Any organism with a brain needs to make decisions about how it’s going to navigate through three-dimensional spaces. That’s why animals have evolved sensory organs in the ears to detect if they’re rotating or moving in a straight line. But how does an animal perceive curved motion, as in turning a corner? One explanation, published April 21 in Cell Reports, from researchers looking at macaques, is that curved motion is detected when sensory neurons in the brain receiving converging information about linear and rotational movement are activated.
The parts of the ear that help macaques and humans detect motion are the same ones that help us stay balanced. Otoliths are sphere-like organs that detect linear motion and gravitational pull. In contrast, semi-circular canals specifically detect rotational movement. Information about an animal’s motion collected by these organs are then sent to the central nervous system in the brain
It’s known that two distinct sets of neurons help us sense linear and rotational movement, but the new study identified a third set of neurons in the macaque sensory cortex that respond optimally to curved motion.
“It’s a very interesting question as to why our brain evolved this way,” says corresponding author Yong Gu, a neuroscientist at the Shanghai Institutes for Biological Sciences and Chinese Academy of Sciences. “We don’t have to have these curved motion neurons in the sensory area of the brain; the information about translation and rotation could have converged at a higher level, e.g. association cortex which is important for sensory-motor transformation and decision making. Our hunch is that representation of curved motion in sensory cortex helps animals rapidly detect this type of movement, and save the working load of the decision centres for other important neural computations.”
Gu and lab member Zhixian Cheng made their discovery by placing macaques in moving platforms and attaching brain electrodes to individual neurons to measure how often and when they fired. “People have known that linear and rotational motion converged in the sensory cortex, and we found that certain neurons fire more spikes when the linear or rotational information are available at the same time for these neurons,” Gu says. “This might have been expected, but we now propose that these neurons could represent curvilinear motion.”
The experiments also mimicked a 1997 human study in which subjects were passively moved in various motion conditions (e.g, curvilinear motion versus moving in a straight line while rotating the body) and reported analogous curved-motion sensation as long as both linear and rotation signals are present simultaneously. The current macaque neurophysiological data show extremely similar patterns, thus could account for the human psychophysical data. “This is surprising,” Gu says. “In nature, we should be able to tell these two different types of motion during active navigation. Other signals in the brain, for example, the motor command signals may help.”
The past decade has seen a surge in papers on how the body senses motion, and Gu believes there are more surprises to come. In particular, he’s interested in learning how other sensory systems play a role in how primates know where they are going.
About this neuroscience research
Funding: This work was supported by grants from the National Natural Science Foundation of China Project; the Strategic Priority Research Program of the Chinese Academy of Sciences; and the Recruitment Program of Global Youth Experts.
Source: Joseph Caputo – Cell Press Image Credit: The image is credited to Cheng and Gu/Cell Reports 2016. Original Research: Full open access research for “Distributed Representation of Curvilinear Self-Motion in the Macaque Parietal Cortex” by Zhixian Cheng, and Yong Gu in Cell Reports. Published online April 21 2016 doi:10.1016/j.celrep.2016.03.089
Distributed Representation of Curvilinear Self-Motion in the Macaque Parietal Cortex
Highlights •Self-motion types are continuously represented in parietal cortex •Convergent cells prefer combined translation and rotation stimuli •Convergent cells subadditively integrate translation and rotation signals •Convergent cells respond similarly to real and illusory curvilinear self-motion
Summary Information about translations and rotations of the body is critical for complex self-motion perception during spatial navigation. However, little is known about the nature and function of their convergence in the cortex. We measured neural activity in multiple areas in the macaque parietal cortex in response to three different types of body motion applied through a motion platform: translation, rotation, and combined stimuli, i.e., curvilinear motion. We found a continuous representation of motion types in each area. In contrast to single-modality cells preferring either translation-only or rotation-only stimuli, convergent cells tend to be optimally tuned to curvilinear motion. A weighted summation model captured the data well, suggesting that translation and rotation signals are integrated subadditively in the cortex. Interestingly, variation in the activity of convergent cells parallels behavioral outputs reported in human psychophysical experiments. We conclude that representation of curvilinear self-motion perception is widely distributed in the primate sensory cortex.
“Distributed Representation of Curvilinear Self-Motion in the Macaque Parietal Cortex” by Zhixian Cheng, and Yong Gu in Cell Reports. Published online April 21 2016 doi:10.1016/j.celrep.2016.03.089