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ScienceWeek
NEUROSCIENCE: ON AUDITORY LOCALIZATION
The following points are made by R.A. Campbell and A.J. King (Current Biology 2004 14:R886):
1) An ability to localize sounds both accurately and rapidly --particularly if they occur outside the field of view -- is of obvious survival value. Because the afferent nerves from the cochlea do not convey spatial information directly, neural computations have to be performed within the brain in order to determine the direction of a sound source. This involves comparing the intensity and time of arrival of the sound at the two ears. Together with the spectral filtering imposed by the external ear, these binaural cues are responsible for auditory localization [1].
2) Psychophysical studies in humans have shown that the principal cue for sound localization in the horizontal plane, at least at low frequencies, is the interaural time difference (ITD) [2]. The maximum ITD encountered depends on the distance between the ears; in adult humans it is about 600 microseconds. Humans can discriminate ITDs as small as 10-20 microseconds [3] -- an astonishing achievement given that the duration of an action potential is two orders of magnitude greater than this.
3) Over the last few years, electrophysiological studies have indicated that the neural basis for ITD coding may vary among different species. Recent modelling data [4], however, suggest that the optimal coding strategy depends primarily on head size and the sound frequency range over which ITDs can be discriminated, rather than a more intrinsic difference between species.
4) Over 50 years ago, Lloyd Jeffress [5] proposed what has become the textbook view of how the brain computes ITDs. He suggested that ITDs could be extracted using a set of binaural coincidence detectors that respond maximally when they receive synchronous excitatory input from each ear. The idea is that different coincidence detectors are tuned to different ITDs within the physiological range -- the range determined by head size -- and therefore to different horizontal directions. This ITD map can be achieved through a series of "delay lines" produced, for example, by systematically varying the relative length of the axonal inputs to the coincidence detectors from each ear. The Jeffress model provides an alluringly simple description of how the brain calculates and represents ITDs, but remained largely hypothetical until the 1980s, when evidence was found for each aspect of the model in the nucleus laminaris of the barn owl, an accomplished auditory predator.
References (abridged):
1. King, A.J., Schnupp, J.W.H. and Doubell, T.P. (2001). The shape of ears to come: dynamic coding of auditory space. Trends Cogn. Sci. 5, 261-270
2. Wightman, F.L. and Kistler, D.J. (1992). The dominant role of low-frequency interaural time differences in sound localization. J. Acoust. Soc. Am. 91, 1648-1661
3. Yost, W.A. (1974). Discrimination of interaural phase differences. J. Acoust. Soc. Am. 55, 1299-1303
4. Harper, N.S. and McAlpine, D. (2004). Optimal neural population coding of an auditory spatial cue. Nature 430, 682-686
5. Jeffress, L.A. (1948). A place theory of sound localization. J. Comp. Physiol. Psychol. 41, 35-39
Current Biology http://www.current-biology.com
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Related Material:
NEURAL "AND" GATES IN BARN OWLS
Notes by ScienceWeek:
In this context, a "logic operation" is an operation performed on quantities (operands) that can be assigned a truth value, the value either "true" or "false". A "truth table" is a table of values that describes a particular logic operation.
An AND operation is a logic operation combining two statements in such a way that the outcome is true only if both statements are true, otherwise the outcome is false.
In this context, the term "gate" refers to a logic gate. In general, a "logic gate" is a device, usually electronic, that is used to control the flow of signals in a computer by performing logic operations on its input, with two or more inputs to the gate and only one output. The term "AND gate" refers to a logic gate whose output is high only when all inputs are high, otherwise the output is low. The AND gate thus performs the AND operation on its inputs and has the same truth table as the AND operation.
The following points are made by Charles Day (Physics Today 2001 June):
1) In a landmark paper in 1943, W. McCulloch and W. Pitts proved theoretically that a network of integrating neurons can perform any computational operation, including multiplication, and their formalism underlies artificial neural networks currently used to predict weather or stock prices. For some time, however, researchers have suspected that individual neurons can multiply. Like an AND gate, a multiplicative neuron fires only when all its neurons are positive. An additive neuron, in contrast, is more like an OR gate, firing whenever the total sum of inputs is above a certain threshold, even if some inputs are negative.
2) Pena and Konishi (Science 2001 292:249) have provided evidence for neural multiplication, uncovering the neural mechanism by which barn owls combine time-difference and intensity cues to locate sound sources. This experiment not only bolsters the case that some neurons are more than simple adding machines, but also adds a final and physiological touch to the model of Stern and Colburn (1978), which proposed to explain how humans localize sound.
3) In general, owls, humans, and other two-eared creatures locate sound sources by exploiting differences in the signals detected at each ear. A sound coming from the right, for example, will reach the right ear before the left ear, and will be less intense in the left ear because the sound has been partially absorbed by the head. The barn owl's ear openings are not at the same level, and this asymmetry heightens the barn owl's ability to localize sound, especially in the vertical dimension. Even in total darkness, an owl can find and snatch a mouse off the ground.
4) Earlier experiments of Konishi involved equipping owls with tiny loudspeakers placed in their ears, and manipulating sound arrival time and intensity differences to trick an owl into believing a sound comes from a direction chosen by the experimenter. Whenever an owl hears a sound, it turns its head to face the source. In the earlier experiments, induction coils fixed to the owl's head and coupled to an external magnetic field recorded the direction of the owl's gaze.
5) In the current experiments, owls were anesthetized, fitted with loudspeakers, and microelectrodes used to make recordings from individual space-specific neurons in the owl's nervous system. Analysis of the data revealed these neurons to be performing multiplication operations, although the cellular mechanism involved is not clear. The quantitative properties of the logic operation are similar to those derived theoretically in 1978 by Stern and Colburn.
Physics Today http://www.physicstoday.org
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Related Material:
ON THE ACCURACY OF SOUND LOCALIZATION IN AN INSECT
Notes by ScienceWeek:
Humans use at least two different strategies to localize the horizontal position of sound sources, depending on the frequencies in the stimulus. For frequencies below 3000 hertz, interaural time differences are used to localize the source; above these frequencies, interaural intensity differences are used as cues. The longest interaural time differences in humans, which are produced by sounds arising directly lateral to one ear, are on the order of only 700 microseconds (the width of the head divided by the speed of sound in air). Experiments, however, demonstrate that humans can actually detect interaural time differences as small as 10 microseconds, and this sensitivity translates into an accuracy for sound localization of approximately 1 degree.
The term "parasitoid" refers to organisms, especially insects, that introduce their eggs into another animal, the eggs hatching and larvae developing in a slow and controlled manner using the resources of the host without killing it. At maturation, the parasitoid emerges and usually does cause the death of the host.
The following points are made by A.C. Mason et al (Nature 2001 410:686):
1) The authors point out that the physics of sound propagation imposes fundamental constraints on sound localization: for a given frequency, the smaller the receiver, the smaller the available cues. Thus, the creation of nanoscale acoustic microphones with directional sensitivity is very difficult. The fly Ormia ochracea possesses an unusual "ear" that largely overcomes these physical constraints, and attempts to exploit principles derived from O. ochracea for improved hearing aids are now in progress.
2) The authors point out that O. ochracea (Diptera: Tachinidae) is a parasitoid fly, with egg-laying (gravid) female flies locating their hosts, male crickets, by homing in on the loud and persistent songs of the crickets. Because of its small body size (less than 1 centimeter in any aspect), this fly must deal with extremely small interaural difference cues to guide directional hearing. The calling song of the host cricket is an amplitude-modulated 5000 hertz tone (6.8 centimeter wavelength). The distance between the eardrums of the fly is approximately 0.5 millimeters, which means that 5 kilohertz sound waves are not diffracted by the body of the fly and generate no interaural intensity difference (indeed, none can me measured). The interaural time difference is frequency independent and depends only on the speed of sound and the distance between the two ears. The maximal interaural time difference in this fly at 90 degrees azimuth is 1.5 microseconds and decreases to zero for a sound source on the midline axis. This minuscule interaural time difference is the only physical cue available for computation of source direction. Nevertheless, this fly can reliably localize cricket song both in nature and in the laboratory.
3) The authors report experiments that demonstrate that O. ochracea can behaviorally localize a salient sound source with a precision equal to that of humans. Despite its small size and minuscule interaural cues, the fly localizes sound sources to within 2 degrees azimuth. As the eardrums of the fly are less than 0.5 millimeters apart, localization cues are of the order of 50 nanoseconds. Directional information is represented in the fly's auditory system by the relative timing of receptor responses in the two ears, and low-jitter, phasic receptor responses are pooled to achieve hyperacute time-coding.
4) The authors suggest that the principle evolutionary innovation responsible for the ability of this fly to overcome its unfavorable auditory physics is a pair of anatomically and functionally couple eardrums. The mechanical resonance of the fly's peripheral auditory apparatus in a directional sound field transforms the minuscule time delay in the free field into two cues that can used by its nervous system: a) the interaural time delay between the eardrums is increased from a maximum of 1.5 microseconds to approximately 55 microseconds; b) the vibration amplitude difference between the two eardrums is as much as 10 decibels for sound sources at 45 to 90 degrees azimuth. Thus, minute interaural time differences in the sound field are converted by eardrum mechanics to interaural differences that are process by the nervous system.
5) The authors suggest these results demonstrate that nanoscale/microscale directional microphones patterned after the fly O. ochracea have the potential for highly accurate directional sensitivity independent of the size of the microphones. In the fly itself this performance is dependent on a newly discovered set of specific coding strategies employed by the fly's nervous system.
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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