Here we provide a brief overview of the role of the SC in the initiation and control of saccadic eye movements (for recent reviews, see Isa 2002; Sparks 2002, 2004; Keller 2004). The SC, a laminated structure forming part of the roof of the midbrain, plays a critical role in triggering and organizing saccadic eye movements. The superficial layers are sensory in function. Cells have visual receptive fields and inputs arrive directly from the retina as well as from visual cortex. In contrast, the intermediate and deep layers have both sensory and motor functions. These layers receive visual, auditory, and somatosensory inputs from many cortical and subcortical regions (for references, see Sparks and Hartwich-Young 1989) and contain neurons generating commands for movements of the eyes, head, and (in some animals) the external ears. When recording in these deeper layers, various types of cells are encountered. Some display only sensory responses, others only motor-related activity; both sensory- and motor-related activity are present in the activity of a third general class of cells (see discussion below and Figure 2.6c-e). Many neurons in the deeper layers generate a high-frequency burst of spike activity that is tightly coupled to saccade onset; the burst begins 18-20 ms before saccade onset. Each neuron with a presaccadic burst has a movement field; that is, the cell discharges before a range of saccades that have particular directions and amplitudes. The burst of collicular cells is not related to moving the eye to aparticular position in the orbit but to changes in eye position, which have a certain direction and a certain amplitude.
Saccade-related burst neurons are arranged topographically within the SC, and this organization forms the basis of the motor map revealed by myostimulation studies. In head-restrained animals, microstimulation in the deeper layers of the primate SC produces contralateral saccadic movements of both eyes with a latency of approximately 20-30 ms. Movements with upward directions are represented medially, and movements with downward directions are represented laterally. Stimulation of rostral regions produces small amplitude movements; larger movements occur with caudal stimulation. The site of stimulation within the colliculus determines the largest movement that can be produced; however, for any stimulation site, movement amplitude depends on the duration of the stimulation train. As train duration increases, movement amplitude increases monotonically until it reaches the site-specific limit. The peak velocity of an evoked movement is influenced by the frequency of stimulation: higher frequencies produce movements with higher velocities. In summary, these findings suggest that the site of collicular activity is the major determinant of saccade direction and amplitude but collicular activity must be sustained until the desired displacement is accomplished. The level of activity influences the speed of the movement.
At one level of analysis, the task of programming a particular saccade involves activating the appropriate region of the collicular map. Once output neurons in this region are driven into high-frequency burst mode, an accurate movement of the desired direction and amplitude will occur. The kinematic rules uncovered by motor psychophysics, such as the relationship between movement duration and amplitude, will be implemented downstream. The collicular command for accurate saccades is produced by neurons with broadly tuned receptive fields and coarsely tuned movement fields. The intrinsic and extrinsic anatomical connections are such that even crude, nonphysiological activation, such as electrical stimulation, is sculpted into a spatial and temporal pattern of activity that evokes movements indistinguishable from those produced by normal physiological inputs. How the pattern of intrinsic connections and the biophysical properties of collicular neurons may be involved in the initiation of saccades, and how accurate movements are produced by neurons with broadly tuned movement fields are discussed below.
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