Neurons in the dorsal subdivision of the medial superior temporal area (MSTd) show directionally selective responses to both visual (optic flow) and vestibular stimuli that correspond to translational or rotational movements of the subject. Previous work has shown that MSTd neurons are clustered within the cortex according to their directional preferences for optic flow, suggesting that there may be a topographic mapping of self-motion vectors in MSTd. If MSTd provides a multisensory representation of self-motion information, then MSTd neurons may also be expected to show clustering according to their directional preferences for vestibular signals, but this has not been tested previously. We have examined clustering of vestibular signals by comparing the tuning of isolated single units (SUs) with the undifferentiated multiunit (MU) activity of several neighboring neurons recorded from the same microelectrode. We find that directional preferences for both translational and rotational vestibular stimuli, like those for optic flow, are clustered within area MSTd. MU activity often shows significant tuning for vestibular stimuli, although this MU selectivity is generally weaker for translation than for rotation. When directional tuning is observed in MU activity, the direction preference generally agrees closely with that of a simultaneously recorded SU. We also examined clustering of visual receptive field properties in MSTd by analyzing receptive field maps obtained using a reverse-correlation technique. We find that both the local directional preferences and overall spatial receptive field profiles are well clustered in MSTd. Overall, our findings have implications for how visual and vestibular signals regarding self-motion may be decoded from populations of MSTd neurons.
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