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The present study was designed to investigate the influence of the level of intrinsic alert- ness on the spatial distribution of attentional weighting, as it is defined by the formalization of TVA. The spatial distribution of attentional weighting is a spatially lateralized component that defines whether one hemi-field receives more attentional weighting than the other. It was as- sessed (1) whether subjects with a low baseline level of intrinsic alertness (indicated by slow RTs) are particularly vulnerable to a rightward lateralization in a low alertness state, (2) whether subjects who get more drowsy under monotone conditions with low external stimula- tion (as assessed by subjective ratings and by RT slowing) are also those who show a more pronounced rightward shift of attentional weighting, and (3) whether the lateralized perform- ance due to lowered intrinsic alertness is a purely attentional effect, or if it could also be as- cribed to a biased change in (non-attentional) sensory effectiveness, or by an impaired (task- elated) top-down control regarding distractors presented in the left visual hemi-field. For re- solving these questions a within-subject design was used. Participants performed a partial- report task twice, once in a state of normal (baseline) alertness and once in a state of low alertness.

The present study therefore provides several findings about the relationship between the level of intrinsic alertness and the extent of changes of spatial attentional lateralization, and gives new insight into the influence of alertness on sensory effectiveness and top-down con- trol. Concerning these parameters, simple changes of the participants´ level of alertness pro- duced different patterns of performance in the TVA based partial-report task, although stim- uli and exposure durations remained the same.

An associated aim was to examine whether the level of alertness has an independent influ- ence on the different attentional components, as expected on the basis of TVA. The present study is the first investigating the influence of a direct (alertness based) manipulation on the

three distinct TVA parameters ‘spatial distribution of attentional weighting’, ‘top-down con- trol’, and ‘sensory effectiveness’.

As expected, in the normal-alertness condition (baseline) target stimuli were found to be weighted higher in the left compared to the right visual hemi-field (leftward bias of attention) – similar to the well-known pseudoneglect effect exhibited by healthy subjects (for a review see, Jewell & McCourt, 2000). The values of the assessed parameters of attentional selection and sensory processing were comparable to those found in previous studies measuring spatial and non-spatial aspects of visual attention under normal-alertness conditions in healthy sub- jects (e.g., Finke et al., 2005).

In contrast to the leftward ‘pseudoneglect’ bias in a state of normal-alertness, in the low- alertness condition a clear rightward lateralization was observed. One drawback of this in- creased salience of right-sided stimuli is a reduction in resources allocated towards the proc- essing of any competing left-sided stimulus. Thus, in a state of low-alertness participants re- ported significantly less targets in the left when accompanied by a second stimulus (target or distractor) in the right hemi-field. This result indicates a biased attentional competition to- wards the right side in a state of low-alertness, with attentional weights re-distributing to the right hemi-field. This re-distribution of attentional weights correlated with direct (RT) and subjective (rating) measures of sleepiness. Slow baseline RTs were correlated to a more pro- nounced spatial re-distributions of attentional weighting indicating a higher vulnerability for a rightward lateralization in subjects with low baseline levels of intrinsic alertness. Thus, it can be assumed that the extend of the rightward lateralization found after a monotonous vigilance task depended on the ability of a subject to maintain an appropriate intrinsic alertness level under conditions with low external stimulation.

The parameter top-down control seemed to be unaffected by changes in the level of alert- ness. Irrespective of alertness states, subjects were able to ignore distractors equally well in

both hemi-fields. In addition, lowered levels of alertness did not have an effect on the non- attentional parameter sensory effectiveness. Thus, subjects were able to report unilaterally presented targets in the left and the right visual hemi-field comparably well under both alert- ness conditions.

Taken together, the present study demonstrated that alertness-deprived healthy partici- pants developed a neglect-like behavior such as a rightward lateralization and (mild) unilat- eral extinction, stressing the relevance of intrinsic alertness in disturbed attentional competi- tion, and thus spatial attentional asymmetries. Interestingly, subjects with a low baseline level of intrinsic alertness showed a higher vulnerability for a more pronounced spatial re- distributions of attentional weighting with decreasing levels of intrinsic alertness. It can be concluded that low levels of intrinsic alertness result in a specific re-distribution of attentional weights, independent of any changes in top-down control or sensory effectiveness. This dem- onstration was only possible by using a method based on Bundesen´s TVA that allowed the different attentional and sensory components to be assessed independently and within the same task and subject.