Disentangling input and output-related components of spatial neglect

Abstract

Spatial neglect is a heterogeneous disorder with a multitude of manifestations and subtypes. Common clinical paper and pencil neglect tests fail to differentiate between these subtypes. For example, neglect patients typically bisect lines to the right. This bias can be caused by an underestimation of the left half of the line (input-related deficit), by the failure to direct actions toward the left side of space (output-related deficit), or by a mixture of these impairments. To disentangle these impairments, we used a test consisting of a line bisection task on a touch screen monitor (manual motor task) and the subsequent judgment of one's own bisection performance (visual perceptual task). It was hypothesized that patients with mainly output-related neglect should be better able to recognize their misbisected lines than patients with purely input-related neglect. In a group of 16 patients suffering from spatial neglect after right brain damage, we found that patients were three times more likely to suffer from a predominantly input-related than from an output-related subtype. The results thus suggest that neglect is typically an input-related impairment. Additional analysis of the line bisection task revealed that temporal (slowness in initiation and execution of contralateral movements) and spatial (insufficient movement amplitude toward the contralesional side) aspects of output-related neglect were mutually unrelated. This independence raises the possibility that a fine-grained differentiation of output-related neglect is required. That is, impairments in lateralized temporal and spatial aspects of movements may underlie different neglect subtypes.

Keywords: attention; motor neglect; neglect subtype; perceptual neglect; proof of concept; rehabilitation; spatial neglect; stroke.

Figure 1

Tasks and procedure. (A) Participants first performed a line bisection task in which they pointed to the lines' midpoint with their index finger. (B) Participants then judged their own bisection errors. Each line, divided into two colored segments at the locations where the participant previously placed the bisection mark, was then re-presented. Participants had to name the color of the longer line segment.

Figure 2

Dependency of correct judgment on deviation error. The percentage of correct judgments is plotted as a function of bisection error. The graph shows the normative data of 71 healthy subjects (black dots) and the corresponding lower bound of the 95% confidence interval (red line). Judgment scores below the lower bound of the fitted confidence interval curve were considered as indicating impaired perceptual judgment abilities.

Figure 3

Disentangling input- and output-related neglect. (A) Shows the performance of the individual patients in the line bisection and error judgment task (split by etiology). Bisection values in the gray colored area indicate the presence of neglect in the line bisection task. Error judgments below the red curve indicate impaired perceptual judgment abilities. See main text for indications of input or output-related neglect (B) Shows the corresponding performance of the control participants.

Figure 4

Relationship between temporal and spatial aspects of neglect. (A) Shows 23 individual patients' temporal (i.e., time required to perform leftward relative to rightward 24 bisections) and spatial (i.e., deviations of leftward relative to rightward bisections) scores 25 in the line bisection task. See main text for details (B) Shows the corresponding 26 performance of the control participants.