Three-dimensional covert attentional functions in Parkinson's disease subjects.

This study assessed the ability of mildly affected Parkinson's disease (PD) subjects (n = 16) to perform attentional cognitive tasks within a three-dimensional object. A hollow cube was displayed on a computer screen and the subject was required to respond as quickly as possible to the highlighting of one of the cube angles by pressing the spacebar of the keyboard. Prior to the appearance of this imperative stimulus, the same ("valid" trials) or an alternative ("invalid" trials) angle was highlighted. For the invalid trials this meant that the subject oriented attention to the cued angle but, on imperative stimulus appearance, was unexpectedly required to redirect attention to another angle, which could be on a different cube face to that which had been cued. For one experimental session the cube was stationary, that is, object-centred and viewer-centred coordinates of a cube angle corresponded. For another session, the cube rotated such that the viewer-centred coordinates of an angle changed between appearance of the cue and appearance of the stimulus, but the angle's object-centred coordinates remained constant. The finding of lower reaction times for the valid than for the invalid trials, even when the cube was rotating, indicated that PD subjects could operate attention using an object-centred coordinate system. However, PD subjects showed exaggerated reaction times when the stimulus appeared in a cube face that was opposite to, rather than the same as, that of the invalidly cued angle. It is suggested that this reflects a dysfunction in the grouping of the structural components of the whole object at an attentional level.