According to new research published in the journal Current Biology, a key area of the hummingbird brain processes motion in a unique, unexpected way.
The Anna’s hummingbird (Calypte anna). Image credit: Kevin Cole / CC BY 2.0.
The brain area in question is called the lentiformis mesencephali (LM), known in mammals as the nucleus of the optic tract.
“In all four-limbed vertebrates studied to date, most of the neurons in this motion-detecting brain area are tuned to detect motion coming from behind, such as would occur for an impending collision or when being attacked from behind by a predator,” said Dr. Douglas Altshuler, an associate professor in the Department of Zoology at the University of British Columbia and corresponding author of the study.
“We found that the LM responds very differently in hummingbirds,” he said.
“Instead of most neurons being tuned to back-to-front motion, almost every neuron we found was tuned to a different direction.”
“We also found that these neurons were most responsive to very fast motion.”
Earlier studies showed that the LM in hummingbirds is enlarged in comparison to that of other birds. Researchers also knew that hummingbirds monitor and correct for any minor drift in their position as they hover.
Those findings had led scientists to suggest that the hummingbird brain might be specially attuned to pick up on slow movements.
To test that hypothesis, Dr. Altshuler and his colleagues recorded neural activity in the LMs of six adult male Anna’s hummingbirds (Calypte anna) and ten adult male zebra finches (Taeniopygia guttata) as the birds watched computer-generated dots move in various directions.
Contrary to expectations, the recordings showed that hummingbirds are most sensitive to fast visual motion.
What’s more, unlike other birds, the hummingbirds responded to movement in any direction about equally.
That is, their LM neurons aren’t specially attuned to movements in the forward direction as in other animals.
The authors suggest that their visual abilities may play a role in dynamic behaviors, including competitive interactions, high-speed courtship displays, and insect foraging.
“This study provides compelling support for the hypothesis that the avian brain is specialized for flight and that hummingbirds are a powerful model for studying stabilization algorithms,” said study co-author Andrea Gaede, also from the Department of Zoology at the University of British Columbia.
The team now plans to investigate the response properties of other nuclei involved in this visual motion-processing pathway, with the ultimate goal of understanding how neural activity in the hummingbird brain is translated into specific flight behaviors.
_____
Andrea H. Gaede et al. Neurons Responsive to Global Visual Motion Have Unique Tuning Properties in Hummingbirds. Current Biology, published online January 5, 2017; doi: 10.1016/j.cub.2016.11.041
