Perception

Helmholtz's famous pincushioned chessboard figure has been used to make the point that straight lines in the world are not always perceived as straight and, conversely, that curved lines in the world can sometimes be seen as straight. However, there is little agreement as to the cause of these perceptual errors. Some authors have attributed the errors to the shape of the retina, or the amount of cortex devoted to the processing of images falling on different parts of the retina, while others have taken the effects to indicate that visual space itself is curved. Helmholtz himself claimed that the ‘uncurved lines on the visual globe’ corresponded to ‘direction circles’ defined as those arcs described by the line of fixation when the eye moves according to Listing's law. Careful re-reading of Helmholtz together with some additional observations lead us to the conclusion that two other factors are also involved in the effect: (i) a lack of information about the distance of peripherally viewed objects and (ii) the preference of the visual system for seeing the pincushion squares as similar in size.

Pictorial space is the 3-D impression that one obtains when looking ‘into’ a 2-D picture. One is aware of 3-D ‘opaque’ objects. ‘Pictorial reliefs’ are the surfaces of such pictorial objects in ‘pictorial space’. Photographs (or any pictures) do in no way fully specify physical scenes. Rather, any photograph is compatible with an infinite number of possible scenes that may be called ‘metameric scenes’. If pictorial relief is one of these metameric scenes, the response may be considered ‘veridical’. The conventional usage is more restrictive and is indeed inconsistent. Thus the observer has much freedom in arriving at such a ‘veridical’ response. To address this ambiguity, we determined the pictorial reliefs for eight observers, six pictures, and two psychophysical methods. We used ‘methods of cross-sections’ to operationalise pictorial reliefs. We find that linear regression of the depths of relief at corresponding locations in the picture for different observers often lead to very low (even insignificant) R²s. Thus the responses are idiosyncratic to a large degree. Perhaps surprisingly, we also observed that multiple regression of depth and picture coordinates at corresponding locations often lead to very high R²s. Often R²s increased from insignificant up to almost 1. Apparently, to a large extent ‘depth’ is irrelevant as a psychophysical variable, in the sense that it does not uniquely account for the relation of the response to the pictorial structure. This clearly runs counter to the bulk of the literature on pictorial ‘depth perception’. The invariant core of interindividual perception proves to be of an ‘affine’ rather than a Euclidean nature; that is to say, ‘pictorial space’ is not simply the picture plane augmented with a depth dimension.

Goodness is a classic Gestalt notion defined as salience or perceptual strength of a given pattern. All operational models of goodness have assigned a central role to mirror symmetry but not much attention has been paid to the distinction between global and local mirror symmetry, and their possible interactions. We designed eight different types of dot patterns (all consisting of 80 dots), combining different numbers (0, 1, and 2) and relative orientations (parallel or orthogonal to each other) of local and global axes of symmetry (affecting 50% or 100% of the dots, respectively) at different absolute orientations (vertical and horizontal). Each of 640 trials consisted of a short presentation of a new dot pattern, which subjects had to classify as regular or random. We hypothesised that the overall goodness of patterns is not the simple sum of the amount of regularity present in them but depends on the cooperation and competition between symmetries. The results confirmed our hypothesis, showing that performance in this regularity-detection task did not increase in a linear way when some symmetries were added to other symmetries.

How humans recognize objects remains a contentious issue in current research on high-level vision. Here, I test the proposal by Wallis and Bülthoff (1999 Trends in Cognitive Sciences 3 22–31) suggesting that object representations can be learned through temporal association of multiple views of the same object. Participants first studied image sequences of novel, three-dimensional objects in a study block. On each trial, the images were from either an orderly sequence of depth-rotated views of the same object (SS), a scrambled sequence of those views (SR), or a sequence of different objects (RR). Recognition memory was assessed in a following test block. A within-object advantage was consistently observed—greater accuracy in the SR than the RR condition in all four experiments, greater accuracy in the SS than the RR condition in two experiments. Furthermore, spatiotemporal coherence did not produce better recognition than temporal coherence alone (similar or less accuracy in the SS compared to the SR condition). These results suggest that the visual system can use temporal regularity to build invariant object representations, via the temporal-association mechanism.

Previous research into the effects of viewpoint change on face recognition has typically dealt with rotations around the head's vertical axis (yaw). Another common, although less studied, source of viewpoint variation in faces is rotation around the head's horizontal pitch axis (pitch). In the current study we used both a sequential matching task and an old/new recognition task to examine the effect of viewpoint change following rotation about both pitch and yaw axes on human face recognition. The results of both tasks showed that recognition performance was better for faces rotated about yaw compared to pitch. Further, recognition performance for faces rotated upwards on the pitch axis was better than for faces rotated downwards. Thus, equivalent angular rotations about pitch and yaw do not produce equivalent viewpoint-dependent declines in recognition performance.

In recent research the change-detection paradigm has been used along with cueing manipulations to show that more attention is allocated to the upper than lower facial region, and that this attentional allocation is disrupted by inversion. We report two experiments the object of which was to investigate how the type of information changed might be a factor in these findings by explicitly comparing the role of attention in detecting change to information thought to be ‘special’ to faces (second-order relations) with information that is more useful for basic-level object discrimination (first-order relations). Results suggest that attention is automatically directed to second-order relations in upright faces, but not first-order relations, and that this pattern of attentional allocation is similar across features.

The face recognition task we perform most often in everyday experience is the identification of people with whom we are familiar. However, because of logistical challenges, most studies focus on unfamiliar-face recognition, wherein subjects are asked to match or remember images of unfamiliar people's faces. Here we explore the importance of two facial attributes—shape and surface reflectance—in the context of a familiar-face recognition task. In our experiment, subjects were asked to recognise color images of the faces of their friends. The images were manipulated such that only reflectance or only shape information was useful for recognizing any particular face. Subjects were actually better at recognizing their friends' faces from reflectance information than from shape information. This provides evidence that reflectance information is important for face recognition in ecologically relevant contexts.

We conducted two experiments to investigate whether the motor preparation of manual aiming to a visual target is affected by either the physical characteristics (size or luminance contrast) or spatial characteristics (location) of the target. Reaction time (RT) of both finger lifting (ie stimulus-detection time) and manual aiming (ie movement-triggering time) to the onset of the target was measured. The difference of RT (DRT) between two tasks (ie the difference of task complexity) was examined to clarify the temporal characteristics of manual aiming per se during visuomotor integration. Results show classical characteristics: RT decreased as either the target size or luminance contrast increased. Furthermore, the task-complexity and target-location factors significantly interacted with each other, where the aiming RT was longer than the finger-lifting RT and the effects of target location on RT differed for each task. However, the task factor did not interact with either the size or luminance-contrast factor, implying that the motor preparation of manual aiming is associated with the spatial characteristics rather than the physical characteristics of the target. Inspection of DRT revealed that the time needed for motor preparation for an ipsilateral target was significantly shorter than that for a contralateral target. This was the case both for the left and for the right hand. Foveal targets required longer processing time, implying a disadvantageous function of motor preparation for the gazed target. The left-hand superiority for the target appearing in the left visual field was also observed. Such lateralised effect and left-hand advantage to the left visual field in manual aiming suggest that visuospatial information processing is activated during the preparation of aiming action, with faster processing in the right hemisphere.

Perceptual integration of audio—visual stimuli is fundamental to our everyday conscious experience. Eye-movement analysis may be a suitable tool for studying such integration, since eye movements respond to auditory as well as visual input. Previous studies have shown that additional auditory cues in visual-search tasks can guide eye movements more efficiently and reduce their latency. However, these auditory cues were task-relevant since they indicated the target position and onset time. Therefore, the observed effects may have been due to subjects using the cues as additional information to maximize their performance, without perceptually integrating them with the visual displays. Here, we combine a visual-tracking task with a continuous, task-irrelevant sound from a stationary source to demonstrate that audio—visual perceptual integration affects low-level oculomotor mechanisms. Auditory stimuli of constant, increasing, or decreasing pitch were presented. All sound categories induced more smooth-pursuit eye movement than silence, with the greatest effect occurring with stimuli of increasing pitch. A possible explanation is that integration of the visual scene with continuous sound creates the perception of continuous visual motion. Increasing pitch may amplify this effect through its common association with accelerating motion.

A new colour-spreading illusion is reported. Illusory colours appear on a white background when three unconnected sinusoidal curves are aligned in parallel when the central line is of a different colour than the other two lines. For some combinations of colours, illusory colours appear not only around the central line but also around the other lines.