| The Brain Barometer: Where to Look Next? |
| Tuesday, 10 June 2008 | |
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Benjamin Pearson follows new efforts to map out thoughts as they happen How we make decisions has recently become one of the hottest topics in brain science. Information enters our senses from the outside world, and the underlying neural mechanisms weigh up probabilities based on this often fuzzy data. Being based on probability, the mechanisms are hard to study, but an approach that has proved very successful is to look at the most frequent decision any of us make: where to look next. The eye movements we use to concentrate on a particular object are called saccades. We make two or three of them every second of our waking lives, so they are more frequent than even a heartbeat. Each is the outcome of a decision to select one of the host of visual objects around us for closer examination. Modern technology allows us to study these tiny movements ever more closely through the use of a non-invasive infrared sensor. In the simplest case, a subject looks at a small visual target on a display screen and saccadic latency-how long they take to respond when it suddenly moves-is measured. Strangely, it is found that this latency is around 200 milliseconds. In neuron time this is an age-enough for a fast nerve to conduct over 20 metres, far longer than the shortest neural path from retina to eye muscles (via a midbrain nucleus called the colliculus). Even more bizarrely, the delay varies dramatically from saccade to saccade, apparently at random. Neuroscientists wondered why this would occur. Although the colliculus does an excellent job of translating the directions of visual targets into appropriate commands to the eye muscles, what it cannot do is decide which target is worth looking at. Only the cerebral cortex (the centre of brain function) has access to the information needed to recognise an object and assess its behavioural importance. Hence, in saccadic latency, rather than the duration of a lowly reflex, we are seeing the grinding gears of high-level cortical decision. Analysing the variability in latency over hundreds of saccades allows investigators to guess at the kind of process that must be giving rise to it, one model for which is called LATER (Linear Approach to Threshold with Ergodic Rate). In this model the reciprocal of latency obeys a normal or Gaussian distribution, so that the mean and variance neatly describe an individual's rate of cerebral function. Precise mathematical description of this kind is rare in behavioural neuroscience. It allows the effects of visual stimulus type, probability, motivation and reward to be studied quantitatively. For the clinician, it provides a tool for identifying and quantifying a wide array of neurological conditions, which may have different effects upon the statistical distribution of the patient's brain function. This brings with it the possibility of improved diagnosis and, more importantly, improved quantitative comparisons between different treatments.Such clinical work, however, requires less expensive and more portable versions of the equipment used in research laboratories. Indeed, what has made much of the recent clinical work on saccades possible is the 'saccadometer', a portable, self-contained infrared eye tracker developed as a collaboration between Dr Roger Carpenter (from the University of Cambridge's Department of Physiology, Development and Neuroscience) and Dr Jan Ober (Director of the Institute of Biocybernetics, Poznañ, Poland). Subjects wear the device like a pair of glasses, and follow targets projected by miniature lasers built into the headpiece. Many hundreds of latencies can be recorded in just a few minutes anywhere there is a blank area of wall or ceiling. This makes saccadometry an attractive proposition compared with neuropsychological tests that can take hours yet generate relatively unreliable data, or the expense of neuroimaging, which in any case does not as yet provide any functional assessment. In fact, it has already been applied in a wide range of clinical situations to gauge latency variation with brain function. Very low levels of anaesthetic, as might occur a few days after surgery, increase latency even when the subject is unaware of any sedative effect. In surgical interventions to improve the cerebral blood supply, latency is either reduced, as blood flow to the cortex is improved, or increased-often dramatically-as debris from surgery lodges in arteries and reduces brain oxygenation. At the Cambridge Brain Repair Centre, saccadometry is being used to quantify the progress of Parkinson's disease and Huntington's disease, whilst at the University of Maastricht it has been shown to provide a good measure of the effectiveness of deep brain stimulation therapy in patients suffering from Parkinson's disease. The portability of the device also makes it ideal for studying concussion. A preliminary trial showed that latency was greatly increased in some university boxers immediately after a fight, though it completely recovered within a few days. Professional jockeys in Newmarket are now regularly having their saccadic latencies measured. They are sometimes known to slow their responses down in traditional pre-season testing, to be sure that later on they will pass even if concussed after a fall, but eye movements cannot be manipulated so easily. Portability was important on a recent expedition to Everest base camp where researchers assessed the effects of altitude on the brain. One can foresee a development of portable altitude sickness warning system for climbers, and possibly something similar detecting the impaired judgement of nitrogen narcosis in SCUBA divers. Other conditions that are not primarily neurological, such as liver damage, can also be assessed by saccadometry. The cumulative effects of ageing take their toll. Latencies rise steadily throughout life, and a recent study suggests that they begin to rise more rapidly a few years before death. Perhaps in the future every family will own a saccadometer, with checks for brain-decay over breakfast. Benjamin Pearson is a PhD student in the Department of Physiology, Development and Neuroscience |
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