An 'automatic pilot' for the hand in human posterior parietal cortex: toward reinterpreting optic ataxia

We designed a protocol distinguishing between automatic and intentional mot or reactions to changes in target location triggered at movement onset. In response to target jumps, but not to a similar change cued by a color switc h, normal subjects often could not avoid automatically correcting fast aimi ng movements. This suggests that an 'automatic pilot' relying on spatial vi sion drives fast corrective arm movements that can escape intentional contr ol. in a patient with a bilateral posterior parietal cortex (PPC) lesion, m otor corrections could only be stow and deliberate. We propose that 'on-lin e' control is the most specific function of the PPC and that optic ataxia c ould result from a disruption of automatic hand guidance.