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Animal echolocation
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==== Neural mechanisms ==== Because bats use echolocation to orient themselves and to locate objects, their auditory systems are adapted for this purpose, highly specialized for sensing and interpreting the stereotyped echolocation calls characteristic of their own species. This specialization is evident from the inner ear up to the highest levels of information processing in the auditory cortex.<ref>{{cite journal |last1=Salles |first1=Angeles |last2=Bohn |first2=Kirsten M. |last3=Moss |first3=Cynthia F. |s2cid=143423009 |title=Auditory communication processing in bats: What we know and where to go |journal=Behavioral Neuroscience |volume=133 |issue=3 |pages=305β319 |date=June 2019 |pmid=31045392 |doi=10.1037/bne0000308 |doi-access=free }}</ref> ===== Inner ear and primary sensory neurons ===== Both CF and FM bats have specialized inner ears which allow them to hear sounds in the ultrasonic range, far outside the range of human hearing. Although in most other aspects, the bat's auditory organs are similar to those of most other mammals, certain bats ([[horseshoe bats]], ''Rhinolophus spp.'' and the [[moustached bat]], ''Pteronotus parnelii'') with a constant frequency (CF) component to their call (known as high duty cycle bats) do have a few additional adaptations for detecting the predominant frequency (and harmonics) of the CF vocalization. These include a narrow frequency "tuning" of the inner ear organs, with an especially large area responding to the frequency of the bat's returning echoes.<ref name="Neuweiler_2003"/> The [[basilar membrane]] within the [[cochlea]] contains the first of these specializations for echo information processing. In bats that use CF signals, the section of the membrane that responds to the frequency of returning echoes is much larger than the region of response for any other frequency. For example, in the greater horseshoe bat, ''[[Rhinolophus ferrumequinum]]'', there is a disproportionately lengthened and thickened section of the membrane that responds to sounds around 83 kHz, the constant frequency of the echo produced by the bat's call. This area of high sensitivity to a specific, narrow range of frequency is known as an "[[acoustic fovea]]".<ref>{{cite journal |last1=Schuller |first1=G. |last2=Pollack |first2=G. |year=1979 |title=Disproportionate frequency representation in the inferior colliculus of Doppler-compensating greater horseshoe bats: Evidence of an acoustic fovea | journal=Journal of Comparative Physiology A |volume=132 |issue=1 | pages=47β54 |doi=10.1007/bf00617731 | s2cid=7176515 |url=https://epub.ub.uni-muenchen.de/3159/ |url-access=subscription }} </ref> Echolocating bats have cochlear hairs that are especially resistant to intense noise. Cochlear hair cells are essential for hearing sensitivity, and can be damaged by intense noise. As bats are regularly exposed to intense noise through echolocation, resistance to degradation by intense noise is necessary.<ref>{{Cite journal |last1=Liu |first1=Zhen |last2=Chen |first2=Peng |last3=Li |first3=Yuan-Yuan |last4=Li |first4=Meng-Wen |last5=Liu |first5=Qi |last6=Pan |first6=Wen-Lu |last7=Xu |first7=Dong-Ming |last8=Bai |first8=Jing |last9=Zhang |first9=Li-Biao |last10=Tang |first10=Jie |last11=Shi |first11=Peng |display-authors=3 |date=November 2021 |title=Cochlear hair cells of echolocating bats are immune to intense noise |url=https://linkinghub.elsevier.com/retrieve/pii/S1673852721001910 |journal=Journal of Genetics and Genomics |volume=48 |issue=11 |pages=984β993 |doi=10.1016/j.jgg.2021.06.007 |pmid=34393089 |s2cid=237094069 |url-access=subscription }}</ref> Further along the auditory pathway, the movement of the basilar membrane results in the stimulation of primary auditory neurons. Many of these neurons are specifically "tuned" (respond most strongly) to the narrow frequency range of returning echoes of CF calls. Because of the large size of the acoustic fovea, the number of neurons responding to this region, and thus to the echo frequency, is especially high.<ref name="Carew_2001">{{cite book |last=Carew |first=T. |date=2004 |orig-year=2001 |title=Behavioral Neurobiology: The Cellular Organization of Natural Behavior |publisher=Oxford University Press |isbn=978-0-8789-3084-5 |chapter=<!--Part II: Sensory Worlds: -->Echolocation in Bats}}</ref> ===== Inferior colliculus ===== In the [[Inferior colliculus]], a structure in the bat's midbrain, information from lower in the auditory processing pathway is integrated and sent on to the auditory cortex. As George Pollak and others showed in a series of papers in 1977, the [[interneurons]] in this region have a very high level of sensitivity to time differences, since the time delay between a call and the returning echo tells the bat its distance from the target object. While most neurons respond more quickly to stronger stimuli, collicular neurons maintain their timing accuracy even as signal intensity changes.<ref name="Pollak Marsh 1977"/> These interneurons are specialized for time sensitivity in several ways. First, when activated, they generally respond with only one or two [[action potential]]s. This short duration of response allows their action potentials to give a specific indication of the moment when the stimulus arrived, and to respond accurately to stimuli that occur close in time to one another. The neurons have a very low threshold of activation β they respond quickly even to weak stimuli. Finally, for FM signals, each interneuron is tuned to a specific frequency within the sweep, as well as to that same frequency in the following echo. There is specialization for the CF component of the call at this level as well. The high proportion of neurons responding to the frequency of the acoustic fovea actually increases at this level.<!--<ref name="Carew_2001"/>--><ref name="Pollak Marsh 1977">{{cite journal |last1=Pollak |first1=George |last2=Marsh |first2=David |last3=Bodenhamer |first3=Robert |last4=Souther |first4=Arthur |title=Echo-detecting characteristics of neurons in inferior colliculus of unanesthetized bats |journal=Science |volume=196 |issue=4290 |pages=675β678 |date=May 1977 |pmid=857318 |doi=10.1126/science.857318 |bibcode=1977Sci...196..675P }}</ref> ===== Auditory cortex ===== The [[auditory cortex]] in bats is quite large in comparison with other mammals.<ref>{{cite journal |last1=Kanwal |first1=Jagmeet S. |last2=Rauschecker |first2=J. P. |title=Auditory cortex of bats and primates: managing species-specific calls for social communication |journal=Frontiers in Bioscience |volume=12 |issue=8β12 |pages=4621β4640 |date=May 2007 |pmid=17485400 |pmc=4276140 |doi=10.2741/2413 }}</ref> Various characteristics of sound are processed by different regions of the cortex, each providing different information about the location or movement of a target object. Most of the existing studies on information processing in the auditory cortex of the bat have been done by [[Nobuo Suga]] on the mustached bat, ''[[Pteronotus parnellii]]''. This bat's call has both CF tone and FM sweep components.<ref name="Suga 1975"/><ref name="Suga 1987"/> Suga and his colleagues have shown that the cortex contains a series of "maps" of auditory information, each of which is organized systematically based on characteristics of sound such as [[frequency]] and [[amplitude]]. The neurons in these areas respond only to a specific combination of frequency and timing (sound-echo delay), and are known as combination-sensitive neurons.<ref name="Suga 1975"/><ref name="Suga 1987"/> The systematically organized maps in the auditory cortex respond to various aspects of the echo signal, such as its delay and its velocity. These regions are composed of "combination sensitive" neurons that require at least two specific stimuli to elicit a response. The neurons vary systematically across the maps, which are organized by acoustic features of the sound and can be two dimensional. The different features of the call and its echo are used by the bat to determine important characteristics of their prey. The maps include:<ref name="Suga 1975"/><ref name="Suga 1987"/> [[File:Bat Auditory Cortex.svg|thumb|upright=1.2|Auditory cortex of a bat {{legend |#BA54C0 |FM-FM area |text=A |textcolor=white}} {{legend |#4190B9 |CF-CF area |text=B |textcolor=white}} {{legend |#4AB8B6 |Amplitude-sensitive area |text=C |textcolor=white}} {{legend |#DC7A5B |Frequency-sensitive area |text=D |textcolor=white}} {{legend |#90C675 |DSCF area |text=E |textcolor=white}}]] *'''FM-FM area''': This region of the cortex contains FM-FM combination-sensitive neurons. These cells respond only to the combination of two FM sweeps: a call and its echo. The neurons in the FM-FM region are often referred to as "delay-tuned", since each responds to a specific time delay between the original call and the echo, in order to find the distance from the target object (the range). Each neuron also shows specificity for one harmonic in the original call and a different harmonic in the echo. The neurons within the FM-FM area of the cortex of ''Pteronotus'' are organized into columns, in which the delay time is constant vertically but increases across the horizontal plane. The result is that range is encoded by location on the cortex, and increases systematically across the FM-FM area.<!--<ref name="Neuweiler_2003"/><ref name="Carew_2001"/>--><ref name="Suga 1975">{{cite journal |last1=Suga |first1=N. |last2=Simmons |first2=J. A. |last3=Jen |first3=P. H. |year=1975 |title=Peripheral specialization for fine analysis of doppler-shifted echoes in the auditory system of the "CF-FM" bat Pteronotus parnellii | journal=Journal of Experimental Biology |volume=63 |issue=1 | pages=161β192 |doi=10.1242/jeb.63.1.161 |pmid=1159359 }}</ref><ref>{{cite journal |last1=Suga |first1=N. |last2=O'Neill |first2=W. E. |s2cid=11840108 |title=Neural axis representing target range in the auditory cortex of the mustache bat |journal=Science |volume=206 |issue=4416 |pages=351β353 |date=October 1979 |pmid=482944 |doi=10.1126/science.482944 |bibcode=1979Sci...206..351S }} </ref> *'''CF-CF area''': Another kind of combination-sensitive neuron is the CF-CF neuron. These respond best to the combination of a CF call containing two given frequencies β a call at 30 kHz (CF1) and one of its additional [[harmonics]] around 60 or 90 kHz (CF2 or CF3) β and the corresponding echoes. Thus, within the CF-CF region, the changes in echo frequency caused by the [[Doppler shift]] can be compared to the frequency of the original call to calculate the bat's velocity relative to its target object. As in the FM-FM area, information is encoded by its location within the map-like organization of the region. The CF-CF area is first split into the distinct CF1-CF2 and CF1-CF3 areas. Within each area, the CF1 frequency is organized on an axis, perpendicular to the CF2 or CF3 frequency axis. In the resulting grid, each neuron codes for a certain combination of frequencies that is indicative of a specific velocity<ref name="Carew_2001"/><ref name="Suga 1975"/><ref name="Suga 1987">{{cite journal |last1=Suga |first1=N. |last2=Niwa |first2=H. |last3=Taniguchi |first3=I. |last4=Margoliash |first4=D. |s2cid=18390219 |title=The personalized auditory cortex of the mustached bat: adaptation for echolocation |journal=Journal of Neurophysiology |volume=58 |issue=4 |pages=643β654 |date=October 1987 |pmid=3681389 |doi=10.1152/jn.1987.58.4.643 }}</ref> *'''Doppler shifted constant frequency (DSCF) area''': This large section of the cortex is a map of the acoustic fovea, organized by frequency and by amplitude. Neurons in this region respond to CF signals that have been Doppler shifted (in other words, echoes only) and are within the same narrow frequency range to which the acoustic fovea responds. For ''Pteronotus'', this is around 61 kHz. This area is organized into columns, which are arranged radially based on frequency. Within a column, each neuron responds to a specific combination of frequency and amplitude. This brain region is necessary for frequency discrimination.<ref name="Carew_2001"/><ref name="Suga 1975"/><ref name="Suga 1987"/>
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