Tonotopy

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When listening , acoustic signals are analyzed at several levels in the brain . They are the basis for subsequent decoding, especially for human understanding of language . The term tonotopy (derived from ancient Greek a)  τόνος tonos , German , “that with which one tightens something” , “string”, “rope”, “belt”; 'Tension', 'emphasis', 'force'; 'Elevation', 'sound of the voice' / b)  τόπος topos , German 'place' , 'place', 'region', 'area', 'locality', 'space') refers to the first main stage of sound analysis.

In the inner ear ( cochlea ), the mechanical vibrations coming from the outside are converted into neural impulses , anatomically ordered according to frequency ( pitch ): high frequencies at the outer end, low frequencies at the inner end. Hence the name "Ton-Ort". In the main strand of the auditory pathway in the brain, the anatomical sorting according to frequency (pitch ) is retained in several areas of the cerebrum ( auditory cortex ).

Mapping the frequency

Relationship between location on the rolled out basilar membrane in mm from the tip of the snail (left), frequency group , frequency f of the sound stimulus in kHz and perceived pitch Z in mel. Entry of the sound waves on the right.

While listening to the recorded from the outside are sound waves by the movements of the stirrup (initially via the oval window to the liquid chambers scala vestibuli ) in the cochlea ( cochlear transmitted). The wave-like propagation of a (minimal) displacement of fluid ( traveling wave ) leads to a wandering deflection of the basilar membrane , which divides the cochlea into two chambers filled with perilymph , which are connected at the tip of the snail ( helicotrema ). The organ of Corti with the hair cells is located on the basilar membrane . These are able to detect shearing of the membrane composite via the finest 'hairs'. The following applies: the stronger the deflection (a strong sound wave was the cause), the stronger the shear effect, the more often the neurons from the hair cells fire, the louder a stimulus or noise is perceived. Right here but is also a weak point of the system that evolutionary biology can not take the high noise level of the present is adjusted and can be damaged, especially at this point: injuries and loss of hairs are in mammals irreversible ( irreversible ), that is incurable.

For anatomical reasons, the system of hair cells is arranged in such a way that each audible acoustic frequency has its specific location of maximum sensitivity. The closer the location of the maximum deflection is to the oval window (here the mechanical vibrations are coupled into the hydraulic system), the higher the tone. The closer the maximum comes to the helicotrema, the lower the tone. This means that a specific frequency is assigned to each location on the basilar membrane. The frequency-location transformation takes place non-linearly (see frequency scale). The frequencies registered on the basilar membrane are mapped in the brain in the form of a ribbon up to the auditory cortex . The high frequencies up to 20,000 Hz are medially behind, the low frequencies up to 200 Hz are represented laterally in the front. So there are still areas where a certain frequency is assigned to each location.

Tonotopy is therefore also referred to as frequency-location mapping and in this respect represents a variant of somatotopics .

Conceptual differentiation from somatotopics

The tonotopy is not a point-to-point mapping of body regions. So this is not about a somatotopia like the sensorimotor cortex . In the case of tonotopy, no body parts are shown, but the range of a physical quantity (acoustic frequency).

If only the physiologically measurable registration of a certain pure frequency (monofrequency sound) at a certain point on the basilar membrane is meant and not the topically structured type of transmission to the central nervous system (CNS), then we do not speak of tonotopy , but of tone localization on the basilar membrane.

These general criteria of the figure have led to the designation “map”, see below section Tonotope maps . The term map, as it is customary in the language of networks , means the spatially-schematically ordered representation or representation of general features in the sense of similarity, frequency and importance (relevance). In the case of tonotopy, it is the continuous representation of acoustic frequencies. In English, the central nervous system is referred to as the topographic organization according to the criteria mentioned .

Frequency and pitch

Pitch is a higher order perception. Several preprocessing steps are necessary in order to extract and synthesize the information from the physical stimulus that is necessary for the representation of a perceived pitch. For electronically generated simple sinusoidal tones (monofrequency sound), the place of excitation on the basilar membrane correlates well with the perceived pitch. In nature, however, there are only complex tones (multi-frequency sound). These generate multiple excitation maxima, in which a correlation to the perceived pitch is often only weak or completely absent. Without further processing in the brain, the tonotopic information in the inner ear would be insufficient for a representation of the perceived pitch.

Tonotopic cards

Tonotopic maps have been created using neurophysiological methods. These, like other neurophysiological mappings, later also served as templates for models of self-organization in artificial neural networks ( Kohonen networks ). Such maps have also been created for animals. You have z. B. With a certain bat species there is a strong spread exactly in the area that corresponds to a narrow frequency band around 61 kHz, in which the animal is specialized for its own orientation and for hunting. Here it uses a neural analysis of the Doppler shift of the reflection of the signals transmitted by itself.

Understanding language

The tonotopic maps in the brain form the basis for the decoding of the smallest units of speech sounds ( phone ) and thus the representation of the smallest abstract sound units ( phonemes ). This is particularly clear in the case of vowels , where signals in certain frequency ranges ( formants ) enable the decoding of the vowels and the registration of personal voice characteristics of the speaker.

See also

Individual evidence

  1. ↑ Peter Duus : Neurological-topical diagnostics. Anatomy, physiology, clinic. Georg Thieme, Stuttgart 5 1990, ISBN 3-13-535805-4 , pp. 156, 367, 373.
  2. ↑ Alfred Benninghoff , Kurt Goerttler : Textbook of Human Anatomy. Shown with preference given to functional relationships. Volume 3: Nervous System, Skin and Sensory Organs. Urban and Schwarzenberg, Munich 7 1964, p. 510.
  3. ↑ Manfred Spitzer : Spirit on the Net. Models for learning, thinking and acting. Spektrum Akademischer Verlag, Heidelberg 1996, ISBN 3-8274-0109-7 , pp. 115 f., 121 (a), 231 f. (b)
  4. ^ CN Woolsey: Multiple auditory maps . tape 3 : Cortical sensory organization. Humana Press, Clifton (NJ) 1982 (English).
  5. ↑ N. Suga, et al .: Disproportionate tonotopic representation for processing CF-FM sonar signals in the mustache bat auditory cortex . In: Science . tape 194 , 1976, pp. 542-544 (English).
  6. ↑ G. Dehaene-Lambetz, et al .: Speed ​​and cerebral correlates of syllable discrimination in infants . In: Nature . tape 370 , 1994, p. 292-295 (English).