SINGLE NEURONS IN POSTERIOR CINGULATE CORTEX OF BEHAVING MACAQUE - EYE-MOVEMENT SIGNALS

1. Posterior cingulate cortex, although widely regarded as a part of t he limbic system, is connected most strongly to parietal and frontal a reas with sensory, motor, and cognitive functions. To gain insight int o the functional nature of posterior cingulate cortex, we have recorde d from its tasks known to activate parietal and frontal neurons. We ha ve found that posterior cingulate neurons fire during periods of ocula r fixation at a rate determined by the angle of gaze and by the size a nd direction of the preceding eye movement. 2. The activity of 530 pos terior cingulate neurons was monitored while rhesus macaque monkeys ma de visually guided eye movements to spots projected on a tangent scree n. 3. In 150/530 neurons, a statistically significant shift in the rat e of discharge occurred around the time of onset of saccadic eye movem ents. The preponderant form of response was an increase in activity (1 42/150 neurons). 4. In 142 neurons exhibiting significant excitation a fter saccades in at least one direction, the level of discharge was an alyzed as a function of time relative to onset of the saccade. Across the neuronal population as a whole, activity increased sharply at the moment of onset of the saccade, rising to a maximum after 200 ms and t hen declining slowly. The net level of discharge remained well above p resaccadic baseline even after >1 s of postsaccadic fixation. 5. In 63 neurons, the postsaccadic rate of discharge was analyzed relative to the angle of the eye in the orbit by monitoring neuronal activity whil e the monkey executed saccades of uniform direction and amplitude to f our targets spaced at 16-deg intervals along a line. The postsaccadic firing level was significantly dependent on orbital angle in 44/63 neu rons. 6. In 45 neurons, the postsaccadic rate of discharge was analyze d relative to the angle of the eye in the orbit by monitoring neuronal activity while the monkey executed 16-deg saccades to a constant targ et from diametrically opposed starting points. The postsaccadic level of activity was significantly dependent on saccade direction in 20/5 n eurons. 7. In 58 neurons, the postsaccadic rate of discharge was analy zed relative to saccade amplitude by monitoring neuronal activity whil e the monkey executed saccades, which varied in amplitude (4, 8, 16, a nd 32 deg) but which were constant in direction and brought the eye to bear on a constant endpoint. The postsaccadic level of activity was s ignificantly dependent on saccade amplitude in 24/58 neurons. In all n eurons exhibiting significant amplitude-dependence, stronger firing ac companied larger saccades. 8 The activity of in 10 neurons was monitor ed during smooth pursuit eye movements (20 deg/s upward, downward, lef tward, and rightward). The level of firing varied as a function of bot h the position of the eye (9 neurons) and the velocity of the eye (6 n eurons). 9. We conclude that posterior cingulate neurons monitor eye m ovements and eye position. It is unlikely that they participate in the generation of eye movements because their shifts of discharge follow the onset of the movements. Eye-movement-related signals in posterior cingulate cortex may reflect the participation of this area in assigni ng spatial coordinates to retinal images.