Good vision requires an oculomotor system that can make rapid and accurate eye movements, called saccades, to visual targets of interest. To do this, the brain must have an internal representation of target location. The superior colliculus (SC), the main source of saccade commands to brainstem circuits driving the eyes, encodes target localization and saccade metrics by place codes: a retinotopic map of locations in register with a map of saccade vectors. It appears that the parabigeminal nucleus (PBN), a small cluster of cells on the lateral edge of the midbrain that is reciprocally interconnected with the SC, also encodes target location, but by a rate code. When cats visually track a moving object, they rely primarily on a series of catch-up saccades to continually intercept it. Preliminary studies have revealed that in the intervals between catch-up saccades, PBN cells fire at rates proportional to the distance between the eye and potential target, or retinal position error (RPE). During saccades, PBN activity abruptly changes to levels appropriate to RPE at saccade end in a fashion that seems too fast for visual feedback, suggesting that activity may be reset by an internally generated signal, such as a resettable integrator of eye velocity, or an open-loop feedforward signal. Its robust anatomical interconnections with the SC imply that the PBN primarily contributes to the saccade system by providing continuous information on target location during intersaccade intervals. Whether or not it contributes to the dynamics or metrics of the saccade is unknown. Since the SC place code must at some point be translated into a rate code to drive eye movements, the PBN may provide a useful model for understanding how a place-to-rate code translation can be accomplished. The goals of this project include better defining the range and topography of RPE encoding, determining if PBN activity provides an index of the currently attended saccade target, examining PBN interactions with the SC, investigating the nature of the signal that resets PBN activity during saccades, and examining the role of the PBN in predictive tracking of moving targets. Historically, there has been a productive interaction between oculomotor physiology and clinical neurology. The PBN appears to be an important part of the oculomotor system, and its incorporation into models of oculomotor circuits will extend this valuable interchange. ? ?
| Ma, Rui; Cui, He; Lee, Sang-Hun et al. (2013) Predictive encoding of moving target trajectory by neurons in the parabigeminal nucleus. J Neurophysiol 109:2029-43 |
