Motor role of parietal cortex in a monkey model of hemispatial neglect

Abstract

Parietal cortex is central to spatial cognition. Lesions of parietal cortex often lead to hemispatial neglect, an impairment of choices of targets in space. It has been unclear whether parietal cortex implements target choice at the general cognitive level, or whether parietal cortex subserves the choice of targets of particular actions. To address this question, monkeys engaged in choice tasks in two distinct action contexts--eye movements and arm movements. We placed focused reversible lesions into specific parietal circuits using the GABAA receptor agonist muscimol and validated the lesion placement using MRI. We found that lesions on the lateral bank of the intraparietal sulcus [lateral intraparietal area (LIP)] specifically biased choices made using eye movements, whereas lesions on the medial bank of the intraparietal sulcus [parietal reach region (PRR)] specifically biased choices made using arm movements. This double dissociation suggests that target choice is implemented in dedicated parietal circuits in the context of specific actions. This finding emphasizes a motor role of parietal cortex in spatial choice making and contributes to our understanding of hemispatial neglect.

Keywords: LIP; attention; choice; decision; spatial cognition.

Fig. 1.

Choice in the context of two distinct actions. In this task, a monkey acquired a central target, which changed color, in blocks of 10 trials, to either red or blue. The red (blue) color instructed the animal to make a subsequent choice using a saccade (reach). After a delay, a white target appeared in the left (right) part of the screen. Following a variable delay (0–107 ms), a second white target appeared in the right (left) part of the screen. To receive a reward, the animal had to select the target that appeared earlier, using the instructed effector.

Fig. 2.

LIP inactivations bias choices specifically made using saccades. (A) Anatomical MR scans following muscimol plus MnCl₂ injections. The contrast agent MnCl₂ indicates the spread of the drug as a white spherical region. The figure gives the loci (S, T, U) of the LIP lesions performed in each monkey. The left and right images for each monkey represent the coronal and transverse views, respectively. See SI Appendix, SI Methods and Table S1, for details. IPS, intraparietal sulcus. A, anterior; P, posterior; D, dorsal; V, ventral; L, left; R, right. (B) Mean ± SEM percentage of choices of the contralesional target as a function of the delay between the onset of the contralesional and the ipsilesional target, separately for control sessions (dark colors), lesions (light colors), and saccade (red) and reach (blue) choices. (Left) Monkey S. (Right) Monkey D. The figure shows all of the individual choices that a monkey made. (C) The difference in the percentage of choices of the contralesional target (inactivation minus control) for each individual session in each monkey, separately for saccade choices (red) and reach choices (blue). (D) Mean ± SEM over the individual session data points shown in C. The P value is the statistical outcome of the paired t test (control versus inactivation) performed on the data of each effector; n.s.: P > 0.05.

Fig. 3.

PRR inactivations bias choices specifically made using reaches. Same format as in Fig. 2 for the effects during PRR lesions. (A) Anatomical MR scans following muscimol plus MnCl2 injections. (B) Mean ± SEM percentage of choices of the contralesional target as a function of the delay between the onset of the contralesional and the ipsilesional target. (C) The difference in the percentage of choices of the contralesional target (inactivation minus control) for each individual session in each monkey, separately for saccade choices (red) and reach choices (blue). (D) Mean ± SEM over the individual session data points shown in C.