Is P300 a kind of SSVEP?
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Not at all. The SSVEP is a stimulus-driven reaction, a fluctuation in the neural signal driven by the fluctuation in the observed visual signal. The SSVEP begins and ends with the presence of the visual signal.
The P300 a reflection of cognitive processing, an increased response in the presence of deviance detection, decision making, attention, categorization... some higher order processing. The sensory stimulation can be long gone from the display, but as long as it's still being processed by the cognitive apparatus, the P300 can arise. The 300 stands for milliseconds, i.e. it roughly begins about that much after stimulus onset. The only connection between the two is that they're electrophysiological measures, i.e. you can capture both with M/EEG, and that you might observe both in the same brain regions.
@Ana is basically correct (possibly baring the fact that the P3 is famous for being latency variant, not stable at 300msec), and I really don't want to contradict her or steal her checkmark, but since it's recent and interesting, I would like to point to some recent research by O'Connell and colleagues looking at correlations and similarities between the two measures, as the primary EEG indexes of attention and evidence accumulation. Incidentally, it also shows the differences in topography, polarity and size for the two, as well as the slight dissociation in time course. (Note also that the P3 is a distinct, monophasic peak - it has an amplitude; the SSVEP is a periodic pattern, strictly speaking, it rather has power.)
In the following picture, the close correspondence between the SSVEP and the P3 is demonstrated; see the figure caption below.