**Figure 1.** A: oculomotor-related discharge pattern of a regular upward efferent fiber of the NIC. Traces from top to bottom illustrate the instantaneous horizontal and vertical eye position and the instantaneous firing rate. B: quantitative analysis of the relationship between the mean intersaccadic firing rate (ordinate) of the same unit and the mean intersaccadic vertical eye position (abscissa). , linear regression line through the data () obeys the expression F = 3.5V + 102.3 (r = 0.96). Insets, right and left: plots of horizontal eye position (abscissa) vs. firing rate (ordinate) and mean interspike interval (meanISI; abscissa) versus standard deviation of interspike intervals [SD(ISI); ordinate]. C: quantitative analysis of the relationship between the mean intersaccadic vertical eye position and the mean intersaccadic firing rate of an irregular downward efferent fiber of the interstitial nucleus of Cajal (NIC). Plot layout as in B. Linear regression line obeys the expression F = 2.0V + 65.8 (r = 0.67).

**Figure 2.** A: linear regression lines describing the relationship between mean intersaccadic firing rate (ordinate) and vertical eye position (abscissa) for 35 bidirectionally modulated upward NIC units. Frequency histograms of the correlation coefficients, slopes, and intercepts of these relationships are illustrated in the inset, B, and C, respectively. D: percentage of fibers already recruited (ordinate) when the eye reaches the downward position indicated on the abscissa. E: plot of the slope of the rate-position curves (ordinate) vs. position threshold (abscissa). , different unit. , least squares regression line that obeys the expression indicated in the text. F: frequency histogram of the coefficient of variation (CV) of the same fibers.

**Figure 3.** Quantitative analysis of the eye-position-related discharge of 31 bidirectionally modulated downward NIC units. Plot layout as in Fig. 2.

**Figure 4.** A: eye-position-related discharge of a unidirectionally modulated upward NIC fiber. Segments of intersaccadic intervals have been arranged from down (left) to up (right). B: quantitative analysis of the relationship between the mean intersaccadic firing rate (ordinate) of the same fiber and the mean intersaccadic vertical eye position (abscissa). , linear regression line through the data () that obeys the expression F = 4.9V + 76 (r = 0.89).

**Figure 5.** A: saccade-related discharge pattern of a burst-tonic NIC fiber that emitted strong bursts for upward saccades. Saccades are arranged from downward (left) to upward (right), and they have been moved vertically so that they all start (upward saccades) or end (downward saccades) at the same vertical position. Note that different calibration bars apply to the instantaneous firing rate that accompanies downward (left) and upward (right) saccades. B: plot of the number of spikes in the burst (N_{b}, ordinate) vs. size of the vertical component of saccades (V, abscissa). Linear regression line obeys the expression N_{b} = 0.56V + 4.9 (r = 0.97). Inset: plot of N_{b} (ordinate) vs. size of the horizontal component of saccades (H, abscissa). C: plot of burst duration (B_{d}; ordinate) vs. saccade duration (S_{d}; abscissa). Linear regression line obeys the expression B_{d} = 0.88S_{d} + 6.96 (r = 0.89). D: plot of peak firing rate (F_{m}; ordinate) vs. peak saccadic eye velocity (_{m}; abscissa). Linear regression line obeys the expression F_{m} = 0.56_{m} + 168.8 (r = 0.93).

**Figure 6.** A: saccade-related discharge pattern of a burst-tonic NIC fiber that emitted weak bursts for upward saccades. Traces as in Fig. 5A. Note the firing rate calibration bar. B: plot of the number of spikes in the burst (N_{b}, ordinate) vs. size of the vertical component of saccades (V, abscissa). Linear regression line obeys the expression N_{b} = 0.3V + 3.8 (r = 0.9). Inset: plot of N_{b} (ordinate) vs. size of the horizontal component of saccades (H, abscissa). C: plot of burst duration (B_{d}; ordinate) vs. saccade duration (S_{d}; abscissa). Linear regression line obeys the expression B_{d} = 0.65S_{d} + 20.3 (r = 0.81). D: plot of peak burst firing frequency (F_{m}; ordinate) vs. peak saccadic eye velocity (_{m}; abscissa). Linear regression line obeys the expression F_{m} = 0.1_{m} + 164.4 (r = 0.4).

**Figure 7.** A: linear regression lines describing the relationship between the number of spikes in the burst (N_{b}; ordinate) and the vertical size of saccades (abscissa) for 38 upward burst-tonic NIC units. B: linear regression lines describing the relationship between burst duration (ordinate) and saccade duration (abscissa) for 36 upward burst-tonic NIC fibers. Note that this plot contains fewer data points because the relationship did not attain statistical significance in 2 units. C: linear regression lines describing the relationship between peak rate of discharge (ordinate) and the peak vertical velocity of saccades (abscissa) for 36 upward burst-tonic NIC units. This plot also contains fewer points because the linear regression did not attain statistical significance in 2 units. Insets: frequency histograms of the slopes and the correlation coefficients of statistically significant relationships.

**Figure 8.** Quantitative analysis of the saccade-related discharge of 27 downward burst-tonic NIC units. Plot layout as in Fig. 7.

**Figure 9.** Discharge pattern of an NIC fiber that emitted neither bursts nor pauses for saccades and the firing rate of which was modulated with downward eye position. Traces as in Fig. 5A.

**Figure 10.** Quantitative analysis of the relationship between vertical eye velocity and firing rate of NIC fibers during smooth-pursuit eye movements. A: plot of the mean firing rate (F, ordinate) vs. mean vertical eye velocity (_{sp}, abscissa) for 1 unit. , least squares linear regression line through the data () obeys the expression indicated in the text. B: frequency histogram of the slopes of the 41 such relationships that attained statistical significance. Inset: frequency histogram of the correlation coefficients of these relationships.

**Figure 11.** Cross-correlation between variables describing NIC unit discharge. Correlation coefficients (r) and F ratios are shown for relationships which attained statistical significance (P = 0.001 or better). k_{V}, slope of the rate-position curve; F_{0}, firing rate at primary position; CV, coefficient of variation; Bi/Uni, bidirectional (Bi) or unidirectional (Uni) modulation of firing rate with vertical eye position; Lat, latency of saccade-related bursts; R_{V}, slope of the relationship between the number of spikes in the burst and the vertical displacement of the eyes; r_{(Bd/Sd)}, correlation coefficient of the relationship between burst duration and saccade duration; G_{(Fm/m)}, slope of the relationship between maximal burst frequency and maximal saccadic eye velocity; , neuron classes in which tonic and burst discharges increase in the same ( = 0°) or in the opposite ( = 180°) directions.

**Figure 12.** Three-dimensional plot of parameters of discharge of 45 burst-tonic NIC efferent fibers (each sphere corresponds to 1 unit). Three axes of the plot correspond to the 3 factors extracted with the help of principal component analysis. Origin of each axis (0) corresponds to the mean of the distribution of each factor, and increments are in units of standard deviation. Positive values indicate higher intensity, decreased regularity and increased sensitivity, on the "burstiness," "regularity," and "sensitivity" axes, respectively. To avoid clutter, some of the data points were removed.