Co-oscillation and synchronization between the posterior thalamus and the caudate nucleus during visual stimulation.

Recent results suggest significant cross-correlation between the spike trains of the suprageniculate nucleus (SG) of the posterior thalamus and the caudate nucleus (CN) during visual stimulation. In the present study visually evoked local field potentials (LFPs) were recorded simultaneously in the CN and the SG in order to investigate the coupling between these structures at a population level. The effect of static and dynamic visual stimulation was analyzed in 55 SG-CN LFP pairs in the frequency range 5-57Hz. Statistical analysis revealed significant correlation of the relative powers of each investigated frequency band (5-8Hz, 8-12Hz, 12-35Hz and 35-57Hz) during both static and dynamic visual stimulation. The temporal evolution of cross-correlation showed that in the majority of the cases the SG was activated first, and in approximately one third of the cases, the CN was activated earlier. These observations suggest a bidirectional information flow. The most interesting finding of this study is that different frequency bands exhibited significant cross-correlation in a stimulation paradigm-dependent manner. That is, static stimulation usually increased the cross-correlation of the higher frequency components (12-57Hz) of the LFP, while dynamic stimulation induced changes in the lowest frequency band (5-8Hz). This suggests a parallel processing of dynamic and static visual information in the SG and the CN. To our knowledge we are the first to provide evidence on the co-oscillation and synchronization of the CN and the SG at a population level upon visual stimulation, which suggests a significant cooperation between these structures in visual information processing.

Neuronal code of spatial visual information in the caudate nucleus.

Earlier reports described huge overlapping visual receptive fields and the absence of retinotopic organization in the dorsolateral, caudal part of the caudate nucleus. In the present study we suggest a possible alternative mechanism for the coding of spatial visual information. Extracellular microelectrode recordings were carried out in halothane-anesthetized, immobilized, artificially ventilated cats. In order to investigate the responsiveness of the single neurons to visual information arriving from different sites of the receptive field, we divided the visual fields to 20 parts of equal size and stimulated the individual parts one-by-one. We found that each single visual caudate nucleus (CN) neuron can carry information about stimulus locations throughout the whole physically approachable visual field of the investigated eye. A large majority (85%) of these neurons exhibited significantly different responses to stimuli appearing in different regions of their huge receptive field. Thus these neurons appear to have the ability to provide information on the site of the stimulus via their discharge rate. The huge receptive fields in combination with the spatial selectivity suggest that these caudate nucleus neurons may serve as panoramic localizers. On the population level, the sites of maximal responsiveness of the visual neurons are distributed over the whole extent of the receptive fields. We argue that groups of these panoramic localizer neurons with different locations of maximal stimulus preference should have the ability to accurately code the locations of visual stimuli. We propose this distributed population code of visual information as an alternative information processing mechanism.

How moving visual stimuli modulate the activity of the substantia nigra pars reticulata.

The orientation of spatial attention via saccades is modulated by a pathway from the substantia nigra pars reticularis (SNr) to the superior colliculus, which enhances the ability to respond to novel stimuli. However, the algorithm whereby the SNr translates visual input to saccade-related information is still unknown. We recorded extracellular single-unit responses of 343 SNr cells to visual stimuli in anesthetized cats. Depending on the size, velocity and direction of the visual stimulus, SNr neurons responded by either increasing or decreasing their firing rate. Using artificial neuronal networks, visual SNr neurons could be classified into distinct groups. Some of the units showed a clear preference for one specific combination of direction and velocity (simple neurons), while other SNr neurons were sensitive to the direction (direction-tuned neurons) or the velocity (velocity-tuned neurons) of the movement. Furthermore, a subset of SNr neurons exhibited a narrow inhibitory/excitatory domain in the velocity/direction plane with an opposing surround (concentric neurons). According to our results, spatiotemporally represented visual information may determine the discharge pattern of the SNr. We suggest that the SNr utilizes spatiotemporal properties of the visual information to generate vector-based commands, which could modulate the activity of the superior colliculus and enhance or inhibit the reflexive initiation of complex and accurate saccades.