Robert D. McIntosh

Do we have independent visual streams for perception and action

The perception–action model proposes that vision-for-perception and vision-for-action are based on anatomically distinct and functionally independent streams within the visual cortex. This idea can account for diverse experimental findings, and has been hugely influential over the past two decades. The model itself comprises a set of core contrasts between the functional properties of the two visual streams. We critically review the evidence for these contrasts, arguing that each of them has either been refuted or found limited empirical support. We suggest that the perception–action model captures some broad patterns of functional localization, but that the specializations of the two streams are relative, not absolute. The ubiquity and extent of inter-stream interactions suggest that we should reject the idea that the ventral and dorsal streams are functionally independent processing pathways.

Prism adaptation improves chronic visual and haptic neglect: A single case study

Visuomotor adaptation to rightward displacing optical prisms is known to induce temporary improvements in the symptoms of left visual neglect. We report a 74 year-old woman with severe and chronic neglect of nine months duration, who underwent three weekly sessions of prism adaptation. Substantial improvements were obtained on tests of visual neglect (cancellation, copying and bisection). Improvement was also observed on a spatial judgement task, with no explicit visual component, in which CS was required to locate the centre of a haptically explored circle. These observations confirm that brief periods of prism exposure can benefit even chronic neglect disorders. Moreover, the improvement observed on the haptic task supports the belief that this procedure can influence higher levels of spatial representation.

The neurological basis of visual neglect

Purpose of reviewA recent study has confirmed the enormous impact of visual neglect on the health services of the western world. Neglect was present in 48% of a sample of 166 right hemisphere stroke patients, and the severity of neglect predicted the extent of functional disability and family burden more accurately than did the extent of brain damage. Given the medical significance of neglect and its tantalizing relevance to understanding human conscious experience, it is unsurprising that the neuropsychological literature concerning the syndrome continues to grow rapidly. Recent findingsWe include brief surveys of six topics currently attracting attention in the field: the anatomical focus of neglect; the visual input pathways implicated; impairments of spatial working memory; the nature of visual extinction; perceptual distortions in neglect; studies on healthy subjects using transcranial magnetic stimulation; and the use of prism adaptation for the rehabilitation of neglect. SummaryThere is steady progress in understanding the essential components of neglect and their brain localization. Every step towards clarity, however, seems to be matched by a new discovery of the inherent complexity of the syndrome. The clinical expression of neglect may reflect the interaction of a variety of spatially lateralized and non-lateralized impairments, not all of which are present in all patients. The quest for an effective technique for the clinical rehabilitation of neglect continues, with prism adaptation emerging as the most promising approach to date.

Sensorimotor effects on central space representation: prism adaptation influences haptic and visual representations in normal subjects

Prism adaptation improves visual and haptic manifestations of left neglect, and can induce a small but reliable simulation of left visual neglect in normal individuals. Here, we present two experiments in which the effects of prism adaptation on the representation of space were explored. In Experiment 1, normal subjects were required to locate the centre of a haptically explored circle, before and after adaptation to leftward displacing prisms. In Experiment 2, a visual circle centring task was used. In both tasks, prism adaptation induced a significant rightward shift of performance. In addition, in both experiments, three classical measures of visuo-manual adaptation were taken: the visual shift, the proprioceptive shift and the total shift. The effects found on the haptic and visual tasks did not correlate with any of these measures. This suggests that the effects of prism adaptation on the circle centring tasks did not depend directly on the sensorimotor consequences of the adaptation. These results imply that prism adaptation can affect noetic levels of space representation in normal subjects, supporting the hypothesis that this low-level sensorimotor intervention can exert a bottom-up structuring influence on higher levels of cognitive integration.

Intact automatic avoidance of obstacles in patients with visual form agnosia

In everyday life our reaching behaviour has to be guided not only by the location and properties of the target object, but also by the presence of potential obstacles in the workspace. Recent evidence from neglect and optic ataxia patients has suggested that this automatic obstacle avoidance is mediated by the dorsal, rather than the ventral, stream of visual processing. We tested this idea in two studies involving patients with visual form agnosia resulting from bilateral ventral-stream damage. In the first study, we asked patient DF to reach out and pick up a target object in the presence of obstacles placed at varying distances to the left or right of the target. We found that both DF and controls shifted their trajectories away from the potential obstacles and adjusted their grip aperture in such a way as to minimize risk of collision. In a second study, we asked DF and a second patient, SB, to either reach between, or to bisect the space between, two cylinders presented at varying locations. We found that both patients adjusted their reach trajectories to account for shifts in cylinder location in the reaching task, despite showing significantly worse performance than control subjects when asked to make a bisection judgement. Taken together, these data indicate that automatic obstacle avoidance behaviour is spared in our patients with visual form agnosia. We attribute their ability to the functional intactness of the dorsal stream of visual processing, and argue that the ventral stream plays no important role in automatic obstacle avoidance.

Preserved obstacle avoidance during reaching in patients with left visual neglect.

We asked 12 patients with left visual neglect to bisect the gap between two cylinders or to reach rapidly between them to a more distal target zone. Both tasks demanded a motor response but these responses were quite different in nature. The bisection response was a communicative act whereby the patient indicated the perceived midpoint. The reaching task carried no imperative to bisect the gap, only to maintain a safe distance from either cylinder while steering to the target zone. Optimal performance on either task could only be achieved by reference to the location of both cylinders. Our analysis focused upon the relative influence of the left and right cylinders on the lateral location of the response. In the bisection task, all neglect patients showed qualitatively the same asymmetry, with the left cylinder exerting less influence than the right. In the reaching task, the neglect group behaved like normal subjects, being influenced approximately equally by the two cylinders. This was true for all bar two of the patients, who showed clear neglect in both tasks. We conclude that the visuomotor processing underlying obstacle avoidance during reaching is preserved in most patients with left visual neglect.

Improving the clinical diagnosis of personal neglect: a reformulated comb and razor test.

Beschin and Robertson (1997) devised a simple clinical test of left personal neglect, which characterises personal grooming behaviour according to the proportion of the total activity that is directed to the left side of the body. Although this test proved highly reliable, and more sensitive than prior diagnostic techniques, its formulation may yet be improved. The present paper reports a reanalysis of Beschin and Robertsons (1997) data, using additional control subjects, and a formula which characterises personal neglect as a lateral bias of behaviour rather than as a lateralised deficit. It is shown that this formula greatly enhances the tests sensitivity to the behavioural abnormalities of brain damaged patients, and it is recommended that this modification be adopted for the future diagnosis of personal neglect.

Vertical gaze angle as a distance cue for programming reaching: insights from visual form agnosia II (of III).

Abstract. It has been shown that a patient with visual form agnosia (DF) relies predominantly on vergence information when gauging target distance in an open-loop pointing task. This finding suggested that the programming of prehension might be severely disrupted if DF viewed target objects through ophthalmic prisms. An initial experiment showed that this prediction was not upheld; DF was able to programme reasonably accurate movements to objects located on a tabletop despite large changes in vergence angle. A second experiment, however, showed that placing the target objects at eye height whilst manipulating vergence angle caused gross disruption to prehension, with DF mis-programming the reach component in a predictable manner. Notably, the evidence for DFs reliance on vergence distance information was obtained in a task where the targets were viewed at eye height. These experiments indicate that DF uses vertical gaze angle to gauge target distance in normal prehension and suggest that this extra-retinal cue may be a useful source of distance information for the human nervous system, especially where pictorial cues are impoverished.

Matching boxes: Familiar size influences action programming

The perception/action model is the dominant account of the primary division of labour in the human visual pathway. Integral to this model is the idea that goal-directed actions are guided spatially by bottom-up vision, independent of perceptual recognition and top-down object knowledge. We question this idea by showing that the expected size of familiar objects (matchboxes) affects the amplitude of reaches made to grasp them, and the pre-shaping of the hand, even when binocular cues are available. This suggests that perceptual recognition routinely influences action programming.

Two visual streams for perception and action: Current trends

This Special Issue of Neuropsychologia is a celebration and evaluation of the work of David Milner, who retired formally from his Chair at the University of Durham at the end of 2008. In the course of a distinguished career, David’s work has informed many topics within cognitive neuroscience. However, his influence has thus far been felt most strongly via his collaboration with Mel Goodale on their perception-action model of primate vision (Goodale & Milner, 1992; Milner & Goodale, 1995; Milner & Goodale, 2006). The model proposes a functional interpretation for the two cortical visual streams, with the (occipito-temporal) ventral stream processing visual information for perceptual purposes, and the (occipito-parietal) dorsal stream providing visual guidance for movement. In the past two decades, the model has had a revolutionary effect within visual neuroscience, elevated the study of motor control to previously unthinkable prominence and inspired thousands of empirical studies. Moreover, its influence has reached beyond the boundaries of neuroscience, fuelling philosophical debates and even capturing the popular imagination (Goodale & Milner, 2004; Ramachandran & Blakeslee, 1998). The perception-action model thus provides an appropriate focus for this Special Issue. To contribute to this Issue, we invited prominent researchers whose work is relevant to the broad scope of the model, many of them friends, former colleagues or students of David. Some have composed timely review articles; many have provided new empirical data. These contributions testify to the diversity of research and the depth of insight inspired by David’s work, whilst showcasing the range of techniques available to modern cognitive neuroscience. The topics range from single-unit properties of parietal neurones to the behavioural organisation of reaching and grasping movements, from face perception to tool use and gesture production, from attentional orienting to the neural correlates of visual awareness. The techniques range from transient disruption of healthy brain function to clinical observations in chronic brain damage, from classical reaction time measures to kinematic analyses of movement, from primate neurophysiology to human functional neuroimaging. There are significant challenges to aspects of the perception-action model as well as evidence supporting its key claims. Our aim in this Editorial is to survey and summarize these contributions and to offer some context by relating them to one another and to the perceptionaction model. 2. Visual form agnosia