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A distracter stimulus may also be used to ensure that the response is due to the target rather than the change from a background pattern.The classic oddball paradigm has seen many variations, but in the end most protocols used to evoke the P300 involve some form of conscious realization or decision making. No subjects have been noted to have fine control over their P300.Candidate structures include deep, closely spaced parts of the brain in the limbic system (e.g.amygdala, hippocampus and parahippocampal gyrus) as well as more widespread regions (e.g.The origins of the component waves (P3a and P3b) are still unclear.f MRI studies suggest that these subcomponents may localize to different regions, with the P3a being mainly a phenomenon of the frontal and insular cortices and the P3b stemming from parietal and inferior temporal regions .In practice, the P300 waveform must be evoked using a stimulus delivered by one of the sensory modalities.One typical procedure is the 'oddball' paradigm, whereby a target stimulus is presented amongst more frequent standard background stimuli.

Effect size estimates were computed using the restricted maximum likelihood model.One hypothesis links the P300 to activation of the locus ceruleus, a noradrenergic center of the brainstem found in the pons .In this view, the role of this noradrenergic circuit is to potentiate significant stimuli for executive decision making.While the neural substrates of this ERP still remain hazy, the reproducibility of this signal makes it a common choice for psychological tests in both the clinic and the laboratory. The P300 wave itself is thought to be comprised of two 'wavelets' known as P3a and P3b signals.

These components respond individually to different stimuli, and it has been suggested that the P3a wave "originates from stimulus-driven frontal attention mechanisms during task processing, whereas P3b originates from temporal–parietal activity associated with attention and appears related to subsequent memory processing." .

These distinct regions are proposed to be separate processing sites for “target” and “distracter” stimuli in the oddball paradigm.