Evidence for fusion and segregation induced by 21 Hz multiple-digit stimulation in humans

Subjects were trained to detect changes in the frequency of 21 Hz tactile stimulation applied to digits 2 + 3 + 4 (fusion group) or 2 + 4 (segregation group) of the right hand. The 21 Hz steady-state response for digit 3 was measured by 64 channel EEG on mapping trials before and after training. Discrimination improved over 3 days, confirming that subjects attended to the training stimuli. The 21 Hz response was larger on training than on mapping trials, indicating sensitivity of the response to the strength of cortical activation. Under these conditions the 21 Hz response for digit 3 decreased after training in both groups on day 1. On day 3 this effect reversed in a subset of fusion subjects while segregation continued to yield decreases. The findings suggest that somatosensory representations are dynamically modified by the sensory input experienced on a task.