Supratemporal Plane Research Articles

Spectral and temporal processing in human auditory cortex.

Hierarchical processing suggests that spectrally and temporally complex stimuli will evoke more activation than do simple stimuli, particularly in non-primary auditory fields. This hypothesis was tested using two tones, a single frequency tone and a harmonic tone, that were either static or frequency modulated to create four stimuli. We interpret the location of differences in activation by drawing comparisons between fMRI and human cytoarchitectonic data, reported in the same brain space. Harmonic tones produced more activation than single tones in right Heschl's gyrus (HG) and bilaterally in the lateral supratemporal plane (STP). Activation was also greater to frequency-modulated tones than to static tones in these areas, plus in left HG and bilaterally in an anterolateral part of the STP and the superior temporal sulcus. An elevated response magnitude to both frequency-modulated tones was found in the lateral portion of the primary area, and putatively in three surrounding non-primary regions on the lateral STP (one anterior and two posterior to HG). A focal site on the posterolateral STP showed an especially high response to the frequency-modulated harmonic tone. Our data highlight the involvement of both primary and lateral non-primary auditory regions.

Functional cerebral asymmetries of pitch processing during dichotic stimulus application: a whole-head magnetoencephalography study

Dichotic listening (DL) studies indicate higher proficiency of the right cerebral hemisphere in processing the pitch of auditory events. Especially, acoustic stimuli of a rich harmonic structure such as square waves (complex tones) elicit a left ear advantage (LEA) under dichotic stimulus application. In order to investigate the timing of early sensory encoding at the level of the supratemporal plane, whole-head magnetoencephalography (MEG; 151 channels) recordings were performed in 20 right-handed subjects using an oddball paradigm based on dichotically applied complex tones. In contrast to electroencephalography (EEG) and event related potentials (ERP), this technique separately measures neuronal activity of left and right auditory cortex. Neuromagnetic responses were obtained both during preattentive stimulus processing, as well as during a pitch detection task. Rare stimuli presented to the left ear elicited a stronger magnetic analogue of mismatch negativity (MMNm) over both hemispheres and gave rise to shorter latencies of the contralateral mismatch fields than right ear deviants. In conclusion, the present data provide first evidence for functional laterality effects even at the level of preattentive pitch processing within the auditory cortex.

Intrinsic connectivity of human auditory areas: a tracing study with DiI.

The human supratemporal plane contains the primary as well as several other auditory areas. We have investigated the intrinsic connectivity of these areas by means of antero- and retrograde labelling with the carbocyanin dye DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). A total of 30 injections was placed in both hemispheres of four freshly fixed postmortem brains. Labelled neurons and axons were found in cortex around the injection. The retrograde labelling varied from faint to Golgi-like; most of the retrograde labelled neurons were layers II-III pyramids and only a few were nonpyramidal neurons. Labelled axons were dense in all layers near the injection site, while they became relatively rare in layer IV further away. The tangential spread of labelling differed among auditory areas. On Heschl's gyrus (corresponding to the primary auditory cortex and cytoarchitectonic areas TD and part of TB) intrinsic connectivity involved a relatively narrow part of cortex. They spread over larger parts of cortex in plana polare and temporale (areas TG, TA and the remaining part of TB). A number of injections also produced anisotropic labelling patterns. These results reveal differences in intrinsic connectivity between auditory areas. They suggest that intrinsic connections within the primary auditory area, area TD and part of TB that is on Heschl's gyrus, involve mainly nearby units or modules, probably with similar coding properties, whereas in surrounding areas, connections spread over more distant units and may play an important role in the integration of different auditory features.

The development of auditory evoked dipole source activity from childhood to adulthood

Objective: Multi-channel recordings show that observed developmental changes of late auditory evoked potentials (LAEP) depend on the location of the scalp electrode. These findings suggest that different LAEP generators have a distinct developmental course. The goal of this study was to investigate the maturational process of cortical LAEP generators. Methods: Eighty-seven healthy children and adolescents with normal hearing, ages 5–16 years, and 21 adults, ages 20–30 years, participated in the study. Pure tone LAEP were recorded from 21 derivations. Dipole source analysis was performed by means of brain electric source analysis (BESA). Peak latencies and amplitudes of dipole source activity were estimated. Results: While the number, location, and direction of dipole sources were similar in children and adults, the course of their activity differed greatly. The latencies shortened and the amplitudes decreased during development. In adolescence a new component appeared in the activity of the tangential dipole, which reflects the generators in the supra-temporal plane. The variability of parameters was greater in children than in adults. Conclusions: Since the dipole source activity of LAEP in childhood differs considerably from that in adulthood, dipole source analysis could be a useful tool for studying both normal and disturbed maturation of the auditory perceptual function.

The interface between the perception and production of speech

Four positron emission tomography studies were reanalyzed to identify anatomically separable, functional subsystems associated with speech perception, speech production, hearing complex, nonspeech sounds, and verbal semantic category verbal fluency. The results from a total of 26, right-handed, male volunteers were included. The results indicated two separate caudal (posterior) ‘‘streams’’ of auditory processing. One, directed along the supratemporal cortical plane, responded to both nonspeech and speech sounds, including the sound of the speaker’s own voice. Activity in response to perception merged at the temporo-parietal junction with a region where activity was linked to speech production. In contrast, the left superior temporal sulcus below (ventrolateral to) the caudal supratemporal plane responded only to an external source of verbal stimuli: but, in addition, it was activated by the recall of lists of words during verbal fluency. The results are compatible with a hypothesis that the caudal superior temporal cortex is specialized for the mimicry of sounds, including repetition, the role of the caudal left superior temporal sulcus being to transiently represent verbal sequences, whether heard or internally generated and rehearsed. These processes are central to the perception, rehearsal (both covert and overt), and eventual acquisition of long-term lexical memories of novel words.

Right-Hemisphere Dominance for the Processing of Sound-Source Lateralization

Cortical processing of change in direction of a perceived sound source was investigated in 12 human subjects using whole-head magnetoencephalography. The German word "da" was presented either with or without 0.7 msec interaural time delays to create the impression of right- or left-lateralized or midline sources, respectively. Midline stimuli served as standards, and lateralized stimuli served as deviants in a mismatch paradigm. Two symmetrically linked dipoles fitted to the mismatch fields showed stronger moments in the hemisphere contralateral to the side of the deviant. The right dipole displayed equal latencies to both left and right deviants, whereas left dipole latencies were longer for ipsilateral than contralateral deviants. Frequency analysis between 20-70 Hz and statistical probability mapping revealed increased induced gamma-band activity at 53+/-2.5 Hz to both types of deviants. Right deviants elicited spectral amplitude enhancements in this frequency range, peaking at latencies of 160 and 240 msec. These effects were localized bilaterally over the angular gyri and posterior temporal regions. Coherence analysis suggested the existence of two separate interhemispheric networks. For left-lateralized deviants, both spectral amplitude enhancements at 110 and 220 msec and coherence increases were restricted to the right hemisphere. In conclusion, both mismatch dipole latencies at the supratemporal plane and gamma-band activity in posterior parietotemporal areas suggested a right hemisphere engagement in the processing of bidirectional sound-source shifts. In contrast, left-hemisphere regions responded predominantly to contralateral events. These findings may help to elucidate phenomena such as unilateral auditory neglect.

Encoding of temporal speech features (formant transients) during binaural and dichotic stimulus application:: A whole-head magnetencephalography study

Spoken-word recognition depends upon the encoding of relevant ‘information bearing elements’ of the acoustic speech signal. For example, relatively rapid shifts of spectral energy distribution (formant transients) cue the perception of stop consonant–vowel (CV) syllables such as /ba/, /ga/, and /da/. A variety of data indicate left-hemisphere superiority with respect to the processing of formant transients. To further delineate the underlying neurophysiological mechanisms, evoked cortical fields in response to CV syllables (oddball design; frequent stimulus=binaural /ga/; four deviant constellations: Binaural /ba/, binaural /da/, left /da/ (left ear deviant)–right /ga/, right /da/ (right ear deviant)–left /ga/) were recorded by means of whole-head magnetencephalography (MEG; 151 channels) under two different conditions of attentional demands (visual distraction versus reaction to prespecified stimuli). (a) During binaural stimulus presentation attention toward target events resulted in a significantly enhanced mismatch field (MMNm, magnetic analogue to the mismatch negativity) over the left as compared to the right hemisphere. In contrast, preattentive processing of the CV syllables failed MMNm lateralization effects. (b) Dichotic application of /da/ elicited a larger contralateral MMNm amplitude in subjects with right ear advantage (REA) at behavioral testing. In addition, right ear deviants yielded a stronger ipsilateral response than the left ear cognates. Taken together, these data indicate bilateral preattentive processing and subsequent attention-related predominant left-hemisphere encoding of formant transients at the level of the supratemporal plane. Furthermore, REA during dichotic application of CV syllables seems to be linked to functional dissociation of the two hemispheres during auditory processing.

Attentive and pre-attentive periodicity analysis in auditory memory: an event-related brain potential study.

In an event-related potential (ERP) study we presented human subjects with streams of repetitive white noise (semi-periodic noise) under attend and ignore conditions to investigate whether the perception of the periodicity with short cycle-lengths is due to lower level, pre-attentive sensory memory processing or higher level, attentive working memory processing. The ERPs of both conditions reveal N1-like deflections that are time locked on the semi-periodic noise suggesting that the processing of the periodicity is due to a pre-attentive rather than an attentive process. The topography of the deflections suggests that its generators are located in the supratemporal plane. Additionally, the ERPs elicited by infrequent disruptions in the periodicity show differences between the conditions suggesting that the detection of disruptions in periodicity is facilitated by attention.

The caudal infrasylvian surface in dyslexia: novel magnetic resonance imaging-based findings.

To detect anatomic abnormalities of auditory association cortex in dyslexia by measuring the area of the perisylvian region known as the caudal infrasylvian surface(s) (cIS) in dyslexic and control subjects. Several quantitative morphometric investigations of cortical areas in dyslexia have focused on the cIS, which encompasses the supratemporal plane and the inferior bank of the posterior ascending ramus of the sylvian fissure. Inconsistencies in the results of these studies may be attributable in part to the use of measurement methods that do not account fully for surface undulations of the cIS. The authors used an MRI-based surface reconstruction technique that models the curvature of the cerebral cortex in three dimensions to obtain whole-hemisphere and regional surface area estimates. Measurements were obtained in both hemispheres of eight right-handed male dyslexic subjects and eight right-handed male control subjects. The cIS area of dyslexic subjects was significantly larger than that of control subjects, and this result was not attributable to a difference in whole-hemisphere surface area. Neither the dyslexic nor control subjects showed a left or right asymmetry in this region, although there was a trend toward less variance of the asymmetry scores in dyslexic subjects. The gross anatomic organization of this region is different in dyslexic subjects, and elucidation of the precise nature of these differences may be aided by surface modeling techniques.

Event-related brain activity associated with auditory pattern processing.

One of the basic properties of the auditory system is the ability to analyze complex temporal patterns. Here, we investigated the neural activity associated with auditory pattern processing using event-related brain potentials. Participants were presented with a continuously repeating sequence of four tones with rare changes in either the frequency or timing of one of the tones. Both frequency- and time-deviant sounds generated mismatch negativity (MMN) waves that peaked at midline central electrode sites and inverted in polarity at inferior temporal and occipital sites, consistent with generators in the supratemporal plane. The MMN scalp topography was similar for the frequency- and time-deviant stimuli, suggesting that both spectral and temporal relations among elements of an auditory pattern are encoded in a unified memory trace.

Hemispheric lateralization of the neural encoding of temporal speech features: a whole-head magnetencephalography study

Using a passive oddball design (randomized series of standard [frequent] and deviant [rare] stimuli), the present study investigated the neural encoding of syllables differing in a duration parameter (/da/=short-lag voice onset time [VOT], /ta/=long-lag VOT) by means of whole-head magnetencephalography (MEG). Dipolar activities at the level of the supratemporal planes allowed to explain the evoked magnetic fields. The N1m/P2m-complex (magnetic equivalent to the N1/P2-wave of the electroencephalogram) in response to standard stimuli showed bilateral symmetric distribution. Furthermore, the latency of P2m significantly depended on VOT. Finally, the mismatch response to the deviant /da/-syllables—which represent in German a very frequent word (English: `here' or `there')— evolved significantly earlier in the left hemisphere as compared to the right side. In conclusion, processing speed may be an important aspect of the hemispheric specialization of language.

Intracerebral Sources of Human Auditory-Evoked Potentials

Evoked potentials to brief 1,000-Hz tones presented to either the left or the right ear were recorded from 30 electrodes arrayed over the head. These recordings were submitted to two different forms of source analysis: brain electric source analysis (BESA) and variable-resolution electromagnetic tomography (VARETA). Both analyses showed that the dominant intracerebral sources for the late auditory-evoked potentials (50–300 ms) were in the supratemporal plane and lateral temporal lobe contralateral to the ear of stimulation. The analyses also suggested the possibility of additional sources in the frontal lobes.

What's right about the neural organization of sign language? A perspective on recent neuroimaging results

To summarize the argument so far, our main points are (1) that the vast majority of behavioral, neuropsychological, and functional imaging data support the hypothesis that the left hemisphere is dominant for lexical and grammatical aspects of sign language perception and production, (2) that because of potential design confounds, the Neville et al. study does not present any serious challenge to existing claims concerning the lateralization of sign language, and (3) that there is evidence from both lesion and functional imaging data which suggests that the within-hemisphere organization of signed and spoken language is in many respects the same—but not in all respects.One difference (which has been overlooked thus far) in the brain regions that were activated in the processing of ASL stimuli compared with those that are activated in the processing of auditorily presented spoken language stimuli concerns the supratemporal plane, the dorsal aspect of the temporal lobe, which includes the transverse temporal (or Heschl's) gyrus and the planum temporale. This region is uniformly activated in hearing subjects listening to spoken language[17xFunctional magnetic resonance imaging of human auditory cortex. Binder, J.R. et al. Ann. Neurol. 1994; 35: 662–672CrossRef | PubMed | Scopus (299)See all References, 18xFunctional magnetic resonance imaging of the central auditory pathway following speech and pure-tone stimuli. Millen, S.J., Haughton, V.M., and Yetkin, Z. Laryngoscope. 1995; 105: 1305–1310CrossRef | PubMedSee all References, 24xFunctional MR imaging of auditorily presented words: a single-item presentation paradigm. Hickok, G. et al. Brain Lang. 1997; 58: 197–201CrossRef | PubMed | Scopus (27)See all References] but was not activated in deaf subjects watching ASL sentences in the Neville et al. study, nor was it activated in an fMRI study of single-sign perception in a native deaf signer[25xSensory mapping in a congenitally deaf subject: MEG and fMRI studies of cross-modal non-plasticity. Hickok, G. et al. Hum. Brain Mapp. 1997; 5: 437–444CrossRef | PubMed | Scopus (17)See all References][25].One potential explanation for this is that supratemporal plane structures are involved in processing non-linguistic auditory information[26xFunction of the left planum temporale in auditory and linguistic processing. Binder, J.T. et al. Brain. 1996; 119: 1239–1247CrossRef | PubMedSee all References][26]: because these are not language processing systems, perception of ASL would not be expected to activate these areas; speech stimuli on the other hand, would produce activation in supratemporal plane as a result of some type of acoustic response. Another possibility, however, is that the supratemporal plane contains systems directly and critically involved in the perception of speech (that is, extracting linguistic information from an auditory signal), as some authors have suggested (Ref. [27xLanguage-specific phoneme representations revealed by electric and magnetic brain responses. Naatanen, R. et al. Nature. 1997; 385: 432–434CrossRef | PubMed | Scopus (723)See all References][27] and D. Poeppel, PhD thesis, MIT, 1995). This hypothesis could explain the presence of supratemporal activation in auditory language perception and its absence in sign language perception. It also predicts that there should be some processing system outside of canonical language areas involved in the extraction of sign information from the visual input. On this view, there are both modality dependent and modality independent components to the neural organization of language perception. Modality dependent components are those involved in extracting linguistic information from the sensory input, modality independent components are those involved in operating on higher-level linguistic representations. Based on available data, it's possible that supratemporal plane structures are part of a modality dependent system involved in speech perception, whereas lateral temporal lobe structures are part of a modality independent system involved in higher-level linguistic operations.But all of this discussion hasn't really answered the question posed at the outset; that is, what is driving the neural organization of language? Well, we don't yet know for sure. In fact, the data reviewed above render this problem a bit more puzzling (and thus perhaps more interesting). What we do know is that modality-specific factors aren't the whole story. Save for the possibility of speech perception, the neural organization of language appears to be largely independent of the modalities through which it is perceived and produced. But notice that this conclusion rules out the most intuitive and probably the oldest answer to the above question, namely that language systems are really just dynamically organized subsystems of the particular sensory and motor channels through which language is used. Instead, the answer will have to be couched in terms that can generalize over modality. Whether such an account will ultimately appeal to genetically constrained domain-specific regional specializations or to some complex interaction of domain-general processing biases (or both) remains to be seen. Provocative issues indeed.

Event-related brain activity associated with auditory pattern processing.

One of the basic properties of the auditory system is the ability to analyse complex temporal patterns. Here, we investigated the neural activity associated with auditory pattern processing using event-related brain potentials. Participants were presented with a continuously repeating sequence of four tones with rare changes in either the frequency or timing of one of the tones. Both frequency- and time-deviant sounds generated mismatch negativity (MMN) waves that peaked at midline central electrode sites and inverted in polarity at inferior temporal and occipital sites, consistent with generators in the supratemporal plane. The MMN scalp topography was similar for the frequency- and time-deviant stimuli, suggesting that both spectral and temporal relations among elements of an auditory pattern are encoded in a unified memory trace.

Corticostriatal connections of the superior temporal region in rhesus monkeys

Corticostriatal connections of auditory areas within the supratemporal plane and in rostral and caudal portions of the superior temporal gyrus were studied by the autoradiographic anterograde tracing technique. The results show that the primary auditory cortex has limited projections to the caudoventral putamen and to the tail of the caudate nucleus. In contrast, the second auditory area within the circular sulcus has connections to the rostral and the caudal putamen and to the body of the caudate nucleus and the tail. The association areas of the superior temporal gyrus collectively have widespread corticostriatal projections characterized by differential topographic distributions. The rostral part of the gyrus projects to ventral portions of the head of the caudate nucleus and of the body and to the tail. In addition, there are connections to rostroventral and caudoventral portions of the putamen. The mid-portion of the gyrus projects to similar striatal regions, but the connections to the head of the caudate nucleus are less extensive. Compared with the rostral and middle parts of the superior temporal gyrus, the caudal portion has little connectivity to the tail of the caudate nucleus. It projects more dorsally within the head and the body and also more dorsally within the caudal putamen. These differential patterns of corticostriatal connectivity are consistent with functional specialization at the cortical level.

Corticostriatal connections of the superior temporal region in rhesus monkeys

Corticostriatal connections of auditory areas within the supratemporal plane and in rostral and caudal portions of the superior temporal gyrus were studied by the autoradiographic anterograde tracing technique. The results show that the primary auditory cortex has limited projections to the caudoventral putamen and to the tail of the caudate nucleus. In contrast, the second auditory area within the circular sulcus has connections to the rostral and the caudal putamen and to the body of the caudate nucleus and the tail. The association areas of the superior temporal gyrus collectively have widespread corticostriatal projections characterized by differential topographic distributions. The rostral part of the gyrus projects to ventral portions of the head of the caudate nucleus and of the body and to the tail. In addition, there are connections to rostroventral and caudoventral portions of the putamen. The mid-portion of the gyrus projects to similar striatal regions, but the connections to the head of the caudate nucleus are less extensive. Compared with the rostral and middle parts of the superior temporal gyrus, the caudal portion has little connectivity to the tail of the caudate nucleus. It projects more dorsally within the head and the body and also more dorsally within the caudal putamen. These differential patterns of corticostriatal connectivity are consistent with functional specialization at the cortical level. J. Comp. Neurol. 399:384–402, 1998. © 1998 Wiley-Liss, Inc.

Combined mapping of human auditory EEG and MEG responses

Auditory electric and magnetic P50(m), N1(m) and MMN(m) responses to standard, deviant and novel sounds were studied by recording brain electrical activity with 25 EEG electrodes simultaneously with the corresponding magnetic signals measured with 122 MEG gradiometer coils. The sources of these responses were located on the basis of the MEG responses; all were found to be in the supratemporal plane. The goal of the present paper was to investigate to what degree the source locations and orientations determined from the magnetic data account for the measured EEG signals. It was found that the electric P50, N1 and MMN responses can to a considerable degree be explained by the sources of the corresponding magnetic responses. In addition, source-current components not detectable by MEG were shown to contribute to the measured EEG signals.

Lateralized processing of speech prosodies in the temporal cortex: a 3-T functional magnetic resonance imaging study.

Prosodic modulation of speech provides information about emotional states of speakers (affective prosodies) or serves as syntactic markers to change linguistic aspects of speech (linguistic prosodies). Previous electrophysiological investigations and studies on patients with right or left hemisphere damage showed nonuniform results with respect to lateralization of prosodic processing. In this study 20 healthy right-handed volunteers were investigated with functional magnetic resonance imaging of the acoustically responsive areas on the supratemporal plane while detecting phonemes as control targets or prosodies in strings of nonsense syllables and adjectives, the latter randomly intonated in a declarative, interrogative, commanding, happy, or sad fashion. In control task A the phoneme /a/ was detected in the syllables. In control task B the phoneme /a/ was detected in the adjectives, and in the experimental task C the sad intonations (affective) and in the experimental task D the interrogative intonations (linguistic) had to be detected in the same material. In task A intensity-weighted volumes of activated voxels were not different in the two hemispheres (laterality index 0). In task B with an irrelevant phoneme detection with respect to prosodic material, the population split into two subgroups with similar right or left hemispheric lateralization of activity leading to an absolute laterality index of 26.8 across all subjects. During detection of affective prosodies (task C), lateralization was maintained yet the absolute laterality index reduced to 14.5, while there was no lateralization during detection of linguistic prosodies. The sum of activations in the two hemispheres was the same across all tasks and subgroups, which suggests that the lateralizations occurring with presentation and detection of prosodic material depend on a redistribution of activity between hemispheres.

Cytochrome Oxidase, Acetylcholinesterase, and NADPH-Diaphorase Staining in Human Supratemporal and Insular Cortex: Evidence for Multiple Auditory Areas

The pattern of cytochrome oxidase, acetylcholinesterase, and NADPH-diaphorase activity was studied in the supratemporal plane, the posterior part of the superior temporal gyrus, and the insula of normal human brains. Five dark cytochrome oxidase regions were found: (i) on Heschl's gyrus (area TC of von Economo and Koskinas); (ii) on the planum polare (area TC/TG); (iii) posterior to Heschl's gyrus (within area TA); (iv) on the posterior convexity of the superior temporal gyrus (within area TA); and (v) on the posterosuperior insula (area IB). More lightly stained cortex separated these regions (areas IA, TD, and part of TB). The laminar distribution of cytochrome oxidase activity varied in different areas. Acetylcholinesterase-positive fibers predominated in area TC and pyramidal neurons in areas TA and IA and in parts of TB; a mixture of fiber and neuronal staining was found in TC/TG, TD, and IB. NADPH-diaphorase positive profiles included large darkly stained nonpyramidal neurons, mostly in infragranular layers and in subcortical white matter, small faintly stained cells, and a dense array of fibers. The NADPH-diaphorase staining pattern did not vary between areas. The present results suggest that the supratemporal plane, the posterior part of the superior temporal gyrus, and the insula contain at least eight putative cortical areas. Comparison with activation studies by others suggest that, apart from the primary auditory area, six other putative areas may be auditory whereas one putative area, on posterior insula, may be vestibular.

Tonotopic organization of auditory cortical fields delineated by parvalbumin immunoreactivity in macaque monkeys

Tonotopic maps, obtained from single and multi-unit recordings in the primary and surrounding areas of the auditory cortex, were related to chemoarchitecture of the supratemporal plane, as delineated by immunoreactivity for parvalbumin. Neurons in the central core were sharply tuned and formed two complete tonotopic representations corresponding to the primary auditory area (AI) and the rostral (R) area. High frequencies were represented posteriorly in AI and anteriorly in R, the representation reversing in the anterior part of the core. Neurons in regions of less dense immunostaining previously described as lateral (L) and posteromedial (P-m) fields, showed broader frequency tuning. Two tonotopic representations were found in L: in an anterolateral (AL) field, corresponding to a field previously reported by others, high frequencies were represented anteriorly and low frequencies posteriorly; in a posterolateral field (PL) the trend reversed. There was a further reversal on entering P-m from the high frequency representation in PL and progressively lower frequencies tended to be represented more medially in P-m, but P-m may contain two representations reported by others. Neurons in the previously described anteromedial (A-m) and medial (M) fields of weaker immunostaining, were even more broadly tuned. A tonotopic progression from low frequency representation posteriorly to high frequency representation anteriorly was observed in the medial field. Frequency representation in A-m remains uncertain. No tonotopic representation could be demonstrated with the stimuli used in the zones of very weak parvalbumin immunostaining outside AL, PL, P-m, A-m, and M. The properties of neurons in the core and surrounding zones are likely to reflect inputs from the ventral and dorsal medial geniculate nuclei, respectively. The fields outside the core seem to be the starting points for separate streams of auditory corticocortical connections passing into association cortex.