Activity of neurons in the medial ponto-medullary recticular formation during orienting movements in alert head-free cats. Tadashi, Isa, and Kimisato Naito. Department of Neurophysiology, Institute for Brain Research, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan.
APStracts 2:0067N, 1995.
SUMMARY AND CONCLUSIONS
1. Single unit activities of 236 neurons were recorded in the medial ponto- medullary reticular formation during visually triggered orienting gaze shifts in 10 alert cats under head-free conditions using movable tungsten needle electrodes attached to the skull. The activities were analyzed mainly in relation to the head movement which was triggered by presentation of an LED in one of eight directions separated radially by 45 degrees after fixation of the center LED. Of these, 120 neurons were recorded in the pontine reticular formation, chiefly in the nucleus reticularis pontis caudalis (RPc), and the remaining 116 were in the medullary reticular formation, chiefly in the nucleus reticularis gigantocellularis (RGc). Activities of 65 pontine and 65 medullary neurons were modulated in relation to the dynamic phase of orienting movements ("orienting-related neurons"). Activities of the remaining neurons were modulated either irregularly or not at all during orienting movement ("irregular or no-response neurons"). Input from the contralateral superior colliculus and cerebral cortex, and projections to the spinal cord were also investigated. 2. Among the "orienting-related neurons", 62 pontine and 55 medullary neurons showed increases in activity preceding the onset of eye and head movement by 0 - 155 ms ("pre- type"). Three pontine and 10 medullary neurons showed increases in activity only following the onset of movement ("post-type"). Of the "pre-type" neurons, 61 pontine and 51 medullary neurons showed directional preference of activity ("directional" neurons). One pontine and 4 medullary neurons were classified as "omnidirectional" because these neurons increased activity preceding movements in all directions tested, and no directional preference was apparent. 3. In the "pre- type/directional" cells, the average firing frequency during bursts was correlated with amplitude and angular velocity of head movements. Activities of the "directional" neurons during movements in the eight different directions could be well fitted with cosine functions in the majority of cases. The preferred directions of most pontine neurons and of about half the medullary neurons, as determined by first degree sinusoidal regression analysis, were distributed around the ipsiversive horizontal axis. However, there were also a considerable number of neurons whose preferred directions were upward, downward, contraversive or oblique in the medulla. 4. Among the directional cells preferring ipsiversive horizontal movements, 11 pontine neurons showed activity the onset of which was locked to visual stimuli with latencies of 40 - 70 ms, in addition to phasic discharges locked to the onset of movement. This "stimulus-locked activity" was sometimes modulated depending on the attentional state of the animal. 5. Among the irregular or no-response neurons, three pontine and 5 medullary neurons showed sustained discharges during active fixation of LEDs. These neurons often stopped firing during the dynamic phase of orienting movements, and 5 increased firing again after the cat fixated the target. These neurons occasionally became active during active searching for the target but did not fire at all while the cat was not attending the target. 6. The effects of stimulation of the contralateral superior colliculus (coSC) and cerebral peduncle (coCP) were investigated. The majority (68/73) of the orienting-related neurons (33/35 pontine and 32/38 medullary) were activated orthodromically from the coSC with a latency of 0.7- 6.8 ms, and many of these neurons responded with monosynaptic latency. Stimulation of the coCP induced orthodromic firing in 44/73 of the "orienting-related" neurons (16/35 pontine and 28/38 medullary) with a latency of 0.8 - 5.5 ms, and the majority were excited with monosynaptic linkage. Thirty-one neurons (29 "orienting-related" neurons) were identified as reticulospinal neurons (RSNs) by antidromic activation. Eight neurons with "stimulus-locked activity" were identified as RSNs. 7. The results of the present study showed two major characteristics of ponto-medullary reticular neurons in relation to orienting movements. Firstly, many of these neurons would function as immediate premotor interneurons generating motor signals which comprise the activation pattern of neck muscles. Especially, the pontine reticular neurons are involved in control of the horizontal component of visually triggered orienting head movements, which may comprise a counterpart of Forel's field H at the mesodiencephalic junction, a presumed vertical center as reported in our previous paper (Isa and Naito, 1994). Secondly, some of the reticular neurons show a visual response which is temporally independent of movement, suggestive of the interactive visuo-motor transformation process in the colliculo- reticular network. 

Received 14 April 1994; accepted in final form 21 February 1995.
APS Manuscript Number J186-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.