Saccadic recordings were obtained from 11 subjects, ages 19–23 yr, who had an unaided Snellen acuity of 6/4, normal binocular single vision, and heterophoria determined with the Maddox rod test of no more than 5Δ. The form of the saccade was fundamentally the same in all subjects in that, after a latent period of ca 180–250 ms, there arose the rapid saccadic eye movement which resulted in fixation of the target. In those experiments employing a moving target, the saccadic movement was followed by a smooth pursuit movement. Examples of saccadic movements in response to a stationary target are shown in Fig. 1A &1B. We saw no evidence of express saccades of very short latency. As shown in Fig. 1A &1B, fixation of the target was always attained smoothly without the generation of secondary saccades.
Saccadic amplitude
In all experiments, the accuracy of the saccadic movements in fixating the target was demonstrated by the highly significant relationship between saccadic amplitude and target displacement. This is illustrated by the montages of increasing saccadic amplitudes generated in response to a stationary target presented at increasing angular displacements from the fixation point (Fig. 1A &1B). Aggregation of the results into plots of saccadic amplitude against target displacement (Fig. 1C &1D) resulted in R2 values of typically 94% (P
slope
< 0.001, F > 1200; d.f. > 772). Similar strong, direct relationships between saccadic amplitude and target displacement for moving targets were also obtained as shown by the results in Fig. 2B, D, F &2H.
Saccadic latency for direct viewing
(a) Stationary target of normal intensity
Initially, a stationary target, which in this case was of normal beam intensity, was tested in 6 subjects with direct viewing of the fixation point. Saccadic latency remained invariant with respect to increasing target displacement in 4 subjects (R2 ≤ 3.4%, P
slope
≥ 0.11) (Fig. 1E). The distribution of the latencies for the subject in Fig. 1E is shown in Fig. 1G in which the data are grouped about a mean value of 226.4 ± 5.4 ms (mean ± S.E.M.). The fifth subject showed a significant but small decrease in the slope of the regression (R2 = 6.8%, P
slope
= 0.045; d.f. = 46), while the sixth subject showed a small significant increase in the slope (R2 = 17.1%, P
slope
= 0.01; d.f. = 35). However, in both cases, the F values of 4.3 and 8.2, respectively, were both well below the threshold value of 25.0 (see Methods), indicating that these R2 values were without importance. From these results, it was concluded that saccadic latency remained invariant with respect to increasing angular displacement of a stationary target of normal intensity.
(b) Stationary red target
Since those investigators who had reported an increase in saccadic latency with increasing target displacement had employed a target in the form of a red light emitting diode or red neon, we repeated the experiments of the previous section using a red target. These experiments were carried out in 3 subjects, an example of which is shown in Fig. 1F, and showed without exception that saccadic latency remained invariant with respect to target displacement (R2 ≤ 6.1%, P
slope
≥ 0.20). The saccadic latencies were grouped about the mean value which, in the example shown, was 227.3 ± 4.1 ms (mean ± S.E.M.)(Fig. 1H). This result thus marks a major difference from previous studies.
(c) Moving target of normal intensity
These experiments were undertaken on the basis that a moving target additionally requires a prediction of the target location in order to achieve fixation. For a total of 7 subjects with direct viewing, the general result was that saccadic latency remained constant with respect to target displacement. In 6 out of the 7 subjects, saccadic latency was not significantly related to target displacement (R2 ≤ 3.1%, P
slope
≥ 0.13). In the seventh subject, saccadic latency increased significantly with target displacement (R2 = 19.7%, P
slope
= 0.001; d.f. = 44), though this was without meaning since the F value of the regression (F = 11.8) was well below that value of 25.0 required for the relationship to be accorded any importance. Typical results are shown in Fig. 2A in which case the latency was most appropriately described by a constant value of 182.8 ± 1.3 ms (mean ± S.E.M.).
Saccadic latency for eccentric directions of gaze
The relationship between saccadic latency and target displacement at different directions of gaze in a leftwards direction with generation of saccades in a rightwards direction was investigated in response to both moving and stationary targets.
(a) Moving target
A typical result is shown in Fig. 2A,C,E,G in which the experiment was carried out at eccentric gazes of 10, 20 and 28 deg, as well as in the straight ahead direction: saccadic latency remained invariant with respect to increasing target displacement at each eccentricity (R2 ≤ 1.1%, P
slope
≥ 0.17). This result was reproduced in 3 more subjects, one of whom provided saccadic latencies at the same eccentric gazes while two subjects undertook the experiment at an eccentric gaze of 20 deg as well as in the straight ahead direction. In all 4 subjects, there was no significant effect of the angle of eye eccentricity on saccadic latency (P > 0.10, ANOVA).
(b) Stationary target
In 2 subjects who viewed at 10, 20 & 40 deg eccentricity and in one subject who viewed at 20 deg eccentricity, as well as in the straight ahead direction, saccadic latency was invariant with respect to eccentricity (P > 0.1, ANOVA). In a fourth subject, the saccadic latency showed a small increase of 15 ms for viewing at 20 deg eccentricity (P = 0.001, ANOVA). Hence, for both sets of experiments, there was no evidence that the saccadic latency was reduced on adoption of an eccentric direction of gaze.
Additional experiments
Saccadic recordings obtained in response to a leftwards as well as a rightwards directed moving target in one subject showed no change in saccadic latency (P = 0.40, ANOVA). Likewise, saccadic latencies were very similar for left and right eye monocular recordings (P = 0.57, ANOVA). In both cases, saccadic latency did not change significantly with increasing target displacement (R2 = 0.0%, P
slope
≥ 0.40). Saccadic recordings in response to angular displacements of 10–38 deg were made in response to a moving target with direct viewing on two further occasions in two subjects. In each of these new sets of data, saccadic latency was again not significantly related to target displacement (R2 ≤ 1.7%, P
slope
≥ 0.07). For the 3 sets of data, the mean values ± S.E.M. for one subject were 182.8 ± 1.33 ms (shown in Fig. 2A), 181.2 ± 0.22 ms and 188.6 ± 1.45 ms, and for the second subject were 195.1 ± 1.58 ms, 199.5 ± 1.46 ms and 192.2 ± 3.52 ms. Hence, the reproducibility of the data on different recording days was very high.