masterThesis
Encoding mechanisms based on fast oscillations in the retina of the cat and their dependencies on anesthesia
Fecha
2013-08-27Registro en:
FREITAG, Fábio Batista. Encoding mechanisms based on fast oscillations in the retina of the cat and their dependencies on anesthesia. 2013. 60 f. Dissertação (Mestrado em Neurobiologia Celular e Molecular; Neurobiologia de Sistemas e Cognição; Neurocomputação Neuroengen) - Universidade Federal do Rio Grande do Norte, Natal, 2013.
Autor
Freitag, Fábio Batista
Resumen
Processing in the visual system starts in the retina. Its complex network of cells
with different properties enables for parallel encoding and transmission of visual
information to the lateral geniculate nucleus (LGN) and 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. Studies in the frog, rabbit, cat and
monkey, have shown strong oscillatory responses to large stimuli which probably
encode global stimulus properties, such as size and continuity (Neuenschwander and
Singer, 1996; Ishikane et al., 2005). Moreover, simultaneous recordings from different
levels in the visual system have demonstrated that the oscillatory patterning of retinal
ganglion cell responses are transmitted to the cortex via the LGN (Castelo-Branco et al.,
1998). Overall these results suggest that feedforward synchronous oscillations
contribute to visual encoding.
In the present study on the LGN of the anesthetized cat, we further investigate
the role of retinal oscillations in 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, as currently most data on retinal oscillations have been
limited to simple, flashed and stationary stimuli. 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 (gray-level and binary movies) oscillations appeared
only for brief moments probably when receptive fields were dominated by large
continuous, flat-contrast surfaces. Moreover, oscillatory responses to a circle stimulus
could be broken with an annular mask indicating that synchronization arises from
relatively local interactions among populations of activated cells in the retina.
A surprising finding in this study was that retinal oscillations are highly
dependent on halothane anesthesia levels. In the absence of halothane, oscillatory
activity vanished independent of the characteristics of the stimuli. The same results
were obtained for isoflurane, which has similar pharmacological properties. These new
and unexpected findings question whether feedfoward oscillations in the early visual
system are simply due to an imbalance between excitation and inhibition in the retinal
networks generated by the halogenated anesthetics. Further studies in awake behaving
animals are necessary to extend these conclusions