Biomedical Engineering Reference
In-Depth Information
450
12
Peak Velocity
400
10
350
Position
300
8
250
Velocity
6
200
150
4
100
2
50
D
u
ration
Latent Period
Time at Peak Velocity
0
0
1
21
41
61
81
101
121
141
161
181
201
Time (ms)
FIGURE 13.5
A10
saccade with various indices labeled.
infrared oculography (shown in Figure 13.4 (right)). A typical saccade is shown in
Figure 13.5, with a latent period of approximately 100 ms, amplitude of 10
, and a duration
of approximately 60 ms. Saccadic eye movements are conjugate and ballistic, with a typical
duration of 30-100 ms and a latency of 100-300 ms. The latent period is thought to be the
time interval during which the CNS determines whether to make a saccade and, if so,
calculates the distance the eyeball is to be moved, transforming retinal error into transient
muscle activity. Also shown in this figure is the velocity of the saccade with a peak velocity
of approximately 400
s
1
.
Generally, saccades are extremely variable, with wide variations in the latent period,
time to peak velocity, peak velocity, and saccade duration. Furthermore, variability is well
coordinated for saccades of the same size; saccades with lower peak velocity are matched
with longer saccade durations, and saccades with higher peak velocity are matched with
shorter saccade durations. Thus, saccades driven to the same destination usually have
different trajectories.
To appreciate differences in saccade dynamics, it is often helpful to describe them with
saccade main sequence diagrams [3, 17, 27]. The main sequence diagrams plot saccade peak
velocity-saccade magnitude, saccade duration-saccade magnitude, and saccade latent
period-saccade magnitude. The saccade size or amplitude is the angular displacement from
the initial position to its destination. The size of a saccade ranges from less than a degree
(microsaccades) to 45
in both the nasal (toward the nose) and temporal (toward the temple)
directions. Peak or maximum velocity occurs at approximately half the duration of the
saccade for small saccades less than 15
[2]. The duration of a saccadic eye movement
is the time from the start to the end of a saccade. Duration is usually hard to determine
