Attentional bias induced by viewing actual or illusory movements

Influence of Moving Background on Line Bisection Performance in the Normal Elderly Versus Patients with Hemispatial Neglect

OBJECTIVE

Previous studies have shown that when healthy young participants bisect stationary lines on a moving background (MB) or optokinetic stimulation, they perceive the stationary line moving in the opposite direction of the MB (illusory motion [IM]), and they displace their bisection mark in the direction of the IM. This study attempted to learn whether IM also influences attentional biases of the healthy elderly and patients with hemispatial neglect.

DESIGN

In experiment 1, healthy elderly participants and patients with neglect bisected lines in conditions where IM was absent or present. To better understand the MB dichotomy between the healthy elderly and neglect patients, in experiment 2, participants' eye movements were recorded using an infrared eye tracker.

RESULTS

In experiment 1, healthy elderly participants' biases occurred in the opposite direction of MB when IM was present but in the same direction of MB when IM was absent. In contrast, neglect patients' biases occurred in the same direction of MB regardless of conditions. Eye movements reflect the spatial direction of attention. In experiment 2, the healthy elderly participants were able to selectively attend to the line, whereas neglect patients were impaired in that they fixated on the line.

CONCLUSIONS

These results suggest that the healthy elderly can selectively fixate on a line, and with MB, they perceive the stationary line moving, resulting in a bisection bias in the direction of the IM. In contrast, when there is an MB, the patients with neglect are impaired in that they fixate on the line. Thus, they do not perceive IM; instead, they are primarily influenced by the MB.

Therapeutic effects of caloric stimulation and optokinetic stimulation on hemispatial neglect

Hemispatial neglect refers to a cognitive disorder in which patients with unilateral brain injury cannot recognize or respond to stimuli located in the contralesional hemispace. Hemispatial neglect in stroke patients is an important predictor for poor functional outcome. Therefore, there is a need for effective treatment for this condition. A number of interventions for hemispatial neglect have been proposed, although an approach resulting in persistent improvement is not available. Of these interventions, our review is focused on caloric stimulation and optokinetic stimulation. These lateralized or direction-specific stimulations of peripheral sensory systems can temporarily improve hemispatial neglect. According to recent functional MRI and PET studies, this improvement might result from the partial (re)activation of a distributed, multisensory vestibular network in the lesioned hemisphere, which is a part of a system that codes ego-centered space. However, much remain unknown regarding exact signal timing and directional selectivity of the network.

The influence of illusory motion on line bisection performance in normal subjects

The present study examines whether illusory movement (IM) of a horizontal line, induced by a moving background (MB), influences line-bisection performance in normal subjects. The first experiment attempted to identify the speeds of MB that induce IM. We found that when speed is increased from 1.53 degrees to 13.3 degrees/sec, IM increases, but that with further speed increases, IM decreases. Leftward MB induces rightward IM, and vice versa. In the second experiment, we had subjects bisect lines at MB speeds that had been shown to induce IM in the first experiment. We found that leftward MB induced a rightward bias, and vice versa. We also found that there was a relationship between the magnitude of IM and the degree of bias. In the third experiment, by making the target line larger than the MB, we made the conditions where IM was presumably absent. Unlike the results of bisection performed with IM, subjects showed a bias in the direction of the MB. Overall, these experiments demonstrated that the perception of motion induces subjects to attend in the direction of movement.