Unveiling contralesional omissions six years after stroke. Effects of top-down and bottom-up manipulations

We examined how spatial processing is affected by attentional load during multitasking in a chronic patient who suffered a right hemisphere stroke six years before the testing. We employed standard paper-and-pencil tests for neglect along with a new version of a well-established computerized dual-task paradigm. The latter combined a spatial processing primary task (reporting lateralized visual targets) with a concurrent secondary task (categorizing visual/auditory stimuli). Attentional load was manipulated through top-down (secondary task) and bottom-up (target size in primary task) factors. Paper-and-pencil tests did not reveal contralesional omissions. In contrast, the dual-task paradigm demonstrated greater sensitivity in detecting asymmetric spatial processing. Surprisingly, contralesional omissions occurred despite primary and secondary task stimuli did not overlap in time (i.e., secondary task stimuli were presented after the disappearance of lateralized visual targets lasting 100 ms). While both top-down and bottom-up manipulations induced contralesional omissions, their effects differed according to target size. Increased attentional load from dual-tasking impaired perception of larger contralesional targets, whereas smaller targets elicited omissions even in single-task conditions without additional multitasking effects. In this patient, very different manipulations, the first involving top-down and exclusively cognitive factors and the second involving bottom-up and purely perceptual aspects, independently modulated the level of processing resources. Both can be exploited to exacerbate very subtle (yet potentially hazardous) spatial processing deficits.