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|Title:||MULTISENSORY CONVERGENCE IN LOW ORDER CORTICAL AREAS: A FINE STRUCTURE ANALYSIS|
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|Authors/Affiliations:||1 Marie-Eve Laramee*; 1 Valerie Charbonneau; 1 Gilles Bronchti; 1 Denis Boire; |
1 Universite du Quebec at Trois-Rivieres, QC, Canada
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|Content:||Objectives : Multisensory integration has long been believed to result from multisensory convergence in high order cortical areas, such as the frontal, parietal an temporal association cortices. Recent evidence also suggests that direct connections exist between primary sensory cortices. In monkeys, auditory projections have been demonstrated to terminate in layers 1 and 6 of the primary visual cortex (V1). Also, axonal terminals in the primary somatosensory cortex (S1) have been observed following auditory cortex tracer injections in the gerbil. Currently, it is still unclear whether these multimodal connections between primary sensory cortices are of a feedforward or feedback type. The goal of this study is to determine if connections between primary sensory cortices in normal mice are typical of feedforward or feedback pathways with respect to the layer specificity of auditory projections. Single axon reconstructions were insvestigated to identify the fine structure of connection patterns between these primary sensory cortices. |
Materials and methods : Columnar iontophoretic injections of BDA 10 000MW were performed in the primary auditory cortex (A1) of normal sighted ZRDCT/RAX/Chumd.j. (YZRDCT) strain mice. Single axons were reconstructed using the Neurolucida software and data analysis will be carried out using the Neurolucida Explorer software (MBF Bioscience). Results : Large injections of BDA 10 000MW covering all cortical layers in A1 in the normal mouse mainly result in anterograde labeling of terminal axonal arbors in the secondary visual lateral (V2L) as well as the secondary visual medial (V2M) areas. In V2L, all cortical layers receive auditory inputs but in V2M, only superficial layers are targeted in a patchy pattern. Moreover, the border between the auditory cortex and V2L is easily noticeable because many terminal axonal arbors travel perpendicular to the pial surface, compared to the other parts of the cortex where they are mainly travelling parallel to the pia. Additionally, axonal terminals following this injection in A1 are also noticeable in V1 and a few can also be found in S1. In V1, axonal fibers are in supragranular and infragranular layers and seem to avoid layer 4. In S1, very few fibers can be observed and they are mostly in infragranular layers.
Conclusion: This study clearly demonstrates that low order sensory cortices are interconnected in the normal mouse. Moreover, it demonstrates that auditory cortex sends projections to other sensory areas in different connectivity patterns depending on the area that receives the input. Interestingly, both V1 and S1 receive feedback connections from A1. Indeed, all non-granular layers in V1 and infragranular layers in S1 are targeted by auditory projections. On the other hand, auditory connections with V2L result from lateral connections because all layers receive non-layer specific inputs while V2M receive auditory inputs only in superficial layers, which is also an indication of a feedback connectivity pattern.
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