Synchronous oscillations in the subcortical visual system

The past few years have seen an explosion of interest in oscillatory phenomena in the central nervous system. The objective of this work was to explore the possibility that fast frequency (10-40Hz) oscillations could exist throughout the subcortical visual system and, further, that their source and transmission could be from the retina. The question of functional significance of the observed activity was addressed by qualitative and quantitative analysis of neuronal responses to various visual stimuli. The results of this study demonstrated that fast frequency oscillations exist in multiple subcortical visual system nuclei: the stratum griseum superficiale of the superior colliculus (SGS-SC), the medial pretectal area (MPA), and the dorsal lateral geniculate nucleus (DLGN). Single unit (SU) activity was observed to be strongly synchronised with multi-neuron clusters (MU) and field potential activity, demonstrating that these oscillations were ubiquitous across the regions under study. They were also found to be synchronised between the different ipsilateral visual system nuclei. These oscillatory phenomena were abolished upon severance of the optic nerve; thus, it was determined that retinofugal outputs were necessary for the appearance of subcortical visual system oscillations. Cross correlation studies provided clear evidence that oscillatory signals are transmitted from the OT to the subcortical nuclei. Therefore, I conclude that these oscillations were retinal derived and subsequently transmitted to the direct retinal receiving targets studied. In order to determine whether these subcortical visual system oscillations (SVSOs) have visual properties, data was gathered from recordings made in the dark vs. during sustained and/or intermittent light stimuli in anaesthetised and awake animals. This revealed a most distinctive feature of the SVSOs: they were prominent in the dark and suppressed by sustained light stimuli to a degree correlated with the area and intensity of the light. Traditional RF analysis of individual neurons revealed small RFs whose optimal responses were highly phasic and greatest during rapid stimulus movement through a highly circumscribed part of the visual field. By contrast, the oscillatory activity of the same single neurons could be inhibited by light stimulation over essentially the whole contralateral visual field; furthermore, these light-induced suppressions were tonic. It was concluded that these neurons had a very large "functional" RF which was distinct from their classic RFs. The data also lend themselves to the question: are SVSOs functionally significant to the visual system? Experimental data demonstrated complex interactions between the oscillatory discharge and temporal patterning of light stimulation. The post-light-ON responses of a single lateral geniculate nucleus (LGN) neuron to 11 different frequencies of light flash from 2-20Hz demonstrated that progressively higher frequencies of light stimulation could: (1) completely drive the oscillations; (2) produce nonlinear frequency bifurcations; and (3) resonate harmonically with the spontaneous oscillations, markedly enhancing responses to stimulation frequencies which were harmonics of baseline oscillation frequencies. My theory is that the spatiotemporal dynamical patterning of SVSOs may carry information about both global and local features of the visual scene. (Abstract shortened by UMI.) Originally published in DAI Vol. 57, No. 11. Reprinted here with corrected bibliographic information.