| dc.description.abstract | Early physiologists were dazzled by the occurrence of high-amplitude, periodic
oscillations, easily discernible in recording traces from the eye, optic tract and optic
ganglia. Numerous studies thereafter pointed to retinal ganglion cell as the
elements responsible for the generation of these fast rhythms, which were known to
propagate to the lateral geniculate and to the cortex. Only recently, however, these early
observations gained renewed interest, mainly in the light of recent theories linking
neuronal oscillations to various cognitive processes, such as perceptual binding, attention
and memory. In this context, fast retinal oscillations have been associated to the binding
of contiguous contours or surfaces, which in principle could support a fast feedforward
segmentation process. In addition, a series of experiments in the cat have shown that fast
oscillations in the retina may convey global stimulus properties, such as size. A limitation in these previous studies, however, was that most of them where were made in the anesthetized and paralyzed cat. Only a few early studies have been performed in the non-anesthetized but still paralyzed cat. Another concern was that, in these latter experiments, visual stimuli were often limited to ganzfeld flashes, far from
natural vision conditions. Moreover, very recently we made the surprising observation
that fast retinal oscillations depend strongly on halothane (and isoflurane) anesthesia. It
was therefore imperative to verify whether oscillatory activity is also present in the
awake cat, under naturalistic conditions, such as during free-viewing of a visual scene.
This is the main goal of the present study. Simultaneous multiple-electrode recordings were made from the lateral geniculate nucleus (LGN) and the retina of anesthetized cats (N= 3) and from the LGN
of an awake cat (N= 1). Comparisons were made for responses to natural movies and
flashed stationary light stimuli. To test specifically the role of retinal oscillations in
encoding stimulus size we designed a protocol made of a light circle of varying size along
the trial. Spike sorting techniques allowed us to study separately the ON- and OFFcomponents
of the responses. Analysis consisted in measuring synchronous oscillations
for single cell spiking activity in the time (sliding correlation analysis) and spectral
domains (multitaper spectral analysis, multitaper coherence). Our present results based
on single-cells extend our previous findings in the anesthetized cat, which were
restricted to an autocorrelation analysis of LGN mutiunitary responses. Both ON- and
OFF-responses to varying size stimuli show that coherent oscillations appear only after
the stimulus attained a minimum size of about 5° (depending on the contrast level),
suggesting that oscillations in the retina are rather limited in encoding subtle changes in
stimulus size. Recordings obtained directly from eye showed that oscillations in the
retina, as in the LGN, are highly correlated with the concentrations level of halothane.
Notably, in a series of sessions we were able to record LGN responses in an awake cat,
which was subsequently anesthetized with halothane, keeping the same recording site.
Oscillations were completely absent in the awake condition and appeared strong as usual
during the halothane anesthesia. Overall these results weaken substantially the notion that fast retinal oscillations are meaningful for vision. Nevertheless, as shown from our single cell analysis, retinal
oscillations share many of the properties of cortical gamma oscillations. In this respect,
oscillations in the retina induced by halothane serve as a valuable preparation, even
though artificial, for studying oscillatory neuronal dynamics. | |
