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PubMed2001

Neural correlates of auditory stream segregation in primary auditory cortex of the awake monkey.

An important feature of auditory scene analysis is the perceptual organization of sequential sound components, or 'auditory stream segregation'. Auditory stream segregation can be demonstrated by presenting a sequence of high and low frequency tones in an alternating pattern, ABAB. When the tone presentation rate (PR) is slow or the frequency separation (DeltaF) between the tones is small (<10%), a connected alternating sequence ABAB is perceived. When the PR is fast or the DeltaF is large, however, the alternating sequence perceptually splits into two parallel auditory streams, one composed of interrupted 'A' tones, and the other of interrupted 'B' tones. The neurophysiological basis of this perceptual phenomenon is unknown. Neural correlates of auditory stream segregation were examined in A1 of the awake monkey using neuronal ensemble techniques (multiunit activity and current source density). Responses evoked by alternating frequency sequences of tones, ABAB, were studied as a function of PR (5, 10, 20 and 40 Hz). 'A' tones corresponded to the best frequency (BF) of the cortical site, while 'B' tones were situated away from the BF by an amount DeltaF. At slow PRs, 'A' and 'B' tones evoked responses that generated an overall pattern of activity at the stimulus PR. In contrast, at fast PRs, 'B' tone responses were differentially suppressed, resulting in a pattern of activity consisting predominantly of 'A' tone responses at half the PR. The magnitude of 'B' tone response suppression increased with DeltaF. Differential suppression of BF and non-BF tone responses at high PRs can be explained by physiological principles of forward masking. The effect of DeltaF is explained by the hypothesis that responses to tones distant from the BF are more susceptible to suppression by BF tones than responses to tones near the BF. These results parallel human psychoacoustics of auditory stream segregation and suggest a cortical basis for the perceptual phenomenon.

Acoustic Stimulation↗
PubMed1989

Neuromagnetic evidence of an amplitopic organization of the human auditory cortex.

It is well known that the location of the source of cortical auditory evoked responses, which can be determined neuromagnetically in humans using the concept of an equivalent current dipole (ECD), shifts with changing stimulus frequency ('tonotopic organization'). Not investigated so far, however, is the question of whether there exists also an 'amplitopic organization' of the human auditory cortex, i.e., a spatial distribution of neurons maximally responsive to respective 'best stimulus intensities.' We measured, in the study presented here, in 3 normally hearing subjects the auditory evoked magnetic field (AEF) in response to tone-burst stimulation with a carrier frequency of 1000 Hz at 6 different intensities (30-80 dB HL in 10 dB steps). The influence of stimulus intensity was quantified in terms of changes in the ECD parameters (amplitude, direction and spatial coordinates) which were determined such that a maximum correspondence between observed and calculated field distributions was obtained. The results of the neuromagnetic measurements presented here prove that the ECD location also shifts with changing stimulus intensity. The depth of wave M100 (latency of about 100 msec) decreases monotonically with increasing stimulus intensity while the horizontal ECD position is slightly shifted in the anterior direction. The results imply that, while topical mechanisms of frequency coding are similar at cortex and at the cochlear level, topical mechanisms of intensity coding are different at these levels.

Acoustic Stimulation↗
PubMed1997

Optical study of spatiotemporal inhibition evoked by two-tone sequences in the guinea pig auditory cortex.

Spatiotemporal response patterns in the anterior and dorsocaudal fields of the guinea pig auditory cortex after two-tone sequences were studied in anesthetized animals (Nembutal 30 mg kg-1) using an optical recording method (voltage-sensitive dye RH795, 12 x 12 photodiode array). Each first (masker) and second (probe) tone was 30 ms long with a 10-ms rise-fall time. Masker-probe pair combinations of the same or different frequencies with probe delays of 30-150 ms were presented to the ear contralateral to the recording side. With same-frequency pairs, responses to the probe were inhibited completely after probe delays of less than 50 ms and the inhibition lasted for more than 150 ms, and the inhibition magnitudes in different isofrequency bands of the anterior field were essentially the same. With different-frequency (octave-separated) pairs, responses to the probe were not inhibited completely even after probe delays as short as 30 ms, and the inhibition lasted only for 110-130 ms. Inhibition magnitudes were different from location to location.

Acoustic Stimulation↗
PubMed2000

Auditory cortex accesses phonological categories: an MEG mismatch study.

The studies presented here use an adapted oddball paradigm to show evidence that representations of discrete phonological categories are available to the human auditory cortex. Brain activity was recorded using a 37-channel biomagnetometer while eight subjects listened passively to synthetic speech sounds. In the phonological condition, which contrasted stimuli from an acoustic /dae/-/tae/ continuum, a magnetic mismatch field (MMF) was elicited in a sequence of stimuli in which phonological categories occurred in a many-to-one ratio, but no acoustic many-to-one ratio was present. In order to isolate the contribution of phonological categories to the MMF responses, the acoustic parameter of voice onset time, which distinguished standard and deviant stimuli, was also varied within the standard and deviant categories. No MMF was elicited in the acoustic condition, in which the acoustic distribution of stimuli was identical to the first experiment, but the many-to-one distribution of phonological categories was removed. The design of these studies makes it possible to demonstrate the all-or-nothing property of phonological category membership. This approach contrasts with a number of previous studies of phonetic perception using the mismatch paradigm, which have demonstrated the graded property of enhanced acoustic discrimination at or near phonetic category boundaries.

Acoustic Stimulation↗
PubMed1984

The pyramidal neurons in layer III of cat primary auditory cortex (AI).

The neuronal architecture of pyramidal cells in layer III of the primary auditory cortex (AI) of adult cats was examined as a prelude to connectional and fine structural studies; in a further paper, the results of parallel studies of non-pyramidal layer III cells are presented. Layer III is about 400 micron thick, comprises about one-quarter of the thickness of AI, and lies some 400-800 micron deep to the pial surface. It is distinguished in Nissl, fiber, and Golgi preparations from layers II and IV, and also on connectional grounds, since its neurons are one of the principal inputs to the contralateral AI. Layer III may be divided into two roughly equal tiers on the basis of its neuronal and cytoarchitecture. Layer IIIa is populated by small cells with oval somata and many tiny pyramidal cells; the fiber architecture is dominated by radial bundles of medium-sized axons interspersed among columns of apical dendrites arising from deeper-lying pyramidal cells. In layer IIIb medium-sized and large pyramidal cells are more numerous, and the fiber architecture has a different, much denser texture, including extensive lateral components which invade layer IV, and large contingents of descending, probably corticofugal, axons. Five kinds of pyramidal neurons occur in Golgi preparations. Most numerous are the small, medium-sized, and large pyramidal cells; the two types of star pyramidal neurons are less common. The small pyramidal cell has a limited dendritic field and rather delicate dendrites; all but the apical one usually end in layer III. The medium-sized pyramidal cell is the most common neurons, and its rich basilar dendritic arbors are conspicuous, with their many dendritic appendages, in the layer III neuropil; their distal dendrites spread into layer IV. The largest pyramidal cells lie mainly in layer IIIb, and their lateral dendrites often mark the layer IIIb-IVa border. The apical dendrites of medium-sized and large pyramidal cells often extend to layer Ib, where they branch obliquely. The axons of these cells branch laterally after descending through layer III and toward the white matter. Often secondary or tertiary branches reascend to layer IV and more superficially; there is considerable stereotypy in this branching pattern. These numerous secondary branches contribute heavily to the layer IIIb-IVa lateral fiber plexus. The fourth variety of pyramidal cell has a round soma and a stellate dendritic field whose distal branches extend from layer V to layer I, but whose axon is chiefly in layer III. Finally, a star pyramidal cell with long lateral basilar arbors but rather smooth dendrites completes the picture.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1984

Topographic and cytoarchitectonic organization of thalamic neurons related to their targets in low-, middle-, and high-frequency representations in cat auditory cortex.

We studied the topographic organization of thalamic projections upon different ranges of cortical frequency representation. Thalamic neurons were labeled by injecting horseradish peroxidase (HRP) or tritiated bovine serum albumin into auditory cortex. Injections in individual brains were confined to the same range of frequency representation, and distributed through three or four tonotopic cortical fields in order to label as much of the thalamic projection upon a limited range of frequency representation as practicable. Low, middle, and high ranges of the frequency representation were injected in different brains. The spatial organizations of arrays of labeled neurons are described, and each array is divided into a ventral division and lateral posterior complex (lateral part of the posterior thalamic group), both composed mainly of small cells; and a medial division, composed mainly of medium and large cells. The ventral and medial divisions (located laterally and medially within the medial geniculate body (MGB), respectively), both contact the lateral posterior complex which is located rostrally. The HRP cytoarchitecture of the three divisions is described, and the portions of the ventral division corresponding with the physiologically and cytoarchitectonically defined ventral nucleus are identified. Relatively few labeled neurons were found within other thalamic areas. The topographic organizations of the ventral division (and its tonotopic subdivision, the ventral nucleus), the lateral posterior complex (also tonotopically organized), and the medial division are described. There are planar and concentric components of the topographic organization in the ventral nucleus. Within the planar component, the low-frequency area is located laterally and the high-frequency area is located rostromedially. Within the concentric component, the low-frequency area is located centrally and the high-frequency area is located peripherally. Low-, middle-, and high-frequency areas course without interruption through the planar and concentric components. In the lateral posterior complex, the low-frequency area is located rostrally, and the high-frequency area is located caudally adjoining the high-frequency area in the ventral nucleus. The topographic organizations of the ventral nucleus and lateral posterior complex are consistent with tonotopic maps of these regions. The medium- and large-cell portion of the medial division is also topographically organized, although there may be more overlap among low-, middle-, and high-frequency arrays than in the ventral nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Temporal modulation transfer functions for AM and FM stimuli in cat auditory cortex. Effects of carrier type, modulating waveform and intensity.

For 167 single units, recorded from primary auditory cortex in 28 cats, we show that tuning to the modulation frequency (MF) of amplitude-modulated (AM) sound is strongly dependent on carrier type. In general AM noise-bursts and click-trains produce good tuning to MFs with repetition rates around 8-10 Hz. Amplitude- or frequency-modulation of tone-carriers resulted largely in low-pass temporal modulation transfer functions (tMTFs) with a best modulation frequency (BMF) around 4 Hz. Individual BMFs for noise carriers ranged from 3-26 Hz, whereas for tone carriers they were mostly below 6 Hz and rarely above 10 Hz. The sharpness of tuning for broad-band stimuli decreased with increasing duty-cycle of the modulation; it was most pronounced for clicks, next best for exponential sine-AM and broadest for sinusoidal AM. In contrast the reverse was found for tone carriers; the better modulation following was found for sinusoidal modulation and was most likely entirely due to a stronger onset response. Decreasing the modulation depth below 100% showed an increasing influence of onset transients and periodic rebounds, however, the average tMTFs for depths between 50-100% are similar. The optimal intensity level for noise carriers was usually higher than for tone carriers. Overall the modulation-sensitivity of cortical neurons regardless of carrier type and modulating waveform was in the range of modulation frequencies found in music, speech and other complex sounds.

Acoustic Stimulation↗
PubMedDate not supplied

Convergent but temporally separated inputs to lateral amygdala neurons from the auditory thalamus and auditory cortex use different postsynaptic receptors: in vivo intracellular and extracellular recordings in fear conditioning pathways.

The lateral nucleus of the amygdala (LA), a key component of the fear conditioning circuitry, receives a rapid but relatively impoverished auditory input from the auditory thalamus and a slower but richer input from the auditory cortex. We examined in urethane anesthetized rats whether individual cells in the LA receive convergent inputs from these two areas, and whether different postsynaptic receptors contribute to the temporally separated excitations over the two pathways. With both extracellular and intracellular recordings, individual cells could be activated by stimulation of each pathway. In extracellular recordings iontophoretic application of the N-methyl-D-aspartate (NMDA) receptor antagonist APV and the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist CNQX demonstrated that synaptic transmission in both pathways depends on AMPA receptors, whereas transmission in the thalamic pathway also depends on the involvement of NMDA receptors. The involvement of NMDA receptors in synaptic activation of the LA from the thalamus but not the cortex was confirmed in intracellular recordings using systemic injections of the NMDA antagonist MK-801. The slow time course of NMDA currents could provide LA cells with a mechanism to integrate the inputs arriving rapidly from the thalamus and somewhat later from the cortex, thus allowing the LA to integrate signals in the two pathways during the acquisition and expression of conditioned fear reactions.

Amygdala↗
PubMed1983

Effects of stimulation of the primary auditory cortex upon colliculogeniculate neurons in the inferior colliculus of the cat.

Electrical stimulation of the primary auditory cortex (AI) of the cat was found to evoke EPSPs, IPSPs or EPSP-IPSP sequences in colliculogeniculate (CG) neurons in the inferior colliculus (IC) which responded antidromically to stimulation of the medial geniculate nucleus. The CG neurons responding to the AI stimulation with short-latency EPSPs (1.0-1.4 msec) were located in the dorsomedial portion of the central nucleus of the IC. On the other hand, latencies of IPSPs elicited in CG neurons by AI stimulation ranged from 2.0 to 4.5 msec.

Animals↗
PubMed2003

Spectral interactions, but no mismatch negativity, in auditory cortex of anesthetized rat.

We used an 'oddball' stimulation paradigm to evoke epidural, event-related potentials over auditory cortex in urethane/xylazine-anesthetized rats, in order to study the neural mechanisms underlying the mismatch negativity (MMN). Oddball stimulus sets included deviants (10 kHz, 10% probability) and one of six standards that ranged from 1.25 to 9.75 kHz (overall stimulus rate 2/s). Controls included 'reference' responses (10 kHz, stimulus rate 1/5 s) obtained before and after each oddball set. We found that the deviant-evoked response was modified by the preceding standards, and the degree of modification varied systematically with the spectral separation between the standard and deviant. However, the modification was apparent relative to the reference, rather than relative to the standard (as with the MMN), and did not resemble the MMN. In fact, we found no evidence for the MMN in this preparation.

Acoustic Stimulation↗
PubMed1998

Net interaction between different forms of short-term synaptic plasticity and slow-IPSPs in the hippocampus and auditory cortex.

Paired-pulse plasticity is typically used to study the mechanisms underlying synaptic transmission and modulation. An important question relates to whether, under physiological conditions in which various opposing synaptic properties are acting in parallel, the net effect is facilitatory or depressive, that is, whether cells further or closer to threshold. For example, does the net sum of paired-pulse facilitation (PPF) of excitatory postsynaptic potentials (EPSPs), paired-pulse depression (PPD) of inhibitory postsynaptic potentials (IPSPs), and the hyperpolarizing slow IPSP result in depression or facilitation? Here we examine how different time-dependent properties act in parallel and examine the contribution of gamma-aminobutyric acid-B (GABAB) receptors that mediate two opposing processes, the slow IPSP and PPD of the fast IPSP. Using intracellular recordings from rat CA3 hippocampal neurons and L-II/III auditory cortex neurons, we examined the postsynaptic responses to paired-pulse stimulation (with intervals between 50 and 400 ms) of the Schaffer collaterals and white matter, respectively. Changes in the amplitude, time-to-peak (TTP), and slope of each EPSP were analyzed before and after application of the GABAB antagonist CGP-55845. In both CA3 and L-II/III neurons the peak amplitude of the second EPSP was generally depressed (further from threshold) compared with the first at the longer intervals; however, these EPSPs were generally broader and exhibited a longer TTP that could result in facilitation by enhancing temporal summation. At the short intervals CA3 neurons exhibited facilitation of the peak EPSP amplitude in the absence and presence of CGP-55845. In contrast, on average L-II/III cells did not exhibit facilitation at any interval, in the absence or presence of CGP-55845. CGP-55845 generally "erased" short-term plasticity, equalizing the peak amplitude and TTP of the first and second EPSPs at longer intervals in the hippocampus and auditory cortex. These results show that it is necessary to consider all time-dependent properties to determine whether facilitation or depression will dominate under intact pharmacological conditions. Furthermore our results suggest that GABAB-dependent properties may be the major contributor to short-term plasticity on the time scale of a few hundred milliseconds and are consistent with the hypothesis that the balance of different time-dependent processes can modulate the state of networks in a complex manner and could contribute to the generation of temporally sensitive neural responses.

Animals↗
PubMed1992

Neural selectivity for interaural frequency disparity in cat primary auditory cortex.

Single-unit responses to interaural frequency disparities (IFDs) were examined in 74 neurons in cat primary auditory cortex (AI). Thirty-three of these cells were classified as EE (binaural facilitators), 39 were classified as EI (binaural inhibitors), and 2 were classified as EO (binaural occluders). The best frequency (BF) was presented to the dominant (usually the contralateral) ear while tones of the same or different frequency (either higher or lower than BF) were presented simultaneously to the nondominant (usually the ipsilateral) ear. Most cells displayed sensitivity to IFDs and thus were classified according to the IFD condition that elicited the strongest facilitatory or inhibitory response. The stimulus condition which evoked the strongest binaural response is referred to as the best IFD. For 50 cells (68%), the best IFD response was obtained when tones of different frequency were presented to each ear. Across the entire sample, binaural IFD responses of cortical neurons were categorized into one of three groups: Those preferring a lower frequency than BF in the ipsilateral ear (referred to as the 'lower IFD group'), those preferring a frequency equal to BF (the 'zero IFD group'), or those preferring a frequency higher than BF (the 'higher IFD group'). Among EE cells, approximately one third were maximally facilitated when the ipsilateral ear frequency was lower than BF, one third when it was equal to BF, and one third when it was higher than BF. Among EI cells, 50% exhibited deepest inhibition for higher IFDs with relatively fewer cells showing inhibition for zero or lower IFDs. Overall, EI cells responded over a broader range of IFD conditions than EE cells. Finally, approximately 50% of all units exhibited bimodal responses such that cells classified as EE displayed some inhibitory response characteristics when stimulated with certain IFD conditions and vice versa.

Acoustic Stimulation↗
PubMed2000

Temporal discharge patterns evoked by rapid sequences of wide- and narrowband clicks in the primary auditory cortex of cat.

The present study investigated neural responses to rapid, repetitive stimuli in the primary auditory cortex (A1) of cats. We focused on two important issues regarding cortical coding of sequences of stimuli: temporal discharge patterns of A1 neurons as a function of inter-stimulus interval and cortical mechanisms for representing successive stimulus events separated by very short intervals. These issues were studied using wide- and narrowband click trains with inter-click intervals (ICIs) ranging from 3 to 100 ms as a class of representative sequential stimuli. The main findings of this study are 1) A1 units displayed, in response to click train stimuli, three distinct temporal discharge patterns that we classify as regions I, II, and III. At long ICIs nearly all A1 units exhibited typical stimulus-synchronized response patterns (region I) consistent with previously reported observations. At intermediate ICIs, no clear temporal structures were visible in the responses of most A1 units (region II). At short ICIs, temporal discharge patterns are characterized by the presence of either intrinsic oscillations (at approximately 10 Hz) or a change in discharge rate that was a monotonically decreasing function of ICI (region III). In some A1 units, temporal discharge patterns corresponding to region III were absent. 2) The boundary between regions I and II (synchronization boundary) had a median value of 39.8 ms ICI ([25%, 75%] = [20.4, 58. 8] ms ICI; n = 131). The median boundary between regions II and III was estimated at 6.3 ms ([25%, 75%] = [5.2, 9.7] ms ICI; n = 47) for units showing rate changes (rate-change boundary). 3) The boundary values between different regions appeared to be relatively independent of stimulus intensity (at modest sound levels) or the bandwidth of the clicks used. 4) There is a weak correlation between a unit's synchronization boundary and its response latency. Units with shorter latencies appeared to also have smaller boundary values. And 5) based on these findings, we proposed a two-stage model for A1 neurons to represent a wide range of ICIs. In this model, A1 uses a temporal code for explicitly representing long ICIs and a rate code for implicitly representing short ICIs.

Acoustic Stimulation↗
PubMedDate not supplied

Information cascade from primary auditory cortex to the amygdala: corticocortical and corticoamygdaloid projections of temporal cortex in the rat.

Corticocortical and corticoamygdaloid connections of temporal cortext and perirhinal cortex (PRh) were examined in the rat with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). Iontophoretic injections of PHA-L into area TE1 resulted in columnar axonal terminations in surrounding and contralateral regions of temporal neocortex and in the striatum, but not in the amygdala. Within temporal neocortex, labeled fibers were present locally in adjacent regions of TE1, as well as in TE2d, TE1v, TE3v, and TE2c. Injection of cortical areas TE1v, TE3v, and TE2c, which received projections from TE1, or injections of perirhinal periallocortex, which received projections from TE1v, TE2v, and TE3v, resulted in projections to the amygdala. The pattern of corticocortical and corticoamygdaloid projections differed among the divisions of auditory cortex. TE1 exhibited extensive ipsilateral and contralateral projections to temporal cortical regions and no projections to the amygdala. In contrast, areas of temporal neocortex ventral and posterior to TE1, including TE1v, TE3v, TE2c, and PRh, had more limited ipsi- and contralateral corticocortical projections but had an increased connectivity with the subcortical forebrain, especially the lateral nucleus of the amygdala (AL). There was a topographic organization to the AL afferents. The dorsal subdivision of AL received projections from TE1v, TE3v, TE2c, and PRh, while the ventrolateral division received projections from TE3v, TE2c, and PRh. The ventromedial division received projections only from PRh, which, unlike other temporal cortical areas, also projected to the basolateral and basomedial nuclei of the amygdala. These findings define the complete sequence of connections linking primary auditory cortex with the amygdala in the rat. In addition, the findings indicate that the ventral portion of TE1, designated TE1v, has connections that distinguish it from dorsal TE1, namely, dense projections to AL and a diminished number of corticocortical projections ipsilaterally and contralaterally. Finally, the results suggest a topographic organization to the cortical terminations within the amygdala.

Amygdala↗
PubMed2005

Representation of the purr call in the guinea pig primary auditory cortex.

Guinea pigs produce the low-frequency purr or rumble call as an alerting signal. A digitised example of the call was presented to anaesthetised guinea pigs via a closed sound system while recording from the primary auditory cortex. The exemplar used in this study had 9 regular phrases each spaced with their centres about 80 ms apart. Low-frequency (1.1 kHz) units responded best to the call but within this population there were four separate groups: (1) cells that responded vigorously to many or all of the 9 phrases; (2) cells that gave an onset response; (3) cells that only responded to a click embedded in the call; (4) cells that did not respond. Particular response types were often grouped together. Thus when orthogonal electrode tracks were used most units gave a similar response. There was no correlation between the type of response and the cortical depth. A similar range of response types was also found in the thalamus and there was no evidence of a distinct response in the cortex that was due to intracortical processing. Cells in the cortex were able to represent the temporal structure of the purr with the same fidelity as cells in the thalamus.

Acoustic Stimulation↗
PubMed2006

Spectrally enhanced acoustic environment disrupts frequency representation in cat auditory cortex.

Sensory environments are known to shape nervous system organization. Here we show that passive long-term exposure to a spectrally enhanced acoustic environment (EAE) causes reorganization of the tonotopic map in juvenile cat auditory cortex without inducing any hearing loss. The EAE consisted of tone pips of 32 different frequencies (5-20 kHz), presented in random order at an average rate of 96 Hz. The EAE caused a strong reduction of the representation of EAE frequencies and an over-representation of frequencies neighboring those of the EAE. This is in sharp contrast with earlier developmental studies showing an enlargement of the cortical representation of EAEs consisting of a narrow frequency band. We observed fewer than normal appropriately tuned short-latency responses to EAE frequencies, together with more common long-latency responses tuned to EAE-neighboring frequencies.

Acoustic Stimulation↗
PubMed1995

Auditory cortex activity changes in long-term sensorineural deprivation during crude cochlear electrical stimulation: evaluation by positron emission tomography.

We studied three right-handed human volunteers who have been prelingually deaf for 16 to 26 years. We measured cerebral regional activity (rA) using 15O labelled water and positron emission tomography (PET) during rest and during electrical cochlear stimulation of the right ear. The stimulus consisted of crude constant current squared pulses, it is currently employed in cochlear implant screening. Two subjects described a subjective auditory sensation under cochlear stimulation, the third did not. An increment of the rA (which is linked to the regional cerebral blood flow) in the auditory cortex was observed in all subjects, activation was ipsilateral to stimulation in one subject and contralateral in two subjects. These findings suggest 1) that auditory pathways to the cortex can remain functional a long time after prelinguistic auditory deprivation, 2) that the auditory cortex can be activated by a crude electrical stimulation of the cochlea in the absence of perception of the auditory stimulus, 3) that PET does not seem to offer any advantage for screening patients who have been prelingually deaf for a long time.

Adult↗
PubMed2003

Anisotropic functional connections between the auditory cortex and area 18a in rat cerebral slices.

We developed a new method to visualize the myeloarchitecture in fresh slices, and investigated the properties of the functional neural connections around the boundary between the primary auditory cortex (area 41) and area 18a in rat cerebral slices. A fresh slice illuminated by near-vertical light was observed with a CCD camera. The translucent images of the slice showed contrast patterns very similar to myeloarchitecture. The boundary between these areas was identified by the well-developed layer IV/V in area 41 but not in area 18a. Antidromic/presynaptic components of the field potentials stimulated and recorded across the areal boundary showed symmetric distribution, while the postsynaptic field potentials in the direction from area 41 to 18a were more prominent than those in the opposite direction in layer II/III. In contrast, the dominant direction of propagation of postsynaptic potentials was from area 18a to 41 in layer V. In the presence of 1 microM bicuculline, an inhibitor of GABA(A) receptors, the polysynaptic activities propagating from area 18a into 41 via layer V were elicited by stimulation of area 18a. The propagation measured by Ca(2+) imaging or field potential recordings was potentiated after both areas 18a and 41 were alternately stimulated several times.

Animals↗