Modulating wheelchair navigation in patients with spatial neglect.

Patients who have had a stroke resulting in the deficit of visuo-spatial neglect are normally not provided with a powered wheelchair, as they are either considered or found to be unsafe navigating about their environment. As these patients are relatively unlikely to regain functional mobility by walking, the denial of alternative forms of mobility is of particular concern. Modest progress has been made over the past two decades with regards to the rehabilitation of neglect but there have been calls for further research which addresses "real world" measures of independence such as wheelchair navigation. In this study, we investigated the ability of patients with neglect to improve their performance when navigating a powered wheelchair by using theoretically-driven strategies that have shown promise in previous studies (spatial cueing and limb activation). Strategies were applied and tested in the most realistic and practical manner for each individual, based on their abilities and concurrent deficits. Performance was improved by the experimental strategies. The data suggest it is possible to apply theoretically-driven strategies to improve wheelchair navigation in patients with neglect and are supportive of further studies that could lead to improved access to powered mobility by this population in the future.

Neuropsychological evidence for an interaction between endogenous visual and motor-based attention.

We report data examining the relations between endogenous allocation of visual attention and effects of motor-preparation on attention. We tested a patient with a spatial bias in perceptual report following damage to the left inferior parietal lobe and superior temporal gyrus. Previously we have shown that the spatial bias in report can be reduced when a movement is planned to where a target falls in the contralesional field, while the bias is exacerbated when a movement is planned to the ipsilesional side (Kitadono & Humphreys, 2007, Cognitive Neuropsychology, 24). Here we pitted the effects of planning a movement to the contra- or ipsilesional side against the effects of endogenous visual attention, manipulated by varying the probability of where a target would fall. In a no-movement baseline, there was better identification of contralesional targets when there was a good likelihood of targets appearing on the contralesional side (the 20-80 and 50-50 conditions) compared with when targets had a low probability of occurring there (the 80-20 condition). This sensitivity to target probability interacted with the effects of planning a movement to the contra- or ipsilesional side. When the target had a good probability of falling in the contralesional field (the 20-80 and 50-50 conditions), planning a movement to the ipsilesional side disrupted contralesional report. When there was a high probability of an ipsilesional target (the 80-20 condition), planning a movement to a congruent side improved contralesional report. The results indicate that effects of endogenous attention and of motor-based attention influence a common system controlling visual orienting.

Attention and its coupling to action.

We discuss two commentaries that we have received on our target article (Humphreys et al., 2010). We elaborate on the evidence for action effects on extinction and discuss whether these effects occur pre or post the selection of a response. In addition, we discuss the neural basis of the effects of action relations on extinction and on the generalization of results on action relations to real-world examples.

The interaction of attention and action: from seeing action to acting on perception.

We discuss evidence indicating that human visual attention is strongly modulated by the potential of objects for action. The possibility of action between multiple objects enables the objects to be attended as a single group, and the fit between individual objects in a group and the action that can be performed influences responses to group members. In addition, having a goal state to perform a particular action affects the stimuli that are selected along with the features and area of space that is attended. These effects of action may reflect statistical learning between environmental cues that are linked by action and/or the coupling between perception and action systems in the brain. The data support the argument that visual selection is a flexible process that emerges as a need to prioritize objects for action.

Sustained interactions between perception and action in visual extinction and neglect: evidence from sequential pointing.

Interactions between perception and action were examined by assessing the effects of action programming on extinction and neglect. In an extension of prior work, effects of sequential motor programming were assessed under conditions in which attention was first directed to an ipsilesional stimulus. Despite pointing and reporting a stimulus on the ipsilesional side first, programming a second action to the contralesional side reduced the spatial deficit on report, improving the report of contralesional stimuli (relative to when the patient just pointed to the ipsilesional side) while decreasing the report of ipsilesional items. The data suggest that perception and action interact through motor feedback to early visual coding, helping a patient overcome a lack of visual awareness to contralesional stimuli. This is effective even when attention has to be disengaged from the ipsilesional side suggesting that motor programming decreases ipsilesional capture and exerts a sustained influence on perception.

Interactions between perception and action programming: evidence from visual extinction and optic ataxia.

We report a series of 7 experiments examining the interaction between visual perception and action programming, contrasting 2 neuropsychological cases: a case of visual extinction and a case with extinction and optic ataxia. The patients had to make pointing responses to left and right locations, whilst identifying briefly presented shapes. Different patterns of performance emerged with the two cases. The patient with "pure" extinction (i.e., extinction without optic ataxia) showed dramatic effects of action programming on perceptual report. Programming an action to the ipsilesional side increased extinction (on 2-item trials) and tended to induce neglect (on 1-item trials); this was ameliorated when the action was programmed to the contralesional side. Separable effects of using the contralesional hand and pointing to the contralesional side were apparent. In contrast, the optic ataxic patient showed few effects of congruency between the visual stimulus and the action, but extinction when an action was programmed. This effect was particularly marked when actions had to be made to peripheral locations, suggesting that it reflected reduced resources to stimuli. These effects all occurred using stimulus exposures that were completed well before actions were effected. The data demonstrate interactions between action programming and visual perception. Programming an action to the affected side with the contralesional limb reduces "pure" extinction because attention is coupled to the end point of the action. However, in a patient with deficient visuo-motor coupling (optic ataxia), programming an action can increase a spatial deficit by recruiting resources away from perceptual processing. The implications for models of perception and action are discussed.

Short-term effects of the 'rubber hand' illusion on aspects of visual neglect.

The 'rubber hand' illusion was induced in a patient showing unilateral visual neglect, with the patient experiencing a shift in the felt position of his right hand towards a contralesional rubber hand. Immediately following the illusion, there were short- lasting reductions in neglect for bisecting about the midline and for cancelling multiple stimuli. No effects were found on the ability to encode briefly presented visual stimuli on the contralesional side. The data demonstrate that the illusion can temporarily alter some aspects of neglect, without altering basic visual encoding. The underlying mechanisms, and the relations to other rehabilitation procedures, are discussed.

The brain's fingers and hands.

The brain keeps track of the changing positions of body parts in space using a spatial body schema. When subjects localise a tactile stimulus on the skin, they might either use a somatotopic body map, or use a body schema to identify the location of the stimulation in external space. Healthy subjects were touched on the fingertips, with the hands in one of two postures: either the right hand was vertically above the left, or the fingers of both hands were interwoven. Subjects made speeded verbal responses to identify either the finger or the hand that was touched. Interweaving the fingers significantly impaired hand identification across several experiments, but had no effect on finger identification. Our results suggest that identification of fingers occurs in a somatotopic representation or finger schema. Identification of hands uses a general body schema, and is influenced by external spatial location. This dissociation implies that touches on the finger can only be identified with a particular hand after a process of assigning fingers to hands. This assignment is based on external spatial location. Our results suggest a role of the body schema in the identification of structural body parts from touch.