First saccades reveal biases in recovered neglect.

Hemispatial neglect affects the ability to explore space on the side opposite a brain lesion. This deficit is also mirrored in abnormal saccadic eye movement patterns. The present study investigated if the recovery of neglect is also reflected in saccadic eye movements. Patient AF, who displayed strong hemispatial neglect 1 month post-right thalamic stroke, had largely recovered 3 months later when tested on visual exploration tasks of the Behavioural Inattention Test. At this stage, AF was tested on a visual search task while his eye movements (direction, latencies and amplitudes of first saccades) and manual reaction times were recorded. The experimental conditions differed with respect to stimulus number and distracter type and increased in difficulty. AF correctly generated saccades into the neglected field when the target was presented alone. In contrast, a considerable left/right difference was present for all multiple-stimulus search displays. Although recovered from neglect in standardized assessment, AF showed a strong rightward bias resulting in highly asymmetric response times and eye movement behaviour. We conclude that eye movement patterns are far more susceptible to remaining spatial impairments and can thus provide a sensitive means to assess the extent of neglect recovery.

The bilateral effect: callosal inhibition or intrahemispheric competition?

The magnification of visual field asymmetry observed with bilateral compared to unilateral tachistoscopic presentation of homologous stimuli (bilateral effect) can be explained by two hypothetical processes: homologous activation with subsequent inhibition of callosal information transfer or intrahemispheric competition for processing resources. A lexical decision task with unilateral and bilateral stimulation and response with the right or left hand was used in an attempt to decide between these hypotheses. Analysis of response time data revealed a bilateral effect, superimposed on a right visual field advantage, and no interaction between visual field and response hand. Results are consistent with the hypothesis of intrahemispheric competition in the left hemisphere.

Illusion processing in hemispatial neglect.

Twelve patients with hemispatial neglect and two control groups were tested to examine the effects of the Müller-Lyer and Judd illusions on bisection behaviour. The studies were designed to investigate whether neglect patients were indeed unaware of the left sides of the illusory figures. In Experiment 1, participants were asked to describe the illusory figures prior to bisection, whereas in Experiment 2, they compared two illusions whose fins, in the critical condition, differed on the left and then performed the bisection. It was found that the illusions worked equally well in all three groups. Interestingly, apart from one exception, almost all neglect patients explicitly reported the left-sided fins in Experiment 1. Only five patients failed to do so but only on an average of 16% of trials. In Experiment 2, six patients made errors in the comparison task but four of these patients did not neglect any left-sided fins in Experiment 1 (with the exception of three overall trials for LC and EdR). This finding seems a good indication that the two tasks differ in their requirements. The comparison task may be perceived as harder as it requires discrimination rather than detection and thus lead to more neglect type errors than the bisection task. In one neglect patient, the illusions consistently failed to work. This patient presented with an occipito-temporal and basal ganglia lesion and the mechanisms responsible for the processing of simple visual features might have possibly been impaired in her case.

Effects of visible and invisible cueing procedures on perceptual judgments in young and elderly subjects.

Healthy subjects were tested in two experiments to examine the effects of lateralized cues on line bisection and landmark judgments. The studies were designed to investigate whether bisection and landmark biases induced by cueing are simply a result of a direct perceptual lengthening of the cued part of the line caused by the fact that the cue is visible, thus creating a composite 'line plus cue' or whether cueing induces an attentional bias operating on judgments of spatial extent by either reducing the magnitude of the parts of the stimulus receiving 'less' attention or magnifying those parts receiving 'more'. Lateralized cues were either visible letter cues or invisible marks drawn with a leadless pencil either by the subject him/herself (Experiment 1) or the experimenter (Experiment 2). Comparable to a previous study (Mattingley, Pierson, Bradshaw, Phillips and Bradshaw, 1993, Neuropsychologia, 31, 1201-1215), the first experiment showed that only visible cues affected line bisection and landmark judgments thus favouring the perceptual explanation. The second study, however, revealed bisection and judgment biases for invisible as well as visible cues with the attended part of the line appearing subjectively longer. These results indicate that attentional modulations can increase the salience of a line in a similar vein to physical changes. It seems likely that the requirement of a motor response in Experiment 1 cancelled out all attentional properties supposedly induced by placing that invisible cue.