Perception

The length of a whole line is overestimated in comparison to the sum of its parts (Künnapas, 1955 Journal of Experimental Psychology 49 134–140). This has been considered to be true for many years, although recent studies have demonstrated that it is not always so. The perception of the length of a whole line is highly dependent on the configuration of its two parts. More precisely, whereas a whole line is perceived as longer than the sum of symmetrically bisected line parts, this overestimation decreases when compared to the sum of the lengths of asymmetrically bisected line parts. Furthermore, the extent of overestimation depends on the degree of asymmetry, so that when the two parts are greatly asymmetric in length, the whole is no longer overestimated (Wolfe et al, 2005 Perception & Psychophysics 67 967–979). Here, two experiments are reported in which a vertical/horizontal line length comparison task was used to investigate how line bisection affects length estimation. The results give rise to a new general principle characterising the mechanisms of visual perception: the sum of the lengths of two asymmetrically bisected parts is perceived as greater than that of two symmetrically bisected parts. Also bisection plays a critical role in length perception by preventing vertical bias.

Mirror reflections and planar rotations of a picture do not result in any variations concerning the internal geometrical layout of the objects depicted in the picture. We examined to what extent these picture plane transformations gave rise to perceptual differences. A large set of pictures was generated by mirror-reflecting and rotating a set of six original photographs in the picture plane. We externalised the percepts of the depicted objects by using a direct perceptual method: the gauge-figure method. Participants had to adjust a gauge figure so that it seemed to be painted on the surface of the depicted object. From an extensive set of settings collected this way, we computed for each picture the three-dimensional interpretation—or pictorial relief—of the depicted object. On the basis of this set of pictorial reliefs, we addressed the effects of mirror reflections and rotations of pictures on the shape percept of the depicted object. Mirror-reflecting a picture around the horizontal axis resulted in large differences in pictorial reliefs, whereas mirror-reflecting pictures around the vertical axis resulted in only small differences in pictorial reliefs. Clockwise 90°, 180°, and 270° rotation affected the pictorial relief significantly. In all cases, the differences between the pictorial reliefs could be resolved by affine transformations, and could thus be ascribed to different solutions of the depth ambiguities inherent in pictures.

Our perception of the space around us is not veridical. It has been reported that the systematic errors in our perception of visual space can be described by a reasonably well-behaving space (the resulting space is approximately projective and complies with an affine geometry). The evidence for this is that the perceived centre of a set of points is independent of the order of the steps taken to construct it. We investigated whether this is also the case in displays with well-known 2-D visual illusions. In two examples (Judd and Poggendorff illusions), we show that the perceptual centre of a set of points depends on how this centre is constructed. The misperceptions induced by visual illusions are thus of a different nature than our everyday misperceptions. We argue that the concept of perceived visual space is not very useful for describing human behaviour. We propose an alternative description whereby illusions do not deform a visual space, but only a single visual attribute, leaving other attributes unaffected.

Outline pictures depict surface edges, and via extensions from this base they can represent sensory and psychological referents. Outline drawings standing for edges of surfaces are relatively ancient, and lines for actions, sounds, and psychological states quite recent—mostly little more than a century. The novel finding here is that lines for psychological states of several kinds are invented by a blind woman, EW, who began making raised-line drawings as an adult. Notably, she invented novel devices representing thoughts and emotional impressions. If lines depict surface edges literally, they depict motion, sensory effects, impressions, and thoughts metaphorically.

We examined whether familiarity with a face influences the spatial frequencies (SFs) required for face matching. Using the psychophysical method of constant stimuli and a 3AFC simultaneous matching paradigm, we obtained SF thresholds for familiar- and unfamiliar-face matching from fourteen observers, of which four were personally familiar with a subset of the faces while the remainder served as controls. SF thresholds from the lower extreme of the spectrum were approximately one octave lower for familiar than for unfamiliar faces, while SF thresholds from the upper extreme of the spectrum were approximately a third of an octave higher. These results highlight a quantitative difference between processing familiar and unfamiliar faces.

In two experiments we investigated the role of eye movements during face processing. In experiment 1, using modified faces with primarily featural (scrambled faces) or configural (blurred faces) information as cue stimuli, we manipulated the way participants processed subsequently presented intact faces. In a sequential same–different task, participants decided whether the identity of an intact test face matched a preceding scrambled or blurred cue face. Analysis of eye movements for test faces showed more interfeatural saccades when they followed a blurred face, and longer gaze duration within the same feature when they followed scrambled faces. In experiment 2, we used a similar paradigm except that test faces were cued by intact faces, low-level blurred stimuli, or second-order scrambled stimuli (features were cut out but maintained their first-order relations). We found that in the intact condition participants performed fewer interfeatural saccades than in low-level blurred condition and had shorter gaze duration than in second-order scrambled condition. Moreover, participants fixated the centre of the test face to grasp the information from the whole face. Our findings suggest a differentiation between featural, configural, and holistic processing strategies, which can be associated with specific patterns of eye movements.

Hand action can bias the perceived direction of ambiguously moving objects (Wohlschlager, 2000 Vision Research 40 925–930). A common coding framework has been proposed to account for action–perception interaction, assuming goal-based feature matching between action and perception [Prinz, 2003, in Agency and Self-Awareness (New York: Oxford University Press)]. I investigated whether and how movement representation and a distal action goal are implicated in biased motion perception using a stream–bounce display as stimulus. By moving a mouse, participants controlled the progression of disks, which were initially separated and then moved toward each other. Mouse movement was found to bias perception toward the stream when the directional motion of the mouse and disk were consistent; however, this biased perception was not observed when the directions were orthogonal. Subsequent experiments revealed that this induced stream percept could not be attributed to the processes of attention, and the action goal of disk control was critical to the occurrence of biased motion perception. These results suggest that feature matching occurs at the level of movement representation, and the action goal is important in linking movement representation to perceptual representation.

Many visual tasks display a well-documented naso-temporal asymmetry (NTA), where sensitivity is greater to stimuli presented in the temporal hemifield. Four-letter strings were presented at various eccentricities under monocular vision conditions, and observers were asked to classify the stimuli as ‘words’ or ‘non-words’ in a lexical decision task (experiment 1). In experiment 2, the same observers had to classify the stimuli as ‘darker’ or ‘lighter’ (contrast discrimination). Apart from the task, the visual conditions and stimuli were identical in both experiments. The typical temporal hemifield advantage was found for a contrast discrimination task in both English and Hebrew readers, but only for lexical decision judgments in Hebrew readers. The lack of the expected NTA in English readers that was observed only for a reading but not a low-level visual task indicates that language lateralisation and reading-related learning can override fundamental, anatomically based, visual asymmetries.

Four duration-discrimination experiments were carried out to compare crossmodal and unimodal timing conditions. For all experiments, participants were presented with two sequences, each consisting of 1 or 4 time intervals (marked by 2 or 5 signals), and asked to indicate whether the interval(s) of the second sequence was (were) shorter or longer than the interval(s) of the first. Markers in the first and second sequences were, respectively, tones and flashes (experiment 1), flashes and tones (experiment 2), both flashes (experiment 3), and both tones (experiment 4). In all modality conditions, except when using only tones (experiment 4), increasing the number of repetitions of the variable interval reduced duration-discrimination thresholds, independently of whether the fixed interval was presented first or second within the sequence pair. Moreover, judgments about sequence timing were best for tones–tones sequence pairs, worst for flashes–flashes sequence pairs, and intermediate for crossmodal (flashes–tones or tones–flashes) sequences. Finally, presenting a fixed interval in the first sequence resulted in better discrimination than presenting a variable interval in the first sequence. Implications for theories of timing are discussed.

While the use of hand tools and other everyday manually controlled devices is naturally accompanied by multisensory feedback, the deployment of fully multimodal virtual interfaces requires that haptic, acoustic, and visual cues be synthesised. The complexity and character of this synthesis will depend on a thorough understanding of the multimodal perceptual experience, including the interrelations between the individual sensory channels during manual interaction. In this study seventy participants were asked to rank the manual operation of ten electromechanical switches according to preference. The participants were randomly assigned in groups of ten to one of seven sensory presentation conditions. These conditions comprised six bimodal and unimodal sensory combinations created by selectively restricting the flow of haptic, auditory, and visual information, plus one condition in which ful sensory information was available. A principal components analysis on the obtained ranking data indicated that the sensory conditions with unimpeded haptic information were clearly distinct from those in which the haptic cues were impeded. The analysis also showed that, for switch use, the unimodal haptic condition most closely approached the condition with combined haptic, auditory, and visual feedback, compared with all of the conditions where haptic feedback was restricted.

Observers cannot accurately discriminate the top halves of two sequentially presented three-letter words. One interpretation of this effect is that words, like faces, are processed holistically. Here we show, in three simple experiments, that this phenomenon is more consistent with the hypothesis that letters, not words, are processed holistically.