Visual stimulus-dependent changes in interhemispheric EEG coherence in ferrets

In recent years, the analysis of the coherence between signals recorded fro m the scalp [electroencephalographic (EEG) coherence] has been used to asse ss the functional properties of cortico-cortical connections, both in anima l models and in humans. However, the experimental validation of this techni que is still scarce. Therefore we applied it to the study of the callosal c onnections between the visual areas of the two hemispheres, because this pa rticular set of cortico-cortical connections can be activated in a selectiv e way by visual stimuli. Indeed, in primary and in low-order secondary visu al areas, callosal axons interconnect selectively regions, which represent a narrow portion of the visual field straddling the vertical meridian and, within these regions, neurons that prefer the same stimulus orientation. Th us only isooriented stimuli located near the vertical meridian are expected to change interhemispheric coherence by activating callosal connections. F inally, if such changes are found and are indeed mediated by callosal conne ctions, they should disappear after transection of the corpus callosum. We perfomed experiments on seven paralyzed and anesthetized ferrets, recording their cortical activity with epidural electrodes on areas 17/18, 19, and l ateral suprasylvian, during different forms of visual stimulation. As expec ted, we found that bilateral iso-oriented stimuli near the vertical meridia n, or extending across it, caused a significant increase in interhemispheri c coherence in the EEG beta-gamma band. Stimuli with different orientations , stimuli located far from the vertical meridian, as well as unilateral sti muli failed to affect interhemispheric EEG coherence. The stimulus-induced increase in coherence disappeared after surgical transection of the corpus callosum. The results suggest that the activation of cortico-cortical conne ctions can indeed be revealed as a change in EEG coherence. The latter can therefore be validly used to investigate the functionality of corticocortic al connections.