How many forms of perseveration? Evidence from cancellation tasks in right hemisphere patients

Neglect patients' performance during cancellation tasks is characterized by left sided omissions and, in many cases, by the production of inappropriate material of various kinds in the ipsilesional space, e.g. additional marks over already cancelled targets, marks drawn away from targets, scribbles, irrelevant drawings. It is unclear whether these behaviours, which have collectively been called perseverative, are functionally and anatomically connected and whether they correlate with the severity of neglect. Here we report a retrospective study on 33 right brain damaged patients with neglect after right hemisphere lesions in whom we measured the intensity of perseveration of the three following kinds: (1) 'additional marks' (AM) perseveration where patients cancelled a target with two or more well separated marks; (2) 'scribble' perseveration, where patients, instead of cancelling the target with a single pen stroke as required by the task, performed multiple pen strokes without breaking the pen-to-paper contact, with the final product being a scribble; (3) 'flying marks' (FM) perseveration where patients produced cancellation marks well away from the targets. We found that AM and FM perseveration correlated with neglect severity, while 'scribble' perseveration did not. The lesion-symptom mapping showed three separate anatomical areas in the right hemisphere: 'scribble' perseveration was associated with lesions of the orbitofrontal cortex and caudate nucleus; AM perseveration was associated with damage to the rolandic operculum, superior temporal gyrus and inferior frontal gyrus; FM perseveration was associated with damage to the dorsal premotor cortex and the temporal pole. Neglect severity followed damage to a region which grossly corresponds to the sum of the regions associated with AM and FM perseveration respectively. This complex behavioural and anatomical pattern is interpreted in terms of a three-factor model, in which AM perseveration is caused by a deficit of disengagement of attention from the right side (also causing omissions), FM perseveration is caused by directional hypokinesia (also causing left-side omissions), and 'scribble' perseveration is the consequence of a failure to inhibit an initiated motor act, which is completely separate (both anatomically and functionally) from the disorder inducing omissions.