Banca de QUALIFICAÇÃO: FÁBIO BATISTA FREITAG

Uma banca de QUALIFICAÇÃO de MESTRADO foi cadastrada pelo programa.
DISCENTE: FÁBIO BATISTA FREITAG
DATA: 09/08/2013
HORA: 17:00
LOCAL: Instituto do Cérebro
TÍTULO:

ENCODING MECHANISMS BASED ON FAST OSCILLATIONS IN THE RETINA OF THE CAT AND THEIR DEPENDENCIES ON ANESTHESIA

PALAVRAS-CHAVES:

retina, oscillation, synchronization, halothane, visual

PÁGINAS: 65
GRANDE ÁREA: Ciências Biológicas
ÁREA: Fisiologia
RESUMO:

Processing in the visual system starts in the retina. It is its complex network of cells with different properties that enables for parallel encoding and transmission of visual information to the lateral geniculate nucleus (LGN), where information is subsequently processed and transmitted to the cortex. In the retina, it has been shown that responses are often accompanied by fast synchronous oscillations (30 - 90 Hz) in a stimulus-dependent manner. As demonstrated in numerous studies in the frog, cat and monkey, synchronous oscillations in the retina are stronger for large as compared to punctiform stimuli and probably encode simple stimulus properties that are global, such as size and continuity (Neuenschwander and Singer, 1996; Ishikane et al., 2005). Moreover, simultaneous recordings from different levels in the visual system have shown that the oscillatory patterning in retinal ganglion cell responses is transmitted faithfully by the LGN to the cortex (Castelo-Branco et al., 1998). Overall these results suggest that feedforward synchronous oscillations are instrumental in encoding visual information.

In the present study on the LGN of anesthetized cat, we further investigate the role of retina oscillations in early visual processing by applying complex stimuli, such as natural visual scenes, light spots of varying size and contrast, and flickering checkerboards. This is a necessary step for understanding encoding mechanisms in more naturalistic conditions, since most studies of retinal oscillations have been obtained with responses to simple stimuli that are flashed and stationary. Correlation analysis of spiking responses confirmed previous results showing that oscillatory responses in the retina (observed here from the LGN responses) largely depend on the size and stationarity of the stimulus. For natural scenes (full gray-level and black-and-white movies) oscillations appeared only for brief moments when receptive fields were dominated by large continuous, even-contrast surfaces. Oscillatory activity seemed to be dependent on a critical mass of activated cells suggesting that it arises from large-scale horizontal interactions in the retina. Moreover, analysis of data obtained for different levels of anesthesia has shown that retinal oscillations in the cat are surprisingly dependent on the concentration of halothane. In the absence of halothane, all oscillatory activity vanished independent of the characteristics of the visual stimulus. The same findings were obtained for isofluorane, which has similar pharmacological properties. Taking into consideration these new and unexpected results we question whether feedfoward oscillations in the early visual system are simply due to an imbalance between excitation and inhibition in the retinal circuitry created by the use of halogenetic anesthetics. Further studies in awake behaving animals are necessary to extend these conclusions.

MEMBROS DA BANCA:
Interno - 1674643 - MARCOS ROMUALDO COSTA
Interno - 1439078 - REGINA HELENA DA SILVA
Presidente - 1842426 - SERGIO TULIO NEUENSCHWANDER MACIEL