Coding of information about rapid eye movements in the pontine reticular formation of alert monkeys

Neurons in the rostral paramedian zone of the pontine reticular formation (PRF) have distinct frequency changes prior to and during quick eye movements, but generally little or no tonic activity associated with eye position. Evidence indicates that horizontal saccades and quick phases of nystagmus are generated in this region. Firing of units activated with eye movements (burst units) and of units which are inhibited (pause units) was analyzed. Eye movements were described by a vector having an amplitude (A) and an angle (a). These parameters were related to position changes in certain planes by the equation delta pos = A - cos alpha. In each of 80 cells in the PRF which were encountered, the activity could be related to some parameter of the above equation: change of position (delta pos), amplitude (A), or the cosine of the angle between the direction of movement and a reference direction (cos alpha). Units coding amplitude of eye movement or change in position in a particular plane conveyed the information by number of spikes. Units coding direction of movement did so by frequency. In many units, the information coding was precise so that the direction or amplitude of single eye movements could be predicted from the frequency changes of single units. In other units, this could be determined from averages of the frequency changes. Cells coding direction or change in position had frequency maxima only in planes corresponding to the pulling direction of the eye muscles. The results suggest that a vector description is not only a convenient mathematical tool, but is the way eye movements are coded in the PRF and possibly elsewhere in the central nervous system.