Subcortical auditory input to the primary visual cortex in anophthalmic mice

Anatomical and imaging studies show ample evidence for auditory activation of the visual cortex following early onset of blindness in both humans and animal models. Anatomical studies in animal models of early blindness clearly show intermodal pathways through which auditory information can reach the primary visual cortex. There is clear evidence for intermodal corticocortical pathways linking auditory and visual cortex and also novel connections between the inferior colliculus and the visual thalamus. A recent publication [L.K. Laemle, N.L. Strominger, D.O. Carpenter, Cross-modal innervation of primary visual cortex by auditory fibers in congenitally anophthalmic mice, Neurosci. Lett. 396 (2006) 108–112] suggested the presence of a direct reciprocal connection between the inferior colliculus and the primary visual cortex (V1) in congenitally anophthalmic ZRDCT/An mice. This implies that this mutant mouse would be the only known vertebrate having a direct tectal connection with a primary sensory cortex. The presence of this peculiar pathway was reinvestigated in the ZRDCT/An mouse with highly sensitive neuronal tracers. We found the connections normally described in the ZRDCT/An mouse between: (i) the inferior colliculus and the dorsal lateral geniculate nucleus, (ii) V1 and the superior colliculus, (iii) the lateral posterior nucleus and V1 and between (iv) the inferior colliculus and the medial geniculate nucleus. We also show unambiguously that the auditory subcortical structures do not connect the primary visual cortex in the anophthalmic mouse. In particular, we find no evidence of a direct projection from the auditory mesencephalon to the cortex in this animal model of blindness.     

Sensory-motor interaction in the primate auditory cortex during self-initiated vocalizations

Little is known about sensory-motor interaction in the auditory cortex of primates at the level of single neurons and its role in supporting vocal communication. The present study investigated single-unit activities in the auditory cortex of a vocal primate, the common marmoset (Callithrix jacchus), during self-initiated vocalizations. We found that 1) self-initiated vocalizations resulted in suppression of neural discharges in a majority of auditory cortical neurons. The vocalization-induced inhibition suppressed both spontaneous and stimulus-driven discharges. Suppressed units responded poorly to external acoustic stimuli during vocalization. 2) Vocalization-induced suppression began several hundred milliseconds prior to the onset of vocalization. 3) The suppression of cortical discharges reduced neural firings to below the rates expected from a unit's rate-level function, adjusted for known subcortical attenuation, and therefore was likely not entirely caused by subcortical attenuation mechanisms. 4) A smaller population of auditory cortical neurons showed increased discharges during self-initiated vocalizations. This vocalization-related excitation began after the onset of vocalization and is likely the result of acoustic feedback. Units showing this excitation responded nearly normally to external stimuli during vocalization. Based on these findings, we propose that the suppression of auditory cortical neurons, possibly originating from cortical vocal production centers, acts to increase the dynamic range of cortical responses to vocalization feedback for self monitoring. The excitatory responses, on the other hand, likely play a role in maintaining hearing sensitivity to the external acoustic environment during vocalization.     

Human cortical activity correlates with stereoscopic depth perception

Stereoscopic depth perception is based on binocular disparities. Although neurons in primary visual cortex (V1) are selective for binocular disparity, their responses do not explicitly code perceived depth. The stereoscopic pathway must therefore include additional processing beyond V1. We used functional magnetic resonance imaging (fMRI) to examine stereo processing in V1 and other areas of visual cortex. We created stereoscopic stimuli that portrayed two planes of dots in depth, placed symmetrically about the plane of fixation, or else asymmetrically with both planes either nearer or farther than fixation. The interplane disparity was varied parametrically to determine the stereoacuity threshold (the smallest detectable disparity) and the upper depth limit (largest detectable disparity). fMRI was then used to quantify cortical activity across the entire range of detectable interplane disparities. Measured cortical activity covaried with psychophysical measures of stereoscopic depth perception. Activity increased as the interplane disparity increased above the stereoacuity threshold and dropped as interplane disparity approached the upper depth limit. From the fMRI data and an assumption that V1 encodes absolute retinal disparity, we predicted that the mean response of V1 neurons should be a bimodal function of disparity. A post hoc analysis of electrophysiological recordings of single neurons in macaques revealed that, although the average firing rate was a bimodal function of disparity (as predicted), the precise shape of the function cannot fully explain the fMRI data. Although there was widespread activity within the extrastriate cortex (consistent with electrophysiological recordings of single neurons), area V3A showed remarkable sensitivity to stereoscopic stimuli, suggesting that neurons in V3A may play a special role in the stereo pathway.     

Combined activation and deactivation of visual cortex during tactile sensory processing

The involvement of occipital cortex in sensory processing is not restricted solely to the visual modality. Tactile processing has been shown to modulate higher-order visual and multisensory integration areas in sighted as well as visually deprived subjects; however, the extent of involvement of early visual cortical areas remains unclear. To investigate this issue, we employed functional magnetic resonance imaging in normally sighted, briefly blindfolded subjects with well-defined visuotopic borders as they tactually explored and rated raised-dot patterns. Tactile task performance resulted in significant activation in primary visual cortex (V1) and deactivation of extrastriate cortical regions V2, V3, V3A, and hV4 with greater deactivation in dorsal subregions and higher visual areas. These results suggest that tactile processing affects occipital cortex via two distinct pathways: a suppressive top-down pathway descending through the visual cortical hierarchy and an excitatory pathway arising from outside the visual cortical hierarchy that drives area V1 directly.     

Auditory cortex projections target the peripheral field representation of primary visual cortex

The purpose of the present study was to identify projections from auditory to visual cortex and their organization. Retrograde tracers were used to identify the sources of auditory cortical projections to primary visual cortex (areas 17 and 18) in adult cats. Two groups of animals were studied. In the first group, large deposits were centered on the lower visual field representation of the vertical meridian located along the area 17 and 18 border. Following tissue processing, characteristic patterns of cell body labeling were identified in extrastriate visual cortex and the visual thalamus (LGN, MIN, & LPl). In auditory cortex, of the four tonotopically-organized regions, neuronal labeling was identified in the supragranular layers of the posterior auditory field (PAF). Little to no labeling was evident in the primary auditory cortex, the anterior auditory field, the ventral posterior auditory field or in the remaining six non-tonotopically organized regions of auditory cortex. In the second group, small deposits were made into the central or peripheral visual field representations of primary visual cortex. Labeled cells were identified in PAF following deposits into regions of primary visual cortex representing peripheral, but not central, visual field representations. Furthermore, a coarse topography was identified in PAF, with neurons projecting to the upper field representation being located in the gyral portion of PAF and neurons projecting to the lower field representation located in the sulcal portion of PAF. Therefore, direct projections can be identified from tonotopically organized auditory cortex to the earliest stages of visual cortical processing.     

Sensitivity to temporal modulation rate and spectral bandwidth in the human auditory system: fMRI evidence

Hierarchical models of auditory processing often posit that optimal stimuli, i.e., those eliciting a maximal neural response, will increase in bandwidth and decrease in modulation rate as one ascends the auditory neuraxis. Here, we tested how bandwidth and modulation rate interact at several loci along the human central auditory pathway using functional MRI in a cardiac-gated, sparse acquisition design. Participants listened passively to both narrowband (NB) and broadband (BB) carriers (1/4- or 4-octave pink noise), which were jittered about a mean sinusoidal amplitude modulation rate of 0, 3, 29, or 57 Hz. The jittering was introduced to minimize stimulus-specific adaptation. The results revealed a clear difference between spectral bandwidth and temporal modulation rate: sensitivity to bandwidth (BB > NB) decreased from subcortical structures to nonprimary auditory cortex, whereas sensitivity to slow modulation rates was largest in nonprimary auditory cortex and largely absent in subcortical structures. Furthermore, there was no parametric interaction between bandwidth and modulation rate. These results challenge simple hierarchical models, in that BB stimuli evoked stronger responses in primary auditory cortex (and subcortical structures) rather than nonprimary cortex. Furthermore, the strong preference for slow modulation rates in nonprimary cortex demonstrates the compelling global sensitivity of auditory cortex to modulation rates that are dominant in the principal signals that we process, e.g., speech.     

Efferent connections of "posterodorsal" auditory area in the rat cortex: implications for auditory spatial processing

We examined efferent connections of the cortical auditory field that receives thalamic afferents specifically from the suprageniculate nucleus (SG) and the dorsal division (MGD) of the medial geniculate body (MG) in the rat [Neuroscience 117 (2003) 1003]. The examined cortical region was adjacent to the caudodorsal border (4.8-7.0 mm posterior to bregma) of the primary auditory area (area Te1) and exhibited relatively late auditory response and high best frequency, compared with the caudal end of area Te1. On the basis of the location and auditory response property, the cortical region is considered identical to "posterodorsal" auditory area (PD). Injections of biocytin in PD revealed characteristic projections, which terminated in cortical areas and subcortical structures that play pivotal roles in directed attention and space processing. The most noticeable cortical terminal field appeared as dense plexuses of axons in area Oc2M, the posterior parietal cortex. Small terminal fields were scattered in area frontal cortex, area 2 that comprises the frontal eye field. The subcortical terminal fields were observed in the pontine nucleus, the nucleus of the brachium inferior colliculus, and the intermediate and deep layers of the superior colliculus. Corticostriatal projections targeted two discrete regions of the caudate putamen: the top of the middle part and the caudal end. It is noteworthy that the inferior colliculus and amygdala virtually received no projection. Corticothalamic projections terminated in the MGD, the SG, the ventral zone of the ventral division of the MG, the ventral margin of the lateral posterior nucleus (LP), and the caudodorsal part of the posterior thalamic nuclear group (Po). Large terminals were found in the MGD, SG, LP and Po besides small terminals, the major component of labeling. The results suggest that PD is an auditory area that plays an important role in spatial processing linked to directed attention and motor function. The results extend to the rat findings from nonhuman primates suggesting the existence of a posterodorsal processing stream for auditory spatial perception.     

Corticofugal amplification of facilitative auditory responses of subcortical combination-sensitive neurons in the mustached bat

Recent studies on the bat's auditory system indicate that the corticofugal system mediates a highly focused positive feedback to physiologically "matched" subcortical neurons, and widespread lateral inhibition to physiologically "unmatched" subcortical neurons, to adjust and improve information processing. These findings have solved the controversy in physiological data, accumulated since 1962, of corticofugal effects on subcortical auditory neurons: inhibitory, excitatory, or both (an inhibitory effect is much more frequent than an excitatory effect). In the mustached bat, Pteronotus parnellii parnellii, the inferior colliculus, medial geniculate body, and auditory cortex each have "FM-FM" neurons, which are "combination-sensitive" and are tuned to specific time delays (echo delays) of echo FM components from the FM components of an emitted biosonar pulse. FM-FM neurons are more complex in response properties than cortical neurons which primarily respond to single tones. In the present study, we found that inactivation of the entire FM-FM area in the cortex, including neurons both physiologically matched and unmatched with subcortical FM-FM neurons, on the average reduced the facilitative responses to paired FM sounds by 82% for thalamic FM-FM neurons and by 66% for collicular FM-FM neurons. The corticofugal influence on the facilitative responses of subcortical combination-sensitive neurons is much larger than that on the excitatory responses of subcortical neurons primarily responding to single tones. Therefore we propose the hypothesis that, in general, the processing of complex sounds by combination-sensitive neurons more heavily depends on the corticofugal system than that by single-tone sensitive neurons.     

Temporal analysis of the flow from V1 to the extrastriate cortex in humans

We previously examined the cortical processing in response to somatosensory, auditory and noxious stimuli, using magnetoencephalography in humans. Here, we performed a similar analysis of the processing in the human visual cortex for comparative purposes. After flash stimuli applied to the right eye, activations were found in eight cortical areas: the left medial occipital area around the calcarine fissure (primary visual cortex, V1), the left dorsomedial area around the parietooccipital sulcus (DM), the ventral (MOv) and dorsal (MOd) parts of the middle occipital area of bilateral hemispheres, the left temporo-occipito-parietal cortex corresponding to human MT/V5 (hMT), and the ventral surface of the medial occipital area (VO) of the bilateral hemispheres. The mean onset latencies of each cortical activity were (in ms): 27.5 (V1), 31.8 (DM), 32.8 (left MOv), 32.2 (right MOv), 33.4 (left MOd), 32.3 (right MOv), 37.8 (hMT), 46.9 (left VO), and 46.4 (right VO). Therefore the cortico-cortical connection time of visual processing at the early stage was 4-6 ms, which is very similar to the time delay between sequential activations in somatosensory and auditory processing. In addition, the activities in V1, MOd, DM, and hMT showed a similar biphasic waveform with a reversal of polarity after 10 ms, which is a common activation profile of the cortical activity for somatosensory, auditory, and pain-evoked responses. These results suggest similar mechanisms of the serial cortico-cortical processing of sensory information among all sensory areas of the cortex.     

Modulation of responses and frequency tuning of thalamic and collicular neurons by cortical activation in mustached bats

In the Jamaican mustached bat, Pteronotus parnellii parnellii, one of the subdivisions of the primary auditory cortex is disproportionately large and over-represents sound at approximately 61 kHz. This area, called the Doppler-shifted constant frequency (DSCF) processing area, consists of neurons extremely sharply tuned to a sound at approximately 61 kHz. We found that a focal activation of the DSCF area evokes highly specific corticofugal modulation in the inferior colliculus and medial geniculate body. Namely a focal activation of cortical DSCF neurons tuned to, say, 61. 2 kHz with 0.2-ms-long, 100-nA electric pulses drastically increases the excitatory responses of thalamic and collicular neurons tuned to 61.2 kHz without shifting their best frequencies (BFs). However, it decreases the excitatory responses of subcortical neurons tuned to frequencies slightly higher or lower than 61.2 kHz and shifts their BFs away from 61.2 kHz. The BF shifts are symmetrical and centrifugal around 61.2 kHz. These corticofugal effects are larger on thalamic neurons than on collicular neurons. The cortical electrical stimulation sharpens the frequency-tuning curves of subcortical neurons. These corticofugal effects named "egocentric selection" last 相似文献   

Convergence of unimodal and polymodal sensory input to the entorhinal cortex in the fascicularis monkey

The hippocampal formation is a key structure in memory formation and consolidation. The hippocampus receives information from different cortical and subcortical sources. Cortical information is mostly funneled to the hippocampus through the entorhinal cortex (EC) in a bi-directional way that ultimately ends in the cortex. Retrograde tracing studies in the nonhuman primate indicate that more than two-thirds of the cortical afferents to the EC come from polymodal sensory association areas. Although some evidence for the projection from visual unimodal cortex to the EC exists, inputs from other visual and auditory unimodal association areas, and the possibility of their convergence with polymodal input in the EC remains largely undisclosed. We studied 10 Macaca fascicularis monkeys in which cortical deposits of the anterograde tracer biotinylated dextran-amine were made into different portions of visual and auditory unimodal association cortices in the temporal lobe, and in polymodal association cortex at the upper bank of the superior temporal sulcus. Visual and auditory unimodal as well as polymodal cortical areas projected to the EC. Both visual unimodal and polymodal association cortices presented dense projections, while those from unimodal auditory association cortex were more patchy and less dense. In all instances, the projection distributed in both the superficial and deep layers of the EC. However, while polymodal cortex projected to all layers (including layer I), visual unimodal cortex did not project to layer I, and auditory unimodal cortex projected less densely, scattered through all layers. Topographically, convergence from the three cortical areas studied can be observed in the lateral rostral and lateral caudal subfields. The present study suggests that unimodal and polymodal association cortical inputs converge in the lateral EC, thereby providing the possibility for the integration of complex stimuli for internal representations in declarative memory elaboration.     

The primary auditory cortex in cetacean and human brain: a comparative analysis of neurofilament protein-containing pyramidal neurons.

To extend our investigation of the anatomy of sensory systems in highly adapted aquatic and terrestrial mammals, we have analyzed the distribution of a particular population of efferent neurons in the cetacean and human primary auditory cortex using an antibody to non-phosphorylated neurofilament protein (SMI32). The neurofilament protein triplet is differentially distributed within neuronal subpopulations in the primate and cetacean neocortex. In primates, it appears that the somatodendritic domain of a subset of pyramidal neurons furnishing specific corticocortical connections contains high concentrations of neurofilament protein. In the human primary auditory cortex these neurons are located in layers III, V and VI, whereas in cetaceans they are concentrated almost exclusively in the cortical efferent layer IIIc/V. Previous analyses have shown that SMI32 immunoreactivity in the cetacean neocortex is uniformly distributed among functionally different areas, while in human neocortex, the distribution of SMI32-positive neurons exhibit a high degree of regional and laminar specialization that is correlated with the functional and anatomical diversity of the cortical areas. In addition, the overall distribution of SMI32-immunoreactive neurons in the cetacean neocortex is comparable to that observed in paralimbic areas of the human, suggesting that the cetacean neocortex has retained many features of phylogenetically older cortical regions.     

Laminar and cytoarchitectonic features of the cerebral cortex in the Risso's dolphin (Grampus griseus), striped dolphin (Stenella coeruleoalba), and bottlenose dolphin (Tursiops truncatus)

The present investigation carried out Nissl, Klüver-Barrera, and Golgi studies of the cerebral cortex in three distinct genera of oceanic dolphins (Risso's dolphin, striped dolphin and bottlenose dolphin) to identify and classify cortical laminar and cytoarchitectonic structures in four distinct functional areas, including primary motor (M1), primary sensory (S1), primary visual (V1), and primary auditory (A1) cortices. The laminar and cytoarchitectonic organization of each of these cortical areas was similar among the three dolphin species. M1 was visualized as five-layer structure that included the molecular layer (layer I), external granular layer (layer II), external pyramidal layer (layer III), internal pyramidal layer (layer V), and fusiform layer (layer VI). The internal granular layer was absent. The cetacean sensory-related cortical areas S1, V1, and A1 were also found to have a five-layer organization comprising layers I, II, III, V and VI. In particular, A1 was characterized by the broadest layer I, layer II and developed band of pyramidal neurons in layers III (sublayers IIIa, IIIb and IIIc) and V. The patch organization consisting of the layer IIIb-pyramidal neurons was detected in the S1 and V1, but not in A1. The laminar patterns of V1 and S1 were similar, but the cytoarchitectonic structures of the two areas were different. V1 was characterized by a broader layer II than that of S1, and also contained the specialized pyramidal and multipolar stellate neurons in layers III and V.     

Effects of stimulus azimuth and intensity on the single-neuron activity in the auditory cortex of the alert macaque monkey

It has been hypothesized that the primate auditory cortex is composed of at least two processing streams, one of which is believed to selectively process spatial information. To test whether spatial information is differentially encoded in different auditory cortical fields, we recorded the responses of single neurons in the auditory cortex of alert macaque monkeys to broadband noise stimuli presented from 360 degrees in azimuth at four different absolute intensities. Cortical areas tested were core areas A1 and rostral (R), caudal belt fields caudomedial and caudolateral, and more rostral belt fields middle lateral and middle medial (MM). We found that almost all neurons encountered showed some spatial tuning. However, spatial selectivity measures showed that the caudal belt fields had the sharpest spatial tuning, A1 had intermediate spatial tuning, and areas R and MM had the least spatial tuning. Although most neurons showed their best responses to contralateral space, best azimuths were observed across the entire 360 degrees of tested space. We also noted that although the responses of many neurons were significantly influenced by eye position, eye position did not systematically influence any of the spatially dependent responses that we measured. These data are consistent with the hypothesis that caudal auditory cortical fields in the primate process spatial features more accurately than the core and more rostral belt fields.     

The effect of orientation adaptation on responses of lateral geniculate nucleus neurons with high orientation bias in cats

Adaptation to stimulus orientation is assumed to have a cortical basis, but few studies have addressed whether it affects the activity of subcortical neurons. Using single-unit recording, we studied the effects of orientation adaptation on the responses of lateral geniculate nucleus (LGN) neurons with high orientation bias (OB) in anesthetized and paralyzed cats. Following adaptation to one stimulus orientation, the response at the adapting orientation was decreased, and the preferred orientation was shifted away from the adapting orientation. This phenomenon was similar to the effects observed for orientation adaptation in the primary visual cortex (V1), and was obvious when the adapting orientation was at an appropriate location relative to the original preferred orientation. Moreover, when the V1 was inactivated, the response at the adapting orientation was also decreased but the preferred orientation did not show a systematic shift after orientation adaptation in LGN. This result indicates that cortical feedback contributes to the effect of orientation adaptation on LGN neurons, which have a high OB. These data provide an example of how the corticothalamic loop modulates the processing of visual information, and suggest that the LGN is not only a simply passive relay but also a modulator of visual information.     

Temporal properties of inputs to direction-selective neurons in monkey V1

Motion in the visual scene is processed by direction-selective neurons in primary visual cortex. These cells receive inputs that differ in space and time. What are these inputs? A previous single-unit recording study in anesthetized monkey V1 proposed that the two major streams arising in the primate retina, the M and P pathways, differed in space and time as required to create direction selectivity. We confirmed that cortical cells driven by P inputs tend to have sustained responses. The M pathway, however, as assessed by recordings in layer 4Calpha and from cells with high contrast sensitivity, is not purely transient. The diversity of timing in the M stream suggests that combinations of M inputs, as well as of M and P inputs, create direction selectivity.     

Behavioral Dependence of Auditory Cortical Responses

Neural responses in the auditory cortex have historically been measured from either anesthetized or awake but non-behaving animals. A growing body of work has begun to focus instead on recording from auditory cortex of animals actively engaged in behavior tasks. These studies have shown that auditory cortical responses are dependent upon the behavioral state of the animal. The longer ascending subcortical pathway of the auditory system and unique characteristics of auditory processing suggest that such dependencies may have a more profound influence on cortical processing in the auditory system compared to other sensory systems. It is important to understand the nature of these dependencies and their functional implications. In this article, we review the literature on this topic pertaining to cortical processing of sounds.     

Neural response properties of primary, rostral, and rostrotemporal core fields in the auditory cortex of marmoset monkeys

The core region of primate auditory cortex contains a primary and two primary-like fields (AI, primary auditory cortex; R, rostral field; RT, rostrotemporal field). Although it is reasonable to assume that multiple core fields provide an advantage for auditory processing over a single primary field, the differential roles these fields play and whether they form a functional pathway collectively such as for the processing of spectral or temporal information are unknown. In this report we compare the response properties of neurons in the three core fields to pure tones and sinusoidally amplitude modulated tones in awake marmoset monkeys (Callithrix jacchus). The main observations are as follows. (1) All three fields are responsive to spectrally narrowband sounds and are tonotopically organized. (2) Field AI responds more strongly to pure tones than fields R and RT. (3) Field RT neurons have lower best sound levels than those of neurons in fields AI and R. In addition, rate-level functions in field RT are more commonly nonmonotonic than in fields AI and R. (4) Neurons in fields RT and R have longer minimum latencies than those of field AI neurons. (5) Fields RT and R have poorer stimulus synchronization than that of field AI to amplitude-modulated tones. (6) Between the three core fields the more rostral regions (R and RT) have narrower firing-rate-based modulation transfer functions than that of AI. This effect was seen only for the nonsynchronized neurons. Synchronized neurons showed no such trend.     

Level dependence of contextual modulation in auditory cortex

Responses of cortical neurons to sensory stimuli within their receptive fields can be profoundly altered by the stimulus context. In visual and somatosensory cortex, contextual interactions have been shown to change sign from facilitation to suppression depending on stimulus strength. Contextual modulation of high-contrast stimuli tends to be suppressive, but for low-contrast stimuli tends to be facilitative. This trade-off may optimize contextual integration by cortical cells and has been suggested to be a general feature of cortical processing, but it remains unknown whether a similar phenomenon occurs in auditory cortex. Here we used whole cell and single-unit recordings to investigate how contextual interactions in auditory cortical neurons depend on the relative intensity of masker and probe stimuli in a two-tone stimulus paradigm. We tested the hypothesis that relatively low-level probes should show facilitation, whereas relatively high-level probes should show suppression. We found that contextual interactions were primarily suppressive across all probe levels, and that relatively low-level probes were subject to stronger suppression than high-level probes. These results were virtually identical for spiking and subthreshold responses. This suggests that, unlike visual cortical neurons, auditory cortical neurons show maximal suppression rather than facilitation for relatively weak stimuli.