A technical tour-de-force uncovers striatal neurons driven by vision and touch
November 18, 2015
I am happy to announce another post by a guest blogger. This time, its Sashank Pisupati, a new graduate student in my lab.
Last week, our lab read a paper by Ramon Reig & Gilad Silberberg titled “Multisensory Integration in the Mouse Striatum”. While studies of multisensory integration have focussed largely on cortical structures and the superior colliculus, this study adds to a growing body of evidence that the striatum may play a key role in this process. Striatal medium-spiny neurons (MSNs) are known to receive convergent projections from multiple sensory cortices, but relatively few studies have reported multisensory responses in these cells.
Here, the authors set out to test whether individual MSNs integrated visual (LED flashes) and tactile (whisker stimulation with air puffs) stimuli in anesthetized mice. In order to observe such synaptic integration, they performed whole-cell patch clamp recordings from striatal neurons. They targeted regions of striatum receiving projections from primary visual (V1) and somatosensory (S1) cortex, as identified by anterograde tracing using BDA.
They found sub-threshold responses to whisker stimulation (purple trace) in all the neurons they recorded from, which were modulated by stimulation strength. More interestingly, in the dorsomedial striatum a subset of these neurons were also responsive to visual stimuli (green trace), with slightly longer peak response latencies. They then presented visual and tactile stimuli together at various relative delays, and observed multisensory responses in these cells (orange & black traces) that were sublinearly additive i.e. less than the linear summation (grey traces) of the visual and tactile responses. Moreover, the peak multisensory response was maximal when the onsets/peaks of the unisensory responses were aligned, suggesting that the neurons summated congruent synaptic inputs.
These findings of multisensory cells in the mouse striatum corroborate similar reports from extracellular recordings in the striatum of rats and cats, and complement them by offering a valuable glimpse of sub-threshold activity. The sub-linear additivity described here contrasts with the super-linear additivity of firing rate responses often emphasized in studies of the superior colliculus.
One of the questions that remained at the end of our discussion, was how this result fit into models of multisensory integration such as divisive normalization4, or Bayes-optimal cue combination. While classical approaches have emphasized the degree of additivity of the unisensory responses, these models make strong predictions about how the weights assigned to each unisensory response in the summation change in accordance to the reliability of that sensory modality4. For example, we expect the contribution of a visual flash to the summation to decrease for weaker, less reliable flashes.
One could test this prediction in the author’s current setup by simply varying the stimulus strength for each modality during multisensory presentation. Combined with the power of the patch clamp approach, this could yield further insight into the sub-threshold computations being performed by these neurons, and we hope to see more such work in the future!