A recent study published in Cerebral Cortex proved that the motion direction signals in macaque middle temporal area are causally involved in heading perception based on visual optic flow cue. This work was performed by researchers in Dr. Gu’s lab at the Institute of Neuroscience, Chinese Academy of Sciences. Combining multiple techniques including psychophysics, in vivo extracellular recording and electrical microstimulation, this authors provides direct neurophysiological evidence showing that MT neurons typically contain small receptive fields compared to the wide field, yet they are highly responsive under optic flow experienced during natural navigation. Furthermore, the responses from MT neurons are weighted pooled by downstream brain areas for global heading computation.

　　When animals navigate in the world, the surrounding objects generate a radial pattern of flow on the retina, the so-called optic flow. Optic flow is informative about the current motion status such that human and nonhuman primates can reliably discriminate heading direction as small as a few degrees based on it. Although this process might be mediated by the MT-MSTd circuits in the dorsal visual pathway, as proposed in many computational works, direct neurophysiological evidence is lacking.

　　To fill this gap, graduate student YU Xuefei and Dr. GU instructed human and monkey subjects to judge heading directions based on global optic flow. They showed that a local perturbation cue applied within only a small part of the visual field could bias the subjects’ heading judgments, and shift the neuronal tuning in the macaque middle temporal (MT) area at the same time. Then electrical microstimulation in MT significantly biased the animals’ heading judgments predictable from the tuning of the stimulated neurons. More interestingly, masking the visual stimuli within these neurons’ receptive fields could not remove the stimulation effect, indicating a sufficient role of the MT signals pooled by downstream neurons for global heading estimation. Specifically, this pooling is proved to be not homogeneous because stimulating neurons with excitatory surrounds produced relatively larger effects than stimulating neurons with inhibitory surrounds. Thus the results not only provide direct causal evidence, but also new insight to understanding the neural mechanisms of pooling local motion information for global heading estimation.

　　This work entitled “Causal evidence of motion signals in macaque middle temporal area weighted-pooled for global heading perception” was published online in Cerebral Cortex on Jan, 5th, 2017.  It was completed by graduate students YU Xuefei and HOU Han under the supervision of Dr. GU Yong. This work was supported by grants from the National Natural Science Foundation of China Project (31471048), the Strategic Priority Research Program of CAS (XDB02010000), National Key Basic Research Project of China (2016YFC1306801), and the Shanghai Key Basic Research Project (16JC1420201).

　　Figure legend: (A) Cartoon illustrates the motion vector when the observer moves (black arrow) with the object moves simultaneously (red arrow). The resultant vector is depicted by the dashed red arrow. In the experiment, the ratio between the red and black arrow is 1/10, leading to the deviated angle (α) about 6？.  （B-D）Interactions between microstimulation and visual perturbation effects. (B) Illustration of 4 experimental conditions interleaved randomly within one block. Upper left: normal self-motion condition without visual perturbation; upper right: perturbation induced by electrical microstimulation; lower left: perturbation by local object motion; lower right: combined effect of simultaneously introducing external (visual perturbation) and internal (microstimulation) perturbations. The red dashed circle represents the receptive field of the recorded/electrically stimulated MU. (C, D) Psychometric functions when microstimulation and local perturbation caused opposite (C) or congruent (D) effects.