David Milner

Why do some perceptual illusions affect visually guided action, when others don't?

In 1995, Aglioti and his colleagues [1] reported that the powerful Ebbinghaus–Titchener size-contrast illusion had no effect on visually guided grasping. Pairs of discs were presented within annular arrays of (respectively) smaller or larger circles, generating a strong perceptualsize illusion; yet the illusion did not affect the extent of hand opening during reaches made to pick up one or other of the discs. Milner and Goodale [2] interpreted these data within their proposed association of the cortical ventral and dorsal visual streams with ‘perceptual’ and ‘visuomotor’ processing, respectively. Ventral-stream processing would be contextually relative, they argued, in order to provide suitably coded visual information for purposes of recognition and storage. In contrast, size and location would need to be coded in absolute metrics in the dorsal stream, in order to be readily translated into motor coordinates. Since 1995, several studies have produced similar dissociations to those of Aglioti et al., but others have found a significant effect, albeit generally a weak one, of perceptual illusions on action [3–5]. Most, if not all, of these results can be encompassed within the two-visual-streams framework. For example, the effect sometimes found of the Ebbinghaus illusion on grasp aperture might be an artefact of the different spaces ‘available’ around the target discs, if the surrounds are treated by the visuomotor system as ‘obstacles’. The preparatory hand posture appears to be highly sensitive to this factor of ‘grasp space’ [6]. When grasp space is equalized between the two targets, the effect of the illusion on grasp disappears [7]. A quite different reason why a perceptual illusion might influence a visually guided action relates to where in the brain the illusion originates. It is likely that ‘contextual’ illusions like the Ebbinghaus have their effects chiefly within the depths of the ventral stream. But other illusions are likely to originate in primary visual cortex (V1), or in one of the other retinotopic areas, which feed not only into the ventral stream but also into the dorsal stream. There are two ways of deceiving the visual system about the orientation of a central stimulus. The rod-and-frame illusion (RFI, Fig. 1a) appears to be due to a ‘contextual’ effect, in which the whole visual frame of reference becomes rotated. The surrounding features of the scene induce a relative percept of the target object, which dominates our conscious judgements [8]. By contrast, the simultaneous-tilt illusion (STI, Fig. 1b) depends on local interactions within the visual field, most probably mediated by short-range inhibitory connections between cortical columns in V1 that respond to different orientations. These interactions would predict a shift in the distribution of neurons responding to a target grating pattern when surrounded by a grating set at an orientation a few degrees away [9]. The two-streams theory [2] must predict a dissociation between perception and action in the RFI, because the frame is most unlikely to influence the target through local interactions in retinotopic visual areas. The STI, however, should not only affect activity in the perceptual system, but

The role of size and binocular information in guiding reaching: insights from virtual reality and visual form agnosia III (of III)

Abstract. Reaching out to grasp an object requires information about the size of the object and the distance between the object and the body. We used a virtual reality system with a control population and a patient with visual form agnosia (DF) in order to explore the use of binocular information and size cues in prehension. The experiments consisted of a perceptual matching task in addition to a prehension task. In the prehension task, control participants modified their reach distance in response to step changes in vergence in the absence of any clear reference for relative disparity. Their reach distance was unaffected by equivalent step changes in size, even though they used this information to modify grasp and showed a size bias in a distance matching task. Notably, DF showed the same pattern of results as the controls but was far more sensitive to step changes in vergence. This finding complements previous research suggesting that DF relies predominantly on vergence information when gauging target distance. The results from the perceptual matching tasks confirmed previous findings suggesting that DF is unable to make use of size information for perceptual matching, including distance comparisons. These data are discussed with regard to the properties of the pathways subserving the two visual cortical processing streams.

Shadows remain segmented as selectable regions in object-based attention paradigms

It is unclear how shadows are processed in the visual system. Whilst shadows are clearly used as an important cue to localise the objects that cast them, there is mixed evidence regarding the extent to which shadows influence the recognition of those objects. Furthermore experiments exploring the perception of shadows per se have provided evidence that the visual system has less efficient access to the detailed form of a region if it is interpreted as a shadow. The current study sought to clarify our understanding of the manner in which shadows are represented by the visual system by exploring how they influence attention in two different object-based attention paradigms. The results provide evidence that cues to interpret a region as a shadow do not reduce the extent to which that region will result in a within-‘object’ processing advantage. Thus, whilst there is evidence that shadows are processed differently at higher stages of object perception, the present result shows that they are still represented as distinctly segmented regions as far as the allocation of attention is concerned. This result is consistent with the idea that object-based attention phenomena result from region-based scene segmentation rather than from the representations of objects per se.

The cognitive and neural bases of visually guided action

Research on the visual guidance of action has developed rapidly over the past 20 years, with many of the seminal contributions to the field being published in Experimental Brain Research. This special issue of the journal on The Cognitive and Neural Bases of Visually Guided Action brings together a number of the strands that currently characterize the field. The contributors to the issue met together to participate in a workshop held in the Agelonde residential complex in Provence, France, in September 2002. The meeting was funded by the European Commission (Research Directorate General, Human Potential Programme, High-Level Scientific Conferences, HPCFCT-1999-00029) and the European Brain and Behaviour Society. The workshop sought to take stock of the achievements made in the past 20 years in the field, covering not only behavioural studies of healthy human subjects, but also studies of patients with brain damage, as well as neurophysiological studies in animals, and functional neuroimaging studies in humans. While most of the plenary speakers at the workshop have contributed papers to this issue, the organizers also wish to thank the large number of other researchers, most of them still at an early stage of their scientific careers, who presented interesting data at the workshop, and who contributed greatly to the stimulating intellectual environment at the meeting. We also gratefully acknowledge the assistance of Valerie Gaveau and Serge Terrones in the organization of the meeting, and of Penny Walker in the preparation of this special issue.