The trajectory taken by the eye rotation is similarly stereotyped although subject to interesting sources of variability.įigure 2: This figure shows the relationship between saccade peak velocity (deg/sec) and magnitude (or amplitude) in degrees of visual angle. The term 'main sequence' has been appropriated from astrophysics to describe these regularities (Bahill, Clark, & Stark, 1975). The velocity with which the eyes move shows a corresponding increase in a similarly stereotyped way, with the maximum velocity increasing from around 10 deg/sec for small microsaccades to over 300 deg/sec for large saccadic movements. Saccadic duration increases in a non-linear manner as the amplitude (size) of the saccade increases, from about 20ms for the smallest movements to over 100ms for the largest possible ones. To a surprisingly good approximation, every saccade made shows a similar pattern. Finally, it is possible to measure the detailed progression of the movement itself, the trajectory made between these points. It is also possible to measure accurately the saccadic metrics, that is the size and direction of the orienting step, using an angular measure to describe the rotation of the eyeball (units degrees visual angle). The term saccadic latency is used to refer to this time delay (units milliseconds). It is possible to measure the time elapsing between the appearance of the target and the initiation of the orienting saccade. Vision is dependent upon the information taken in during fixation pauses between saccades: no useful visual information is taken in while the eyes are making a saccadic movement.Ī considerable amount has been learned about the control of saccades by studying in detail the simple situation in which an observer is asked to move their eyes to orient to a newly appearing target. In normal viewing, several saccades are made each second and their destinations are selected by cognitive brain process without any awareness being involved. Saccades direct the fovea onto an object or region of interest which enables subsequent high-acuity detailed visual analysis at that location. These step movements are known as saccades or saccadic eye movements. The brain commands sent to the eye muscles result in the eyes making a rapid step-like rotation following which the eyes remain stationary at their new position. Large orienting movements involve co-ordinated action of the eyes, head and body, but smaller movements, such as those made when looking at a picture (See Figure 1) are made with the eyes alone (Yarbus, 1967). Looking is achieved by orienting the eyes, that is to say directing their visual axes to point to a new location. The ability to look at things is a familiar part of the process of seeing.
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