Interhemispheric Interactions between the Human Primary Somatosensory Cortices

In the somatosensory domain it is still unclear at which processing stage information reaches the opposite hemispheres. Due to dense transcallosal connections, the secondary somatosensory cortex (S2) has been proposed to be the key candidate for interhemispheric information transfer. However, recent animal studies showed that the primary somatosensory cortex (S1) might as well account for interhemispheric information transfer. Using paired median nerve somatosensory evoked potential recordings in humans we tested the hypothesis that interhemispheric inhibitory interactions in the somatosensory system occur already in an early cortical processing stage such as S1. Conditioning right S1 by electrical median nerve (MN) stimulation of the left MN (CS) resulted in a significant reduction of the N20 response in the target (left) S1 relative to a test stimulus (TS) to the right MN alone when the interstimulus interval between CS and TS was between 20 and 25 ms. No such changes were observed for later cortical components such as the N20/P25, N30, P40 and N60 amplitude. Additionally, the subcortically generated P14 response in left S1 was also not affected. These results document the existence of interhemispheric inhibitory interactions between S1 in human subjects in the critical time interval of 20–25 ms after median nerve stimulation

The contribution of pre-existing depression to the acute cognitive sequelae of mild traumatic brain injury

Primary objective: To determine the effect of pre-existing depression on the cognitive sequelae of mild traumatic brain injury ( mTBI) within 24 hours of injury. Research design: A 2 x 2 between-subjects design was used to examine the effect of depression and injury type on neuropsychological test performance. The independent variables were the injury type ( mTBI or control) and the presence of depression ( depressed or not depressed). Methods and procedures: Participants who had sustained mTBI ( 30 with depression, 30 without depression) within the previous 24 hours and control participants ( 19 with depression, 30 without depression) were assessed on the Digit Symbol Substitution Test ( DSS), Hopkins Verbal Learning Test ( HVLT) and the Speed of Comprehension Test. Results: Participants with mTBI performed worse than controls on the tests, particularly HVLT delayed recall and DSS total correct. Participants with depression did not perform worse than participants without depression. However, there was a significant univariate interaction for HVLT recognition, participants who had sustained mTBI and were classified in the depressed group exhibited worse recognition compared to mTBI participants without depression. Conclusions: The results indicate that depression may interact with mTBI to impair word recognition during the acute phase after a head injury