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Abstract:
Much contemporary work is trying to elucidate how the brain integrates the information provided by the different senses into a coherent percept. To reveal causal contributions of individual areas in this process model systems are required that allow i) simultaneous assessment of neural activity across sensory streams, ii) use of genetic techniques to manipulate neural function, and iii) to perform all this in awake behaving animals. Using a combination of functional imaging and electrophysiology we highlight the rat parietal cortex (region PtA) as one promising candidate area.
We first studied the convergence of visual and somatosensory-evoked responses using high-resolution intrinsic imaging in urethane anaesthetized rats. We found a consistent (n=11) overlap of significant responses to both modalities in an elongated region between the presumed unisensory visual and somatosensory cortices. This region showed properties of an association area and overlapped with region PtA as defined by Paxinos and Watson1. Subsequent to localizing this multisensory region we inserted a multisite electrode at the point of largest co-activation. This confirmed the multisensory nature of this region as both current source densities (CSD) and multi-unit activity (MUA) revealed significant responses to both stimuli. In addition, we directly tested for functional signs of multisensory integration, such as non-additive response interactions23. Intriguingly, we found that the multisensory interaction depends on the temporal sequence of the stimuli: CSD-sinks interacted super-additively when the somatosensory stimulus preceded (21±4 enhancement, mean and s.e.m.), but sub-additively when the visual stimulus preceded (-43±11). For MUA, both stimulus sequences resulted in sub-additive interactions (-32±9 and -46±16, respectively). Control experiments revealed no bi-modal responses or non-additive interactions in visual cortex, confirming the specificity of the multisensory response patterns to the parietal association region.
Our results reveal a region in the parietal cortex which features the key criteria of multisensory processing2. Previous studies have shown the ability of rats to combine visual and somatosensory information, and the highlighted area constitutes a promising model to elucidate underlying neural mechanisms.
