in subjects with normal and disturbed binocular vision.

Binocular mixtures of equiluminous components of different wavelengths were matched with additive monoptic mixtures of the same components. After a satisfactory match had been achieved, the luminance of each colour in the monoptic mixture was measured photometrically. After presentation of an orthogonal grating superimposed on the colour shown to one eye, the colour matching was repeated. The grating induced a strong dominance of the colour with which it was combined. Yet the uncontoured colour was not entirely suppressed, but contributed to the binocular colour to various degrees. In three subjects with anisometropic amblyopia in one eye the colour presented to the amblyopic eye contributed little or nothing to the haploscopic colour mixture, depending on the degree of amblyopia. This diminished contribution could not be enhanced by a grid. In three cases with strabismic amblyopia and in one case with strabismus alternans no haploscopic colour mixture effects could be demonstrated. The observations are discussed in the context of neurophysiological findings in the visual system of primates, and it is suggested that colour and contour are not transmitted through independent channels from the retina to the cortex.

Adult↗
PubMed1985

Perceptual organization and apparent brightness in subjective contour figures.

According to a number of theories subjective contours arise from brightness contrast and/or assimilation. The apparent brightness gradients generated by these effects are assumed to give rise to the perception of contours delineating the gradients. A study is reported in which naive observers were shown a subjective contour display and asked to report what they saw. They were then asked to judge whether the center or the surround of the display appeared brighter. Subjects whose reports indicated that they had seen the subjective contour figure showed an overwhelming preference for the center of the display being brighter than the surround. However, subjects who did not see the subjective contour figure did not differ significantly in their selection of the center over the surround. This finding presents difficulties for any theory which derives subjective contours from the apparent brightness difference.

Color Perception↗
PubMed1986

Encoding speed of visual features and the occurrence of illusory conjunctions.

Earlier studies have found that people show characteristic errors when asked to observe a rapid stream of events and to name the colour of a certain letter, or the letter in a certain colour. They tend to report the letter or colour from an event later in time than the correct one. This is not true, however, if they are asked to name the colour of a digit amongst letters, without knowing in advance the identity of the digit. There is also some evidence that the errors are symmetric if the location of the target is not known in advance. Three experiments are reported which show that this symmetry results from slow encoding of the colour as compared with the digit. It is still true that uncertainty about target identity does reduce the tendency to see the target in a later than an earlier colour. This effect is, however, superimposed on asymmetries resulting from coding speed, and the effect of uncertainty of location may be due only to this.

Color Perception↗
PubMed1986

Recognition of positive and negative bandpass-filtered images.

A study is reported in which the significance for vision of low- and high-spatial-frequency components of photographic positive and negative images was investigated by measuring recognition of bandpass-filtered photographs of faces. The results show that a 1.5 octave bandpass-filtered image contains sufficient visual information for good recognition performance, provided the filter is centred close to 20 cycles facewidth-1. At low spatial frequencies negatives are more difficult to recognize than positives, but at high spatial frequencies there is no difference in recognition, implying that it is the low-frequency components of negatives which present difficulties for the visual system.

Color Perception↗
PubMed1988

A new visual problem: phenomenic folding.

It is possible to produce outline drawings that are perceived as representations of sheets or plates folded over themselves. However, only some of the many possible representations are immediately and necessarily perceived as such. Investigations were carried out to find out which elements must be included in a drawing if a subject is to perceive folding. Four necessary, though not individually sufficient, factors were detected. Other factors which are not necessary but which can intensify the perception of folding were also found. The four necessary factors are: (i) the existence of two phenomenically overlapping figures; (ii) at least one side of the upper figure must perfectly coincide with one side of the lower figure, this common side being defined as the folding line; (iii) the two phenomenically overlapping areas must be on the same side of the folding line; (iv) three segments must converge at the ends of the folding line. Some cognitive processes which appear to be involved in the phenomenon are also discussed.

Adolescent↗
PubMed1992

Amodal completion, depth stratification, and illusory figures: a test of Kanizsa's explanation.

Subjective contours have been explained by Kanizsa as being a consequence of amodal completion of incomplete figures. According to the theory of amodal completion, figural incompleteness triggers the emergence of an illusory object superimposed on the gaps in the inducers, which in turn hide parts of the pattern, thus suggesting that the plane of the illusory object must always be seen to be above the plane of the inducers. A figure was created in which subjective contours are seen despite the fact that the perceived depth relationships run counter to that required by the theory of amodal completion. In four experiments, this depth relationship is confirmed by using direct and indirect measures which assess both registered and apprehended depth. By emphasizing a logical inconsistency in the explanation based on amodal completion, the results show that amodal completion, at least in Kanizsa-like patterns, cannot be considered as a causal factor for subjective contour figures.

Adult↗
PubMed1992

Visual search: detection, identification, and localization.

In two studies, observers searched for a single oblique target in a field of vertical distractors. In one experiment, target detection and identification (left versus right tilt) were compared. In another experiment, detection and localization were compared for the left versus the right half of the display. Performance on all three tasks was virtually identical: if a target could be detected, it could also be identified and localized. A review of previous studies generally supports the conclusion that performance on the three tasks is similar. This argues against current search theories, which rest heavily on data showing differences in identification and localization.

Attention↗
PubMed1993

Perceived rigidity and nonrigidity in the kinetic depth effect.

Stroboscopic simulations of three-dimensional rotating rigid structures can be perceived as highly nonrigid. To investigate this nonrigidity effect a sequence of either three (experiment 2 and 3) or thirty six frames (experiment 4) was used, each consisting of a set of dots with location on the horizontal axis corresponding to the parallel projection of a nominally defined helix. Observers were asked to judge the angle of rotation of eighty helices defined by the factorial combination of eight phase (phi) values (ie difference between the sinusoidal path of one dot and its neighbours) and ten different angular displacement values (alpha). When in each static frame the dots can be organized into curved dotted line (small values of phi), the perceived 3-D helices are highly nonrigid. But when shape information is not available in each static frame (high values of phi), the helices are perceived as rigid and rotation judgement is possible providing that alpha < 15 degrees. It appears that at small values of phi observers fail to recover the rigid structure of the helices since the input to the structure from the motion process may be distorted.

Adult↗
PubMed1993

An inquiry into relational concepts of colour, based on incremental principles of colour coding for minimal relational stimuli.

Centre-surround stimuli evoke colour appearances (resembling surface colours) which cannot be produced by a single homogeneous spot of light alone (eg brown or grey). Although this seems of great impact to a general theory of colour (including 'colour constancy'), the psychophysics of these 'minimal relational stimuli' is still less well understood than often assumed. On the basis of empirical as well as theoretical observations concerning centre-surround-type stimuli we introduce a relational model of colour coding. At the core of this model is the concept of a three-dimensional linear incremental colour code which behaves differently for increments and decrements. This model takes into account results on 'discounting the background' mechanisms and it is closely related to ratio-based relational concepts and to certain opponent-colour theories. In addition, it provides an analogue to the classical distinction between light and object colours, and covers colour appearances related to object colours as well. Within the conceptual framework offered, problems of complex colour perception (eg 'colour constancy') and judgmental modes are discussed. Conclusions regarding general limitations of three-dimensional modelling in colour perception are derived and corresponding refinements of the relational perspective are briefly outlined.

Color Perception↗
PubMed1993

New depth to the Müller-Lyer illusion.

It is demonstrated that when the two halves of the Müller-Lyer illusion are shown separately to the two eyes this can result in a perception of the binocularly viewed shaft as slanted in depth. For short fin lengths, the magnitude of this slant can be predicted by the extent of the ordinary two-dimensional illusion. This result is discussed in relation to Julesz's claim that stereoscopic matching of the images in the two eyes must precede whatever process is responsible for the Müller-Lyer illusion (whereas the reported illusion seems to imply the reverse). The cyclopean Müller-Lyer demonstration on which Julesz's argument is based is reexamined. It is suggested that the matching of coarse-scale features in the images in the two eyes may help to explain both the cyclopean and the reported three-dimensional Müller-Lyer illusion.

Depth Perception↗
PubMed1994

The perception of spatial structure with oblique viewing: an explanation for Byzantine perspective?

Earlier work has confirmed that (i) observers can judge divergent receding lines, placed directly in front of them, to be parallel, and (ii) converging lines which are displaced laterally, so that they are viewed obliquely, can also be judged to be parallel. The former observation is in accord with traditional views of perspective while the latter, which is in accord with the depictions of objects found in Byzantine painting, is not in accord with perspective but is predicted by the relative magnitude of the visual angles subtended by the near and far ends of the pair of lines. To investigate whether these effects occurred when the stimulus was clearly three-dimensional, experiments were conducted with a novel apparatus, consisting of a framework of computer-controlled motor-driven luminous rods. This could be remotely adjusted so that all visible sides appeared to be parallel, ie to resemble a cube. Results showed that observers set the sides of this trapezohedron framework as diverging when it was viewed immediately in front of them, a result which is concordant with linear perspective, ie they see the normal projection of a cube as having converging edges. When the framework was displaced from the median plane so that it was viewed obliquely, the sides were set as converging and the magnitude of this effect was significantly related to angle of view, ie observers see the normal projection of a cube as having diverging sides.(ABSTRACT TRUNCATED AT 250 WORDS)

Art↗
PubMed1994

Three-dimensional illusory objects produced by rotation in depth.

Rotation of a Kanizsa triangle in depth around its vertical axis causes a perception of a three-dimensional object with a flat, rigid illusory triangle between the inducing discs. When the inducing discs of a Kanizsa triangle were made thicker, the illusory triangle between the discs also became thicker. In the experiments both computer animation and real inducers made of plastic were used. The method promoted border perception in a three-dimensional illusory figure. We suggest that the perception of three-dimensional illusory objects is due to a process which is also used in the perception of real three-dimensional objects.

Adult↗
PubMed1996

Evidence for good recovery of lengths of real objects seen with natural stereo viewing.

Six experiments are described in which good performance of the task of matching the lengths of two stationary real objects, gnarled wooden sticks, under a variety of binocular viewing conditions, including variations in viewing distances was demonstrated. Relatively poor matching performance was observed when the sticks were viewed monocularly in oscillatory motion, or monocularly and stationary. The results suggest that stereo can support good representations of metric scene structure when length judgments of natural objects are required under (quasi-)natural viewing. The implications of these results for theories of structure from stereo and structure from motion are discussed.

Adolescent↗
PubMed1997

The effect of depth rotation on object identification.

Five experiments are reported in which the time to verify the name of different three-dimensional common objects shown rotated in depth was investigated. Views of computer-generated images of elongated objects rotated in steps of 30 degrees along six axes of rotation were used as stimuli. A significant main effect of view was found in all experiments. This effect was initially attributed to the relatively slower verification times to the end-on views of objects but further analysis revealed that views 30 degrees off the end-on views were significantly slower to verify than other views. Objects with gravitational uprights yielded the same effects as objects without gravitational uprights. The results were not dependent on practice with the stimuli prior to the experiment or on repeated exposure of the views during the experiment. Also, there was no benefit found for the identification of shaded over silhouetted images of objects when shown in more-conventional views but unconventional views were more recognisable when shaded than when silhouetted. Last, initial verification times for familiar views of a set of novel objects were faster than for unfamiliar views even when the views were unconventional. With practice on unfamiliar views, however, the same function relating view to verification time found for familiar objects was found for the novel objects. The results suggest that for recognition purposes visual memory stores discrete views of objects but it characteristically favours a canonical range of views of elongated objects that are based on the salient geometry of the objects so that more unconventional or foreshortened views are less readily recognised.

Adult↗