THE ACTIVATION PATTERN IN NORMAL HUMANS DURING SUPPRESSION, IMAGINATION AND PERFORMANCE OF SACCADIC EYE-MOVEMENTS

The distribution of activated cerebral regions was examined in nine no rmal subjects during four different eye movement-related conditions: ( 1) fixation - fixation on a central light emitting diode; (2) saccadic suppression - fixation on a diode in the presence of flashing lateral targets, (3) reflexive/ volitional saccades - performance of overt ey e movements to two laterally lit targets and back to the centre; and ( 4) imagined saccades -imagining, but not performing, the same eye move ments. The regional neural activity was measured indirectly using repe titive bolus injections of oxygen-15-labelled water and positron emiss ion tomography (PET) to yield time-integrated images of the normalized count distribution. These were aligned and anatomically normalized to a standard stereotactic space and the averages of each condition were analysed categorically using statistical parametric mapping. Compared to central fixation, reflexive/volitional saccades significantly acti vated regions in the classically known cortical ocutomotor regions. Th e most notable activation during the saccade suppression task, compare d to central fixation alone, was a bilateral activation of the parieta l cortex with a right-sided preponderance, activation of the supplemen tary eye field/caudal cingulate regions and activation of frontal regi ons close to the frontal eye fields. Imagined performance of eye movem ents without overt eye movements activated the supplementary eye field and frontal eye fields identically to regions involved in overt eye m ovements, thus demonstrating that overt eye movements are not a prereq uisite of the activation of these regions in normal humans.