ORIENTING IN 3D SPACE: BEHAVIORAL AND NEUROPHYSIOLOGICAL STUDIES IN BIG BROWN BATS

In their natural environment, animals engage in a wide range of behavioral tasks that require them to orient to stimuli in three-dimensional space, such as navigating around obstacles, reaching for objects and escaping from predators. Echolocating bats, for example, have evolved a high-resolution 3D acoustic orienting system that allows them to localize and track small moving targets in azimuth, elevation and range. The bat’s active control over the features of its echolocation signals contributes directly to the information represented in its sonar receiver, and its adaptive adjustments in sonar signal design provide a window into the acoustic features that are important for different behavioral tasks. When bats inspect sonar objects and require accurate 3D localization of targets, they produce sonar sound groups (SSGs), which are clusters of sonar calls produced at short intervals and flanked by long interval calls. SSGs are hypothesized to enhance the bat’s range resolution, but this hypothesis has not been directly tested. We first, in Chapter 2, provide a comprehensive comparison of SSG production of bats flying in the field and in the lab under different environmental conditions. Further, in Chapter 3, we devise an experiment to specifically compare SSG production under conditions when target motion is predictable and unpredictable, with the latter mimicking natural conditions where bats chase erratically moving prey. Data from both of these studies are consistent with the hypothesis that SSGs improve the bat’s spatio-temporal resolution of target range, and provide a behavioral foundation for the analysis and interpretation of neural recording data in chapters 4 and 6. The complex orienting behaviors exhibited by animals can be understood as a feedback loop between sensing and action. A primary brain structure involved in sensorimotor integration is the midbrain superior colliculus (SC). The SC is a widely studied brain region and has been implicated in species-specific orienting behaviors. However, most studies of the SC have investigated its functional organization using synthetic 2D (azimuth and elevation) stimuli in restrained animals, leaving gaps in our knowledge of how 3D space (azimuth, elevation and distance) is represented in the CNS. In contrast, the representation of stimulus distance in the auditory systems of bats has been widely studied. Almost all of these studies have been conducted in passively listening bats, thus severing the loop between sensing and action and leaving gaps in our knowledge regarding how target distance is represented in the auditory system of actively echolocating bats. In chapters 4, 5 and 6, we attempt to fill gaps in our knowledge by recording from the SC of free flying echolocating bats engaged in a naturalistic navigation task where bats produce SSGs. In chapter 4, we provide a framework to compute time-of-arrival and direction of the instantaneous echo stimuli received at the bats ears. In chapters 5 and 6, we provide an algorithm to classify neural activity in the SC as sensory, sensorimotor and premotor and then compute spatial receptive fields of SC neurons. Our results show that neurons in the SC of the free-flying echolocating bat respond selectively to stimulus azimuth, elevation and range. Importantly, we find that SC neuron response profiles are modulated by the bat’s behavioral state, indicated by the production of SSG. Broadly, we use both behavior and electrophysiology to understand the action-perception loop that supports spatial orientation by echolocation. We believe that the results and methodological advances presented here will open doors to further studies of sensorimotor integration in freely behaving animals

Action Enhances Acoustic Cues for 3-D Target Localization by Echolocating Bats

<div><p>Under natural conditions, animals encounter a barrage of sensory information from which they must select and interpret biologically relevant signals. Active sensing can facilitate this process by engaging motor systems in the sampling of sensory information. The echolocating bat serves as an excellent model to investigate the coupling between action and sensing because it adaptively controls both the acoustic signals used to probe the environment and movements to receive echoes at the auditory periphery. We report here that the echolocating bat controls the features of its sonar vocalizations in tandem with the positioning of the outer ears to maximize acoustic cues for target detection and localization. The bat’s adaptive control of sonar vocalizations and ear positioning occurs on a millisecond timescale to capture spatial information from arriving echoes, as well as on a longer timescale to track target movement. Our results demonstrate that purposeful control over sonar sound production and reception can serve to improve acoustic cues for localization tasks. This finding also highlights the general importance of movement to sensory processing across animal species. Finally, our discoveries point to important parallels between spatial perception by echolocation and vision.</p></div

Action Enhances Acoustic Cues for 3-D Target Localization by Echolocating Bats - Fig 4

<p>(A) Top, normalized inter-pinna separations for one-target simple motion (blue) versus two-target simple motions (green) combined across bats. Plotted are the mean +/- s.e.m. of the inter-pinna separation distances as a function of target distance. Bottom, dʹ calculation, or discriminability index, between one-target simple motion and two-target simple motion inter-pinna separation. Red shaded region indicates dʹ values above the 95% confidence interval as determined by a permutation test, indicating time points of significant differences in inter-pinna separation. (B) Top, normalized inter-pinna separations for one-target simple motion (blue) versus two-target pass motions (black) combined across bats. Bottom, dʹ calculation between one-target simple and two-target pass motion conditions. Red shaded region indicates significant differences. (C) Top, normalized inter-pinna separations for two-target simple motion (green) versus two-target pass motions (black) combined across bats. Bottom, dʹ calculation between two-target simple and two-target pass motion conditions. Red shaded region indicates significant differences. Data for this figure can be found at <a href="http://dx.doi.org/10.7281/T1W66HPZ" target="_blank">http://dx.doi.org/10.7281/T1W66HPZ</a>.</p

Changes in pinna separation with target distance for each target motion condition (colors as in Fig 1).

<p>(A) Top left, change in inter-pinna separation as a function of target distance for one-target simple motion. Plotted is the mean +/- s.e.m. for all trials. Values are normalized to zero for the starting position at the beginning of each trial. Top right, normalized data for two-target same motions; bottom left, normalized data for two-target pass motions (asterisks indicate when the second target begins to move, and when it overtakes the first target); bottom right, normalized data for one-target complex motions (asterisks indicate times when the target changes motion direction). (B) Normalized inter-pinna separation as a function of target distance for all varieties of target motion. There is a significant correlation between decreasing target distance and increasing pinna separation (Pearson’s correlation, r = 0.45, <i>p</i> < 0.0001). Inset details inter-pinna separation measurement. Data for this figure can be found at <a href="http://dx.doi.org/10.7281/T1W66HPZ" target="_blank">http://dx.doi.org/10.7281/T1W66HPZ</a>.</p

Behavioral setup and adaptive echolocation call changes.

<p>(A) Bats are trained to rest on a platform and track a moving target (tethered mealworm) by echolocation in the dark. The target(s) is moved via a rotary stepper motor attached to monofilament wire that is looped around a set of four pulleys. While the bat tracks the target(s), one microphone records the bat’s vocalizations, and a second microphone records the returning echoes. The 3-D positions of the head and pinna are recorded with a set of four infrared (IR) motion capture cameras. (B) Example sonar oscillograms (top) and target distance versus time plot (bottom) for each target motion condition presented to the bat (top to bottom): one-target-simple motion, two-target simple motion, two-target-pass motion, and one-target complex motion. (C) Top, mean +/- standard error of the mean (s.e.m) change in pulse interval over target distance for different varieties of target motion (colors as in B, significant differences are indicated in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002544#pbio.1002544.s001" target="_blank">S1 Fig</a>). Bottom, mean +/- s.e.m. change in pulse duration over target distance for each target motion condition (colors as in B, significant differences are indicated in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002544#pbio.1002544.s002" target="_blank">S2 Fig</a>). Data for this figure can be found at <a href="http://dx.doi.org/10.7281/T1W66HPZ" target="_blank">http://dx.doi.org/10.7281/T1W66HPZ</a>.</p

Head waggles and target motion.

<p>(A) Incidence of head waggle for Bat 1 during one-target simple (left) and one-target complex (right) motion. Displayed are the z-positions (height) of right ear (black) and left ear (red), with times of head waggles displayed in green. Notice the increase in head waggles for one-target complex motion. Far right panel displays a cartoon of the head waggle motion. (B) Summary across all bats for normalized head waggles per second for four different types of target motion. There is a significantly higher number of head waggles per second for one-target complex motions than all other types of target motion (permutation test, <i>p</i> < 0.0001). (C) Mean +/- s.e.m. of head waggles per second at the conclusion of a sonar sound group. The <i>zero</i> time point is the offset of the last sonar pulse in a sonar sound group. Plotted over these data are the mean waggles per second outside the context of sonar sound group production (black) and the 95% confidence interval on the mean in red. (D) Left to right, normalized head waggles per second for one-target simple, two-target simple, two-target pass, and one-target complex motions. The top row is the target position throughout trials; the bottom row is the target distance aligned normalized mean +/- s.e.m. of head waggles per second for each variety of target motion. Data for this figure can be found at <a href="http://dx.doi.org/10.7281/T1W66HPZ" target="_blank">http://dx.doi.org/10.7281/T1W66HPZ</a>.</p

Local changes in inter-pinna separation are tied to sonar signals.

<p>(A) Example of changes in inner-pinna distance for a bat tracking a tethered insect in a one-target complex motion trial. On a global scale, the bat changes inter-pinna separation with changing target distance; locally, there is also a small increase in inter-pinna separation near vocal onset time. Red dots indicate time of sonar call emission; green dots indicate time of sonar echo arrival. (B) Top, normalized individual pulse aligned (top panel) and echo aligned (bottom panel) inter-pinna separations. Red indicates the largest separation between the pinna, blue the smallest, in a 60 ms window aligned to the time of pulse emission or echo arrival. Bottom, mean +/- s.e.m. of normalized local peaks in inter-pinna distance aligned to either the sonar pulse onset (red) or echo arrival time (green). (C) All shifts in the peak of inter-pinna separation aligned to pulse onset (red) and echo arrival (green). Asterisks indicate the average shift in the peak displacement of inter-pinna separation around the time of pulse emission/echo arrival. The two distributions are significantly different (permutation test, <i>p</i> < 0.001). (D) Velocity of pinna motion during pulse production (red) and echo arrival (green). Pinna velocity is significantly higher during echo reception than during pulse production (permutation test, <i>p</i> < 0.0001). Data for this figure can be found at <a href="http://dx.doi.org/10.7281/T1W66HPZ" target="_blank">http://dx.doi.org/10.7281/T1W66HPZ</a>.</p

Motion parameters and number of trials for each variety of target motion.

<p>Motion parameters and number of trials for each variety of target motion.</p