Saccadometry
What is it?
Saccades are fast eye movements that allow for the voluntary change the direction of gaze quickly. A saccadometer has become the technology of choice for evaluating saccades, because it is non-invasive and objectively analyzes and graphs eye movements moving at several hundred degrees per second (1).
Why is it important?
Saccades are triggered by objects seen or heard, from memory, or as a part of an involuntary natural strategy to scan the environment (1). It is important to study saccades because of the many areas of the brain it takes to generate not only fast, but, appropriate, purposeful, and accurate movements. Saccades are analyzed using three different characteristics including, latency, amplitude, and velocity. Studying these is a way to gain insight into the function of specific parts of the brain and neural pathways.
How does it work?
From the initial presentation of a stimulus, it takes approximately 200 milliseconds for the inhiation of an eye movement to occur, during which time, neural processing amongst the retina, cerebral cortex (frontal lobe), superior colliculus, cerebellum and vestibular system is occurring (1,3). This latency, or time it takes to process the target presentation, is used to understand aspects of saccade programming associated with visual processing, target selection, and motor programming (1-3), strongly associated with the integrity of the frontal lobe (executive center) and superior colliculus.
To achieve clear, stable, single vision, the control of eye movements must be accurate, relying heavily on the vestibular system (balance and coordination center) and cerebellum (fine motor movement center), to reference spatial maps in order to generate appropriate velocity and position commands (1). Because saccades are brief, vision cannot be relied on to guide the eye to the target (4), therefore, the speed, or degrees per second (velocity), in which the eyes move to the intended target becomes important so that no disorientation occurs during this time.
How does it help?
One of the most impressive aspects of ocular motor control is the way in which the brain constantly monitors its performance, and in the face of disease and aging, adjusts its strategies accordingly (5). Therefore, the evaluation of saccades, due to the immense coordination and integration needed, is a valuable tool to identify different disorders that can have pathognomonic abnormality shifts (5). For example, diseases targeting the cerebellum, may have subclinical findings that may be identified in areas affecting associations with spatial orientation, memory, cognition, or motor control (1). Other conditions affecting these aforementioned association areas that may have an effect on saccadic function are, but not limited to, Multiple sclerosis (6), Parkinson’s Disease (7,8) and/or stroke (9), neurodevelopmental disorders, degenerative diseases, etc.
References
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J. Leigh and D. S. Zee, The neurology of eye movements, 4th ed., New York, NY, Oxford Univ Press, 2006
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Leigh RJ, Zee DS. The neurology of eye movements. 3rd ed. New York: Oxford University Press: 1999. ,Google Scholar
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Fischer B, Ramsperger E. Human express saccades: extremely short reaction times of goal directed eye movements. Exp Brain Res 1984; 57: 191–5.Google Scholar
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Lewis RF, Zee DS, Hayman MR, Tamargo RJ. Oculomotor function in the rhesus monkey after deafferentation of the extraocular muscles. Exp Brain Res 2001; 141: 349–58.Google Scholar
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Wong, A. Eye movement disorders. New York, NY: Oxford University Press: 2009.
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Serra A, Derwenskus J, Downey DL, Leigh RJ. Role of abnormal eye movement examination and subjective visual vertical in clinical evaluation of multiple sclerosis. J Neurol 200;250: 569–75. Google Scholar
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Crawford T, Goodrich S, Henderson L, Kennard C. Predictive responses in Parkinson’s disease: manual keypresses and saccadic eye movements to regular stimulus events. J Neurol Neurosurg Psychiatry 1989; 52: 1033–42.Google Scholar
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Vermersch AI, Müri RM, Rivaud S, Vidailhet M, Gaymard B, Agid Y, et al. Saccade disturbances after bilateral lentiform nucleus lesions in humans. J Neurol Neurosurg Psychiatry 1996; 60: 179–184. Google Scholar
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Sprenger A, Kompf D, Heide W. Visual search in patients with left visual hemineglect. Prog Brain Res 2002; 140: 395–416. Google Scholar
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