We studied the effects of small lesions of the oculomotor vermis of the cerebellar cortex on the ability of monkeys to execute and adapt saccadic eye movements. For saccades in one horizontal direction, the lesions led to an initial gross hypometria and a permanent abolition of the capacity for rapid adaptation. Mean saccade amplitude recovered from the initial hypometria, although variability remained high. A series of hundreds of repetitive saccades in the same direction resulted in gradual decrement of amplitude. Saccades in other directions were less strongly affected by the lesions. We suggest the following.(1) The cerebellar cortex is constantly recalibrating the saccadic system, thus compensating for rapid biomechanical changes such as might be caused by muscle fatigue. (2) A mechanism capable of slow recovery from dysmetria is revealed despite the permanent absence of rapid adaptation.

The cerebral cortex must have access to an eye position signal, as humans can report passive changes in eye position in total darkness, and visual responses in many cortical areas are modulated by eye position. The source of this signal is unknown. Here we demonstrate a representation of eye position in monkey primary somatosensory cortex, in the representation of the trigeminal nerve, near cells with a tactile representation of the contralateral brow. The neurons have eye position signals that increase monotonically with increasing orbital eccentricity from near the center of gaze, with directionally selectivity tuned in a Gaussian manner. All directions of eye position are represented in a single hemisphere. The signal is proprioceptive, because it can be obliterated by anesthetizing the contralateral orbit. It is not related to foveal or peripheral visual stimulation, and it represents the position of the eye in the head and not the angle of gaze in space.

The influence of cognitive context on orienting behaviour can be explored using the mixed memory-prosaccade, memory-antisaccade task. A symbolic cue, such as the colour of a visual stimulus, instructs the subject to make a brief, rapid eye movement (a saccade) either towards the stimulus (prosaccade) or in the opposite direction (antisaccade). Thus, the appropriate sensorimotor transformation must be switched on to execute the instructed task. Despite advances in our understanding of the neuronal processing of antisaccades, it remains unclear how the brain selects and computes the sensorimotor transformation leading to an antisaccade. Here we show that area LIP of the posterior parietal cortex is involved in these processes. LIP's population activity turns from the visual direction to the motor direction during memory-antisaccade trials. About one-third of the visual neurons in LIP produce a brisk, transient discharge in certain memory-antisaccade trials. We call this discharge 'paradoxical' because its timing is visual-like but its direction is motor. The paradoxical discharge shows, first, that switching occurs already at the level of visual cells, as previously proposed by Schlag-Rey and colleagues; and second, that this switching is accomplished very rapidly, within 50 ms from the arrival of the visual signals in LIP.

The lateral intraparietal area (LIP) contains neurons that are active during the memory interval of memory saccades. We call these "persistent neurons." Here we study the activity of the persistent neurons in memory antisaccades, "motor" (the saccade is made toward the response field, although the response field is not stimulated visually) and "visual" (the response field is stimulated visually, but the movement is away from the field). Most persistent neurons are active during parts of the memory intervals of both visual and motor memory-antisaccades. Typically, these parts significantly overlap each other and together span the entire memory interval. The amplitude of the activity changes systematically during the memory intervals of visual and motor memory antisaccades. These changes are reflected in an antisaccade differential activity, which turns first to the visual direction and then crosses over to the motor direction. Some persistent neurons appear to show the paradoxical activity previously characterized in visual neurons; paradoxical activity accelerates the transition of the neuron's activity from visual to motor. These observations suggest that the persistent neurons reflect working memory for the computation of the antisaccade sensorimotor transformation. Ensembles of persistent neurons with different response fields may make up modules of working memory.

Humans and monkeys have access to an accurate representation of visual space despite a constantly moving eye. One mechanism by which the brain accomplishes this is by remapping visual receptive fields around the time of a saccade. In this process a neuron can be excited by a probe stimulus in the current receptive field, and also simultaneously by a probe stimulus in the location that will be brought into the neuron's receptive field by the saccade (the future receptive field), even before saccade begins. Here we show that perisaccadic neuronal excitability is not limited to the current and future receptive fields but encompasses the entire region of visual space across which the current receptive field will be swept by the saccade. A computational model shows that this receptive field expansion is consistent with the propagation of a wave of activity across the cerebral cortex as saccade planning and remapping proceed.