黃雪宜

Abstract： The sound we hear as speech is generated from a sound source and filtered through the vocal tract. Interestingly， human brain can process speech signals into meaningful representation and reproduce it， even the perceptual interpretation of speech differs radically by the variation of acoustic organization of speech sounds. The dual-route model of speech processing consists of auditory-conceptual stream and auditory-motor stream which was proposed by Hickok & Poeppel （2000， 2004， 2007）. The current study made a brief introduction on Dual-Stream Model， which initiated from the early cortex stages of speech processing to the final conclusion that how dual-stream works on speech perception.

Key words： Speech Perception Dual-Stream Model Language Processing

1.Introduction

Human speech is a complex phenomenon which involves with complicated acoustic pattern and adjustable articulatory organs. Air exhales from the lungs through trachea into the larynx （Adams apple）. Vocal folds are glottis are parts of the larynx. Vocal folds are stretched over the glottis and vibrate rapidly because of the pressure from the lungs （Kemmerer， 2015）. How do people transform the sound patterns of speech into a representation of the meaning and how do people produce those sound patterns with vocal tract？ Generally， perception of speech is realized by acoustic information in combination with articulatory， linguistic， semantic， and circumstantial cues. One classic model is the Dual Stream Model which is the most influential theory on speech perception contemporary. It was developed by Gregory Hickok and David Poeppel that concerning the neural substrates of speech perception. The dual-stream model including a ventral stream and a dorsal stream. The ventral stream involves with superior and middle portions of the temporal lobe structures that relates with processing speech signals. The dorsal stream contains structures in the posterior planum temporale region which is at the parietal-temporal junction， also including the posterior frontal lobe. Ventral stream is bilaterally organized with different extent of computational importance in two hemispheres. （Schirmer et al.， 2012， Hickok & Poeppel， 2015）， while the dorsal stream represents as traditionally indicated that left-dominant.

2. Early Cortex Stages of Speech Processing

Several fMRI studies showed that superior temporal regions in both hemispheres are activated by the speech stimuli. （Binder et al.， 2000， Hickok & Poepppel， 2007， Rimol et al.，2005）. Either hemisphere is capable of perceiving speech by itself， generally， only bilateral lesion may cause severe impairment of speech perception function. In the mostly cited study conducted by Binder et al. （2000）， they prepared five kinds of auditory stimulus including a “white” noise； frequency-modulated pure sine wave tone within 50-2400 Hz； monosyllabic， medium frequency， concrete English words （e.g. desk， fork）； monosyllabic pronounceable pseudo words transformed form true （e.g.， korf，sked） and reversed speech that the temporally reversed version of the word stimuli. Each stimulus grouped into one block and last 12 seconds each block. Participants were required to press a button at the beginning and the end of the block. Binder et al. recorded blood oxygenation signals of the temporal lobes by fMRI under different kinds of stimuli. The result indicates that dorsal areas surrounding Heshilgyrus （transverse temporal） bilaterally， especially the planum temporal and dorsolateral superior temporal gyrus， were more strongly activated by frequency-modulated tones than by noise， indicating they play a role in processing simple temporally encoded auditory information. Moreover， regions centered in the superior temporal sulcus bilaterally were more activated by speech stimulus than by FM tones.

3. A Double Dissociation Between Comprehension and Repetition

After the early cortical stages of speech perception have been completed， further processing is along ventral stream pathway and the dorsal stream pathway. Ventral stream pathway is the “what” pathway since it transmits into the ventral temporal cortex to represent information that essential to identify an object such as color， shape and texture. The other stream is the “how” dorsal pathway which funnels trough the superior parietal cortex to the premotor cortex. Based on previous studies， there seems a double dissociation in speech processing. The most supportive evidence was from aphasia patients. In transcortical sensory aphasia who are difficult as understanding the meaning of words and sentences while they can repeat them. In contrast， conduction aphasia patients can perform comprehension tasks quite well while failed in repletion tasks.

Kümmerer et al. （2013） had conducted a study among 100 aphasic stroke patients to identify brain regions engaged in repetition and comprehension. Patients were required to finish a task consist of 6 repetitions and 6 comprehensions， and data were obtained by Magnetic Resonance Imaging. The result in support of the dual dorsal-ventral stream pathway， the dorsal stream relates frontal premotor regions to temporoparietal through the superior longitudinal and arcuate fasciculus， then integrates the sensorimotor processing to perform activity such as repeating speeches. It supports that language is processing along two separated pathways.

4. Ventral Stream and Dorsal Stream

The ventral stream has been assumed as “what” stream， which is heavily related with lexical interface （bilateral posterior middle temporal gyrus and posterior inferior temporal gyrus） that function in mapping phonological structures onto semantic representation. It helps to detect semantic anomalies and syntactic anomalies in the processing of speech perception. In a recent study conducted by Efthymiopoulou et al. （2017） has investigated in the role of ventral stream among aphasic patients. They recruited 41 left hemisphere stroke-induced aphasic patients and required them to finish two tasks. One asked the patients to recount their own stoke incidents while the other required them to describe the picture that depicting a “cookie theft” story. CT and/or MRI were applied to obtain the data from the patients. The result of the experiments showed that speech rate in the stroke story had significantly contradictory association with total number of lesioned areas as well as lesion in the inferior frontal gyrus， while without associating with external capsule region.

The dorsal stream is considered as the “how” pathway is mainly concerned with sensorimotor interface that maps phonological structures onto moto representation. It plays an important role in articulatory network （left posterior frontal lobe） which is essential for speech production. In 2015， Murakami et al. had designed a series of experiments to investigate the effects of repetitive TMS on articulatory motor cortex excitability， CTBS effects on behavioral measures and their relationship to speech-related facilitation of articulatory motor cortex excitability， temporal evolution and curation CTBS effects and sentence repetition. They recruited 24 normal German subjects and obtain the data by fMRI. Results of those experiments showed that speech-related MEP facilitation was affected by the intervention rTMS of the posterior superior temporal sulcus， furthermore， the sylbis parieto-temporal region， while not by rTMS of the ventral speech-processing stream.

5. Conclusion

Dual-stream model concerns with the neural basis of speech perception has provided much insights on understanding speech perception. Based on the Dual Stream Model， on the one hand， ventral stream pathway mainly concerns with computational channel of comprehending speeches， on the other hand， the dorsal stream pathway subserves the construction of semantic representation of the meanings of words and sentences. moreover， the dual-stream model also offers an explanation for classic language disorders. It serves as a framework for integrating different fields researches and riches the context of cognitive researches.

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