Caracterização funcional de pares e assembleias de neurônios intra- e inter-hemisféricos no córtex visual primário

• Ano de defesa: 2023
• Autor(a) principal: Frota, Beatriz Carvalho
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal do Rio Grande do Norte; Brasil; UFRN; PROGRAMA DE PÓS-GRADUAÇÃO EM NEUROCIÊNCIAS
• Palavras-chave em Português: Conexões calosas; Córtex visual primário; Atividade neuronal de disparo; Integração perceptiva visual; CNPQ::OUTROS::CIENCIAS
• Link de acesso: https://repositorio.ufrn.br/handle/123456789/57903

In primates and carnivores, the primary visual cortex contains long-range selective horizontal connections which are thought to contribute to the perceptual grouping of local image elements according to their orientation, direction of movement and collinear alignment. These selective circuits could be also decisive in maintaining biases in the ongoing cortical activity that condition neuronal responses by facilitating the likely conjunctions of forms or movement trajectories before such a stimulus arrives. Thus, ongoing functional interactions should reflect the selective cortical connectivity. Close to the border of the primary visual cortex, these connections extend through the corpus callosum with the same selectivity for orientation, direction of movement and collinearity to link homotopic sites in the two hemispheres and unify the image split in two halves at the vertical midline of the visual field. Because of this central action of visual callosal connections, it is possible to compare the ongoing and stimulus-evoked inter-hemispheric and intra-hemispheric interactions, and investigate whether they represent shapes and directions of movement crossing the vertical midline. To address the neuronal signatures of those functional interactions we studied the activity of neurons representing midline crossing stimuli in the two visual cortices by recording simultaneously from 2 x 16 spatially separated electrodes. These electrodes were placed into homotopic parts of the visual cortex, i.e. the transition zone between area 17 and 18, in cats (n = 4). In order to implant electrodes into cortical sites that are directly or indirectly linked by the corpus callosum, we performed intrinsic signal imaging to precisely define the areal border. Subsequently, we obtained extracellular electrophysiological recordings of single unit spiking activity during grating and natural visual stimulation designed to study integration. As an efficient measure of functional interaction we used ongoing and stimulus-evoked coherence between pairs of firing rates in the low-Gamma frequency band, focusing on pairs of neurons with overlapping receptive fields. To restrict the analysis to optimally responding neurons we applied a Sign test and a criterion of orientation- and direction selectivity. We observed significant ongoing and evoked low Gamma band (30-59 Hz) coherence in the majority of interhemispheric (n=131) and intra-hemispheric pairs (n=132) of single units with overlapping RFs (at least in one stimulus condition), but with higher amplitudes between intra-hemispheric units. Intra-hemispherically, the ongoing pairwise coherence decayed with cortical distance between neurons of the same orientation preference (ISO) and peaked again at an interval of about 800μm in accordance with the clustering of horizontal long-range connections linking sites of iso-orientation preference. In accordance with the latter, both intra- and interhemispheric ongoing and stimulus-evoked coherence was higher for pairs of single units with the same orientation or direction selectivity (ISO) than with the opposite selectivity (CROSS). Overall, horizontal gratings (± 45 degree) evoked higher coherence than vertical ones (± 45 degree). Surprisingly, this bias was the same for intra- and interhemispheric pairs. With natural scene stimulation, intra-hemispheric coherence was higher for pairs of the opposite direction selectivity (CROSSdir), in contrast to gratings, but for interhemispheric pairs of the same direction selectivity (ISOdir). Also in contrast to gratings, coherences were higher for directions of movement crossing (left, right) the vertical midline than for directions running parallel to it (up and down). During grating and natural scene stimulation, we observed both types of assemblies, either restricted to the population of intra-hemispheric neurons or across the hemispheres. With gratings, intra-hemispheric assemblies were significantly more often activated than interhemispheric ones. However, similar to the bias in pairwise coherence, interhemispheric assemblies were significantly more often activated during stimulation with horizontal gratings (± 45 degree) than with vertical ones. Interestingly, with natural scene stimulation intra- and interhemispheric assemblies do neither differ in absolute activation rate nor stimulus selectivity indicating that under these conditions both populations are functionally linked within one horizontal circuit which includes both hemispheres. In conclusion, we confirm that both ongoing and stimulus-driven activity of long-range intra- and interhemispheric networks reflects the selectivity of long-range axons to link neurons of similar response properties over longer distances. Selective neuronal interactions can be seen in both, pairwise coherence activity in the low-Gamma range and in the activity of simultaneously active populations of neurons spanning one or both visual cortices. The results are compatible with the interpretation that those connections serve to anticipate and mediate likely grouping operations between neurons representing the vertical meridian, such as those caused by crossing movements, frequently observed in natural scenes, or shapes such as in gratings. They also confirm that intra- and interhemispheric connections in the primary visual cortex are most likely not functionally separate circuits but rather form one horizontal network continuing through the corpus callosum.