Bat navigation and the human brain
Whoever coined the phrase “blind as a bat” probably didn’t know that bats see almost as well as humans. Bats also hear well, orienting themselves via “echolocation”—a system based on sound. But it turns out that, even without such sensory input, bats can successfully find their way to a target, thanks to a special class of neurons in the bat brain, recently identified by scientists at the Weizmann Institute.
The discovery of these neurons, located in a part of the brain associated with memory, reveals a previously unknown link between memory and navigation in the brains of mammals. Interestingly, the bat-based study may help explain how memory loss and “getting lost” tend to go together in human Alzheimer’s patients.
The study was performed by Prof. Nachum Ulanovsky and research student Ayelet Sarel, both of the Department of Neurobiology, together with their departmental colleagues Drs. Arseny Finkelstein and Liora Las. Their results were published in Science in January.
A calculated flight path
To clarify how bats perform the complex neural calculations needed to navigate to a specific destination, Prof. Ulanovsky and his team trained Egyptian fruit bats to fly in highly complex trajectories within a specially-designed flight room. The room included a single landing site—the navigational goal—where the bat could eat and rest. As the bat flew, the researchers used a wireless device to record the activity of 309 single neurons in a defined area of the bats’ hippocampus.
The scientists discovered that about a fifth of these hippocampal neurons were significant goal-direction cells; they were maximally active when the bat was heading at a particular angle relative to the goal—with most neurons showing the strongest activation when the bat headed directly towards the goal. In flying sessions in which the goal was obscured by an opaque curtain that blocked sensory input based on vision, echolocation, and olfaction, a substantial fraction of the neurons continued to exhibit directional firing—indicating that this directional signal was triggered by information about the target that was stored in memory.
The scientists also discovered something else: “distance” neurons that fired maximally when the bat was a short distance away from the target. Rather than being entirely separate from the directional neurons, a statistically significant number of these “goal-distance” neurons were seen to encode for “goal-direction” as well. This suggests that such individual neurons are capable of what Prof. Ulanovsky calls “vectorial” representation: they encode complex computational information based on the measurements of direction and distance to the goal.
The Alzheimer’s connection
The Science study helps clarify the connection between navigation and memory—and not just in bats.
“The previously unrecognized neural mechanism is localized to the hippocampus, a part of the brain involved in memory loss in Alzheimer’s disease,” says Prof. Ulanovsky, adding that brain studies in human subjects attest to the involvement of the hippocampus in the performance of navigational tasks. “Damage to the hippocampus has been shown to impact navigational functioning, something that may explain the tendency of people with Alzheimer’s disease to get disoriented or lost.”
But when it works, the mechanism—like a neural GPS—helps us get where we need to go.
“Just like the bat wants to reach the fruit hidden behind the curtain, humans may want to find a campsite over the hill, or a coffee shop tucked just behind an office building,” Prof. Ulanovsky says. “In such cases, even if there are no clear cues coming in via our senses, we can rely on the neural mechanism in our brain to help us locate the target. In this experiment, the bats showed us how it’s done.”
Prof. Nachum Ulanovsky is supported by the Lulu P. & David J. Levidow Fund for Alzheimers Diseases and Neuroscience Research, the Adelis Foundation, Mike and Valeria Rosenbloom through the Mike Rosenbloom Foundation, and the Harold and Faye Liss Foundation.