Useful data were obtained from 14 adult squirrel monkeys of either sex, weighing 500-950 g. Animals were treated in accordance with the guidelines of the National Institutes of Health as stated in the Guide for the Care and Use of Laboratory Animals (DHEW Publication NIH85-23 1985) and with European Union directive 86/609 (Presidential Decree 160/1991). The methods employed have been extensively described before (Moschovakis et al. 1988, 1991a,b; Scudder et al. 1996a,b). Briefly, the animals were prepared for recording under sterile conditions and pentobarbital anesthesia (15 mg/kg). A stainless steel bolt was cemented on the occipital bone for head fixation, and preformed search coils, made of teflon-insulated stainless steel wire (Cooner), were sutured on the sclera of one or both eyes. In a second surgery 2 wk later, a parietal craniotomy was performed, and part of the cortex was aspirated to place a plastic chamber over the exposed left superior colliculus. The animals were alert and fully active within hours after surgery and showed no obvious neurological deficits.
Two days after surgery animals were placed in a primate chair with their heads fixed. Otherwise, they were free to move their body and limbs and appeared comfortable during the recording sessions. Glass micropipettes filled with either a 10% solution of horseradish peroxidase or a 5% solution of biocytin in 0.5 M KCl and 100 mM tris(hydroxymethyl)aminomethane buffer (pH = 7.4), and beveled to ) were inserted into the brain stem through the superior colliculus, and advanced toward the posterior commissure. Axon penetration was signaled by a 30- to 80-mV DC shift and the presence of 5- to 50-mV action potentials. We used the eye-coil method (Robinson 1963) to record the instantaneous position of the eyes with a resolution of 0.5°. Eye position was digitized on-line at a sampling rate of 2 ms, and the time between action potentials was measured with 100-µs resolution using the Spike2 software (Cambridge Electronics Design) running on a personal computer. Eye velocity was calculated off-line through software differentiation.
Digitized data were analyzed off-line. To evaluate the relationship between firing rate and eye position, we marked the beginning and the end of segments during which the eyes were fairly stationary (peak eye velocities = " src="http://jn.physiology.org/math/12pt/normal/ge.gif" align="baseline" />200 ms long and did not include pre- and postsaccadic intervals. The computer stored the horizontal and vertical eye position at the beginning (H1 and V1) and at the end (H2 and V2) of the segment, the mean horizontal [H = (H1 + H2)/2] and vertical [V = (V1 + V2)/2] eye position as well as the intervals between all spikes within the segment. An estimate of the regularity of neuronal discharge was obtained from the coefficient of variation (CV = SD/mean) of interspike intervals for spikes emitted while the eyes were at or close to the primary position (within 2°).
To evaluate the relationship between parameters of the bursts of NIC fibers and parameters of saccades, we first marked the beginning and the end of saccades and the beginning and the end of related bursts. The computer then stored values for the following variables: saccade latency (Lat), saccade duration (Sd), burst duration (Bd), horizontal (H) and vertical (V) eye displacement, horizontal (m) and vertical (m) peak eye velocity, peak firing rate (Fm), and the number-of-spikes in the burst (Nb). Linear regressions between parameters of discharge and parameters of the movement included those between Nb and V (H), between Fm and m (m), and between Sd and Bd. To estimate the relationship between the firing rate of NIC fibers and the mean eye velocity during smooth-pursuit eye movements, we used a procedure previously employed by Skavenski and Robinson (1973). Briefly, we selected segments during which the monkey pursued smoothly an object of interest. For most neurons, we found several segments during which the eyes moved at average velocities up to ±60°/s. In these segments, we marked six interspike intervals symmetrically bracketing a certain eye position. The computer then stored the horizontal and vertical eye position that corresponded to the first (H1 and V1) and to the last (H2 and V2) spike, the mean horizontal [(H1 + H2)/2] and vertical [(V1 + V2)/2] eye position, the time of occurrence of the first (t1) and of the last (t2) spike, and the intervals between all seven spikes in the series. The average vertical (horizontal) eye velocity during such segments then was estimated as the ratio (V2 V1)/(t2 t1) [(H2 H1)/(t2 t1)]. To estimate the average frequency of a neuron's discharge associated with vertical (horizontal) eye velocity, we subtracted the firing rate attributed to the mean vertical (horizontal) eye position during the same segment (from the rate-position curve that had been established previously for the same fiber).