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Phasic alerting of neglect patients overcomes their spatial deficit in visual awareness

Abstract

Patients with extensive damage to the right hemisphere of their brain often exhibit unilateral neglect of the left side of space. The spatial attention of these patients is strongly biased towards the right1, so their awareness of visual events on the left is impaired2. Extensive right-hemisphere lesions also impair tonic alertness (the ability to maintain arousal)3,4,5. This nonspatial deficit in alertness is often considered to be a different problem from spatial neglect5,6, but the two impairments may be linked7,8. If so, then phasically increasing the patients' alertness should temporarily ameliorate their spatial bias in awareness. Here we provide evidence to support this theory. Right-hemisphere-neglect patients judged whether a visual event on the left preceded or followed a comparable event on the right. They became aware of left events half a second later than right events on average. This spatial imbalance in the time course of visual awareness was corrected when a warning sound alerted the patients phasically. Even a warning sound on the right accelerated the perception of left visual events in this way. Nonspatial phasic alerting can thus overcome disabling spatial biases in perceptual awareness after brain injury.

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Figure 1: The sequence of events for a single trial, with time running from bottom left to top right.
Figure 2: Eight patients with right-hemisphere ischaemic lesions were tested, and were selected if they showed left neglect and a bias of at least 144 ms towards the right in the PEST procedure in our visual task (see Methods) in the unwarned condition (indicating a pathological rightwards bias of spatial attention).
Figure 3: Examples of data from four concurrent staircases for one of the patients (F.F.), showing the PEST staircase procedure which determines thresholds for phenomenal simultaneity in warned (squares) and unwarned (circles) trials.
Figure 4: Mean thresholds of subjective simultaneity for each of the eight right-hemisphere-damaged patients, as derived from the PEST staircases they underwent.

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Acknowledgements

This research was supported by the Medical Research Council (UK) and also by a Wellcome Trust grant to J.D. J.B.M. was supported by a National Health and Medical Research Council (Australia) Neil Hamilton Fairley Fellowship, and by AMRAD Australia.

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Robertson, I., Mattingley, J., Rorden, C. et al. Phasic alerting of neglect patients overcomes their spatial deficit in visual awareness. Nature 395, 169–172 (1998). https://doi.org/10.1038/25993

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