The lability of the late slow biphasic deflection (i.e. N1-P2) of the vertex EP is examined in relation to selective attention and repetitive stimulation in both the visual and auditory modalities. Amplitude effects are of primary Interest and a capacity theory of attention is applied, with some neurophysiological Justification, throughout the work in order to account for such effects. A range of data analytic techniques relevant to EP research are also evaluated and. In particular, a correlation method for determining the amplitude and latency of individual EPs is described and assessed. Unlike the auditory modality, the amplitude of the N1 component of the visual EP was found to remain invariant with respect to selective attention, although the subsequent P2 component was enhanced to stimuli on the attended channel. The N1 correlate of auditory selective attention was shown to be a reflection of the greater 'mental work' associated with attended stimuli. Various aspects of the fast habituation of the visual EP were explored, including its interaction with slow habituation (using a single trial analysis) and the effect of presentation rate. The role of pupillary mechanisms was also evaluated. The generic term 'fast response decrement' (FRD), which subsumes both the fast habituation and the temporal recovery of the late componentry of the EP, was introduced. Using paradigms based on pairs of stimuli, the mechanisms of both the visual and auditory PRD were investigated. The visual. PRD was not affected by dichoptic presentation and exhibited extensive stimulus generalisation. A refractoriness mechanism for the FRD was finally rejected in favour of a psychological one which represented a marriage of Sokolov's concept of a neuronal model with a capacity theory of attention. Using a novel technique. In which an oscilloscope 'clock' enabled experimental control over temporal uncertainty, it was demonstrated that the Inadequacy of the neuronal model in its temporal aspects was the key variable under pinning the FRD, although in the auditory modality a second variable, designated 'firstness', also appeared to be important
