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Saccadometry The saccadometer Product range Downloads About ACI Contact us |
Saccadometry is the measurement and analysis of saccadic reaction time (latency) distributions.
Although such factors as nerve conduction speed and receptor transduction obviously contribute to reaction time, it is clear that latencies are far longer than can be explained by simple physiological considerations. They appear in fact to represent a process of procrastination, in which higher, predominantly cortical, levels prevent the brainstem from responding as fast as it is capable of, by a process of tonic inhibition. Though these lower levels may be perfectly capable of detecting stimuli and initiating directed responses to them, what they lack is the ability to ascertain the meaning of the stimuli, and therefore whether, and how, to respond. In general, this requires cortical mechanisms of decision, that typically require a long time - around 100 or 200 ms - to reach completion. As a result, reaction time is essentially decision time, and by making accurate measurements of latency, and its variability from trial to trial, we obtain a window into the neural mechanisms underlying decision, that are at the very top of the hierarchy of neural control.
Saccades are ideal for this purpose: with modern devices such as the saccadometer, hundreds of saccadic latencies can be measured in a matter of few minutes, providing the quality and range of data needed to characterise these cerebral decision processes. Since we make two or three saccades every second of our waking life, performing an intensive series of saccadic trials is not particulary tiring or unnatural, and with head-mounted projection of the visual targets, the subject's head can be completely unconstrained.
Over the last decade or so, there has been an enormous focus of interest on these neural decision mechanisms, and much of this information has come from studying saccades. In particular, there is a simple model of reaction time, LATER (linear approach to threshold with ergodic rate) that explains the main characteristics of distributions of reaction times, and enables them to be fully described in just two or three parameters. This in turn has led to sensitive, quantitative measures of cerebral performance, leading to improved measures of disease progression and diagnosis, and the accurate, objective evaluation of different therapies.
Current areas of application include migraine, neurodegenerative disorders (Parkinson's and Huntington's diseases), sleep deprivation, the sedative effects of residual anaesthetic, population studies in relation to intelligence and personality, schizophrenia, hepatic encephalopathy, Gaucher's and Sandhof diseases, hypoxia, mild traumatic brain injury in sports and combat, and the effects of deep brain stimulation of the subthalamus. A saccadometer was taken to the top of Everest as part of the 2007 Medical Everest Expedition!
Some publications, and further information about the scientific and clinical aspects of saccadometry and the LATER model, can be found
here.