Reaching a moveable visual target: dissociations in brain tumour patients

Area of Science:

• Neuroscience
• Cognitive Psychology
• Neuro-ophthalmology

Background:

• Damage to the posterior parietal cortex (PPC) is linked to Optic Ataxia (OA), characterized by reaching errors to peripheral targets.
• Emerging evidence suggests the PPC's role extends beyond simple reaching to include real-time trajectory adjustments when targets move.

Purpose of the Study:

• To investigate selective impairments in patients with prefrontal, premotor, or parietal lesions across three pointing tasks: peripheral targets, fixatable targets, and moved targets (online corrections).
• To determine if deficits in reaching to peripheral targets and correcting movements to moved targets are independent processes.

Main Methods:

• Experimental study involving patients with lesions from tumor operations (prefrontal, premotor, parietal).
• Assessment of pointing accuracy in three conditions: peripheral targets, fixatable targets, and targets that moved mid-movement.
• Correlation analysis of reaching errors and 'shift cost' for online corrections.

Main Results:

• Parietal cortex lesions selectively impaired performance in all three pointing tasks.
• Peripheral reaching errors strongly correlated with errors to fixatable targets in parietal patients.
• No correlation was found between peripheral reaching errors and the 'shift cost' for online movement corrections, indicating double dissociations.

Conclusions:

• The parietal cortex is crucial for reaching to peripheral and fixatable targets, as well as for online movement corrections.
• Deficits in reaching to peripheral targets and correcting movements to shifted targets appear to involve at least partly independent processes.
• Clinical understanding of Optic Ataxia may need refinement regarding the distinct neural underpinnings of reaching errors and online movement adjustments.