[讨论]听觉处理 auditory processing in human&a

Subcortical route

The auditory system is the sensory system for the sense of hearing. The cochlea inside the ear transduces sound waves into action potentials. This information travels up the auditory nerve through parts of the thalamus and brainstem to auditory areas of the. Sound waves are collected by the external ear and channelled along the ear canal to the eardrum or tympanic membrane. When sound hits the tympanic membrane, the impact creates vibrations that cause three bones in the middle ear, the auditory ossicles, to move. The smallest of these bones, the stapes, fits into the oval window between the middle and inner ear. When the oval window vibrates, fluid in the inner ear transmits the vibrations into a delicate, snail-shaped structure called the (Fig. 1.1). As a receptive organ, cochlea may transduce the mechanical energy of sound into electrical signals suitable for the processing by the nervous system.

Fig. 1.1 Four major divisions of auditory system by function (download from internet)

Hair cells, in cochlea, are tonotopically organized along the basilar membrane, and the distance from the cochlear apex is proportional to the logarithm of the best frequency (Hudspeth, 2000a and 2000b). Information from hair cells is transmitted to neuronal cells from the spiral ganglion. The number of active neurons in the spiral ganglion and their firing rate encode the information about sound intensity. Sound frequency is coded by a “place code” represented by a tonotopic map, which retains in all specific auditory structures including cortical, and a “frequency code”, necessary because tonotopic maps do not contain neurons with a characteristic frequency below 200 Hz. The intermediate frequency (up to about 4 kHz) is encoded by both tonotopy and phase locking represented in a pooled activity of several neurons. At very high frequencies, tonotopy is the main factor for frequency encoding (Hudspeth, 2000a; Bear et al., 2001).

Axons of spiral ganglion neurons contribute to the VIII vestibulo-cochlear cranial nerve. The auditory part of this nerve transmits information to the ipsilateral cochlear nuclear complex situated in the medullo-pontine junction and consisted of three tonotopically organized main parts: dorsal, anteroventral and posteroventral nuclei (Fig 1.2). The cochlear nucleus (CN) is a switchboard of sorts, distributing auditory information to several different areas in the auditory pathway.  It also does a substantial amount of processing of both time and frequency information. The neurons of the cochlear nuclei respond either tonically (and may take part in the frequency encoding) or phasically (cells which fire a single spike to the stimulus onset; they are thought to provide accurate information about the timing of acoustic stimuli and hence take part in sound localization in the horizontal plane). Cells which respond to a broad frequency range are suggested to play role in the localization of sounds along the elevation axis (Altman, 1990; Hudspeth, 2000a; Bear et al., 2001).

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Fig. 1.2 Auditory pathway. Abbreviations for this figure: CN cochlear nuclei, SOC superior olivary complex, IC inferior colliculi, MGB medial geniculate bodies (download from internet)

［此帖子已被 zhixl 在 2005-11-2 14:29:42 编辑过］

The efferents of both the anteroventral and posteroventral nuclei contribute to the trapezoid body, which terminates at the pontine level in the complex of the superior olives complex (SOC)(Fig 1.2). The SOC processes information about interaural delays and amplitudes.  It also acts as a crossover site for spatially oriented auditory information. Receiving both ipsi- and contralateral inputs, the medial and lateral olives represent the first level of binaural convergence and play an essential role in sound localization using two localization cues: interaural time and intensity differences (ITD and IID). Some superior olivary neurons respond selectively to a certain direction of frequency modulation (Watanabe et al., 1968; Vartanjan, 1978). Furthermore, some olivary neurons were found to synchronize their activity with relatively high modulation frequencies (150 – 200 Hz) of amplitude-modulated sounds (Andreeva and Vasil’ev, 1977).fficeffice" />

The outputs from medial and lateral superior olives extend to the midbrain auditory structure, the inferior colliculi (IC) (Fig 1.2), via the lateral lemniscus (LL). The LL is primarily a tract of axons ascending the brainstem.  The LL however are sensitive to changes in both the timing and the amplitude of sound. The IC appears to be an integrative station as well as a switchboard.  It is responsive to interaural delay and amplitude differences and may provide a spatio-topic map of the auditory environment. The IC consists of two main components: the multi-layer central nucleus, which receives most of its projections from lower auditory structures, and the dorsal part, which receives both auditory and somato-sensory input. Within the IC there are areas in which axons from different brainstem nuclei converge integrating information from the lower level (Oliver et al., 1997). The inferior colliculi obviously play an important role in sound localization since this structure contains numerous neurons sensitive to the interaural time and intensity difference (Altman, 1990; Hudspeth, 2000a; Bear et al., 2001). In response to amplitude- or frequency-modulated sounds, inferior collicular neurons are able to synchronize their firing rate with modulation frequency up to 30 – 100 Hz. This synchronization pattern becomes more robust as the stimulus carrier frequency gets closer to the characteristic frequency of a given neuron (Vartanjan, 1978). Such selectivity to particular combinations of complex stimulus parameters may result in an orthogonal representation of timing and spectral information in the IC (Langner and Schreiner, 1988). While the central nucleus is the main origin of cochleotopic projections to the thalamic level, nuclei of the dorsal part constitute multiple diffuse ascending pathways (Andersen et al., 1980; Calford and Aitkin, 1983).

The most prominent pathways connecting midbrain and thalamic auditory structures, the medial geniculate bodies (MGB), are the brachia of inferior colliculi (Fig 1.2). The functions of MGB are relative intensity comparison, duration comparison and relay to cortex. The thalamus is the last relay site on the way to the cortex for almost all sensory information, including auditory, visual, and somatosensory. It is shaped like a football and has been functionally and morphologically divided into a large number of nuclei.  the primary nucleus involved in the auditory pathway is the MGB, but several other nuclei are at least partially responsive to auditory stimulation.  The MGB has itself been divided into three subneclei based on functional as well as morphological distinctions.  These divisions are ventral (VMBG), Medial (MMGB), and dorsal (DMGB). The VMGB is thought to be primarily responsible for relaying frequency, intensity and binaural information to the cortex.  The responses in the VMGB appear to be organized in a tonotopically similarly way to those in the IC. The MMGB seems to functionally be responsible for detection of the relative intensity and duration of a sound. It shows a wide range of responses to auditory stimuli. Many types of responses are present in the DMGB which appear to vary by sub-nuclei.  Generally, the responses are broadly tuned, but some cells appear to respond only to complex stimuli. Other cells are multi modal, often responding to somatosensory as well as auditory stimuli (see http://serous.med.buffalo.edu/hearing/thalamus.html). The main MGB efferents project tonotopically through the acoustic radiation to the primary auditory cortex. The projections from MGB to auditory cortex preserve the segregation into tonotopic, diffuse and polysensory pathways. The tonotopic pathway goes from ICC to MGBv to the four fields of tonotopic cortex. The diffuse pathway projects from dorsal cortex of the IC to MGBd to secondary cortical areas. The polysensory pathway goes from IC paracentral nuclei to medial MGB to all cortical areas.

At the mesencephalic level, there are connections between the MGB and associative thalamic nuclei: dorso-medial (DM), dorso-lateral (DL), postero-lateral (PL) and Pulvinar. These nonspecific thalamic nuclei, in turn, constitute extensive projections to frontal and parietal associative cortical areas. The DM nucleus sends efferents to Brodmann areas 8 – 12 and 45 – 47, the DL and PL nuclei to parietal areas 5 and 7, while the Pulvinar is connected to areas 39 and 40 (Andreeva et al., 1985). It should be emphasized that some auditory information circumvents the primary auditory cortex and is transferred directly to polymodal areas. However, the main MGB efferents project tonotopically through the acoustic radiation to the primary auditory cortex.

 

［此帖子已被 zhixl 在 2005-10-31 14:29:22 编辑过］

Subcortical route并不是我兴趣所在,而且听觉的这一部分已经被人研究的很透了.

有一个结论需要提及,就是人耳对3kHz的声音很敏感,他的物理学解释很简单,但生理学意义呢?我没读过这类文章(绝对不是说不存在这类文章).10月份去日本的八景岛远足,忽然好象找到了答案.

当时我站在海边,很远处有个娱乐场,场内有过山车.娱乐场很远,用我那个25倍光学变焦的摄像机能看清,但肉眼看不清楚(不过我近视).大家都知道,过山车的噪音很大,足可以淹没附近两个人正常讲话时的声音.我站在海边,距离过山车很远,根本听不到它的噪音,一点也听不到,海边很静,但却能听到过山车上游客的尖叫声.目测距离足有几公里远,过山车的噪音早已经衰减没了,却能听到"人"的尖叫.所以我推测人的减叫声音近似等于3kHz,而通常人尖叫是由于恐惧发出的,所以人类对3kHz敏感的生理学(更确切说是生物学)意义很可能是人类利用3kHz这个自然环境很少出现的声音来警报和求助信号,是人类进化的结果

听觉信息从thalamus开始被传递到听觉皮质和联合皮质,那才是我的兴趣所在,我会介绍些世界最前沿的东西给大家,请接着关注

 Superb!

Please continue your introduction for us.

期待！

小插曲尤其精彩，虽然只是猜想。

1.2    Cortical processingfficeffice" />

 

1.2.1          Functional anatomy of multiple auditory areas

 

Despite numerous studies stretching over the last 100 years there is still no general agreement on the number of auditory areas or how to locate them on the human cortex. Usually, we may reference the results of non-human primates study. The auditory cortex of nonhuman primates has been extensively studied, and the concept of dividing the auditory cortex into core, belt, and parabelt areas has been proposed based on several different neuroanatomical techniques, including retrograde tracer injections and histochemical staining (Pandya and Sanides, 1973; Hackett et al., 1998). The core areas are located inside the lateral sulcus, laterally surrounded by the belt areas. The parabelt areas are further lateral to the belt and located on the exposed surface of the superior temporal gyrus. There is general agreement that in humans the primary auditory field (AI) is located deep within the lateral fissure on a small patch of the transverse gyrus of Heschl (HG); secondary auditory area (A2) is located along Heschl’s sulcus (HS) and within BA 42, divided into medial secondary auditory cortex (A2m) and lateral secondary auditory cortex (A2l). Several investigators consider A1 corresponds to the core area (Pandya and Sanides, 1973), the belt area corresponds to A2, though a subdivision has been suggested (Hackett et al., 1998).  Hashimoto and co-workers further consider that A2m and A2l are related to the belt area, and the part of planum temporale (PT) and anterior superior temporal gyrus (STa) are related to the belt and parabelt areas.

Cortical mechanisms for hearing include anatomically and functionally distributed pathways that have been interpreted as evidence of different streams of information processing (for recent reviews see Kaas & Hackett 2000, Rauschecker & Tian 2000). The thalamocortical auditory projections in monkeys provide a basis for hierarchical transformation, as the auditory signal is processed sequentially through core, belt and parabelt regions. Subsequently, the signal is distributed to multi-modal areas of the temporal, parietal and frontal lobes (as introduced in last section). It is noteworthy that the belt is never bypassed as information is relayed from the core to higher brain levels. Consistent with our expectations of a hierarchy, responses in belt areas are to some extent transformations of the properties of the core. Within this framework there is also a clear basis for parallel processing. Parallel processing begins with the parallel projection from the primary auditory thalamus to multiple core cortical fields. It culminates with parabelt projections within temporal and parietal areas near the superior temporal plane, and also to frontal lobe targets that are implicated in visual gaze, working memory and multimodal stimulus recognition. The physiological evidence shows that different fields within the same level of the processing hierarchy (e.g. AI and the rostral field, or different lateral belt fields) process information at least partly in parallel, and with some specialization.

这一部分需要记住的有几点

1. 人类听觉皮质的确切解剖定位还没有定论(猴的已经确定了)

2.个体差异,特别是初级听觉皮质的位置个体差异很大,所以参考一些经过加算平均处理后的研究结果时一定要慎重. 自己写report的时候也要尽量避免写出"定位在primary auditory area"的字样,否则会遇到麻烦的.\

3.如果是用eeg做工具,定位a1 a2的时候就尽量谦虚些,meg就不需要有顾虑了.

Frequencyfficeffice" />

 

Results from several PET and fMRI studies suggest that the location of activated volume within the temporal lobes depends on sound frequency (Lauter et al., 1985; Wessinger et al., 1997; Bilecen et al., 1998; Talavage et al., 2000; Yang et al., 2000; Wessinger et al., 2001; Hall et al., 2003), which reflects underlying tonotopic organization of the auditory cortex.

Results from lesion studies suggest that simple frequency discrimination can be accomplished at subcortical level. Unilateral (Zatorre, 1988; Zatorre and Samson, 1991) or even large bilateral lesions of the auditory cortices (Peretz et al., 1994) do not generally result in a permanent impairment in frequency discrimination tests.

In normal subjects, perception of sound frequency was shown to elicit asymmetrical activation of the temporal cortex favoring the right hemisphere (Tzourio et al., 1997). Auditory attention to tone frequency enhanced the activity in the auditory areas of the superior temporal cortex predominantly in the hemisphere contralateral to the attended direction (Tzourio et al., 1997; Alho et al., 1999), and in the prefrontal (Tzourio et al., 1997; Alho et al., 1999) and parietal (Stevens et al., 2000) cortical areas, which appear to be involved in controlling attention (Tzourio et al., 1997; Alho et al., 1999). Activation within the parietal cortex had an earlier onset than activation in the prefrontal areas (Stevens et al., 2000). Thus, two networks are involved during selective attention to sound frequency: a local temporal network, responsible for the perceptual analysis of frequency and a fronto-parietal network modulating temporal cortex activity and its functional lateralization: a decrease of general rightward dominance and appearance of lateralization dependent on the side of attended stimulation (Tzourio et al., 1997).