Vying for dominance: dynamic interactions control visual fixation and saccadic initiation in the superior colliculus.
By the time you have reached this point, your daily count of alternating saccades and fixations will have increased considerably. So too will have your understanding of the dynamic interactions model. In the superior colliculi, visual fixation and saccadic initiation may be viewed as independent motor plans that compete for dominance across the intermediate layers. Extrinsic input modifies a point location on the retinotopic motor map that is shaped into a motor plan through the intrinsic circuitry of the superior colliculi. Independent motor plans compete for selection in a push-pull fashion and when a saccadic plan ultimately reaches threshold, it produces a strong burst of action potentials that shuts down the remaining regions of the intermediate layers. Modifying the activity of the intermediate layers changes these dynamic interactions in predictable ways. Enhancing the activity of one region facilitates nearby locations and inhibits distant locations. Diminishing the activity of one region inhibits nearby locations and facilitates distant locations. Such effects have been demonstrated in the neurophysiological activity of single cells (Munoz and Istvan, 1998; Olivier et al., 1999) and in behavior (Hikosaka and Wurtz, 1985; Munoz and Wurtz, 1993b). In addition to explaining visual fixation and saccadic initiation during basic saccadic tasks, the dynamic interactions model can explain changes in the timing of saccadic initiation that are observed when this task is modified. Namely, the gap effect, or decreased saccadic reaction times as a consequence of a gap period, occurs because removing fixation decreases the activity of fixation regions and, correspondingly, increases the excitability of saccadic regions. Express saccades, are a special instance of such dynamic interactions, in which decreased fixation activity and heightened motor preparation signals cause the target-related activity to be translated into a saccadic signal immediately. Finally, the slowing of saccadic initiation for antisaccades, can be interpreted as the consequence of multiple competing signals across the intermediate layers. It should be emphasized that the dynamic interactions that we have described in this chapter are not limited to the superior colliculi. On the contrary, similar interactions take place at many levels of the neuraxis (Moschovakis et al., 1996; Leigh and Zee, 1999; Schall and Thompson, 1999; Hikosaka et al., 2000; Munoz et al., 2000; Glimcher, 2001; Scudder et al., 2002). At this juncture, however, the dynamic interactions involved in producing visual fixation and saccadic initiation are better understood in the superior colliculi because of its well-organized motor map and its well-characterized neuronal elements. Although we are a long way from understanding how the brain controls visual fixation and saccadic initiation, we have made substantial progress in understanding these behaviors in the superior colliculi.