Multisensory integration in the rat: behavioral benefits and neural correlates in parietal cortex

The complementary information provided by our different senses greatly enhances our ability to perceive and interact with the environment. Rodent models offer the possibility to study the underlying neural mechanisms and computations using a range of methodologies. However, suitable behavioral tasks and cortical candidate areas for the rodent remain to be elucidated. We developed a two-response forced-choice stimulus detection paradigm where rats (Long Evans) were required to detect lateralized audio-visual targets presented in either uni- or multisensory configuration. After training, the animals exhibit faster reaction times and enhanced detection rates in congruent multisensory conditions and this multisensory response enhancement is strongest for weak unisensory stimuli. These multisensory behavioral benefits mirror those described for similar tasks in humans. To localize target areas of multisensory convergence, we performed high-resolution intrinsic imaging experiments in urethane anaesthetized rats. We found a consistent overlap of responses to visual, somatosensory and auditory stimuli in an elongated region which had the cytoarchitectonic properties of an association area (sparse layer IV) and which overlapped well with parietal region PtA, as defined by the Paxinos atlas. Laminar recordings confirmed the functional convergence of unisensory inputs both in current source densities and multi-unit activity. These recordings also demonstrated multisensory response interactions and the magnitude and sign of response enhancement / suppression was dependent on temporal stimulus order. Control experiments confirmed the specificity of the multisensory response patterns to the parietal region (in comparison to visual cortex). We developed a rodent model of behavioral multisensory integration similar to paradigms known from human psychophysics and we show the presence of key criteria of multisensory processing in a region in the parietal cortex. Ongoing experiments directly study the neural underpinnings of behavioral benefits for enhanced stimulus detection in the behaving animal