Periodicity and frequency coding in human auditory cortex

Understanding the neural coding of pitch and frequency is fundamental to the understanding of speech comprehension, music perception and the segregation of concurrent sound sources. Neuroimaging has made important contributions to defining the pattern of frequency sensitivity in humans. However, the precise way in which pitch sensitivity relates to these frequency-dependent regions remains unclear. Single-frequency tones also cannot be used to test this hypothesis as their pitch always equals their frequency. Here, temporal pitch (periodicity) and frequency coding were dissociated using stimuli that were bandpassed in different frequency spectra (centre frequencies 800 and 4500 Hz), yet were matched in their pitch characteristics. Cortical responses to both pitch-evoking stimuli typically occurred within a region that was also responsive to low frequencies. Its location extended across both primary and nonprimary auditory cortex. An additional control experiment demonstrated that this pitch-related effect was not simply caused by the generation of combination tones. Our findings support recent neurophysiological evidence for a cortical representation of pitch at the lateral border of the primary auditory cortex, while revealing new evidence that additional auditory fields are also likely to play a role in pitch coding.     

Contrast tuned responses in primary auditory cortex of the awake ferret

Auditory neurons are often characterized by their spectro-temporal receptive field (STRF), a linear measure that captures overall trends of neural responses to modulations of the spectro-temporal envelopes of sounds. We have previously shown that primary auditory cortex neurons of the awake ferret are better characterized by STRFs followed by a non-trivial non-linearity. This non-linearity is a half-wave rectification followed by a squaring function, indicating that cortical neurons probably encode higher-order statistics of the spectrum of sounds. In this article, we introduce the concept of a contrast receptive field (CRF) and show that neurons in the auditory cortex encode quadratic statistics of the spectro-temporal envelope of sounds, which we call auditory contrast. We reveal phase-dependent contrast tuning in single units. Most units with a reliable STRF also possess a reliable CRF, such that the response to stimulus contrast complements the linear response described by the STRF. The relationship between the STRF and the CRF is analyzed in terms of orthogonality, co-localization in time-frequency, feature orientation selectivity, and output non-linearity interdependence. Our study shows that contrast can be used by auditory cortex neurons to sharpen, in a noise-resistant fashion, their responses to dynamic spectral profiles.     

Modulation of auditory cortex activation by sound presentation rate and attention

We studied the effects of sound presentation rate and attention on auditory supratemporal cortex (STC) activation in 12 healthy adults using functional magnetic resonance imaging (fMRI) at 3 T. The sounds (200 ms in duration) were presented at steady rates of 0.5, 1, 1.5, 2.5, or 4 Hz while subjects either had to focus their attention to the sounds or ignore the sounds and attend to visual stimuli presented with a mean rate of 1 Hz. Consistent with previous observations, we found that both increase in stimulation rate and attention to sounds enhanced activity in STC bilaterally. Further, we observed larger attention effects with higher stimulation rates. This interaction of attention and presentation rate has not been reported previously. In conclusion, our results show both rate-dependent and attention-related modulations of STC indicating that both factors should be controlled, or at least addressed, in fMRI studies of auditory processing.     

The effects of stimuli on the activity and functional connectivity of local neuronal groups in the cat auditory cortex

Simultaneous extracellular recordings from one electrode of ‘local’ groups of 3–6 neurons were obtained from the auditory cortex of unanesthetized, paralyzed cats. The activity and functional connectivity of local microenvironments were examined under various auditory stimuli. Single cell response patterns were examined using peri-stimulus (PST) histograms and functional connectivity among neighboring cells by the cross renewal density (CRD) histograms.Analysis of the PST histograms suggested that a high percentage of single cells demonstrated different response patterns of different stimuli. Analysis of the CRD histograms suggested, on the one hand, that only small numbers of neighboring cells behaved as if there were direct connections from one cell to another, and that these direct connections appeared to be excitatory. On the other hand, many cell pairs shared input from shared sources which lay outside the local groups. The majority of functional connections were altered by at least one of the stimuli delivered, thus demonstrating the system's plasticity. It is suggested that long-term gates at the synaptic level are responsible for this phenomenon.     

Thalamic and callosal connections of the rat auditory cortex

This study was designed to assess the relative distributions of two entrinsic afferent fiber systems in the rat auditory cortex as indicated by the patterns of specific lesion-induced degeneration evident in Fink-Heimer preparations. The auditory cortex consists of cytoarchitectural areas 41, 20 and 36. Lesions were made in the medial geniculate body (MGB) or the corpus callosum in some rats, while in other rats, lesions were made in both the MGB and the corpus callosum. Following the thalamic lesions, degenerating terminals occur throughout the auditory region of cortex, principally in layer IV and deep layer III, but also in layer VI and in the superficial part of layer I. With the exception of the band of degenerationin layer I, the density of the thalamic degeneration is uneven, such as that patches of increased density of degeneration are seperated by regions with few degenerating terminals. Following lesions of the corpus callosum, degenerating callosal terminals are also evident thoughout the auditory region of cortex and they occur in deep layer I through layer III, superficial layer V and layer VI. The dennsity of the degenerating callosal terminals is not uniform throughout most of area 41, to the extent that there are radially-oriented bands of increased density which appears within the continuous callosal projection. Following the double lesions, degenerating terminals throughout the auditory region are distributed homogenoously within all cortical layers with the exception of deep layer Vwhish is relatively free of degeneration. The results indicate that all regions within the rat auditory cortex are subject to both thalamic and callosal influence, although the input is not completely uniform, for the zones in layers IV and VI which have decreased thalamic input appear to have increased callosal input.     

Roles of the auditory cortex in discrimination learning by rats

We investigated the roles of the auditory cortex in sound discrimination learning in Wistar rats. Absolute pitch or relative pitch can be used as discrimination cues in sound frequency discrimination. To clarify this, rats were trained to discriminate between rewarded (S+) and unrewarded (S-) test stimuli (S+ frequency>S- frequency). After learning was acquired by rats, performance was tested in a new test in which S+ frequency was constant but S+ frequencyS- frequency but both frequencies were increased. If the discrimination cue of the first test was preserved in the new test, performance following change of testing procedures was expected to remain high. The measured performance suggested that rats used relative pitch in half octave discrimination (difference between S+ and S- frequencies, 0.5 octave), and absolute pitch in octave discrimination (difference between S+ and S- frequencies, 1.0 octave). Bilateral lesions in the auditory cortex had almost no effect on performance before procedure change. Furthermore, performance following procedure change was not affected by lesions in the auditory cortex when the discrimination cue was preserved. However, performance was impaired by lesions in the auditory cortex when a new discrimination cue was used following procedure change. Lesions in the auditory cortex also impaired multimodal discrimination between sound and sound plus light. The present findings suggest that the auditory cortex plays a role as a sensory interface of the higher cortices required for flexible learning and multimodal discrimination.     

The cortex of the primary auditory area in Alzheimer''s disease

The cortex of the superior temporal gyrus has been examined in two brains with Alzheimer's disease. Numerous neurofibrillary tangles and neuritic plaques that are characteristic of the disease, were present in area 38 in the anterior part of the gyrus and in area 22 more posteriorly but the primary auditory cortex, area 41, was virtually unaffected by these pathological changes. This relatively minor involvement of the primary auditory cortex, like that of the primary somatic and visual areas, again emphasises the uniqueness of the olfactory system in being severely degenerate. The findings are considered to support the suggestion that the distribution of the pathological changes in Alzheimer's disease has an anatomical basis due to spread of the disease process along certain well-defined sets of cortical fibre connections.     

Nonuniform impacts of forward suppression on neural responses to preferred stimuli and nonpreferred stimuli in the rat auditory cortex

In natural conditions, human and animals need to extract target sound information from noisy acoustic environments for communication and survival. However, how the contextual environmental sounds impact the tuning of central auditory neurons to target sound source azimuth over a wide range of sound levels is not fully understood. Here, we determined the azimuth‐level response areas (ALRAs) of rat auditory cortex neurons by recording their responses to probe tones varying with levels and sound source azimuths under both quiet (probe alone) and forward masking conditions (preceding noise + probe). In quiet, cortical neurons responded stronger to their preferred stimuli than to their nonpreferred stimuli. In forward masking conditions, an effective preceding noise reduced the extents of the ALRAs and suppressed the neural responses across the ALRAs by decreasing the response strength and lengthening the first‐spike latency. The forward suppressive effect on neural response strength was increased with increasing masker level and decreased with prolonging the time interval between masker and probe. For a portion of cortical neurons studied, the effects of forward suppression on the response strength to preferred stimuli was weaker than those to nonpreferred stimuli, and the recovery from forward suppression of the response strength to preferred stimuli was earlier than that to nonpreferred stimuli. We suggest that this nonuniform forward suppression of neural responses to preferred stimuli and to nonpreferred stimuli is important for cortical neurons to maintain their relative stable preferences for target sound source azimuth and level in noisy acoustic environments.     

Modified synaptic dynamics predict neural activity patterns in an auditory field within the frontal cortex

Frontal areas of the mammalian cortex are thought to be important for cognitive control and complex behaviour. These areas have been studied mostly in humans, non‐human primates and rodents. In this article, we present a quantitative characterization of response properties of a frontal auditory area responsive to sound in the brain of Carollia perspicillata, the frontal auditory field (FAF). Bats are highly vocal animals, and they constitute an important experimental model for studying the auditory system. We combined electrophysiology experiments and computational simulations to compare the response properties of auditory neurons found in the bat FAF and auditory cortex (AC) to simple sounds (pure tones). Anatomical studies have shown that the latter provides feedforward inputs to the former. Our results show that bat FAF neurons are responsive to sounds, and however, when compared to AC neurons, they presented sparser, less precise spiking and longer‐lasting responses. Based on the results of an integrate‐and‐fire neuronal model, we suggest that slow, subthreshold, synaptic dynamics can account for the activity pattern of neurons in the FAF. These properties reflect the general function of the frontal cortex and likely result from its connections with multiple brain regions, including cortico‐cortical projections from the AC to the FAF.     

Cholinergic modulation incorporated with a tone presentation induces frequency-specific threshold decreases in the auditory cortex of the mouse

Learning-induced or experience-dependent auditory cortical plasticity has often been characterized by frequency-specificity. Studies have revealed the critical role of the cholinergic basal forebrain and acoustic guidance. Cholinergic facilitation of specific thalamocortical inputs potentially determines such frequency-specificity but this issue requires further clarification. To examine the cholinergic effects on thalamocortical circuitry of specific frequency channels, we recorded the responses of cortical neurons while pairing basal forebrain activation or acetylcholine (ACh) microiontophoresis with tone presentations at 10 dB below the neuronal response threshold. We found that both basal forebrain activation and acetylcholine microiontophoresis paired with a tone induced a significant decrease in response threshold of the recorded cortical neurons to the frequency of the paired tone, and that this threshold decrease could be eliminated by atropine microiontophoresis. Our data suggest that cortical acetylcholine specifically facilitates thalamocortical circuitry tuned to the frequency of a presented tone; it is the first, fundamental step towards frequency-specific cortical plasticity evoked by auditory learning and experience.     

The relationship between the auditory cortex and the claustrum in the cat

The relationship between the primary auditory cortex and the claustrum has been re-examined in the cat with axoplasmic flow and axonal degeneration methods. Labelled cells are found in a restricted part of the claustrum after injections of HRP or HRP-WGA in the primary auditory cortex, but they are relatively few in number and are palely stained. The number of labelled cells and their depth of staining are greatest at 72 h (the longest survival time used here), and this survival period an occasional labelled cell is also present in the claustrum of the contralateral hemisphere. No labelled cells are seen after 24 h. After small lesions in the primary auditory cortex, fibre and terminal degeneration are present in the part of the claustrum where labelled cells are seen with axoplasmic flow techniques. It is concluded that there are reciprocal connections between the primary auditory cortex and the claustrum, but the rate of axoplasmic flow is unusually slow.     

Invariance of firing rate and field potential dynamics to stimulus modulation rate in human auditory cortex

The effect of stimulus modulation rate on the underlying neural activity in human auditory cortex is not clear. Human studies (using both invasive and noninvasive techniques) have demonstrated that at the population level, auditory cortex follows stimulus envelope. Here we examined the effect of stimulus modulation rate by using a rare opportunity to record both spiking activity and local field potentials (LFP) in auditory cortex of patients during repeated presentations of an audio‐visual movie clip presented at normal, double, and quadruple speeds. Mean firing rate during evoked activity remained the same across speeds and the temporal response profile of firing rate modulations at increased stimulus speeds was a linearly scaled version of the response during slower speeds. Additionally, stimulus induced power modulation of local field potentials in the high gamma band (64–128 Hz) exhibited similar temporal scaling as the neuronal firing rate modulations. Our data confirm and extend previous studies in humans and anesthetized animals, supporting a model in which both firing rate, and high‐gamma LFP power modulations in auditory cortex follow the temporal envelope of the stimulus across different modulation rates. Hum Brain Mapp, 2011. © 2010 Wiley‐Liss, Inc.     

大鼠听皮层GABAA受体亚单位mRNA年龄相关的表达变化

应用原位杂交技术,研究了大鼠出生后发育过程中,听皮层GABAA受体a2及β3亚单位mRNA年龄相关的表达变化。特异性DIG标记寡核苷酸探针检测显示,a2亚单位mRNA阳性神经元数量从出生后第1天开始逐渐增加,到第12d达到一个高峰,而后逐渐减少,出生后21d降至检测水平以下;β3亚单位mRNA阳性神经元数量从出生后第12d,一直保持较高表达水平,第12d后急剧降低,第21d后降至检测水平以下。研究结果为进一步在皮层水平上探讨出生后听觉功能发育可塑性的分子机制提供了重要资料。     

Multiple brain systems generating the rat auditory evoked potential. I. Characterization of the auditory cortex response

The objectives of this study were to characterize the auditory cortex response in the rat and to examine its contributions to the auditory evoked potentials (AEPs) recorded from the dorsal and lateral skull. This was accomplished by simultaneously recording AEPs from the cortical surface and from skull screw electrodes in anesthetized animals. The initial positive-negative response (P17-N32) was largely restricted to the cortical region corresponding to area 41. More detailed examination of the AEP mapping revealed multiple subcomponents (P9, P14, P17, P19) underlying the initial positivity, with differing topographies. Stimulus-response properties further dissociated the multiple positive subcomponents Reversible local neurochemical suppression confirmed the auditory cortical origin of these AEPs. The auditory cortex-generated AEPs were refractory to barbiturate anesthesia which eliminated all dorsal skull AEPs, indicating that primary auditory cortical AEPs do not make a significant contribution to the dorsal skull-recorded (‘vertex’) AEPs. The findings raise issues regarding multiple parallel auditory processing systems and their associated AEPs.     

Contrast coding in the primary visual cortex depends on temporal contexts

Contrast response function in the primary visual cortex (V1) has long been described as following a sigmoid curve. However, this is mainly based on measuring neural responses to drifting contrast grating in a stable stimulation, a model that does not consider the effects of motion or length of stimulus presentation. During natural viewing, the visual system can obtain sufficient information for identifying the shapes defined by contrast from a single glance; acquiring greater knowledge of the neuronal response properties to contrast in such a short timescale is necessary to understand the underlying mechanisms. We investigated responses of cat V1 neurons to contrast presented by static grating for 40 ms without pause compared to drifting grating presented continuously for 2000 ms. The neuronal response to transiently presented contrast could be well described by a linear function. Further examination of the effects of motion and presentation duration on contrast responses demonstrated that motion increased response sensitivity in the low‐contrast range, while brief presentation increased response sensitivity in the high‐contrast range. Motion and prolonged presentation (adaptation) together resulted in an asymptotic sigmoid curve with a saturation response in the high‐contrast range. These results suggest that motion mainly enhance the neural response sensitivity to low‐contrast objects, while short and rapid presentation mainly enhance the neural sensitivity to high‐contrast stimulus. Our findings indicate that multiple factors influence the properties of contrast response functions, suggesting that V1 neuron contrast coding is flexible and depends on the temporal contexts.     

Neuronal connections in the primary auditory cortex: an electrophysiological study in the cat

Neuronal connections in the primary auditory cortex (AI) of the cat were studied electrophysiologically by using intracellular recording techniques. Fast-conducting fibers from the medial geniculate nucleus (MG) projected monosynaptically onto AI neurons in layers III-VI (mainly in layer IV), whereas slow-conducting MG-fibers projected monosynaptically onto AI neurons in layer I. AI neurons which received monosynaptic inputs from the auditory association cortices (AII and Ep) and/or from the contralateral AI were distributed in all layers of the AI; the commissural fibers from the contralateral AI were divided into fast- and slow-conducting ones. AI neurons were categorized into seven types: type I neurons which received monosynaptic inputs from slow-conducting MG-fibers were located in layer I. Type II neurons which received polysynaptic inputs from the MG were located in layers II-VI. Type III neurons which sent their axons to the AII or Ep were mainly located in layer III. Type IV neurons which sent their axons to the contralateral AI were located mainly in layer III. Type V neurons which received monosynaptic inputs from fast-conducting MG-fibers were located mainly in layer IV. Type VI neurons which projected onto the inferior colliculus were located in the upper part of the layer V. Type VII neurons which projected onto the MG were located in layers V and VI.     

Parcellation of human temporal polar cortex: A combined analysis of multiple cytoarchitectonic,chemoarchitectonic, and pathological markers

Although the human temporal polar cortex (TPC), anterior to the limen insulae, is heavily involved in high‐order brain functions and many neurological diseases, few studies on the parcellation and extent of the human TPC are available that have used modern neuroanatomical techniques. The present study investigated the TPC with combined analysis of several different cellular, neurochemical, and pathological markers and found that this area is not homogenous, as at least six different areas extend into the TPC, with another area being unique to the polar region. Specifically, perirhinal area 35 extends into the posterior TPC, whereas areas 36 and TE extend more anteriorly. Dorsolaterally, an area located anterior to the typical area TA or parabelt auditory cortex is distinguishable from area TA and is defined as area TAr (rostral). The polysensory cortical area located primarily in the dorsal bank of the superior temporal sulcus, separate from area TA, extends for some distance into the TPC and is defined as the TAp (polysensory). Anterior to the limen insulae and the temporal pyriform cortex, a cortical area, characterized by its olfactory fibers in layer Ia and lack of layer IV, was defined as the temporal insular cortex and named as area TI after Beck (J. Psychol. Neurol. 1934;41:129–264). Finally, a dysgranular TPC region that capped the tip with some extension into the dorsal aspect of the TPC is defined as temporopolar area TG. Therefore, the human TPC actually includes areas TAr and TI, anterior parts of areas 35, 36, TE, and TAp, and the unique temporopolar area TG. J. Comp. Neurol. 514:595–623, 2009. © 2009 Wiley‐Liss, Inc.     

Desynchronization in the right auditory cortex during musical hallucinations: A MEG study

Desynchronization in the right auditory cortices, including the transverse gyrus of Heschl, planum temporale and supramarginal cortex, occurred during musical hallucinations in a 78‐year‐old woman with hearing impairment and depression. This phenomenon was assessed using a novel, spatially filtered magnetoencephalography (MEG) analysis, termed synthetic aperture magnetometry (SAM). In general, the affected areas are consistent with neuroimaging studies of normal musical perception and imagery, suggesting that musical hallucinations involve abnormal spontaneous activity in the neural substrate dedicated to musical perception and imagery in which false imagery occurs.     

Subdivisions of auditory cortex and ipsilateral cortical connections of the parabelt auditory cortex in macaque monkeys

Auditory cortex of macaque monkeys can be divided into a core of primary or primary-like areas located on the lower bank of the lateral sulcus, a surrounding narrow belt of associated fields, and a parabelt region just lateral to the belt on the superior temporal gyrus. We determined patterns of ipsilateral cortical connections of the parabelt region by placing injections of four to seven distinguishable tracers in each of five monkeys. Results were related to architectonic subdivisions of auditory cortex in brain sections cut parallel to the surface of artificially flattened cortex (four cases) or cut in the coronal plane (one case). An auditory core was clearly apparent in these sections as a 16- to 20-mm rostrocaudally elongated oval, several millimeters from the lip of the sulcus, that stained darkly for parvalbumin, myelin, and acetylcholinesterase. These features were most pronounced caudally in the cortex assigned to auditory area I, only slightly reduced in the rostral area, and most reduced in the narrower rostral extension we define as the rostrotemporal area. A narrow band of cortex surrounding the core stained more moderately for parvalbumin, acetylcholinesterase, and myelin. Two regions of the caudal belt, the caudomedial area, and the mediolateral area, stained more darkly, especially for parvalbumin. Rostromedial and medial rostrotemporal, regions of the medial belt stained more lightly for parvalbumin than the caudomedial area or the lateral belt. The parabelt region stained less darkly than the core and belt fields. Injections confined to the parabelt region labeled few neurons in the core, but large numbers in parts of the belt, the parabelt, and adjacent portions of the temporal lobe. Injections that encroached on the belt labeled large numbers of neurons in the core and helped define the width of the belt. Caudal injections in the parabelt labeled caudal portions of the belt, rostral injections labeled rostral portions, and both caudal and rostral injections labeled neurons in the rostromedial area of the medial belt. These observations support the concept of dividing the auditory cortex into core, belt, and parabelt; provide evidence for including the rostral area in the core; suggest the existence of as many as seven or eight belt fields; provide evidence for at least two subdivisions of the parabelt; and identify regions of the temporal lobe involved in auditory processing. J. Comp. Neurol. 394:475–495, 1998. © 1998 Wiley-Liss, Inc.