Abstract
Two main types of cortical terminals have been identified in the cat thalamus. Large (type II) have been proposed to drive the response properties of thalamic cells while smaller (type I) are believed to modulate those properties. Among the cat’s visual cortical areas, the anterior ectosylvian visual area (AEV) is considered as one of the highest areas in the hierarchical organization of the visual system. Whereas the connections from the AEV to the thalamus have been recognized, their nature (type I or II) is presently not known. In this study, we assessed and compared the relative contribution of type I and type II inputs to thalamic nuclei originating from the AEV. The anterograde tracer BDA was injected in the AEV of five animals. Results show that (1) both type I and II terminals from AEV are present in the Lateral Posterior- Pulvinar complex, the lateral median suprageniculate complex and the medial and dorsal geniculate nuclei (2) type I terminals significantly outnumber the type II terminals in almost all nuclei studied. Our results indicate that neurons in the AEV are more likely to modulate response properties in the thalamus rather than to determine basic organization of receptive fields of thalamic cells.
Highlights
Studies from our laboratory and others have shown that neurons in the pulvinar contribute to the processing of basic and complex visual information[5,6,7] and there are assumptions that trans-thalamic pathways can be used to facilitate the cortical flow of information[8] and regulate cortical activity according to attentional demand[9]
We investigated the morphology and distribution of axon terminals originating from the anterior ectosylvian visual area (AEV), an associative area located in the anterior ecstoylvian sulcus (AES) considered as one of the highest areas along the hierarchical organization of the cat visual system[24]
The present results indicate that most CT axons of the visual and multisensory subregions located in the AES targeting higher-order nuclei exhibit type I terminals
Summary
Studies from our laboratory and others have shown that neurons in the pulvinar contribute to the processing of basic and complex visual information[5,6,7] and there are assumptions that trans-thalamic pathways can be used to facilitate the cortical flow of information[8] and regulate cortical activity according to attentional demand[9]. Type I axons are thin and possess long thin branches with occasional en passant swellings and “drumstick-like” side branches with small terminal endings[13] considered to be equivalent to the round small (RS) presynaptic terminals observed at the ultrastructural level[14,15] These small axons arise from layer VI cortical neurons[16,17]. In the cat, the vast majority of axons coming from the primary visual cortex have type II terminals while those from the posteromedial lateral cortex (PMLS, an extrastriate area considered as the homologue of the primate area MT) exhibit type I terminals[15] These results suggest that the nature of the cortical projections to extrageniculate thalamic nuclei varies as a function of cortical hierarchy, characterized by an increase of the modulatory/driver inputs ratio. Our results indicate that most CT axons are type I terminals, supporting the assumption that the proportion of driver/modulator inputs vary along the cortical hierarchy
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