Characteristics of the Steady State Visual Evoked Potentials

Fig. 1

Instantaneous power of SSVEP for individual subjects exhibiting: fast habituation (left), slow habituation (middle) and no habituation (right). The black line is the average across 50 trials, and the blue lines mark the standard error. The vertical scale of each subplot was adjusted to optimally present the power time course

Habituation was quantified as 100 % ⋅ (E1-E2)/E1, where E1 is SSVEP energy between seconds 2 and 3 after the stimulation onset, E2 is SSVEP energy between seconds 3 and 2 before the stimulation offset. The habituation was 0.49 % + −0.09 % on average across subjects and it reached maximal value of 71 % + −0.10 % in a single subject.

4 Discussion

In this study we showed that for most of the subjects the strength of SSVEP decreases during long term stimulation. The habituation mechanism is not well understood. It is generally considered that attention modulates the SSVEP response [3]. Therefore, one possible explanation would be that a subject cannot maintain the same level of attention during whole stimulation period. Alternatively, the habituation may be caused by neuronal, plastic changes at the level of synapses. More experimental work is needed to provide physiological explanation for habituation phenomenon.

In most of the BCI systems the stimulation light flickers continuously during the entire period when the device is used by the subject. Habituation processes may decrease the SSVEP response affecting their proper classification. Possible solution to the problem would be to divide the stimulation with short rest periods allowing SSVEP to recover. Influence of length of rest period for restoring SSVEP strength should be examined in further studies.

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