Stefan Smaczny

Arithmetic fact retrieval deficits in acute stroke patients can arise after the severing ofwhite matter connections within left temporo-parietal areas (Smaczny et al., 2023). Thecurrent study examined white matter disconnections in multiplication, subtraction, andaddition in patients in the chronic stage of stroke. The strongest effects were found inmultiplication, where the disconnection of long-term memory parahippocampal areaswas strongly associated with lower multiplication scores. Lower subtraction scoreswere related to interhemispheric thalamus disconnections and weakly withdisconnections within the left hemisphere language network. The results for additionproblems were equivocal, yet they were in line with the literature, stating that additioncan be solved by either fact retrieval or magnitude processing. The multiplicationresults are in line with the view that the neuroanatomical correlates of arithmetic factretrieval deficits in chronic stroke point at a relearning deficit, the subtraction results arein line with bilateral redundancy of the magnitude processing network, and all resultstogether suggest that number operations are more in line with strategy-dependentprocessing than operation-dependent processing.

The static magnetic field of MRI scanners induces magneto-hydrodynamic stimulation of the vestibular organ (MVS) in the inner ear. This not only causes a horizontal vestibular nystagmus but also induces a horizontal bias in spatial attention. In this study, we aimed to determine the time course of MVS-induced biases in both VOR and spatial attention inside a 3T MRI-scanner as well as their respective aftereffects after participants left the scanner. Eye movements and overt spatial attention in a visual search task were assessed in healthy volunteers before, during and after a one-hour MVS period. All participants exhibited a VOR inside the scanner, which declined over time but never vanished completely. Importantly, there was also an MVS-induced horizontal bias in spatial attention and exploration, which persisted throughout the entire hour within the scanner. Upon exiting the scanner, we observed aftereffects in the opposite direction manifested in both the VOR and in spatial attention, lasting for about 6 minutes. Sustained MVS effects on spatial attention have important implications for the design and interpretation of fMRI-studies and for the development of therapeutic interventions counteracting spatial neglect.

Alertness, or one’s general readiness to respond to stimulation, has previously been shown to affect spatial attention. However, most of this previous research focused on speeded, laboratory-based reaction tasks, as opposed to the classical line bisection task typically used to diagnose deficits of spatial attention in clinical settings. McIntosh et al. (Cogn Brain Res 25:833–850, 2005) provide a form of line bisection task which they argue can more sensitively assess spatial attention. Ninety-eight participants were presented with this line bisection task, once with and once without spatial cues, and both before and after a 50-min vigilance task that aimed to decrease alertness. A single participant was excluded due to potentially inconsistent behaviour in the task, leaving 97 participants for the full analyses. While participants were, on a group level, less alert after the 50-min vigilance task, they showed none of the hypothesised effects of reduced alertness on spatial attention in the line bisection task, regardless of with or without spatial cues. Yet, they did show the proposed effect of decreased alertness leading to a lower level of general attention. This suggests that alertness has no effect on spatial attention, as measured by a line bisection task, in neurotypical participants. We thus conclude that, in neurotypical participants, the effect of alertness on spatial attention can be examined more sensitively with tasks requiring a speeded response compared to unspeeded tasks.

Alertness, or one’s general readiness to respond to stimulation, has previously been shown to affect spatial attention. However, most of this previous research focused on speeded, laboratory-based reaction tasks, as opposed to the classical line bisection task typically used to diagnose deficits of spatial attention in clinical settings. McIntosh et al. (2005) provide a form of line bisection task which they argue can more sensitively assess spatial attention. Ninety-eight participants were presented with this line bisection task, once with and once without spatial cues, and both before and after a 50-minute vigilance task that aimed to decrease alertness. A single participant was excluded due to potentially inconsistent behaviour in the task, leaving 97 participants for the full analyses. While participants were, on a group level, less alert after the 50-minute vigilance task, they showed none of the hypothesised effects of reduced alertness on spatial attention in the line bisection task, regardless of with or without spatial cues. Yet, they did show the proposed effect of decreased alertness leading to a lower level of general attention. This suggests that alertness has no effect on spatial attention, as measured by a line bisection task, in neurotypical participants. We thus conclude that, in neurotypical participants, the effect of alertness on spatial attention can be examined more sensitively with tasks requiring a speeded response compared to unspeeded tasks.

It is still a matter of scientific debate whether the line bisection bias frequently observed in patients with spatial neglect is due to attentional underawareness of the left end of the line, attentional hyperattention towards the right end, or a logarithmically compressed perception of the line. To address this question, neglect patients who showed a line bisection bias were administered additional tasks involving horizontal lines (e.g., number line estimation tasks). Their performance was compared to neglect patients not showing a line bisection bias, patients with right hemisphere damage without neglect, and healthy controls. Results indicated that patients with a line bisection bias tended to overestimate lefthand segments when they had to dissect lines into three or four equal parts. This is congruent with both the notions of an underawareness of lefthand segments as well as a logarithmic compression of the line. However, when these patients had to imagine the lines as bounded fraction number lines ranging from 0-1, the results were mixed. When the number lines ranged from 0-10, these patients showed rightward overestimation biases for the numbers 4 and 5. Additionally, all patient groups, but not healthy controls, tended to place number 1 too far to the left and number 9 too far to the right, suggesting a general bias towards endpoints. In sum, this seems more congruent with attentional accounts than a perceptual one. Spatial-numerical associations could be ruled out, as all participants showed a verbal number bisection bias towards smaller numbers (i.e., the ‘left’ of the mental number line). Therefore, these findings seem to indicate that the line bisection bias is most likely due to underawareness of the left end rather than hyperattention towards the right or a logarithmic perception of the line.

Recently, it was discovered that the static magnetic field of an MRI scanner not only causes horizontal vestibular nystagmus in healthy individuals but, in addition, leads to a horizontal bias of spatial orienting and exploration that closely resembles the one observed in stroke patients with spatial neglect (a disorder of spatial attention and exploration). The present study asked whether the behavioral effects of this magnetic vestibular stimulation (MVS) can be inverted and thus be used to reduce the pathological bias of stroke patients with spatial neglect. Indeed, when patients with left-sided spatial neglect entered the scanner with their feet first, i.e., with the magnetic field vector pointing from head to toes, MVS inside the scanner reduced the ipsilesionally biased distribution of overt attention and the corresponding neglect of the left parts of the search-space. Thus, an intervention as simple as lying in a 3T MRI scanner bears the potential to become an integral part of a future strategy for treating spatial neglect.