{"id":53039,"date":"2019-11-22T00:00:00","date_gmt":"2019-11-21T21:00:00","guid":{"rendered":"https:\/\/prohoster.info\/blog\/blog_prohoster\/lokalizatsiya-zvukov-kak-mozg-raspoznaet-istochniki-zvukov"},"modified":"2020-02-18T14:00:53","modified_gmt":"2020-02-18T11:00:53","slug":"lokalizatsiya-zvukov-kak-mozg-raspoznaet-istochniki-zvukov","status":"publish","type":"post","link":"https:\/\/prohoster.info\/en\/blog\/news\/lokalizatsiya-zvukov-kak-mozg-raspoznaet-istochniki-zvukov","title":{"rendered":"Sound Localization: How the Brain Recognizes Sound Sources","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

\"Sound
\n
\nThe world around us is filled with all sorts of information that our brain continuously processes. It receives this information through our senses, each responsible for its share of signals: eyes (sight), tongue (taste), nose (smell), skin (touch), vestibular apparatus (balance, spatial orientation, and sense of weight), and ears (sound). By gathering signals from all these organs, our brain can construct an accurate picture of the environment. However, not all aspects of processing external signals are known to us. One of these mysteries is the mechanism of localizing sound sources.<\/p>\n

Scientists from the Speech and Hearing Neuroengineering Laboratory (New Jersey Institute of Technology) have proposed a new model of the neural process of sound localization. What processes occur in the brain during sound perception, how does our brain understand the position of the sound source, and how can this research assist in combating hearing impairments? We will learn about this from the research group's report. Let's go.<\/a><\/noindex><\/p>\n

The foundation of the research<\/h3>\n

\nThe information that our brain receives from the senses differs from one another both in terms of the source and in terms of its processing. Some signals immediately present themselves to our brain as precise information, while others require additional computational processes. Roughly speaking, we feel touch right away, but when we hear a sound, we still need to determine where it is coming from. <\/p>\n

The basis of sound localization in the horizontal plane is interaural*<\/i> the time difference (ITD from interaural time difference<\/i>) of sounds reaching the listener's ears.<\/p>\n

Interaural base*<\/b> is the distance between the ears.<\/p><\/blockquote>\n

There is a specific area in the brain (medial superior olive or MSO) that is responsible for this process. When a sound signal is received, the MSO converts interaural time differences into the response speed of neurons. The shape of the output signal speed curves from the MSO as a function of ITD resembles the cross-correlation function of the input signals for each ear.<\/p>\n

The way information is processed and interpreted in the MBO remains unclear, leading to several rather contradictory theories. The most well-known and, in fact, classic theory of sound localization is the Jeffress model (Lloyd A. Jeffress<\/i>). It is based on the labeled line*<\/i> detector neurons that are sensitive to the binaural synchronization of neural input signals from each ear, with each neuron being maximally sensitive to a specific value of ITD (1A<\/b>).<\/p>\n

The Labeled Line Principle*<\/b> is a hypothesis explaining how different nerves, all of which use the same physiological principles to transmit impulses along their axons, can generate different sensations. Structurally similar nerves can generate various sensory perceptions if they are connected to unique neurons in the central nervous system capable of decoding similar neural signals in different ways.<\/p><\/blockquote>\n

\n\"Sound
\n Image No. 1<\/i><\/p>\n

This model is computationally similar to neural coding based on unlimited mutual correlations of sounds reaching both ears. <\/p>\n

There is also a model suggesting that sound localization can be simulated based on differences in the reaction times of certain populations of neurons from different brain hemispheres, i.e., the interhemispheric asymmetry model (1B<\/b>).<\/p>\n

It has been difficult to definitively state which of the two theories (models) is correct, considering that each predicts different dependencies of sound localization on sound intensity. <\/p>\n

In the study we are discussing today, researchers decided to combine both models to understand whether the perception of sounds is based on neural coding or the difference in the responses of individual populations of neurons. Several experiments were conducted with participants aged 18 to 27 (5 women and 7 men). The audiometry (hearing acuity measurement) of participants was 25 dB or higher at frequencies from 250 to 8000 Hz. Participants were placed in a soundproof room equipped with precision-calibrated special equipment. Upon hearing a sound signal, participants had to indicate the direction from which it originated.<\/p>\n

Research Results<\/h3>\n

\nTo assess the dependence lateralization*<\/i> of brain activity on sound intensity in response to labeled neurons, data on the reaction speed of neurons in the laminar nucleus of the owl monkey's brain were used.<\/p>\n

Laterality*<\/b> is the asymmetry of the left and right halves of the body.<\/p><\/blockquote>\n

To evaluate the dependence of lateralization of brain activity on the reaction speed of specific populations of neurons, data on the activity of the lower colliculus of the rhesus macaque's brain were used, after which differences in the speed of neurons from different hemispheres were additionally calculated.<\/p>\n

The model of labeled line detector neurons suggests that as sound intensity decreases, the laterality of the perceived source will converge around average values similar for quiet and loud sounds (1C<\/b>).<\/p>\n

The model of interhemispheric asymmetry, in turn, suggests that as sound intensity decreases to near-threshold levels, perceived laterality will shift towards the midline (1D<\/b>).<\/p>\n

At higher overall sound intensity, it is assumed that lateralization will be invariant with respect to intensity (inserted at 1C<\/b> and 1D<\/b>).<\/p>\n

Therefore, analyzing how sound intensity influences the perceived direction of sound allows for a clearer determination of the nature of the processes occurring at that moment \u2014 whether neurons from a single region or neurons from different hemispheres.<\/p>\n

It is clear that a person's ability to discern ITD may vary depending on sound intensity. However, researchers state that it is quite challenging to interpret previous findings linking sensitivity to ITD and the listener's assessment of the direction of the sound source as a function of sound intensity. Some studies suggest that as sound intensity approaches a threshold limit, the perceived laterality of the source decreases. Other studies indicate that there is no effect of intensity on perception at all.<\/p>\n

In other words, scientists are \"gently\" hinting that there is relatively little information in the literature regarding the relationship between ITD, sound intensity, and determining the direction of its source. There are theories that exist as a sort of axiom, widely accepted by the scientific community. Therefore, it was decided to thoroughly examine all theories, models, and possible mechanisms of auditory perception in practice.<\/p>\n

The first experiment was based on the use of a psychophysical paradigm, which allowed studying lateralization based on ITD as a function of sound intensity in a group of ten normally hearing participants.<\/p>\n

\"Sound
\nImage #2<\/i><\/p>\n

The sound sources were specifically tuned to cover a wide range of frequencies within which humans can recognize ITD, i.e., from 300 to 1200 Hz (2A<\/b>).<\/p>\n

In each trial, the listener was required to indicate the perceived laterality, measured as a function of sensation levels, in the range of ITD values from 375 to 375 ms. To determine the effect of sound intensity, a nonlinear mixed-effects model (NMLE) was used, which included both fixed and random sound intensity.<\/p>\n

Chart 2B<\/b> demonstrates the estimated lateralization with spectrally flat noise at two sound intensities for a representative listener. And the graph 2C<\/b> shows the raw data (circles) and fitted to the NMLE model (lines) for all listeners.<\/p>\n

\"Sound
\nTable No. 1<\/i><\/p>\n

The table above shows all parameters of NLME. It is evident that perceived laterality increased with an increase in ITD, as the scientists expected. As the intensity of sound decreased, perception shifted more towards the midline (inset in the graph 2C<\/b>).<\/p>\n

These trends were supported by the NLME model, which showed a significant effect of ITD and sound intensity on the maximum degree of laterality, confirming the model of interhemispheric differences.<\/p>\n

Moreover, there was a slight influence on perceived laterality from the average audiometric thresholds of pure tones. However, sound intensity did not significantly affect the indicators of psychometric functions.<\/p>\n

The main goal of the second experiment was to determine how the results obtained in the previous experiment would change when taking into account the spectral characteristics of stimuli (sounds). The necessity of testing spectrally flat noise at low sound intensity lies in the fact that parts of the spectrum may be inaudible, which can affect the determination of sound direction. Therefore, the results of the first experiment might mistakenly suggest that the width of the audible part of the spectrum can decrease with the reduction in sound intensity. <\/p>\n

Thus, it was decided to conduct another experiment, but this time using reverse A-weighted*<\/i> noises.<\/p>\n

A-weighting*<\/b> is applied to sound levels to account for the relative loudness perceived by the human ear, as the ear is less sensitive to low-frequency sounds. A-weighting is implemented by arithmetically adding a table of values listed in octave bands to the measured sound pressure levels in dB.<\/p><\/blockquote>\n

On the graph 2D<\/b> The unprocessed data (circles) and model-fitted NMLE data (lines) for all participants in the experiment are shown. <\/p>\n

Data analysis showed that when all parts of the sound are approximately equally audible (as in both the first and second experiments), the perceived laterality and slope on the graph explaining the change in laterality with ITD decrease with the drop in sound intensity.<\/p>\n

Thus, the results of the second experiment confirmed the results of the first. That is, it has been practically shown that the model proposed by Jeffress back in 1948 is incorrect.<\/p>\n

It turns out that sound localization worsens with the decrease in sound intensity, whereas Jeffress believed that sounds are perceived and processed by humans uniformly, regardless of their intensity. <\/p>\n

For a more detailed understanding of the nuances of the research, I recommend checking out the the scientists' report<\/a><\/noindex>.<\/p>\n

Epilogue<\/h3>\n

\nTheoretical assumptions and supporting practical experiments have shown that neurons in the brains of mammals are activated at different rates depending on the direction of the sound signal. Subsequently, the brain compares these rates among all neurons involved in the process to dynamically construct a map of the auditory environment.<\/p>\n

The Jeffress model is not completely incorrect, as it can perfectly describe sound source localization in owls. Yes, for owls, sound intensity does not matter; they will identify the source's location regardless. However, this model does not apply to rhesus macaques, as previous experiments have shown. Therefore, the Jeffress model cannot describe sound localization for all living beings. <\/p>\n

Experiments involving humans have further confirmed that sound localization occurs differently across various organisms. Many participants were unable to accurately determine the location of sound sources due to low sound intensity. <\/p>\n

Scientists believe their work shows a certain similarity between how we see and how we hear. Both processes are related to the speed of neurons in different areas of the brain, as well as the assessment of this difference to determine both the positions of the objects we see in space and the location of the sounds we hear.<\/p>\n

In the future, researchers plan to conduct a series of experiments for a more detailed examination of the connection between human hearing and vision, which will provide better insights into how our brain dynamically maps the surrounding world.<\/p>\n

Thank you for your attention, stay curious, and have a great working week, everyone! \ud83d\ude42<\/p>\n

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\n
Source: habr.com<\/a><\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"

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