Prof. WANG's team at IBP. First row from left: YANG yan, YANG yang, Wang Shurong, CHEN Yan, MEI Yuling, NIU Yuqiong; Second row from left: WU Leqing, LIU Ruifeng, LI Dapeng.

To search for an object of interest, our eyes frequently make quick movements (saccades) from here to there. Studies have shown that when observing a large field motion, the eyes alternately perform slow tracking and rapid resetting, i.e. optokinetic nystagmus (OKN). It is known that this OKN is necessary to keep the images on the retina clear and stable; nevertheless its underlying neural mechanism has long been elusive, and therefore been quite attractive to neuroscientists.

Encouragingly, a group of scientists at the CAS Institute of Biophysics (IBP) has recently made some progress in this field. Prof. WANG Shurong and Ph.D students YANG Yang and YANG Yan report in the 15 Oct. issue of The Journal of Neuroscience their discovery of the neuronal circuitry in the pigeon controlling its eye movements, and revealed in detail how the shift and oscillation components of a horizontal saccade are initiated.

In human and other vertebrates, the eye is driven by three pairs of antagonistic muscles. Different combinations of their contraction and relaxation activate various movements of eyes. These muscles are controlled by optokinetic neurons located in the abducens, oculomotor and trochlear nuclei. For example, horizontal eye movements are jointly actuated by the lateral and medial rectus muscles, which are controlled by the abducens and oculomotor nuclei in the brain. Birds have highly developed visual sense, and their saccades consist of shift and oscillation components, thus providing a good model for research on the neural mechanism of eye movements. Based on the understanding of the visual neural networks in model birds like the pigeon and the similarity between them and those of mammals, scientists can get some clues about the neural mechanism of eye movements of the latter.

Wang and his students record 297 neurons in the abducens nucleus in the pigeon, and discover that the neurons differ in discharge patterns, generating different electric signals to activate the shift and/or oscillation components of a horizontal saccade. The recorded neurons fall into at least three types, namely the shift-related neurons (ShNs) that make sustained firing around the saccadic shift, the oscillation-related neurons (OsNs) that produce bursts accompanying saccadic oscillations, and the saccade-related neurons (SaNs) that discharge both sustained firing and bursts around the saccades. The researchers find that the ShNs begin to discharge 20 ms before the saccade and generate continuous firing until the end of the saccadic movement. The OsNs give off five to six bursts, with each corresponding to one cycle of oscillations of the horizontal saccade. As for the SaNs, their bursts also correspond to the oscillation component of the saccade, and its sustained firing helps the eyes to stay at a new position.

As reported by the team, the OsNs and SaNs can be further divided into two subgroups, one of which make bursting activity before the onset of a nasotemporal saccade (a saccade away from the nose toward the temple) and the other after. The former, named the leading group, begins to discharge bursts 8.1 ms before the onset of the saccade, and the latter, named the lagging group, begins to discharge bursts 7.9 ms after the onset. The team finds that when a temporonasal saccade (a saccade away from the temple toward the nose) occurs, the two groups change their discharge time courses: the leading group discharges after, yet the lagging group before the saccade. When examining the responses of the neurons to antidromic activation of the contralateral oculomotor nucleus with electric current, the team finds that the leading group makes no response, and the lagging group responds by giving off antidromic spikes. This suggests that the latter projects to the contralateral brain to coordinate the activity of the medial rectus muscles of the contralateral eye; whereas the leading group of neurons directly projects to the lateral rectus muscles of the eye on the local side.

"Therefore, the two eyes are able to synchronize their movements, if the two groups coordinate with each other in terms of the timing of discharge." Explains Wang.

Further research by the team has demonstrated that the abducens nucleus is controlled by the oculomotor signals from certain areas in the brain. Chemical blockade of the nucleus lentiformis mesencephali and the nucleus of the basal optic root, both of which are involved in optokinetic nystagmus, can stop the sustained firing generated by abducens neurons, and as a consequence, the shift component of saccades and the slow-phase of optokinetic nystagmus also disappear. Chemical blockade of the brainstem raphe complex, on the other hand, can stop the bursting activity and the oscillation component of saccades, and meanwhile eliminate the quick-phase of optokinetic nystagmus.

"Based on the experiments we can say that the optokinetic nuclei and the saccade-related raphe complex respectively send signals to the abducens nucleus, and the signals travel through motoneurons and innervate the lateral and medial rectus muscles to contract or relax, hence triggering and controlling the saccades or optokinetic nystagmus of the eyes." Introduces Wang.

"It is traditionally believed that optokinetic nystagmus contains slow and quick phases. Our research clearly demonstrates that the quick phase is essentially a saccade that lasts for a shorter time, if judged from the velocity of movement and its neural origin." Continues Wang: "As we know that the nucleus lentiformis mesencephali and the nucleus of the basal optic root are respectively comparable to the nucleus of the optic tract and the terminal nuclei of the accessory optic tract in mammals, and the brainstem raphe complex in birds might be equivalent to the nucleus raphe interpositus in primates, this discovery might provide some insights into neuronal circuitry controlling eye movements in mammals."

The finding of the neuronal circuitry underlying eye movements is highly evaluated by reviewers of the Journal of Neuroscience, and it is just the latest one of the exciting advances made by the IBP team in the field of neuronal circuitry controlling eye movements and its influence on visual perception. Five months ago, they reported their discovery of corollary discharge circuits for saccadic suppression in Nature Neuroscience and have since earned lots of acclaims in the neuroscience circle.