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

Auditory detection is modulated by theta phase of silent lip movements

Audiovisual speech perception relies, among other things, on our expertise to map a speaker's lip movements with speech sounds. This multimodal matching is facilitated by salient syllable features that align lip movements and acoustic envelope signals in the 4–8 ​Hz theta band. Although non-exclusive, the predominance of theta rhythms in speech processing has been firmly established by studies showing that neural oscillations track the acoustic envelope in the primary auditory cortex. Equivalently, theta oscillations in the visual cortex entrain to lip movements, and the auditory cortex is recruited during silent speech perception. These findings suggest that neuronal theta oscillations may play a functional role in organising information flow across visual and auditory sensory areas. We presented silent speech movies while participants performed a pure tone detection task to test whether entrainment to lip movements directs the auditory system and drives behavioural outcomes. We showed that auditory detection varied depending on the ongoing theta phase conveyed by lip movements in the movies. In a complementary experiment presenting the same movies while recording participants' electro-encephalogram (EEG), we found that silent lip movements entrained neural oscillations in the visual and auditory cortices with the visual phase leading the auditory phase. These results support the idea that the visual cortex entrained by lip movements filtered the sensitivity of the auditory cortex via theta phase synchronization.

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Subjects entrain to visual activity conveyed by speakers' lip movements.

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Visual entrainment modulates auditory perception and performances.

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Silent lips perception recruits both visual and auditory cortices.

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Visual and auditory cortices synchronize via theta phase coupling.

Synaptic properties of corticocortical connections between the primary and secondary visual cortical areas in the mouse

Despite the importance of corticocortical connections, few published studies have investigated the functional, synaptic properties of such connections in any species, because most studies have been purely anatomical or aimed at functional features other than synaptic properties. We recently published a study of synaptic properties of connections between the primary and secondary cortical auditory areas in brain slices from the mouse, and, in the present study, we aimed to extend this by performing analogous studies of the primary and secondary visual areas (V1 and V2). We found effectively the same results. That is, connections between V1 and V2 in both directions were quite similar; in each case, the glutamatergic inputs could be classified as one of two types, Class 1B (formerly "driver") and Class 2 (formerly "modulator"). There is a clear laminar correlation for these different inputs, in terms of both the laminae of origin and those in which the recorded cells were located. Our data suggest a common pattern to the functional organization of corticocortical connectivity in the mouse cortex.

Spatiotemporal dynamics of epileptiform propagations: imaging of human brain slices

Seizure activities often originate from a localized region of the cerebral cortex and spread across large areas of the brain. The properties of these spreading abnormal discharges may account for clinical phenotypes in epilepsy patients, although the manner of their propagation and the underlying mechanisms are not well understood. In the present study we performed flavoprotein fluorescence imaging of cortical brain slices surgically resected from patients with partial epilepsy caused by various symptomatic lesions. Elicited neural activities in the epileptogenic tissue spread horizontally over the cortex momentarily, but those in control tissue taken from patients with brain tumors who had no history of epilepsy demonstrated only localized responses. Characteristically, the epileptiform propagation comprised early and late phases. When the stimulus intensity was changed gradually, the early phase showed an all-or-none behavior, whereas the late phase showed a gradual increase in the response. Moreover, the two phases were propagated through different cortical layers, suggesting that they are derived from distinct neural circuits. Morphological investigation revealed the presence of hypertrophic neurons and loss of dendritic spines, which might participate in the aberrant activities observed by flavoprotein fluorescence imaging. These findings indicate that synchronized activities of the early phase may play a key role in spreading abnormal discharges in human cortical epilepsies.

Synaptic properties of connections between the primary and secondary auditory cortices in mice

Little is known regarding the synaptic properties of corticocortical connections from one cortical area to another. To expand on this knowledge, we assessed the synaptic properties of excitatory projections from the primary to secondary auditory cortex and vice versa. We identified 2 types of postsynaptic responses. The first class of responses have larger initial excitatory postsynaptic potentials (EPSPs), exhibit paired-pulse depression, are limited to ionotropic glutamate receptor activation, and have larger synaptic terminals; the second has smaller initial EPSPs, paired-pulse facilitation, metabotropic glutamate receptor activation, and smaller synaptic terminals. These responses are similar to the driver and modulator properties previously identified for thalamic and thalamocortical circuitry, suggesting that the same classification may extend to corticocortical inputs and have an implication for the functional organization of corticocortical circuits.

Visual activation and audiovisual interactions in the auditory cortex during speech perception: intracranial recordings in humans

Hemodynamic studies have shown that the auditory cortex can be activated by visual lip movements and is a site of interactions between auditory and visual speech processing. However, they provide no information about the chronology and mechanisms of these cross-modal processes. We recorded intracranial event-related potentials to auditory, visual, and bimodal speech syllables from depth electrodes implanted in the temporal lobe of 10 epileptic patients (altogether 932 contacts). We found that lip movements activate secondary auditory areas, very shortly (approximately equal to 10 ms) after the activation of the visual motion area MT/V5. After this putatively feedforward visual activation of the auditory cortex, audiovisual interactions took place in the secondary auditory cortex, from 30 ms after sound onset and before any activity in the polymodal areas. Audiovisual interactions in the auditory cortex, as estimated in a linear model, consisted both of a total suppression of the visual response to lipreading and a decrease of the auditory responses to the speech sound in the bimodal condition compared with unimodal conditions. These findings demonstrate that audiovisual speech integration does not respect the classical hierarchy from sensory-specific to associative cortical areas, but rather engages multiple cross-modal mechanisms at the first stages of nonprimary auditory cortex activation.

Long-term depression induced by local tetanic stimulation in the rat auditory cortex

In sensory cortices, synaptic plasticities such as long-term potentiation (LTP) and long-term depression (LTD) have important roles in the development of neural circuits and sensory information processing. However, the differential roles and mechanisms of the various types of LTP and LTD are not clear. In the present study, we investigated LTP and two types of LTD in slices obtained from the rat auditory cortex. Supragranular field potentials elicited by layer VI stimulation were recorded through a metal electrode. Transsynaptic field potentials exhibited marked LTP after tetanic stimulation (TS, 100 Hz for 1 s) was applied to layer VI. The same field potential components exhibited LTD after low-frequency stimulation (LFS, 1 Hz for 900 s) was applied to layer VI. LTD of supragranular field potentials was also induced by local TS applied to supragranular layers 0.3 mm from the recording site. Neither LTP nor LTD of either type was induced in the presence of 50 muM d-(-)-2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist. However, 500 muM (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of metabotropic glutamate receptors, had no effect. LTD induced by LFS and that induced by local TS were suppressed in the presence of 3 muM bicuculline, an antagonist of GABA(A) receptors. Each of these forms of LTD occluded the other. These results and intracellular recordings in supragranular pyramidal neurons during LFS and local TS strongly suggest that the two types of LTD share common neural circuits for their induction.

Experience-dependent formation of activity propagation patterns at the somatosensory S1 and S2 boundary in rat cortical slices

Somatosensory information is serially processed by the primary (S1) and secondary (S2) cortices, which can be identified in fresh cortical slices. We visualized activity propagation between S1 and S2 in rat cortical slices using flavoprotein fluorescence imaging. When S1 was stimulated, fluorescence responses extended into S2, while responses hardly propagated to S1 following S2 stimulation. The dominant activity propagation pattern from S1 to S2 was not affected by antagonists of glutamate or GABA(A) receptors. Ca(2+) imaging and electrophysiological recordings confirmed the anisotropic activity propagation pattern. This pattern could be formed as a result of serial information processing in S1 and S2. To test this hypothesis, activity propagation was investigated in cortical slices prepared 2 weeks or 3 days after trimming contralateral whiskers that provide massive inputs to S1. Supragranular activities in the barrel cortex were clearly suppressed. Furthermore, activities elicited in the rostral small vibrissae/mouth area of S1 near the border between S1 and S2 spread into the adjacent barrel cortex rather than into S2. Behavioral effects of whisker trimming were evaluated using a test, in which rats chose one of two bridges that had a wall on the right or left side only. Immediately after hemilateral whisker trimming, rats preferred to use the bridge with a wall close to the intact side. However, this preference disappeared 3 days after trimming. Modified activities observed in cortical slices after whisker trimming might be mechanisms for this behavioral compensation. These findings suggest experience-dependent formation of activity propagation patterns in the somatosensory cortex.

Functional local connections with differential activity-dependence and critical periods surrounding the primary auditory cortex in rat cerebral slices

Sensory information is processed in neural networks connecting the primary sensory cortices with surrounding higher areas. Here, we investigated the properties of local connections between the primary auditory cortex (area 41) and surrounding areas (areas 20, 36, 18a and 39) in rat cerebral slices. Neural activities elicited by repetitive electrical stimulation were visualized using the activity-dependent changes in endogenous fluorescence derived from mitochondrial flavoproteins, which mostly reflect activities produced by polysynaptic glutamatergic transmission. Polysynaptic feedforward propagation was dominant compared with the corresponding polysynaptic feedback propagation between the primary (area 41) and secondary (areas 20 and 36) auditory cortices, while such a tendency was less clear in other pathways. Long inter-areal (>1 mm) propagation with the same dominancy was observed after layer V stimulation between areas 41 and 20, and was not affected by cutting the underlying white matter. Activity-dependent changes in neural activities induced by low-frequency stimulation in the presence of 1 microM bicuculline were investigated using Ca2+ imaging. Significant potentiation of the polysynaptic Ca2+ activities was only observed in polysynaptic feedforward pathways from the primary to secondary auditory cortices. Experience-dependence of the connections between areas 41 and 20 was investigated using flavoprotein fluorescence imaging. The activities from areas 41 to 20 were reduced by cochlear lesions produced at P12 but not at P28, while the activities from areas 20 to 41 were reduced by the lesions at P28, suggesting the critical period for the polysynaptic feedforward connection was before P28, while for the polysynaptic feedback connection was after P28.

Transcranial fluorescence imaging of auditory cortical plasticity regulated by acoustic environments in mice

Functional brain imaging using endogenous fluorescence of mitochondrial flavoprotein is useful for investigating mouse cortical activities via the intact skull, which is thin and sufficiently transparent in mice. We applied this method to investigate auditory cortical plasticity regulated by acoustic environments. Normal mice of the C57BL/6 strain, reared in various acoustic environments for at least 4 weeks after birth, were anaesthetized with urethane (1.7 g/kg, i.p.). Auditory cortical images of endogenous green fluorescence in blue light were recorded by a cooled CCD camera via the intact skull. Cortical responses elicited by tonal stimuli (5, 10 and 20 kHz) exhibited mirror-symmetrical tonotopic maps in the primary auditory cortex (AI) and anterior auditory field (AAF). Depression of auditory cortical responses regarding response duration was observed in sound-deprived mice compared with naïve mice reared in a normal acoustic environment. When mice were exposed to an environmental tonal stimulus at 10 kHz for more than 4 weeks after birth, the cortical responses were potentiated in a frequency-specific manner in respect to peak amplitude of the responses in AI, but not for the size of the responsive areas. Changes in AAF were less clear than those in AI. To determine the modified synapses by acoustic environments, neural responses in cortical slices were investigated with endogenous fluorescence imaging. The vertical thickness of responsive areas after supragranular electrical stimulation was significantly reduced in the slices obtained from sound-deprived mice. These results suggest that acoustic environments regulate the development of vertical intracortical circuits in the mouse auditory cortex.

Controlling synaptic input patterns in vitro by dynamic photo stimulation

Recent experimental and theoretical work indicates that both the intensity and the temporal structure of synaptic activity strongly modulate the integrative properties of single neurons in the intact brain. However, studying these effects experimentally is complicated by the fact that, in experimental systems, network activity is either absent, as in the acute slice preparation, or difficult to monitor and to control, as in in vivo recordings. Here, we present a new implementation of neurotransmitter uncaging in acute brain slices that uses functional projections to generate tightly controlled, spatio-temporally structured synaptic input patterns in individual neurons. For that, a set of presynaptic neurons is activated in a precisely timed sequence through focal photolytic release of caged glutamate with the help of a fast laser scanning system. Integration of synaptic inputs can be studied in postsynaptic neurons that are not directly stimulated with the laser, but receive input from the targeted neurons through intact axonal projections. Our new approach of dynamic photo stimulation employs functional synapses, accounts for their spatial distribution on the dendrites, and thus allows study of the integrative properties of single neurons with physiologically realistic input. Data obtained with our new technique suggest that, not only the neuronal spike generator, but also synaptic transmission and dendritic integration in neocortical pyramidal cells, can be highly reliable.

Short-term plasticity visualized with flavoprotein autofluorescence in the somatosensory cortex of anaesthetized rats

In the present study, short-term plasticity of somatosensory neural responses was investigated using flavoprotein autofluorescence imaging in rats anaesthetized with urethane (1.5 g/kg, i.p.) Somatosensory neural activity was elicited by vibratory skin stimulation (50 Hz for 1 s) applied on the surface of the left plantar hindpaw. Changes in green autofluorescence (lambda = 500-550 nm) in blue light (lambda = 450-490 nm) were elicited in the right somatosensory cortex. The normalised maximal fluorescence responses (deltaF/F) was 2.0 +/- 0.1% (n = 40). After tetanic cortical stimulation (TS), applied at a depth of 1.5-2.0 mm from the cortical surface, the responses elicited by peripheral stimulation were significantly potentiated in both peak amplitude and size of the responsive area (both P < 0.02; Wilcoxon signed rank test). This potentiation was clearly observed in the recording session started 5 min after the cessation of TS, and returned to the control level within 30 min. However, depression of the responses was observed after TS applied at a depth of 0.5 mm. TS-induced changes in supragranular field potentials in cortical slices showed a similar dependence on the depth of the stimulated sites. When TS was applied on the ipsilateral somatosensory cortex, marked potentiation of the ipsilateral responses and slight potentiation of the contralateral responses to peripheral stimulation were observed after TS, suggesting the involvement of commissural fibers in the changes in the somatosensory brain maps. The present study clearly demonstrates that functional brain imaging using flavoprotein autofluorescence is a useful technique for investigating neural plasticity in vivo.

Activity-dependent persisting modification of polysynaptic neural circuits involving layer V pyramidal neurons in rat auditory cortex in vitro

Synaptic plasticity in polysynaptic neural circuits permits modulation of the dynamic properties of these circuits. We investigated the properties of polysynaptic potentiation in pyramidal neurons in layer V of rat auditory cortex (AC) slices using the perforated patch clamp technique. The GABAA receptor inhibitor bicuculline was used to facilitate polysynaptic activity. The amplitude and duration of the polysynaptic activity were both gradually potentiated with repetitive stimulation (RS) at 12 s intervals. Potentiation was saturated within 10 min of the onset of RS. After the cessation of RS, the polysynaptic responses returned to control levels within 30 min. RS-induced potentiation was confirmed by fluorescence imaging of slices loaded with the Ca2+ indicator rhod-2. Such potentiation was not induced by stimulation at 60 s intervals. The magnitude of the RS-induced potentiation in layer V pyramidal neurons in the AC was greater than that in either layer II/III pyramidal neurons in the AC or layer V pyramidal neurons in the visual cortex. The NMDA receptor antagonist APV (100 microm), inhibited RS-induced potentiation. When stimulated at 1 Hz, the potentiated response appeared rapidly. In the absence of bicuculline, RS consisting of five pulses at 30 ms intervals, repeated at 12 s intervals for 10 min, elicited potentiation of firing activity, suggesting that the potentiation is independent of bicuculline. The present study demonstrates the dynamic properties of polysynaptic circuits involving layer V pyramidal neurons in the AC are strongly affected by activity-dependent synaptic potentiation.