Visual stimulus-dependent changes in interhemispheric EEG coherence in ferrets

In recent years, the analysis of the coherence between signals recorded from the scalp [electroencephalographic (EEG) coherence] has been used to assess the functional properties of cortico-cortical connections, both in animal models and in humans. However, the experimental validation of this technique is still scarce. Therefore we applied it to the study of the callosal connections between the visual areas of the two hemispheres, because this particular set of cortico-cortical connections can be activated in a selective way by visual stimuli. Indeed, in primary and in low-order secondary visual 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. Thus only isooriented stimuli located near the vertical meridian are expected to change interhemispheric coherence by activating callosal connections. Finally, if such changes are found and are indeed mediated by callosal connections, 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 lateral suprasylvian, during different forms of visual stimulation. As expected, we found that bilateral iso-oriented stimuli near the vertical meridian, or extending across it, caused a significant increase in interhemispheric coherence in the EEG beta-gamma band. Stimuli with different orientations, stimuli located far from the vertical meridian, as well as unilateral stimuli 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 connections can indeed be revealed as a change in EEG coherence. The latter can therefore be validly used to investigate the functionality of cortico-cortical connections.     

On nature and limits of cortical developmental plasticity after an early lesion,in a child

MS is a little girl who suffered severe, bilateral destruction of her primary visual areas at six weeks, after premature birth at 30 weeks. Between the ages of 4.5 and 5.5 years she partially recovered different aspects of visual function, and, in particular, the ability to segregate figures from background, based on texture cues. The recovery might have been due to the compensatory role of the remaining visual areas that could have acquired response properties similar to those of the primary visual areas. This is not supported by the available FMRI (functional magnetic resonance imaging) responses to visual stimuli. Instead, abnormalities in the pattern of stimulus-induced changes of interhemi-spheric EEG-coherence in this patient suggest that her visual callosal connections, and possibly other cortico-cortical connections have re-organized abnormally. Since cortico-cortical connections, including the callosal ones appear to be involved in perceptual binding and figure-background segregation, their reorganization could be an important element in the functional recovery after early lesion, and/or in the residual perceptual impairment.     

Visual stimulus-dependent changes in interhemispheric EEG coherence in humans

We analyzed the coherence of electroencephalographic (EEG) signals recorded symmetrically from the two hemispheres, while subjects (n = 9) were viewing visual stimuli. Considering the many common features of the callosal connectivity in mammals, we expected that, as in our animal studies, interhemispheric coherence (ICoh) would increase only with bilateral iso-oriented gratings located close to the vertical meridian of the visual field, or extending across it. Indeed, a single grating that extended across the vertical meridian significantly increased the EEG ICoh in normal adult subjects. These ICoh responses were obtained from occipital and parietal derivations and were restricted to the gamma frequency band. They were detectable with different EEG references and were robust across and within subjects. Other unilateral and bilateral stimuli, including identical gratings that were effective in anesthetized animals, did not affect ICoh in humans. This fact suggests the existence of regulatory influences, possibly of a top-down kind, on the pattern of callosal activation in conscious human subjects. In addition to establishing the validity of EEG coherence analysis for assaying cortico-cortical connectivity, this study extends to the human brain the finding that visual stimuli cause interhemispheric synchronization, particularly in frequencies of the gamma band. It also indicates that the synchronization is carried out by cortico-cortical connection and suggests similarities in the organization of visual callosal connections in animals and in man.     

Physicochemical Properties and Pharmacological Activity of Mn(II), Fe(II), Co(II), Cu(II), AND Zn(II) Gluconates

Pharmaceutical Chemistry Journal -     

Synthesis of a New Structural Analog of the Natural Thyrotropin-Releasing Hormone

Pharmaceutical Chemistry Journal -     

Effect of electrical stimulation of the mamillary nuclei of the hypothalamus on skin vessel permeability in intact and capsaicin-treated rats

Stimulation of the mamillary nuclei of the rat hypothalamus induced increases in the permeability of forelimb skin microvessels. This effect was not seen in rats stimulated after administration of capsaicin at a dose (150 mg/kg) sufficient to deplete vasoactive neuropeptides from the peripheral nerve terminals of capsaicinsensitive neurons. These data indicate a role for the mamillary nuclei in central mechanisms modulating the effector functions of primary capsaicin-sensitive neurons. Laboratory of Functional Neuromorphology, Institute of Physiology, Siberian Division, Russian Academy of Medical Sciences, 6 Academician Timakov Street, 630117 Novosibirsk, Russia. Translated from Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 82, No. 12, pp. 1–5, December, 1996.