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Comparison of auditory stream segregation in sighted and early blind individuals
Accepted Manuscript Title: Comparison of auditory stream segregation in sighted and early blind individuals Author: Fatemeh Moghadasi Boroujeni Masoumeh Rouzbahani Fatemeh Heidari Mohammad Kamali PII: DOI: Reference:
S0304-3940(16)30957-0 http://dx.doi.org/doi:10.1016/j.neulet.2016.12.022 NSL 32491
To appear in:
Neuroscience Letters
Received date: Revised date: Accepted date:
31-7-2016 4-12-2016 12-12-2016
Please cite this article as: Fatemeh Moghadasi Boroujeni, Masoumeh Rouzbahani, Fatemeh Heidari, Mohammad Kamali, Comparison of auditory stream segregation in sighted and early blind individuals, Neuroscience Letters http://dx.doi.org/10.1016/j.neulet.2016.12.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparison of Auditory Stream Segregation in Sighted and Early Blind Individuals
Fatemeh Moghadasi Boroujeni, Department of Audiology, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran. Department of Audiology, Faculty of Rehabilitation, Isfahan University of Medical Sciences, Isfahan, Iran. Masoumeh Rouzbahani, Department of Audiology, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran. Fatemeh Heidari, Department of Audiology, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran. Mohammad Kamali, Department of Basic Sciences in Rehabilitations, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran.
Highlights
Early blindness leads to lower fission boundary (FB) threshold.
Basis of ERBs number, the FB threshold is independent of the frequency of the tone A.
Visual deprivation can increase auditory stream segregation (ASS) capability.
Abstract An important characteristic of the auditory system is the capacity to analyze complex sounds and make decisions on the source of the constituent parts of these sounds. Blind individuals compensate for the lack of visual information by an increase input from other sensory modalities, 1
including increased auditory information. The purpose of the current study was to compare the fission boundary (FB) threshold of sighted and early blind individuals through spectral aspects using a psychoacoustic auditory stream segregation (ASS) test. This study was conducted on 16 sighted and 16 early blind adult individuals. The applied stimuli were presented sequentially as the pure tones A and B and as a triplet ABA-ABA pattern at the intensity of 40 dBSL. The A tone frequency was selected as the basis at values of 500, 1000, and 2000Hz. The B tone was presented with the difference of a 4-100 percent above the basis tone frequency. Blind individuals had significantly lower FB thresholds than sighted people. FB was independent of the frequency of the tone A when expressed as the difference in the number of equivalent rectangular bandwidths (ERBs). Early blindness may increase perceptual separation of the acoustic stimuli to form accurate representations of the world.
Keywords: Blindness; Auditory stream segregation; Fission boundary threshold; Plasticity; Equivalent rectangular bandwidths; Compensation
1. Introduction Human beings experience different auditory environments throughout their lives. In each auditory environment, an individual simultaneously or sequentially receives various sounds from different sources. The individual has to differentiate sounds received from different sources to attain adequate auditory and spatial understanding. The auditory system separates and groups components of different sounds and makes decisions about their sources. In auditory science, this phenomenon is referred to as auditory scene analysis[1-5].
2
Auditory scene analysis involves perceptive organization of the sound simultaneously or succssively. The ability to organize sounds sequentially is called auditory stream segregation (ASS)[6, 7]. Defects in ASS include difficulties in identifying the direction from which a sound originates, perceiving melodies, and others[8-13]. The fission boundary (FB) threshold can be used to assess ASS. Carlyon (2004)[14] stated that primary auditory cortex contributes to the organization of streaming. Moreover, he points out that the role of non-auditory areas in streaming and attention also affects streaming. Streaming develops as a result of both bottom-up and top-down processes[15]. For an individual, an accurate perception of the surrounding environment and of the sound sources in the environment is essential, particularly when loss of a sense, such as vision, is involved. Blind individuals depend on other senses, such as hearing, to understand the environment. Previous studies indicate that blind individuals possess better hearing than sighted individuals [16-25]. Blind individuals compensate for their vision deprivation by using their other senses more[22, 26]. Even short-term visual deprivation in sighted individuals can improve frequency discrimination and sound location[23]. Blind individuals have better auditory capabilities in terms of orientation[17], auditory attention[18], discovery of peripheral sounds[19], acoustic stimulus frequency discrimination [16], judgement in auditory stimulus sequences [21], and verbal skills [20]. In a brain imaging study, auditory cortical activity was greater in early blind individuals than in sighted people [27]. Visual regions in blind people, including occipital cortex, are active during proccessing auditory stimuli [26, 28, 29]. However, most studies suggest neural plasticity in early blind individuals; but not all of them confirm the superior auditory skills of
3
blind individuals. For example, Wan et al. (2010)[23] reported that blind individuals have better perceptive skills than sighted people, although this is not the case in higher-level processing skills, such as memorization. Since blind individuals have a better tonotopic map than sighted individuals[30], we hypothesized that they have better ASS capability than sighted people. To our knowledge, this is the first report of ASS using spectral signs in early blind individuals. This study aimed to compare FB threshold in sighted and blind individuals via spectral aspects using an ASS as psychoacoustic and behavioral test. 2. Materials and Methods 2.1. Participants This comparative, cross-sectional study icluded 32 right-handed adults (aged: 18-35 years) with normal hearing sensitivity. Participation was voluntary, and all participants provided written informed consent. Control group contained 16 sighted individuals (eight males and eight females). Blind group included 16 early blind individuals who were age- and gender- matched with the control group. Also, all blind subjects had no reactivity to light, and the age of blindness onset was under 6 months after birth (11/16 were congenital blind). Pure-tone audiometric threshold and tympanometry were meaured for two groups. All subjects had a normal hearing thresold (15dBHL or better at octave frequencies from 250 to 8000 Hz in both ears) and normal type A tympanometry findings[31]. 2.2. ASS Measurement Stimuli were as the pure tones A and B sequentially and in the triplet pattern of ABA-ABA. The "-" indicates the silence or the interval between triplets, which was set at 100-ms (Fig. 1). The
4
duration of stimuli A and B was 100-ms (the 10-ms \rise and fall\ time and the time interval between them in each triplet was 20-ms) [32, 33]. The amplitude of A and B stimuli was considered equal. Their intensity level was also considered comparable owing to the effect of the increase in intensity on the FB value[33]; the stimuli were presented binaurally at the intensity of 40 dBSL to minimize the effect of the intensity level on the results of the study. The A tone frequency with the values of 500, 1000,and 2000 Hz was selected as the basis in the sequence of the presented triplets. The B tone was presented with the difference of 4 to 100 percent above the basis tone frequency. Each trail contains five triplets[35]. At the end of each trail, the individuals were asked whether they heard the presented sounds as a fluctuational rhythm or perceived them as two separate sounds (AA…and B-B…). We instructed participants to raised the right hand if they heard the stimulus as a continuous sound and the left hand if they perceived it as two separate sounds. A five-minute-long break was intended between in order to enhance the test validity. If the subject feels exhausted, the test was discontinued and deferred to the next day. Herein FB was measured for each frequency independently. Thus, FB is considered the minimum frequency difference (ΔF) between A and B frequencies at which an individual can recognize the presented stimuli as two streams[15].Then, the FB thresholds were calculated as difference between the number of equivalent rectangular bandwidths (ERBs) or E using the following formula: E=21.4 log(4.37F+1)[33]. The scale E is a logarithmic scale of frequency for matching the internal representation of the sound[34]. When the difference between the center frequencies of consecutive tones in FB is expressed in terms of E, ΔE remains almost constant at different frequencies. The prediction is based on the fact that when ΔE has been fixed, the overlap between stimulation patterns of the two tones remains virtually unchanged.
5
Test stimuli were constructed using MATLAB10[35]. Stimuli were played on a DVD player, and presented to participants through headphones. All sounds were calibrated before presentation. 2.3.Statistical analysis Mean and standard deviation (SD) were obtained for all data. Repeated measure analyses of variance (RM-ANOVAs) were performed to compare FB thresholds in two groups at three tested frequencies. P0.05 was considered statistically significant. 3.Results Herein, the FB thresholds were measured for sighted and early blindind ividuals in the frequency range 500, 1000, and 2000 Hz in terms of ΔE. Fig.2 indicates that FB thresholds in the early blind group were lower than that in the sighted group for three frequencies. RM-ANOVAs showed that this discrepancy was statistically significant (P0.001) implying that early blind individuals required a smaller frequency difference (ΔF) between the pure tones A and B for ASS. Besides, the separation between the tones at the FB was independent of the frequency of the tone A when expressed as the difference in ERB number between A and B (P= 0.451). 4.Discussion This study aimed to compare auditory stream segregation in sighted and early blind individuals using the ASS psychoacoustic test. The results of the present study revealed that FB thresholds for all tested frequencies differed significantly between two groups. Early blind individuals had lower FB thresholds than sighted individuals. The FB thresholds were independent of the tone frequency for A, when ΔF between the sounds A and B was expressed in terms of ΔE. FB threshold probably depends on frequencyplace map in the cochlea and auditory filters. It may not correspond to a point where consecutive tones are equally apart at the basilar membrane. This can explain why ΔE is almost constant for
6
people with normal hearing at different frequencies. The scale properly matches the distance scale along the basilar membrane for normal individuals[36]. This is consistent with the model proposed by Beauvois and Meddis, which predicts that in calculating FB, the difference between the frequencies of consecutive tones is expressed in terms of E; ΔE remains almost constant at different frequencies[34]. Possible reasons forlower FB threshold in early blind group based on cochlear and neural pathways participating in the auditory streaming areas follows: There are two hypothesis regarding ASS. The first hypothesis which deals with bottom-up pathways stating that the auditory system performs streaming based on signs in the acoustic stimulus is primitive[15]. Scholes et al.(2015) indicated that the auditory system was able to segregate auditory streams as a result of the cochlear frequency tuning formed for each stimulus based on its characteristics. When characteristics of the two stimuli are highly similar owing to an overlap in the frequency tunings, the stimuli are heard as a single stream[37]. If frequency tuning is more acute and frequency selectivity is stronger, two streams will be heard with the lower frequency difference according to the primitive auditory streaming hypothesis. The second hypothesis regarding ASS is the schema-based hypothesis, which deals with topdown pathways. This hypothesis, which assumes higher brain development, states that the segregation of certain sounds, such as familiar words, melodies, and peers’ sounds, occurs via learning. This segregation may be attentional ( e.g., when we attempt to hear a familiar voice in a noisy room) or pre-attentional ( e.g., when our attention is drawn to a voice calling our name among background sounds)[15]. Most neurons in the auditory system above the auditory nerve have frequency selectivity. This simple fact shows that some aspects of streaming can be seen at any point in the neural pathway_
7
from the auditory nerve to the A1 area_ that has frequency selectivity. When the frequency difference between the tones A and B is small, the neuron responds to both tones A and B. As a result, one fluctuational rhythm is heard. With an increase in the frequency difference, the neuron responds only to the A tone[14]. Auditory streaming can be strongly influenced by attention. Schema-based processing of auditory streaming is both attentional and pre-attentional. Previous studies indicate that blind individuals have better auditory attention[18] and divided bimodal attention[38] compared to sighted individuals. Studies conducted on blind individuals indicate that they show better performance in frequency discrimination tasks[16, 23].Wan et al. (2010)[23] stated that congenitally blind individuals performed better than sighted individuals when thefrequency difference between two tones was as small as 0.5-1 percent. Thus, they are better at discovering of frequencies despite impaired or absent vision at birth. Both auditory and non-auditory areas contribute to the processing of auditory streaming. Activity in three intra-parital sulcus(IPS) areas is related to the perception of two auditory streams[39]. IPS may have a common perceptive organization in some supra-modal aspects as streaming changes with time according to the sound sequence and IPS activity changes during the same sequence, depending on whether the person hears one or two streams at time [14]. Evidence of extrasensory or intersensory plasticity has been observed in the brains of blind individuals[23, 26]. Taken together, early blind subjects had lower FB thereshold probably due to changes in the bottom-up and top-down auditory pathways resulting from prolonged deprivation of vision and more use of auditory system. These factors could be the possible explanations of the superior performance of early blind individuals in ASS: more accurate frequency tuning curves and the
8
stronger tonotopic map lead to better frequency selectivity, reinforced auditory attention, and participation of non-auditory cortex areas in auditory processing as a result of neural plasticity subsequent to compensate early loss of vision. However, the present behavioral study cannot distinguish among different alternatives. Conclusion The results of the present study indicated that early blind individuals have lower FB thresholds than sighted ones at the three tested frequencies, and the FB thresholds at different frequencies based on E were identical. Therefore, early blind individuals have better ability to segregate auditory streams in comparison to sighted people. It seems that frequency selectivity in these subjects is more accurate. This capability is highly practical in blind individuals, and can be used for better identification of incoming acoustic information in order to compensation of visual deprivation and designing alternative sensory rehabilitation approaches. Acknowledgement This paper is an extract from Fatemeh Moghadasi Borujeni’s MSc thesis. It has been supported by Iran University of Medical Sciences grant(IR.IUMS.REC.1394.9211301202). The authors arealso grateful to all subjects especially blind individuals who participated in the study. No conflict of interest was present.
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A.S.Bregman. Auditory Scene Analysis A2 - Squire, Larry R, in Encyclopedia of Neuroscience. (2009), Academic Press: Oxford. 729-736.
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F.Gougoux, , et al. Neuropsychology: pitch discrimination in the early blind. Nat. 430(2004) 309-309.
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N.Lessard, et al. Early-blind human subjects localize sound sources better than sighted subjects. Nat. 395(1998) 278-280.
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M.Liotti, K. Ryder, and M.G. Woldorff, Auditory attention in the congenitally blind: where, when and what gets reorganized?. Neuroreport.9(1998) 1007-1012.
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B.Röder, F. Rösler. Memory for environmental sounds in sighted, congenitally blind and late blind adults: evidence for cross-modal compensation. Int. J. Psychophysiol.50(2003) 27-39.
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A. Rokem, M. Ahissar. Interactions of cognitive and auditory abilities in congenitally blind individuals. Neuropsychol.47(2009) 843-848.
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Z.Jafari, S. Malayeri. Effects of congenital blindness on the subcortical representation of speech cues. Neurosci.258(2014) 401-409.
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13
Fig. 1. Schematic representation of stimuli pattern to ASS measurement (ΔF: frequency difference between the pure tones A and B)[35].
Fig.2. FB thresholds in adult sighted and blind individuals at frequencies of 500, 1000, and 2000 Hz in terms of E. The minimum frequency difference between A and B frequencies at which an individual can recognize the presented stimuli as two streams in the blind group was smaller than in sighted individuals (P0.001).Error bars indicate one standard deviation (SD). 1.2 1
FB. ΔE
0.8 0.6
Sighted
0.4
Blind
0.2 0 500
1000
Frequency (in Hz)
14
2000
S0304-3940(16)30957-0 http://dx.doi.org/doi:10.1016/j.neulet.2016.12.022 NSL 32491
To appear in:
Neuroscience Letters
Received date: Revised date: Accepted date:
31-7-2016 4-12-2016 12-12-2016
Please cite this article as: Fatemeh Moghadasi Boroujeni, Masoumeh Rouzbahani, Fatemeh Heidari, Mohammad Kamali, Comparison of auditory stream segregation in sighted and early blind individuals, Neuroscience Letters http://dx.doi.org/10.1016/j.neulet.2016.12.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparison of Auditory Stream Segregation in Sighted and Early Blind Individuals
Fatemeh Moghadasi Boroujeni, Department of Audiology, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran. Department of Audiology, Faculty of Rehabilitation, Isfahan University of Medical Sciences, Isfahan, Iran. Masoumeh Rouzbahani, Department of Audiology, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran. Fatemeh Heidari, Department of Audiology, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran. Mohammad Kamali, Department of Basic Sciences in Rehabilitations, School of Rehabilitation Sciences, Iran University of Medical Sciences, Tehran, Iran.
Highlights
Early blindness leads to lower fission boundary (FB) threshold.
Basis of ERBs number, the FB threshold is independent of the frequency of the tone A.
Visual deprivation can increase auditory stream segregation (ASS) capability.
Abstract An important characteristic of the auditory system is the capacity to analyze complex sounds and make decisions on the source of the constituent parts of these sounds. Blind individuals compensate for the lack of visual information by an increase input from other sensory modalities, 1
including increased auditory information. The purpose of the current study was to compare the fission boundary (FB) threshold of sighted and early blind individuals through spectral aspects using a psychoacoustic auditory stream segregation (ASS) test. This study was conducted on 16 sighted and 16 early blind adult individuals. The applied stimuli were presented sequentially as the pure tones A and B and as a triplet ABA-ABA pattern at the intensity of 40 dBSL. The A tone frequency was selected as the basis at values of 500, 1000, and 2000Hz. The B tone was presented with the difference of a 4-100 percent above the basis tone frequency. Blind individuals had significantly lower FB thresholds than sighted people. FB was independent of the frequency of the tone A when expressed as the difference in the number of equivalent rectangular bandwidths (ERBs). Early blindness may increase perceptual separation of the acoustic stimuli to form accurate representations of the world.
Keywords: Blindness; Auditory stream segregation; Fission boundary threshold; Plasticity; Equivalent rectangular bandwidths; Compensation
1. Introduction Human beings experience different auditory environments throughout their lives. In each auditory environment, an individual simultaneously or sequentially receives various sounds from different sources. The individual has to differentiate sounds received from different sources to attain adequate auditory and spatial understanding. The auditory system separates and groups components of different sounds and makes decisions about their sources. In auditory science, this phenomenon is referred to as auditory scene analysis[1-5].
2
Auditory scene analysis involves perceptive organization of the sound simultaneously or succssively. The ability to organize sounds sequentially is called auditory stream segregation (ASS)[6, 7]. Defects in ASS include difficulties in identifying the direction from which a sound originates, perceiving melodies, and others[8-13]. The fission boundary (FB) threshold can be used to assess ASS. Carlyon (2004)[14] stated that primary auditory cortex contributes to the organization of streaming. Moreover, he points out that the role of non-auditory areas in streaming and attention also affects streaming. Streaming develops as a result of both bottom-up and top-down processes[15]. For an individual, an accurate perception of the surrounding environment and of the sound sources in the environment is essential, particularly when loss of a sense, such as vision, is involved. Blind individuals depend on other senses, such as hearing, to understand the environment. Previous studies indicate that blind individuals possess better hearing than sighted individuals [16-25]. Blind individuals compensate for their vision deprivation by using their other senses more[22, 26]. Even short-term visual deprivation in sighted individuals can improve frequency discrimination and sound location[23]. Blind individuals have better auditory capabilities in terms of orientation[17], auditory attention[18], discovery of peripheral sounds[19], acoustic stimulus frequency discrimination [16], judgement in auditory stimulus sequences [21], and verbal skills [20]. In a brain imaging study, auditory cortical activity was greater in early blind individuals than in sighted people [27]. Visual regions in blind people, including occipital cortex, are active during proccessing auditory stimuli [26, 28, 29]. However, most studies suggest neural plasticity in early blind individuals; but not all of them confirm the superior auditory skills of
3
blind individuals. For example, Wan et al. (2010)[23] reported that blind individuals have better perceptive skills than sighted people, although this is not the case in higher-level processing skills, such as memorization. Since blind individuals have a better tonotopic map than sighted individuals[30], we hypothesized that they have better ASS capability than sighted people. To our knowledge, this is the first report of ASS using spectral signs in early blind individuals. This study aimed to compare FB threshold in sighted and blind individuals via spectral aspects using an ASS as psychoacoustic and behavioral test. 2. Materials and Methods 2.1. Participants This comparative, cross-sectional study icluded 32 right-handed adults (aged: 18-35 years) with normal hearing sensitivity. Participation was voluntary, and all participants provided written informed consent. Control group contained 16 sighted individuals (eight males and eight females). Blind group included 16 early blind individuals who were age- and gender- matched with the control group. Also, all blind subjects had no reactivity to light, and the age of blindness onset was under 6 months after birth (11/16 were congenital blind). Pure-tone audiometric threshold and tympanometry were meaured for two groups. All subjects had a normal hearing thresold (15dBHL or better at octave frequencies from 250 to 8000 Hz in both ears) and normal type A tympanometry findings[31]. 2.2. ASS Measurement Stimuli were as the pure tones A and B sequentially and in the triplet pattern of ABA-ABA. The "-" indicates the silence or the interval between triplets, which was set at 100-ms (Fig. 1). The
4
duration of stimuli A and B was 100-ms (the 10-ms \rise and fall\ time and the time interval between them in each triplet was 20-ms) [32, 33]. The amplitude of A and B stimuli was considered equal. Their intensity level was also considered comparable owing to the effect of the increase in intensity on the FB value[33]; the stimuli were presented binaurally at the intensity of 40 dBSL to minimize the effect of the intensity level on the results of the study. The A tone frequency with the values of 500, 1000,and 2000 Hz was selected as the basis in the sequence of the presented triplets. The B tone was presented with the difference of 4 to 100 percent above the basis tone frequency. Each trail contains five triplets[35]. At the end of each trail, the individuals were asked whether they heard the presented sounds as a fluctuational rhythm or perceived them as two separate sounds (AA…and B-B…). We instructed participants to raised the right hand if they heard the stimulus as a continuous sound and the left hand if they perceived it as two separate sounds. A five-minute-long break was intended between in order to enhance the test validity. If the subject feels exhausted, the test was discontinued and deferred to the next day. Herein FB was measured for each frequency independently. Thus, FB is considered the minimum frequency difference (ΔF) between A and B frequencies at which an individual can recognize the presented stimuli as two streams[15].Then, the FB thresholds were calculated as difference between the number of equivalent rectangular bandwidths (ERBs) or E using the following formula: E=21.4 log(4.37F+1)[33]. The scale E is a logarithmic scale of frequency for matching the internal representation of the sound[34]. When the difference between the center frequencies of consecutive tones in FB is expressed in terms of E, ΔE remains almost constant at different frequencies. The prediction is based on the fact that when ΔE has been fixed, the overlap between stimulation patterns of the two tones remains virtually unchanged.
5
Test stimuli were constructed using MATLAB10[35]. Stimuli were played on a DVD player, and presented to participants through headphones. All sounds were calibrated before presentation. 2.3.Statistical analysis Mean and standard deviation (SD) were obtained for all data. Repeated measure analyses of variance (RM-ANOVAs) were performed to compare FB thresholds in two groups at three tested frequencies. P0.05 was considered statistically significant. 3.Results Herein, the FB thresholds were measured for sighted and early blindind ividuals in the frequency range 500, 1000, and 2000 Hz in terms of ΔE. Fig.2 indicates that FB thresholds in the early blind group were lower than that in the sighted group for three frequencies. RM-ANOVAs showed that this discrepancy was statistically significant (P0.001) implying that early blind individuals required a smaller frequency difference (ΔF) between the pure tones A and B for ASS. Besides, the separation between the tones at the FB was independent of the frequency of the tone A when expressed as the difference in ERB number between A and B (P= 0.451). 4.Discussion This study aimed to compare auditory stream segregation in sighted and early blind individuals using the ASS psychoacoustic test. The results of the present study revealed that FB thresholds for all tested frequencies differed significantly between two groups. Early blind individuals had lower FB thresholds than sighted individuals. The FB thresholds were independent of the tone frequency for A, when ΔF between the sounds A and B was expressed in terms of ΔE. FB threshold probably depends on frequencyplace map in the cochlea and auditory filters. It may not correspond to a point where consecutive tones are equally apart at the basilar membrane. This can explain why ΔE is almost constant for
6
people with normal hearing at different frequencies. The scale properly matches the distance scale along the basilar membrane for normal individuals[36]. This is consistent with the model proposed by Beauvois and Meddis, which predicts that in calculating FB, the difference between the frequencies of consecutive tones is expressed in terms of E; ΔE remains almost constant at different frequencies[34]. Possible reasons forlower FB threshold in early blind group based on cochlear and neural pathways participating in the auditory streaming areas follows: There are two hypothesis regarding ASS. The first hypothesis which deals with bottom-up pathways stating that the auditory system performs streaming based on signs in the acoustic stimulus is primitive[15]. Scholes et al.(2015) indicated that the auditory system was able to segregate auditory streams as a result of the cochlear frequency tuning formed for each stimulus based on its characteristics. When characteristics of the two stimuli are highly similar owing to an overlap in the frequency tunings, the stimuli are heard as a single stream[37]. If frequency tuning is more acute and frequency selectivity is stronger, two streams will be heard with the lower frequency difference according to the primitive auditory streaming hypothesis. The second hypothesis regarding ASS is the schema-based hypothesis, which deals with topdown pathways. This hypothesis, which assumes higher brain development, states that the segregation of certain sounds, such as familiar words, melodies, and peers’ sounds, occurs via learning. This segregation may be attentional ( e.g., when we attempt to hear a familiar voice in a noisy room) or pre-attentional ( e.g., when our attention is drawn to a voice calling our name among background sounds)[15]. Most neurons in the auditory system above the auditory nerve have frequency selectivity. This simple fact shows that some aspects of streaming can be seen at any point in the neural pathway_
7
from the auditory nerve to the A1 area_ that has frequency selectivity. When the frequency difference between the tones A and B is small, the neuron responds to both tones A and B. As a result, one fluctuational rhythm is heard. With an increase in the frequency difference, the neuron responds only to the A tone[14]. Auditory streaming can be strongly influenced by attention. Schema-based processing of auditory streaming is both attentional and pre-attentional. Previous studies indicate that blind individuals have better auditory attention[18] and divided bimodal attention[38] compared to sighted individuals. Studies conducted on blind individuals indicate that they show better performance in frequency discrimination tasks[16, 23].Wan et al. (2010)[23] stated that congenitally blind individuals performed better than sighted individuals when thefrequency difference between two tones was as small as 0.5-1 percent. Thus, they are better at discovering of frequencies despite impaired or absent vision at birth. Both auditory and non-auditory areas contribute to the processing of auditory streaming. Activity in three intra-parital sulcus(IPS) areas is related to the perception of two auditory streams[39]. IPS may have a common perceptive organization in some supra-modal aspects as streaming changes with time according to the sound sequence and IPS activity changes during the same sequence, depending on whether the person hears one or two streams at time [14]. Evidence of extrasensory or intersensory plasticity has been observed in the brains of blind individuals[23, 26]. Taken together, early blind subjects had lower FB thereshold probably due to changes in the bottom-up and top-down auditory pathways resulting from prolonged deprivation of vision and more use of auditory system. These factors could be the possible explanations of the superior performance of early blind individuals in ASS: more accurate frequency tuning curves and the
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stronger tonotopic map lead to better frequency selectivity, reinforced auditory attention, and participation of non-auditory cortex areas in auditory processing as a result of neural plasticity subsequent to compensate early loss of vision. However, the present behavioral study cannot distinguish among different alternatives. Conclusion The results of the present study indicated that early blind individuals have lower FB thresholds than sighted ones at the three tested frequencies, and the FB thresholds at different frequencies based on E were identical. Therefore, early blind individuals have better ability to segregate auditory streams in comparison to sighted people. It seems that frequency selectivity in these subjects is more accurate. This capability is highly practical in blind individuals, and can be used for better identification of incoming acoustic information in order to compensation of visual deprivation and designing alternative sensory rehabilitation approaches. Acknowledgement This paper is an extract from Fatemeh Moghadasi Borujeni’s MSc thesis. It has been supported by Iran University of Medical Sciences grant(IR.IUMS.REC.1394.9211301202). The authors arealso grateful to all subjects especially blind individuals who participated in the study. No conflict of interest was present.
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Fig. 1. Schematic representation of stimuli pattern to ASS measurement (ΔF: frequency difference between the pure tones A and B)[35].
Fig.2. FB thresholds in adult sighted and blind individuals at frequencies of 500, 1000, and 2000 Hz in terms of E. The minimum frequency difference between A and B frequencies at which an individual can recognize the presented stimuli as two streams in the blind group was smaller than in sighted individuals (P0.001).Error bars indicate one standard deviation (SD). 1.2 1
FB. ΔE
0.8 0.6
Sighted
0.4
Blind
0.2 0 500
1000
Frequency (in Hz)
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2000