By comparing the norm values of WINP test using SSN for Persian speakers aged 18 to 22 years (at the SNRs of -5 dB = 53 ± 1.93, 0 dB = 69 ± 2.22, + 5 dB = 82 ± 2.43, + 10 dB = 90 ± 1.50 and + 15 dB = 95 ± 1.44) with the norm values obtained from the WINP test using white noise for Persian speakers aged 18 to 22 years (at the SNRs of 0 dB = 53 ± 14.80, + 5 dB = 68.15 ± 13.20, and + 10 dB = 88 ± 12.60) in recent study [4] can be seen that the mean of the WINP test using SSN in both ears are higher and the standard deviations are lower.
Previous researchers have reported that the high standard deviation of WINP using white noise in their studies [3–5], may be due to the occurrence of central auditory processing disorder (CAPD) in young adults [4], elderly [3] and patients with renal failure [5]. In this study, it can be assumed that the lower limit of the mean WINP test scores using SSN (in SNRs of -5 dB = 48%, 0 dB = 44%, + 5 dB = 64%, + 10 dB = 68% and + 15 dB = 84% likely to occur as a hidden cognitive lesion that could not be detected.
Audiological tests that are used to diagnose CAPD do not have the ability to determine the site of lesions, which is due to the simultaneous participation of central auditory neural centers in different functions of auditory processing [1, 2, 6]. For example, word-in-noise discrimination, dichotic digit, and pitch pattern tests that can detect abnormalities of the auditory brain and brainstem [2]. For the diagnosis of CAPD, various tests are usually employed, and since the WINP test minimizes the participation of the brainstem in the detection of vowels, it can be one of the appropriate tests to assessment the function of the auditory brain in the detection of speech consonants and lexical-semantic processing [3–5].
In silent conditions, the left hemisphere of the auditory brain prefers verbal stimuli [6, 8]. The left frontal pole, left dorsolateral prefrontal cortex, and right posterior parietal lobe are activated in any function associated with the attention mechanism, especially speech perception [9, 10]. In the presence of noise, each hemisphere works with the participation of all areas of the cerebral cortexes [11, 12]. Two widespread lexical-semantic processing streams are described for speech-in-noise perception: Bilaterally and largely symmetrically flows, which organized in superior temporal gyrus. Dorsal stream or posterior portion of superior temporal gyrus for sensory-motor interaction, that is left dominant and involves structures at the parietal-temporal-frontal junction [13].
When the process of speech-in-noise perception, the intraparietal sulcus is activated and also the planum temporale acts as a computational hub and plays an important role in distinguishing two simultaneous speech signals. Left insula cortex processes fundamentals frequency sounds containing semantic information, right insula cortex processes fundamentals frequency sounds sounds containing phonetic information. Ventral premotor cortex also increases the ability to distinguish vowels, especially when the signal-to-noise ratio decreases. Mirror neurons of the medial frontal cortex, which are active in imitation functions, are also involved in speech-in-noise perception [2, 14, 15].
When signsl to noise ratio decrease, and speech-in-noise perception in becomes difficult, brain areas associated with semantic processing and speech production are employed and possibly designating the use of overt tactics by the listeners, e.g. using overt vocalization to attempt to help perception [2, 10]. Wernicke's area's principal function is speech perception, but it as well cooperates to production. Broca's area's main role is in speech production, it is also involved in perception [2, 9]. When a person read a text aloud, the imaging of the words is created in the occipital cortex. Each word is shaped in a spatial form, which is different for each person. Then, created visual signals transmit from the occipital cortex to the angular gyrus and go to Wernicke's area [16].
Regardless of whether CAPD and other auditory impairments are unilateral or bilateral, speech-in-noise perception impairments appear to be due to damages of the superior temporal lobe of the language-dominant hemisphere, suggesting that speech inputs are processed asymmetrically at early stages [2, 8]. Bilateral impairment of the superior temporal lobe produce word deafness, a condition in which pure tone hearing thresholds are within normal limits. But the ability to speech perception is effectively nil. Damage to posterior temporal lobe areas are most predictive of speech perception deficits (in aphasia), [12].
Damage to the posterior portion of superior temporal gyrus (in certain forms of aphasia) does produce deficits in speech perception. However, these conditions involve only mild phonological impairments [13]. Anterior temporal lobe regions have also been implicated both in lexical-semantic and sentence-level processing. Patients with semantic dementia have atrophy involving the anterior temporal lobe bilaterally, along with deficits on lexical tasks such as naming, semantic association and single-word perception [17]. The anterior and inferior temporal lobes seems to be more cooperated in meaning procedure, whereas the anterior temporal lobe involved in integrating certain forms of semantic knowledge across modalities [18].
Damage to posterior temporal lobes, particularly along the middle temporal gyrus is associated with speech perception deficits [14]. It is well-established that auditory inputs can produce rapid and automatic effects on speech production. For example, delayed auditory feedback of one’s own voice disrupts speech fluency. The adult onset deafness is associated with articulatory decline, indicating that auditory feedback is important in maintaining articulatory tuning [15, 18].
Damage to the left dorsal posterior superior temporal gyrus is associated with production disorders [12]. In particular, such a lesion is associated with conduction aphasia, which is a syndrome typically caused by stroke that is characterized by good speech perception, but frequent phonemic errors in production, naming difficulties and struggle with verbatim repetition [16].
Finally, since patients with CAPD and various types of peripheral hearing loss have difficulty in speech-in-noise perception [2, 10], it is necessary to perform WINP test using SSN.