Bats, the Brain, and Behavior
The fact that bats are able to use sound alone to navigate and hunt in complete darkness is fascinating in its own right, but Dr. Moss and her students hope that what they learn about how bats perform these amazing tasks will also help us to understand more about brain-behavior relationships in general.Dr. Cynthia Moss has bats in her belfry – but we're not trying to say she's crazy. The belfry is what they call the section of her lab where the research animals are kept. These animals, as you may have guessed, are bats. The official name of the lab is the auditory neuroethology lab, but most people just call it the batlab. Research in Dr. Moss' lab focuses on bat echolocation. The fact that bats are able to use sound alone to navigate and hunt in complete darkness is fascinating in its own right, but Dr. Moss and her students hope that what they learn about how bats perform these amazing tasks will also help us to understand more about brain-behavior relationships in general. The projects in her lab range from those that explore the most fundamental aspects of echolocation, how bats determine the location of objects from the echoes they receive, to more complex questions such as how bats direct their attention in flight, and how they use spatial memory to navigate.
Cranium-Echo Interaction in Bats
Murat Ayetkin wants to know what a bat's head does to sounds. He measures the changes in a sound at a bat's eardrum as the sound source moves relative to the bat's head. These changes, taken across all directions are called the head-related-transfer-function (HRTF), and are due to the acoustic interaction of sound with the bat's ears and head. The HRTF provides the cues bats (and humans) use to determine the direction of a sound source. Conventional scientific wisdom breaks down the localization process into different cues for different components of sound localization. Differences in the sounds intensity at each ear (interaural level difference, or ILD) are related to the horizontal direction of the sound, and spectral changes (in particular, spectral 'notches') are related to the vertical direction of the sound. Murat's work in measuring the HRTFs of bats has revealed very different results, however. His data indicate that in bats, ILD can be used for determining both the horizontal and vertical components of a sound's location. For the lower frequencies of bat echolocation, ILD corresponds with the horizontal position of a sound, as described by the classical view. However, at higher frequencies, ILD can be used to localize sounds in the vertical direction. Thus, bats may be capable of fully determining the direction of an object using ILD cues alone.
While it is one thing to say that bats use their HRTF to determine sound direction, a question remains about how bats learn that mapping between changes in sound structure and sound direction. Much like human heads, all bat heads are different. While some overall trends my apply to all HRTFs in bats, it is unlikely that bats are able to perform at the level observed using a hardwired mechanism. It seems likely that some degree of learning is involved. To explore how this learning process might occur, Murat is currently developing a model of HRTF learning that would allow a bat (or any organism) to learn it's HRTF using only knowledge of it's own head movements and how sounds change in response to those movements, without any prior knowledge of the sound source's location.
Observing Bats in Flight
Several of the projects in Dr. Moss' lab focus on the behavior of bats in flight. Within the lab is a large anechoic chamber set aside for flight experiments. The chamber is equipped with infrared cameras which can monitor the bat's position in the room, as well as an innovative array of microphones developed by Kaushik Ghose which allow the direction of the bat's sonar beam to be determined. By measuring the intensity of the bat's vocalization at several points around the room, Kaushik is able to reconstruct the sonar beam pattern of the bat. Knowing the aim of the sonar beam allows him to make inferences about the attentional state of the bat, much as one can do with humans by observing where their gaze is directed. Initial studies of the bat's beam aim have shown that bats lock onto targets with about 7 degrees of accuracy, and that sonar beam aim is a good predictor of flight path (the bat "looks" where it is going). Further uses for beam aim data include segregating flights into reliable search and intercept phases, allowing for better analysis of target intercept strategies. Beam aim data could also potentially be useful in observing how and when bats choose to deal with obstacles in flight.
Spatial Memory in Bats
Nachum Ulanovsky, a postdoc in Dr. Moss' lab, is interested in how bats remember how to get home. He studies the hippocampus, a region of the mammalian brain known to play an important role in spatial memory. Bats have a well developed spatial memory, both over short distances (e.g. returning every night to the same roosting tree or to the same feeding location) and over long distances (e.g. homing over tens or hundreds of kilometers, and in some species also long-range annual migrations).
The physiology of the hippocampus has been extensively studied for many decades, mostly in rats. Rats have neurons in their hippocampus
which peak in activity when the animal passes through a certain part of the environment.
Many researchers believe that these "place cells" form a map of the environment which the rat uses to navigate. However, some data from monkeys have called into question whether or not place cells are a general feature of hippocampus, or are only specific to certian species. In studies of hippocampal neurons in monkeys, researchers discovered "spatial view cells," neurons that responded to the part of the environment where the animal was looking rather than where it was located
(although other researchers demonstrated place cells in monkey hippocampus).
The studies in rodents and monkeys represent nearly all of the available data on the activity of hippocampal neurons in freely-moving mammals. Understanding how bats' hippocampal neurons respond during navigation should improve our overall understanding of hippocampal neurons and the consistency of their responses across species. In his experiments, Dr. Ulanovsky places a bat within an open arena which contains landmarks, and mealworms in random locations. As the bat searches for mealworms, he records neural activity in the bat's hippocampus. This data is analyzed to see if the neurons respond like place cells, spatial view cells, or something else.
