Mohammed T. Abou-Saleh

NEUROPHYSIOLOGY OF AGEING 59‘‘endogenous’’ because their latency and amplitude are hardlyinfluenced by the physical characteristics of the stimulus such asits intensity or frequency, but reflect how attentive the subject is.They also give an indication of the degree of complexity inherentin a cerebral cognitive or memory operation performed onexposure to a stimulus. The P300 response is one endogenousERP that has been extensively studied, as it is thought to reflectthe mental processes of selective attention, learning and memory.To generate a P300 response to an auditory stimulus, the subject isrequired to attend to a series of low-pitched (non-target) tones,randomly interspersed with high-pitched (target) tones. Therecognition of these target tones generates a positive potential,which can be recorded widely over the scalp at approximately300 ms after the auditory stimulus. The P300 response to thetarget stimuli is much more explicit, and it is generated only whenthe subject concentrates on the task; its amplitude is thought toreflect the level of attention and its latency the processing timeinvolved in the recognition of a target tone.EFFECT OF AGE ON LONG LATENCYEVENT-RELATED POTENTIALSThe latencies of all endogenous event-related potential componentsappear to increase with age from the second decade onward,a fact that led to extensive research activity in relation to theauditory P300 component 35–41 . Some authors have reported alinear increase of P300 latency with age up to senescence with anincrease of 1–2 ms/year. However, comparative studies, comprisinglarge numbers of controls, have found an exponential ageingeffect with a much higher rate of increase (of up to 4 ms/year) inP300 latency in subjects older than 60 years as compared toyounger adults 37,39,41 . P300 latency is also increased in the presenceof a variety of brain pathologies, including the dementias ofAlzheimer’s disease and cerebrovascular disease. In elderlysubjects, it is difficult to separate the effect of ageing per se fromthe effects of subclinical degenerative or vascular changes onendogenous event-related potentials, and this is equally true forroutine EEG. In a clinical environment, where P300 measurementsmay be useful, for example in dementia and schizophrenia,it is essential to carefully match individuals for age, particularly ifelderly individuals are concerned.REFERENCES1. Binnie CD. Recording techniques: montages, electrodes, amplifiers andfilters. In Halliday AM, Butler SR, Paul R, eds, A Textbook of ClinicalNeurophysiology. Chichester: Wiley, 1987; 3–22.2. Jasper HH. Report of the Committee on Methods of ClinicalExamination in Electroencephalography. Electroencephalogr ClinNeurophysiol 1958; 10: 370.3. Jasper HH. 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