Quick detection of changes in the sensory environment is very important for survival, resulting in automatic shifts of attention to the event and the facilitation of subsequent processes to execute appropriate behaviors. The abrupt onset or offset of a sensory stimulus should also activate the neural network detecting changes. To test this hypothesis, we compared cortical on- and off-responses using somatosensory-evoked potentials (SEPs) elicited by a train of electrical pulses delivered to the right hand in eight healthy volunteers. SEPs were recorded from 15 electrodes on the scalp at three different interstimulus intervals (ISIs, 50, 20, and 10 ms) under two sets of conditions (attended and unattended). Both the onset and offset of stimulation evoked two similar components, P100 and N140, in the attended and unattended conditions. The latency of P100 and N140 in response to stimulus onset did not differ among the three ISIs, while the latency of both components in response to stimulus offset was significantly longer for the longer ISI; that is, detection of the cessation of the stimulation was based on short-term memory of the stimulus frequency. The present results supported a cortical network triggered by both the onset and offset of sensory stimulation. In this network, the change is automatically detected using a memory trace by comparing the abrupt event (on or off) with the preceding condition (silent or repetitive stimuli).
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