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Previous issue date: 2014-03-31 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Desde os descobrimentos pioneiros de Hubel e Wiesel acumulou-se uma vasta literatura
descrevendo as respostas neuronais do c?rtex visual prim?rio (V1) a diferentes est?mulos
visuais. Estes est?mulos consistem principalmente em barras em movimento, pontos ou
grades, que s?o ?teis para explorar as respostas dentro do campo receptivo cl?ssico (CRF
do ingl?s classical receptive field) a caracter?sticas b?sicas dos est?mulos visuais como a
orienta??o, dire??o de movimento, contraste, entre outras. Entretanto, nas ?ltimas duas
d?cadas, tornou-se cada vez mais evidente que a atividade de neur?nios em V1 pode ser
modulada por est?mulos fora do CRF. Desta forma, ?reas visuais prim?rias poderiam estar
envolvidas em fun??es visuais mais complexas como, por exemplo, a separa??o de um
objeto ou figura do seu fundo (segrega??o figura-fundo) e assume-se que as conex?es
intr?nsecas de longo alcance em V1, assim como as conex?es de ?reas visuais superiores,
est?o ativamente envolvidas neste processo. Sua poss?vel fun??o foi inferida a partir da
an?lise das varia??es das respostas induzidas por um est?mulo localizado fora do CRF de
neur?nios individuais. Mesmo sendo muito prov?vel que estas conex?es tenham tamb?m
um impacto tanto na atividade conjunta de neur?nios envolvidos no processamento da
figura quanto no potencial de campo, estas quest?es permanecem pouco estudadas.
Visando examinar a modula??o do contexto visual nessas atividades, coletamos potenciais
de a??o e potenciais de campo em paralelo de at? 48 eletrodos implantados na ?rea visual
prim?ria de gatos anestesiados. Estimulamos com grades compostas e cenas naturais,
focando-nos na atividade de neur?nios cujo CRF estava situado na figura. Da mesma forma,
visando examinar a influ?ncia das conex?es laterais, o sinal proveniente da ?rea visual
isot?pica e contralateral foi removido atrav?s da desativa??o revers?vel por resfriamento.
Fizemos isso devido a: i) as conex?es laterais intr?nsecas n?o podem ser facilmente
manipuladas sem afetar diretamente os sinais que est?o sendo medidos, ii) as conex?es
inter-hemisf?ricas compartilham as principais caracter?sticas anat?micas com a rede
lateral intr?nseca e podem ser vistas como uma continua??o funcional das mesmas entre os
dois hemisf?rios e iii) o resfriamento desativa as conex?es de forma causal e revers?vel,
silenciando temporariamente seu sinal, permitindo conclus?es diretas a respeito da sua
contribui??o. Nossos resultados demonstram que o mecanismo de segmenta??o figurafundo
se reflete nas taxas de disparo de neur?nios individuais, assim como na pot?ncia do
potencial de campo e na rela??o entre sua fase e os padr?es de disparo produzidos pela
popula??o. Al?m disso, as conex?es laterais inter-hemisf?ricas modulam estas vari?veis
dependendo da estimula??o feita fora do CRF. Observamos tamb?m uma influ?ncia deste
circuito lateral na coer?ncia entre potenciais de campo entre eletrodos distantes. Em
conclus?o, nossos resultados d?o suporte ? ideia de um mecanismo complexo de
segmenta??o figura-fundo atuando desde as ?reas visuais prim?rias em diferentes escalas
de frequ?ncia. Esse mecanismo parece envolver grupos de neur?nios ativos
sincronicamente e dependentes da fase do potencial de campo. Nossos resultados tamb?m
s?o compat?veis com a hip?tese que conex?es laterais de longo alcance tamb?m fazem
parte deste mecanismo / Since Hubel and Wiesel s pioneer finding a vast body of literature has accumulated describing neuronal responses in the primary visual cortex (V1) to different visual stimuli. These stimuli mainly consisted of moving bars, dots or gratings which served to explore the responses to basic visual features such as orientation, direction of motion or contrast, among others, within a classical receptive field (CRF). However, in the last two decades it became increasingly evident that the activity of V1 neurons can be modulated by stimulation outside their CRF. Thus, early visual areas might be already involved in more complex visual tasks like, for example, the segmentation of an object or a figure from its (back)-ground. It is assumed that intrinsic long-range horizontal connections within V1 as well as feedback connections from higher visual areas are actively involved in the figure-ground segmentation process. Their possible role has been inferred from the analysis of the spike rate variations induced by stimuli placed outside the CRF of single neurons. Although it is very likely that those connections also have an impact on the joined activity of neurons involved in processing the figure and on their local field potentials (LFP), these issues remain understudied.
In order to examine the context-dependent modulation of those activities, we recorded spikes and LFPs in parallel from up to 48 electrodes in the primary visual cortex of anesthetized cats. We stimulated with composite grating and natural scene stimuli focusing on populations of neurons whose CRFs were situated on the foreground figure. In addition, in order to examine the influence of horizontal connections we removed the inter-hemispheric input of the isotopic contralateral visual areas by means of reversible cooling deactivation.
We did so because i) the intrinsic horizontal connections cannot be easily manipulated without directly affecting the measured signals, ii) because inter-hemispheric connections share the major anatomical features with the intrinsic lateral network and can be seen as a functional continuation of the latter across the two hemispheres and iii) because cooling causally and reversibly deactivates input connections by temporarily silencing the sending neurons and thus enables direct conclusions on their contribution.
Our results demonstrate that the figure-ground segmentation mechanism is reflected in the spike rate of single neurons, as well as in their LFP power and its phase-relationship to the spike patterns produced by the population. In addition "lateral" inter-hemispheric connections modulate spike rates and LFP power depending on the stimulation of the neurons CRF surround. Further, we observe an influence of this lateral circuit on field-
field coherences between remote recording sites. In conclusion, our findings support the idea of complex figure-ground segmentation mechanism acting already in early visual areas on different time scales. This mechanism seems to involve groups of neurons firing synchronously and dependent on the LFP s phase. Our results are also compatible with the hypothesis that long-range lateral connections contribute to that mechanism
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufrn.br:123456789/17032 |
| Date | 31 March 2014 |
| Creators | Ocazionez, Sergio Andr?s Conde |
| Contributors | CPF:70024141437, http://lattes.cnpq.br/4902752912395893, Tort, Adriano Bretanha Lopes, CPF:96602767020, http://lattes.cnpq.br/3181888189086405, Neuenschwander, Sergio, CPF:45548927604, http://lattes.cnpq.br/9217956361436464, Houzel, Jean Christophe, CPF:05594296735, http://lattes.cnpq.br/0010117558043568, Schmidt, Kerstin Erika |
| Publisher | Universidade Federal do Rio Grande do Norte, Programa de P?s-Gradua??o em Neurociencias, UFRN, BR, Neurobiologia Celular e Molecular; Neurobiologia de Sistemas e Cogni??o; Neurocomputa??o Neuroengen |
| Source Sets | IBICT Brazilian ETDs |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/doctoralThesis |
| Format | application/pdf |
| Source | reponame:Repositório Institucional da UFRN, instname:Universidade Federal do Rio Grande do Norte, instacron:UFRN |
| Rights | info:eu-repo/semantics/openAccess |
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