Horizontal saccades induced by stimulation of the central mesencephalic reticular formation

Abstract

The central mesencephalic reticular formation (cMRF) was electrically stimulated in the alert monkey. Saccadic eye movements were induced to the contralateral side in the horizontal plane at latencies of 18-35 ms. Smooth or slow eye deviations were not produced by cMRF stimulation. If the stimulus was given during slow phases of nystagmus, rapid eye movements were elicited, and the velocity of the slow phases was not affected. The function of cMRF neurons and/or of pathways that lie within it appear primarily related to generation of rapid eye movements in the horizontal plane. The amplitude of induced saccadic eye movements depended solely on the region of cMRF that was activated. When the stimulation frequency was lower, the latency was longer, but the size and characteristics of the induced movement were the same. The product of latency and stimulus frequency was approximately constant, suggesting that saccades had been triggered after a fixed number of pulses had been given. Stimulation of cMRF at frequencies that were too low to elicit rapid eye movements had a tonic effect on saccade generation. When the animal was having optokinetic nystagmus (OKN), stimulation modulated beat frequency according to the direction of the nystagmus: contralateral quick phases were facilitated and ipsilateral quick phases were suppressed. The frequencies of stimulation necessary to suppress ipsilateral quick phases increased as slow phase eye velocity increased. This demonstrates that both cMRF activity and slow phase velocity affect quick phase triggering. When the cMRF on both sides were simultaneously stimulated, the eyes were fixed in place, and no further rapid movements occurred until the stimulus had ended. Thus, activity in pathways and/or cells in cMRF is not only able to trigger saccades, but can also change the excitability of saccade generating mechanisms and promote fixation by suppressing eye movements. Two types of rapid eye movements were elicited from cMRF. From dorsal portions of cMRF saccades were induced whose size was relatively constant and not dependent on the initial position of the eyes in the orbit. The size of saccades increased from small to large as the stimulating electrode was advanced through cMRF from dorsal to ventral. This suggests that the tecto-bulbo-spinal efferents coursing through cMRF and/or cMRF neurons related to this input, are organized in a topographic fashion, with cells and fibers related to eye movements of increasing size being layered one beneath another.(ABSTRACT TRUNCATED AT 400 WORDS)

Anatomical Evidence that the Superior Colliculus Controls Saccades through Central Mesencephalic Reticular Formation Gating of Omnipause Neuron Activity

“…Alternatively, they may truly represent excitatory reticuloraphe projections, as GABA-negative terminals are also present following PhaL injections. In line with this, unilateral electrical microstimulation of some sites in the cMRF can suppress saccades, and bilateral stimulation can induce fixation (Cohen et al, 1985). Thus, the cMRF may also contain excitatory projections to RIP that help maintain fixation (Fig.…”

“…Pressure injections of BDA were made with a 1.0 l Hamilton microsyringe attached to a micromanipulator that was driven through the pulvinar, as previously described (Wang et al, 2010). The coordinates used were based on previous descriptions (Cohen et al, 1985;Chen and May, 2000;Paxinos et al, 2000). For these BDA injections, 0.1-0.2 l of a 10.0% solution of BDA (Invitrogen) was delivered into the left cMRF along each of one or two penetrations.…”

“…The central mesencephalic reticular formation (cMRF) is also part of the saccade circuitry (Cohen and Büttner-Ennever, 1984;Cohen et al, 1985). Retrograde tracer studies suggest the cMRF provides the RIP with input (Langer andKaneko, 1984, 1990), its activity is dominated by its inputs from the SC (Moschovakis et al, 1988a,b;Chen and May, 2000;Waitzman et al, 2000a,b;Cromer and Waitzman, 2006), and it contains GABAergic neurons (Araki et al, 1984, Wang et al, 2010Appell and Behan, 1990).…”

“…Electrical stimulation of this region produces contraversive horizontal saccades (Cohen and Büttner-Ennever, 1984;Cohen et al, 1985), and three-fourths of the cells Waitzman et al (1996) recorded in the cMRF have firing patterns that display saccade-related activity similar to those of pontine long-lead burst neurons (LLBNs). Scudder et al (1996) recorded saccade-relate activity in primate MRF neurons, which they subsequently intra-axonally injected.…”

Saccade-like behavior in the fast-phases of optokinetic nystagmus: An illustration of the emergence of volitional actions from automatic reflexes.

“…Furthermore the SC is ideally situated to engage in fast-phase related processing, as it has substantial connections between reticular formation areas known to be involved in the generation of the fast-phase of nystagmus (Cohen, Matsuo, Fradin, & Raphan, 1985;Grantyn & Grantyn, 1976;Hikosaka & Kawakami, 1977;Kitama, Ohki, Shimazu, Tanaka, & Yoshida, 1995). We do not claim that the SC is the main site in which fast-phases are generated, as ablation of the SC has little influence on basic fast-phases elicited in standard nystagmus paradigms (Albano & Wurtz, 1982;Pierrot-Deseilligny, Rosa, Masmoudi, Rivaud, & Gaymard, 1991;Schiller, True, & Conway, 1980).…”