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Role and development of the corpus callosum for the interhemispheric transfer of visual motion

Periodic Report Summary 1 - DIRCALLOSDVPT (Role and development of the corpus callosum for the interhemispheric transfer of visual motion)

Our perception of the world is mainly dynamic. In order to interact adequately with the environment, our brain must analyse rapidly and efficiently various kinds of motion present in the visual world, including complex motions such as the optic flow induced by our own movements. The primary visual cortex, through its direction-selective cells, is the first cortical step toward such integration. Each hemifield is analysed by one hemisphere, and the corpus callosum is thought to link these two representations into a single dynamic scene. Using cat as a model animal and optical imaging as a technique, the goal of this project is to elucidate the role of the corpus callosum in the interhemispheric transfer of visual motion.

Generally, callosal connections in the early visual cortex are expected to link cells with similar receptive field properties. Such a view appears to be however challenged with the new perceptual roles that have been assigned to the primary visual cortex. In particular, the optic flow created by self-motion, exhibits different properties in the left and right visual fields. An organisation of callosal connections between cells of similar direction preference is therefore not suited for the processing of such stimulus. On the other hand, if a target moves from the left to the right visual field, one should expect the perceived direction to be continuously coded through the two hemispheres. In that case, we should expect an interhemispheric transfer of motion between cells that code for similar direction preferences.

We investigated this issue with the use of optical imaging of intrinsic signals, which allows visualising a large cortical surface of direction preference domains in cat primary visual cortex. We focused on a cortical region located at the border between area 17 and 18, where terminations of callosal connections are located. Advanced state-of-art paradigms of stimulation were applied for visualising direction preference domains with a great precision. Prior the experiment, the optic chiasm of the animal was cut in order to visualise the geniculo-cortical and transcallosal activation following the stimulation of each eye in succession.

For instance, when the left eye was stimulated, we could observe a clear direction preference map in the left hemisphere. This map reflected the geniculo-cortical activation following left eye stimulation. On the other hand, in the right hemisphere, the organisation of direction preference domains into patches reflected the transcallosal activation from the left hemisphere.

The transcallosal direction map in the right hemisphere was compared to the geniculo-cortical direction map for the same region of interest obtained with stimulating the other eye. For most of direction selective domains, we found a good accordance between the direction coded through geniculo-cortical and transcallosal activation. However, at some domains, typically coding for downward directions, we found a mismatch of a few dozens degrees between the angles of the two direction preference maps. These particular directions are important, since they encode for the same directions than the optic flow induced by forward locomotion in our region of interest.

At present time, our data suggest that corpus callosum could play a role in the processing of self-motion by modifying the perceived direction in the opposite hemisphere. We plan to confirm these results by coupling optical imaging with anatomical tracer injection, and study the functional selectivity of interhemispheric connections with respect to direction preference domains. These experiments will lead to new insights on how complex type movements can be processed at the level of the primary visual cortex.