Perception

Grid illusions, including the Hermann grid and scintillating grid (in which light disks are superimposed upon the grid intersections), are diminished by curving the alleys that limn the repeating pattern. Curvature might either disrupt the processes that induce the illusion, or simply make the illusory effects harder to see. To determine which mechanism might be invoked, we examined the effects of curving the alleys upon the vanishing-disk illusion, a phenomenon in which a single disk in a grid intersection is rendered less detectable. This illusion is of reduced visibility, rather than generating an illusory apparition as in the Hermann grid or scintillating grid. Thus, inhibition of illusory influence would enhance disk visibility, while a general reduction of visibility would render disks even harder to detect. We find that thresholds for both scintillation and the disk itself increase in a graded manner with increased curvature. Measuring the effect of curvature upon the vanishing disk with traditional forced-choice staircase methods demonstrates that the effect of curvature is upon detection, not subjective criterion. Furthermore, disks that are easy to detect within a rectilinear grid are more difficult to detect when the alleys are curved. Thus, curvature of the alleys induces a general tendency to inhibit the visibility of features, and is not specifically a repression of illusory effects.

A single experiment was designed to investigate perceptual learning and the discrimination of 3-D object shape. Ninety-six observers were presented with naturally shaped solid objects either visually, haptically, or across the modalities of vision and touch. The observers' task was to judge whether the two sequentially presented objects on any given trial possessed the same or different 3-D shapes. The results of the experiment revealed that significant perceptual learning occurred in all modality conditions, both unimodal and cross-modal. The amount of the observers' perceptual learning, as indexed by increases in hit rate and d', was similar for all of the modality conditions. The observers' hit rates were highest for the unimodal conditions and lowest in the cross-modal conditions. Lengthening the inter-stimulus interval from 3 to 15 s led to increases in hit rates and decreases in response bias. The results also revealed the existence of an asymmetry between two otherwise equivalent cross-modal conditions: in particular, the observers' perceptual sensitivity was higher for the vision—haptic condition and lower for the haptic—vision condition. In general, the results indicate that effective cross-modal shape comparisons can be made between the modalities of vision and active touch, but that complete information transfer does not occur.

A vertical bar is salient among horizontal ones by orientation contrast, and, traditionally, bottom—up salience is viewed as caused only by feature contrast. Recently, it has been proposed that the primary visual cortex (V1) creates a bottom—up salience map in its outputs, which depends on direct inputs and on contextual inputs by intra-cortical interactions (Li Zhaoping, 2002 Trends in Cognitive Sciences 6 9–16). Since the interactions include iso-feature (eg iso-orientation) suppression, responsible for salience by feature contrast, and collinear facilitation, the V1 proposal predicts that collinear grouping contributes to salience additionally. Accordingly, in orientation textures, texture bars are more salient near texture borders owing to orientation contrast, and are even more salient when they are additionally parallel to the border by collinear grouping. We show that colour changes in texture bars are more detectable when the bars are parallel to the texture border. Since changes are more detectable at salient locations, our finding supports V1 mechanisms for bottom—up salience.

Line drawings of everyday objects were modified into silhouettes by filling-in the complete area enclosed by boundary contours, and outline versions were created by extracting the contours from the silhouettes. A large number of participants was asked to try to identify these silhouette and outline versions in experiment 1. Identifiability ranged from 0% to 100% correct responses with a large range in-between. Several kinds of errors and several reasons for difficulties with identification emerged in our data set. In experiment 2, we compared the original identification rates to those of inverted silhouettes (white figures on a black background), and in experiment 3 we compared the original identification rates of objects with filled-in holes or background parts to those of versions without filling-in. These stimuli and identification norms are useful for additional research on priming and context effects of object identification, neuropsychological deficits of object identification, and all kinds of studies with silhouettes where the role of top down knowledge could be of interest.

Attneave (1954 Psychological Review 61 183–193) demonstrated that a line drawing of a sleeping cat can still be identified when the smoothly curved contours are replaced by straight-line segments connecting the positive maxima and negative minima of contour curvature. Using the set of line drawings by Snodgrass and Vanderwart (1980 Journal of Experimental Psychology: Human Learning and Memory 6 174–215) we made outline versions (with known curvature values along the contour) that can still be identified and that can be used to test Attneave's demonstration more systematically and more thoroughly. In five experiments (with 444 subjects in total), we tested identifiability of straight-line versions of 184 stimuli with different selections of points to be connected (using 24 to 28 subjects per stimulus per condition). Straight-line versions connecting curvature extrema were easier to identify than those based on inflections (where curvature changes sign), and those connecting salient points (determined by 161 independent subjects) were easier than those connecting midpoints. However, identification varied considerably between objects: some were almost always identifiable and others almost never, regardless of the selection criterion, whereas identifiability depended on the specific shape attributes preserved in the straight-line version of the outline in other objects. Results are discussed in relation to Attneave's original hypotheses as well as in the light of more recent theories on shape perception and object identification.

Although Attneave (1954 Psychological Review 61 183–193) and Biederman (1987 Psychological Review 94 115–147) have argued that curved contour segments are most important in shape perception, Kennedy and Domander (1985 Perception 14 367–370) showed that fragmented object contours are better identifiable when straight segments are shown. We used the set of line drawings published by Snodgrass and Vanderwart (1980 Journal of Experimental Psychology: Human Learning and Memory 6 174–215), to make outline versions that could be used to investigate this issue with a larger and more heterogeneous stimulus set. Fragments were placed either around the ‘salient’ points or around the midpoints (points midway between two salient points), creating curved versus relatively straight fragments when the original outline was fragmented (experiment 1), or angular and straight fragments when straight-line versions were fragmented (experiment 2). We manipulated fragment length in each experiment except the last one, in which we presented only selected points (experiment 3). While fragmented versions were on average more identifiable when straight fragments were shown, certain objects were more identifiable when the curved segments or the angles were shown. A tentative explanation of these results is presented in terms of an advantage for straight segments during grouping processes for outlines with high part salience, and an advantage for curved segments during matching processes for outlines with low part salience.

We examined absolute-pitch (AP) and short-term musical memory abilities of five musical savants with congenital blindness, seven musicians, and seven non-musicians with good vision and normal intelligence in two experiments. In the first, short-term memory for musical phrases was tested and the savants and musicians performed statistically indistinguishably, both significantly outperforming the non-musicians and remembering more material from the C major scale sequences than random trials. In the second experiment, participants learnt associations between four pitches and four objects using a non-verbal paradigm. This experiment approximates to testing AP ability. Low statistical power meant the savants were not statistically better than the musicians, although only the savants scored statistically higher than the non-musicians. The results are evidence for a musical module, separate from general intelligence; they also support the anecdotal reporting of AP in musical savants, which is thought to be necessary for the development of musical-savant skill.

The texture of a rolling circle depicts the translational and rotational components of its motion. In the case of a homogeneous circle, however, visual cues to the rotational component of motion are absent. To examine how the visual system resolves undetermined motion cues, optically neutral circles were displaced so that changes in their orientation were invisible. Contextual cues systematically triggered the perception of illusory rotation, suggesting that the visual system uses contextual cues along with intrinsic surface cues to compute percepts of rolling objects. This might also explain why people rarely experience the perception of ambiguous motion.

Previous studies have shown that conscious awareness of hill slant is overestimated, but visually guided actions directed at hills are relatively accurate. Also, steep hills are consciously estimated to be steeper from the top than the bottom, possibly because they are dangerous to descend. In the present study, participants stood at the top of a hill either on a skateboard or a wooden box of the same height. They gave three estimates of the slant: a verbal report, a visually matched estimate, and a visually guided action. Fear of descending the hill was also assessed. Those participants who were scared (by the skateboard) consciously judged the hill to be steeper than unafraid participants. However, the visually guided action measure was accurate across conditions. These results suggest that explicit awareness of slant is influenced by the fear associated with a potentially dangerous action that could be performed on the hill.

Most cognitive scientists know that an airplane tends to lose its engine when the display is flickering. How does such prior experience influence visual search? We recorded eye movements made by vision researchers while they were actively performing a change-detection task. In selected trials, we presented Rensink's familiar ‘airplane’ display, but with changes occurring at locations other than the jet engine. The observers immediately noticed that there was no change in the location where the engine had changed in the previous change-blindness demonstration. Nevertheless, eye-movement analyses indicated that the observers were compelled to look at the location of the unchanged engine. These results demonstrate the powerful effect of prior experience on eye movements, even when the observers are aware of the futility of doing so.