システムニューロサイエンス講座 １１． 上丘による眼球運動制御

Abstract

The appearance of a novel or attractive stimulus induces saccadic eye movements and head movements towards the stimulus (orienting response). The mammalian superior colliculus (SC) has been shown to be an important structure for generation of the orienting response. This paper reviews the function of the SC in relation to saccade in the monkey and the cat. Functionally, the SC can be divided into dorsal (superficial) and ventral (intermediate and deep) layers. The former serves primarily as a visual structure receiving input directly from the retina and the striate cortex. Neurons in this layer respond to visual stimuli in the contralateral visual field. These visually responsive neurons are arranged topographically forming a retinotopic map over the SC's surface. The latter receives convergent inputs of multi-sensory modalities (auditory, somatosensory and visual). The auditory, somatosensory and visual receptive fields are arranged in a topographic fasion within the intermediate and deep layers corresponding well to the retinotopic map of the superficial layer. It also receives inputs from the motor-relating areas such as frontal and parietal association cortices and basal ganglia. Electrical stimulation in a given point of the intermediate layer evokes a saccade with a given direction and amplitude. A map of the direction and amplitude can be developed. This motor map is aligned with the retinotopic map. Neurons discharge in relation to the saccade directed to the motor field. It has been suggested that these neurons send descending axons to the contralateral reticular formation including the saccade-generation center. Recent studies reveal that another class of neurons, fixation neurons, are located in the intermediate layer of the rostral SC. These show sustained activities during active fixation and pause during saccade. It is suggested that saccade-related and the fixation neurons have connected mutually inhibitory roles : when the former fires the latter pauses and vice versa. Thus, both the visual and motor maps for the saccade are represented within the SC, suggesting that the SC serves as an important center for sensorimotor transformation for the saccade.