Perception

The McCollough effect (ME) is a colour aftereffect contingent on pattern orientation. This effect is generally thought to be mediated by primary visual cortex (V1) although this has remained the subject of some debate. To determine whether V1 is in fact sufficient to subserve the ME, we compared McCollough adaptation in controls to adaptation in two patients with damage to ventrotemporal cortex, resulting in achromatopsia, but who have spared V1. Each of these patients has some residual colour abilities of which he is unaware. Participants performed a 2AFC orientation-discrimination task for pairs of oblique and vertical/horizontal gratings both before and after adaptation to red/green oblique induction gratings. Successful ME induction would manifest itself as an improvement in oblique-orientation discrimination owing to the additional colour cue after adaptation. Indeed, in controls oblique grating discrimination improved post-adaptation. Further, a subdivision of our control group demonstrated successful ME induction despite a lack of conscious awareness of the added colour cue, indicating that conscious colour awareness is not required for ME induction. The patients, however, did not show improvement in oblique-orientation discrimination, indicating a lack of ME induction. This suggests that V1 must be connected to higher cortical colour areas to drive ME induction.

It is well-established that distractors interfere with goal-directed responses. Our recent findings indicate that the presence of corners in degraded line drawings of distractor stimuli modulates response times and accuracy to non-degraded targets (Kritikos and Pavlis 2007, Experimental Brain Research 183 159–170). In the present study we asked whether non-degraded distractors may facilitate responses to degraded targets (corners or line segments missing). We presented targets at fixation and accompanied by identical, category-congruent, or category-incongruent distractors. Participants responded to two object categories (musical instruments and tools) consisting of four line drawings. Corners-missing targets in particular were associated with greater interference from distractors than non-degraded targets. This interference was modulated when distractor locations were endogenously or exogenously cued. Findings are discussed in the context of additional processing of object features that are crucial to action.

Two experiments were carried out to study the perception of parallelepipeds. In the first, the subjects were shown images of parallelepipeds and were asked to judge the 3-D orientations of the faces of the parallelepipeds, as well as the shapes of the faces. These two types of judgments were found to be inconsistent. Specifically, the parallelepipeds reconstructed from judgments of 3-D orientations of the faces were systematically different from the parallelepipeds reconstructed from judgments of the shape of the faces. In the second experiment, the subjects were asked to choose which reconstruction was closer to their percept. In most trials the subjects chose the 3-D parallelepiped reconstructed from judgments of the shapes of the faces. These results suggest that the percept of the shape of a 3-D object is not based on the judgments of the 3-D orientations of the object's surfaces. Instead, the 3-D shape percept is based on simplicity constraints. A new computational model is presented, which generalizes Perkins's law (Perkins 1972). Instead of orthogonality, the new model uses mirror-symmetry and planarity constraints, in conjunction with maximum 3-D compactness and minimum surface-area constraints. The parallelepipeds recovered by the model are very close to the parallelepipeds perceived by the subjects.

Previous research has shown that changes to the body can influence the perception of distances in near space (Witt et al, 2005 Journal of Experimental Psychology: Human Perception and Performance 31 880–888). In this paper, we question whether changes to the body can also influence the perception of extents in extrapersonal space, namely the perception of aperture widths. In experiment 1, broad-shouldered participants visually estimated the size of apertures to be smaller than narrow-shouldered participants. In experiment 2, we questioned whether changes to the body, which included holding a large object, wearing a large object, or simply holding out the arms would influence perceived width. Surprisingly, we found that only when participants' hands were widened was extrapersonal space rescaled. In experiment 3, we explored the boundaries of the effect observed in experiment 2 by asking participants to hold their arms at four different positions in order to determine the arm width at which apertures appeared smaller. We found that arm positions that were larger than the shoulder width made apertures appear smaller. The results suggest that dimensions of the body play a role in the scaling of environmental parameters in extrapersonal space.

Bodily illusions offer an experimental method to investigate the origins and functional role of the sense of one's own body. Using the rubber hand illusion (RHI) we show that a representation of one's own body is implicitly used to calibrate perception of external objects. Twelve participants experienced the RHI while watching stimulation of a large or small glove simultaneously with stimulation of their own hand. They then grasped cylinders of identical size but varying weight. RHI with the large glove caused the cylinders to feel heavier. We suggest that an illusory increase in hand size made the subsequently grasped cylinder feel correspondingly small, evoking a size–weight illusion. Self-representation thus influenced exteroception. The sense of one's own body provides a fundamental reference for perception in general.

Bodily illusions differently affect body representations underlying perception and action. We investigated whether this task dependence reflects two distinct dimensions of embodiment: the sense of agency and the sense of the body as a coherent whole. In experiment 1 the sense of agency was manipulated by comparing active versus passive movements during the induction phase in a video rubber hand illusion (vRHI) setup. After induction, proprioceptive biases were measured both by perceptual judgments of hand position, as well as by measuring end-point accuracy of subjects' active pointing movements to an external object with the affected hand. The results showed, first, that the vRHI is largely perceptual: passive perceptual localisation judgments were altered, but end-point accuracy of active pointing responses with the affected hand to an external object was unaffected. Second, within the perceptual judgments, there was a novel congruence effect, such that perceptual biases were larger following passive induction of vRHI than following active induction. There was a trend for the converse effect for pointing responses, with larger pointing bias following active induction. In experiment 2, we used the traditional RHI to investigate the coherence of body representation by synchronous stimulation of either matching or mismatching fingers on the rubber hand and the participant's own hand. Stimulation of matching fingers induced a local proprioceptive bias for only the stimulated finger, but did not affect the perceived shape of the hand as a whole. In contrast, stimulation of spatially mismatching fingers eliminated the RHI entirely. The present results show that (i) the sense of agency during illusion induction has specific effects, depending on whether we represent our body for perception or to guide action, and (ii) representations of specific body parts can be altered without affecting perception of the spatial configuration of the body as a whole.

We investigated the idea that our memory for familiar faces involves an accurate representation of their unique spatial configuration and, further, whether this configuration may be caricatured in memory. In separate experimental blocks, thirty-five Irish participants were presented with a series of photographic images of their own face and of the face of a close friend, and were asked to choose the image which looked most like themselves or their friend. Both sets of images included an original full-face colour photograph, and photographic distortions ranging from a highly caricatured (+100%) to a highly anti-caricatured (–100%) version of the original, generated with reference to newly created average male and female Irish faces. Contrary to suggestions that we hold a slightly caricatured version of a familiar face in memory, the mean ‘best-likeness’ image, calculated across both self and friend trials, was an anti-caricature of −13.88% which was significantly different from 0 (t₆₉ = −5.34, p < 0.0001). The difference in the mean ‘best-likeness’ image chosen for self (–12.06%) and friend (–15.7%) was not significant (t₃₄ = 0.715, p = 0.48). These results are discussed with reference to our ability to discriminate facial shape, together with the possibility that we idealise the attractiveness of faces of those close to us.

Five experiments are reported in which the relative importance of internal and external features for unfamiliar face identification are examined by a matching task. In experiments 1–3, Egyptian adults showed a robust internal-feature advantage for matching photographs of Egyptian faces. In experiment 4, a cross-cultural comparison between the ability of Egyptian and British adults to match the internal and external features of unfamiliar Egyptian and British faces was made. Once again, Egyptians showed an internal-feature advantage, for all faces. In contrast, British observers—and also Egyptian children in experiment 5—showed external-feature advantages consistent with previous research. We attribute this contrast to the long-term experience of Egyptians in perceiving and recognising faces with headscarves, which might develop more expertise in processing the internal than the external features of unfamiliar faces.

It has been repeatedly shown that face inversion affects the recognition of emotional faces. However, previous results are heterogeneous concerning the affected emotions and the influence of presentation time is unclear. We examined the impact of limited presentation time (200 ms) on the face-inversion effect during recognition of basic emotions in 128 healthy young adults. Data analysis revealed differential inversion effects for emotional expressions, further modified by limitation of presentation time: when presentation was limited, we observed inversion effects for angry and neutral faces which were absent in the unlimited trials. In the unlimited condition, inversion particularly affected recognition of disgust and sadness. No general inversion effect occurred for neutral expressions. Error analysis highlighted specific confusions for the inverted condition, except for happy and neutral expressions. Hence, emotion recognition is affected by inversion—an indicator for configural processing, and presentation time—an indicator for cognitive effort of processing.

Previous research has demonstrated that changing perceivers' action capabilities can affect their perception of the extent over which an action is performed. In the current study, we manipulated jumping ability by having participants wear ankle weights and examined the influence of this manipulation on the perception of jumpable and un-jumpable extents. When wearing ankle weights, jumpable gaps appeared longer than when not wearing ankle weights; however, for un-jumpable gaps, there was no difference in the apparent gap extent, regardless of whether the participant was wearing ankle weights. This suggests that the perception of a jumpable extent is affected by one's action boundary for jumping, but only if jumping is an action that can be performed over the extent.

Stereopsis is largely unperturbed by various types of eye, head, and target movements. Here, we used a simple setup to investigate the limits of a previously untested type of stimulus motion on stereoscopic depth perception. Clockwise and counterclockwise rotations of an auto-stereogram were used to describe the limits of stereopsis to roll-tilt. The result showed intact depth perception with stimulus rotation up to ∼ 12°, regardless of rotation direction and viewing distance, indicating a tolerant mechanism for stereoscopic processing by the human neural system.