Development of cortical somatosensory evoked potentials in rats

The development of the contra- and ipsilateral cortical potential evoked by electrical sciatic nerve stimulation was studied in 77 male albino rats aged 5 to 45 days. A contralateral response was already recorded, as double negativity, in the youngest animals, while an ipsilateral evoked potential was not reliably present until the 10th day. At this time, however, both responses started with an inconstant positive wave and their shape was practically the same. During subsequent development the responses differed only in respect to their dominant component: in the contralateral response, the N1 wave had the highest amplitude for most of the time, while in the ipsilateral response the delayed N2 wave was the largest component. The latent periods of contralateral responses were somewhat shorter than those of ipsilateral evoked potentials. During development we noticed a phase of abrupt shortening of the latent period, which took place before the 15th day in the contralateral response and before the 20th day in the ipsilateral response. We also found a difference in the fatigability of the responses, which was greater in immature rats than in adult animals; in the ipsilateral evoked potential it approached adult values more slowly. The development of the ipsilateral response is thus delayed compared with the development of the contralateral response.