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Wednesday, September 30, 2009

Echolocation, Science & Power

The Italian scientist Lazzaro Spallanzani  attained fame beyond his country already in his life time. He studied philosophy at the University of Bologna, became member of the clergy, and taught logic, metaphysics, and Greek at the universities of Reggio, Modena and and eventually assumed the chair in natural history at Pavia. His studies contributed profoundly to the understanding of a broad range of natural phenomena spanning geology, biology and physiology. Late in his career, he became fascinated with bats, wondering how they could navigate so elegantly in full darkness complicated environments wrought with obstacles.

Spallanzani proved himself as an observant experimenter. He attacked his question methodically with a series of tedious behavioral experiments, systemically ruling out one sense after another. He observed bats flying skillfully passed nooks and crannies in a L-shaped basement. Occlusion of the eyes did not appear to degrade the bats' performance, neither did covering the skin with a paste nor occluding the nose. His findings were first published as a collection of letters by Anton-Maria Vasalli (1794). A few years later, the Swiss zoologist Charles Jurine observed that occlusion of the ears rendered bats entirely disoriented. Spallanzani confirmed this observation, but was unable to explain how bats would use hearing for navigation. He had speculated earlier that the animals perhaps possessed a sixth sense unbeknownst to human kind.

Professor Spallanzani's hypothesis was met by strong and powerful resistance in the scientific community. One of the most eminent zoologists of his time, Georges Cuvier, argued against the validity of the experiments (Cuvier, 1795).  Interestingly, Cuvier's arguments were entirely based on conjecture. He did not conduct a single experiment to disprove Spallanzani's results. Instead, he appealed to common knowledge. He reasoned that everybody knows that bats, in as much as blind people, orient themselves with the sense of touch.

History would prove Cuvier wrong on both accounts. Neither bats (Griffin, 2001) nor blind people ( Wall Emerson and Ashmead, 2008) use their sense of touch for navigation in space. However, Cuvier's reputation was domineering. His influence on science was overarching. Roughly for the next century and a half researchers devoted their attention on the bats' sense of touch, until the Americans Donald R. Griffin and Robert Galambos and the Dutchman Sven Dijkgraaf discovered echolocation. That is, they unequivocally demonstrated that bats use the echos of ultrasound they emit to navigate their environment in flight and catch prey (Griffin, 2001). Griffin wrote an informative popular book about his findings entitled "Listening in the Dark: The Acoustic Orientation of Bats and Men". Vigorous research continues to the day to elucidate the nerve cell mechanisms that underlie this fascinating behavior.


I was exposed to some aspects of bat research when I was a student. The processing of ultrasound frequencies used for echolocation constitutes a prominent feature in the auditory pathway of echolocating bats (Neuweiler and others, 1980). My pilot study helped visualize this prominence with a functional imaging method (Melzer P, 1985).

The colors in the picture show ultrasound-related activation in a transverse slice through the brain of an echolocating bat. The ear on the opposite side was exposed to sound pips of this bat's individual echolocation frequency. Two regions in the auditory midbrain known as inferior colliculus (white circle) responded to the ultrasound most prominently.

Spallanzani's observations were influenced by the fact that he used pipistrelle bats (Pipistrellus pipistrellus) for his study. They emit their echolocation calls through the mouth, and his attempts to occlude the mouth noticeably compromised navigation. Had he used the more common horseshoe bat (Rhinolophus ferrumequinum), he would have been surprised to discover that the nose was doing the job.

Addendum
  • The co-discoverer of bat echolocation Robert Galambos, PhD, MD, passed away a month ago at the age of 96.  Douglas Martin provides a concise summary of his career in The New York Times today with the title "Robert Galambos, Neuroscientist Who Showed How Bats Navigate, Dies at 96".  He was a profound experimentalist. In an inseminating recent paper (Galambos, 2003), he described four elegant experiments that demonstrate the principles of empirical neuroscience in most illustrative fashion (07/16/10).
References
  • Cuvier G (1795) Conjectures sur le sixiéme sens qu'on a cru remarquer dans les chauve-souris. Mag. Encyclopéd 6:297-301.
  • Galambos R (2003) Four favorite experiments and why I like them. Int J Psychophysiol 48:133-140.
  • Griffin DR (2001) Return to the magic well: Echolocation behavior of bats and responses of insect prey. BioScience 51:555–556.
  • Griffin DR (1958) Listening in the dark: The acoustic orientation of Bats and men. Yale Univ Press.
  • Melzer P (1985) A deoxyglucose study on auditory responses in the bat Rhinolophus rouxi. Brain Res Bull 15:677-681.
  • Neuweiler G, Bruns V, Schuller G (1980) Ears adapted for the detection of motion, or how echolocating bats have exploited the capacities of the mammalian auditory system. J Acoust Soc Am 68:741-753.
  • Vasalli A-M (1794) Lettere sopra il Sospetto di un Nuovo Senso nei Pipistrelli . . . Con le Risposte dell’Abate. Stamperia Reale (Torino).
  • Wall Emerson R, Ashmead D (2008) Visual Experience and the concept of compensatory spatial hearing abilities. In: Blindness and brain plasticity in navigation and object perception (Rieser JJ, Ashmead DH, Ebner FF, Corn AL, eds). Taylor & Francis (New York):pp367-380.
Meet Bert the kitchen bat!




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