The characteristics of evoked potentials due to omission of stimuli in a sequence of rhythmic auditory tones

INTRODUCTION
The detection of absence of a stimulus from a sequence of rhythmic stimuli generates potentials which may express different cognitive processes from the P300 wave.

PATIENTS AND METHODS
In nine healthy subjects we studied the differences between the P300 wave obtained by the 'odd ball' paradigm and the potentials evoked by random omission of an auditory tone in a rhythmic sequence of tones of 1,000 Hz. Stimulation was biaural at a frequency of 0.7 Hz and a proportion of stimuli were frequent/infrequent or omitted 3/1. The subjects indicated appearance of an infrequent stimulus or its omission by moving a finger. The potentials were registered on all the points of the international system 10-20.

RESULTS
All the subjects evoked potentials P300 and P3o with a range of maximum latencies between 212 and 424 ms, without any differences between the two conditions being observed. The amplitude of P3o was significantly less than that of the P300 for all the electrodes. The topographical distribution of both waves, although predominantly in the midline, was more posterior in the case of P3o with lateralization to the left parietal region.

CONCLUSION
The potentials omitted show some differences from the P300 wave and this may be derived from the process of estimation and production of a time interval, which is essential in detecting omission of a stimulus, and is useful in the study of time perception and the generation of internal rhythms.

CITA DEL ARTÍCULO Rev Neurol. 1999 Sep 1-15;29(5):403-7