Saccade Parameters
Saccade Parameters
Amplitude - the size of the saccade, usually measured in
degrees or mins. of arc. The amplitude determines the saccade accuracy. This is sometimes
denoted using "gain". The gain is the ratio of the actual saccade amplitude
divided by the desired saccade amplitude (usually determined by the size of a target
step). Gains of <1 indicate the saccade was too small or hypometric; gains of >1
indicate the saccade was too large or hypermetric. A number of congenital conditions,
diseases lesions or drugs cause saccade dysmetria (saccades of the wrong size) to a lesser
or greater extent.
Peak Velocity - this is the highest velocity reached during
the saccade. Saccade velocity profiles are usually symmetrical at least for small and
medium size saccades. So determining the peak is usually straight forward. In some
conditions while the size of saccades remains reasonably accurate, the saccade velocity is
greatly reduced - so-called slowed saccades.
Duration - the time taken to complete the saccade. This is
most easily measured from the velocity profile. Partly because of the high velocities
involved, most saccades are complete within a few tens of milliseconds (ms). As it takes
about 80ms for visual information to reach the visual cortex, these short durations mean
that saccades cannot be modified "in-flight" by visual information. If a target
moves, new information is used to trigger a corrective saccade.
Latency - this is the time taken from the appearance of a
target to the beginning of a saccade in response to that target. Whereas the other
parameters tend to fall in a narrow range for a given set of circumstances, latency is
extremely variable. The latency for most medium amplitude saccades (5°-10°) is usually
around 200ms. However, it can be as low as 100ms, or as high as 350ms. The distribution of
saccade latency has attracted a great deal of attention recently, as it may indicate
alterations in processing in the oculomotor system above the level of the brainstem and
superior colliculus.
It turns out that saccades are extremely stereotyped, and that there
are relatively fixed relationships between the amplitude, duration and peak velocity. The
relationships between saccade amplitude and saccade duration and velocity are so fixed,
that is is possible to use them to show whether a particular eye movement is a normal
saccade or not. Thus there is a "main sequence" for saccades, and these
parameters and their relationships are sometimes 
called main
sequence parameters and relationships. For normal subjects the relationship between
saccade amplitude and duration is fairly linear. The equation of the line through normal
subject data is usually 2.2A+21 (where A is the saccade amplitude). There is a
fairly fixed relationship between peak velocity and amplitude (see Fig. 4.3, pg72,
in Carpenter, 1988).
Given that these relationships are all fairly stereotyped, one can
use quantitative measurements of saccades to assess the function of the oculomotor system,
to investigate the effects of drugs or lesions, and in some cases to aid diagnosis of
disease or locating of lesions.