Functional organization of auditory cortex in the Mongolian gerbil (Meriones unguiculatus). III. Anatomical subdivisions and corticocortical connections

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.

Functional subregions in primary auditory cortex defined by thalamocortical terminal arbors: an electrophysiological and anterograde labeling study

Several functional maps have been described in primary auditory cortex, including those related to frequency, tuning, latency, binaurality, and intensity. Many of these maps are arranged in a discontinuous or patchy manner. Similarly, thalamocortical projections arising from the ventral division of the medial geniculate body to the primary auditory cortex are also patchy. We used anterograde labeling and electrophysiological methods to examine the relationship between thalamocortical patches and auditory cortical maps. Biotinylated dextran-amine was deposited into physiologically characterized sites in the ventral division of the medial geniculate body of New Zealand white rabbits. Approximately 7 d later, the animal was again anesthetized and the ipsilateral auditory cortex was mapped with tungsten microelectrodes. Multi-unit physiological data were obtained for the following characteristics: best frequency (BF), binaurality, response type, latency, sharpness of tuning, and threshold. Immunocytochemical methods were used to reveal the injection site in the ventral division of the medial geniculate body as well as the anterogradely labeled thalamocortical afferents in the auditory cortex. In 86% of the cases (12 of 14), entry into a thalamocortical patch was associated with a marked change in physiological responses. A consistent BF and binaural class were usually observed within a patch. The patches appear to innervate distinct functional regions coding frequency and binaurality. A model is presented showing how patchy thalamocortical projections participate in the formation of tonotopic and binaural maps in primary auditory cortex.

The medial pain system: neural representations of the motivational aspect of pain

In this article, we propose that the pathways mediating the motivational aspect of pain originate in laminae VII and VIII of the spinal cord, and in the deep layers of the spinal trigeminal complex, and projections from these areas reach three central structures where pain motivation is represented, the ventrolateral quadrant of the periaqueductal gray, posterior hypothalamic nucleus, and intralaminar thalamic nuclei. A final representation of the motivational aspect of pain is located within the anterior cingulate cortex, and this representation receives inputs from the intralaminar nuclei. Outputs from these representations reach premotor structures located in the medulla, striatum, and cingulate premotor cortex. We discuss pathways and structures that provide inputs to these representations, including those involved in producing involuntary (innate) and instrumental responses which occur in response to the recognition of stimuli associated with footshock and other nociceptive stimuli.

Memory consolidation for the discrimination of frequency-modulated tones in mongolian gerbils is sensitive to protein-synthesis inhibitors applied to the auditory cortex

Differential conditioning of Mongolian gerbils to linearly frequency-modulated tones (FM) has recently received experimental attention. In the study of the role of cerebral protein synthesis for FM discrimination memory, gerbils received post-training bilateral injections of anisomycin into the auditory cortex under light halothane anesthesia. Compared with saline-treated controls, anisomycin-treated gerbils showed a discrimination decrement during the subsequent three days of training. They markedly improved their performance within training sessions, but started each session at low levels. When repeatedly trained gerbils received post-session injections of anisomycin, discrimination performance during subsequent sessions was similar to the pre-injection performance, indicating that retention, retrieval, reconsolidation, and expression of the established reaction were not affected. However, the improvement of a partially established discrimination reaction was impaired after this treatment. Intracortical injections of emetine confirmed this finding. Neither drug affected FM discrimination learning when given several days before the initial training. Our results suggest that protein-synthesis inhibitors applied to the auditory cortex of gerbils during the post-acquisition phase interfered with learning and memory-related aspects of FM processing. The resulting deficit was evident for a number of post-injection training days. This effect was probably due to impaired consolidation, i.e., processes required for long-term stabilization or retrieval of the memory trace while leaving short-term memory intact.

Auditory activation of "visual" cortical areas in the blind mole rat (Spalax ehrenbergi)

The mole rat (Spalax ehrenbergi) is a subterranean rodent whose adaptations to its fossorial life include an extremely reduced peripheral visual system and an auditory system suited for the perception of vibratory stimuli. We have previously shown that in this blind rodent the dorsal lateral geniculate nucleus, the primary visual thalamic nucleus of sighted mammals, is activated by auditory stimuli. In this report we focus on the manifestation of this cross-modal compensation at the cortical level. Cyto- and myeloarchitectural analyses of the occipital area showed that despite the almost total blindness of the mole rat this area has retained the organization of a typical mammalian primary visual cortex. Application of the metabolic marker 2-deoxyglucose and electrophysiological recording of evoked field potentials and single-unit activity disclosed that a considerable part of this area is activated by auditory stimuli. Previous neuronal tracing studies had revealed the origin of the bulk of this auditory input to be the dorsal lateral geniculate nucleus which itself receives auditory input from the inferior colliculus.

Sound-level-dependent representation of frequency modulations in human auditory cortex: a low-noise fMRI study

Recognition of sound patterns must be largely independent of level and of masking or jamming background sounds. Auditory patterns of relevance in numerous environmental sounds, species-specific vocalizations and speech are frequency modulations (FM). Level-dependent activation of the human auditory cortex (AC) in response to a large set of upward and downward FM tones was studied with low-noise (48 dB) functional magnetic resonance imaging at 3 Tesla. Separate analysis in four territories of AC was performed in each individual brain using a combination of anatomical landmarks and spatial activation criteria for their distinction. Activation of territory T1b (including primary AC) showed the most robust level dependence over the large range of 48-102 dB in terms of activated volume and blood oxygen level dependent contrast (BOLD) signal intensity. The left nonprimary territory T2 also showed a good correlation of level with activated volume but, in contrast to T1b, not with BOLD signal intensity. These findings are compatible with level coding mechanisms observed in animal AC. A systematic increase of activation with level was not observed for T1a (anterior of Heschl's gyrus) and T3 (on the planum temporale). Thus these areas might not be specifically involved in processing of the overall intensity of FM. The rostral territory T1a of the left hemisphere exhibited highest activation when the FM sound level fell 12 dB below scanner noise. This supports the previously suggested special involvement of this territory in foreground-background decomposition tasks. Overall, AC of the left hemisphere showed a stronger level-dependence of signal intensity and activated volume than the right hemisphere. But any side differences of signal intensity at given levels were lateralized to right AC. This might point to an involvement of the right hemisphere in more specific aspects of FM processing than level coding.

Parvalbumin and calbindin are differentially distributed within primary and secondary subregions of the mouse auditory forebrain

The calcium binding proteins parvalbumin and calbindin are thought to differentially regulate physiological functions and often show complementary distributions in the CNS. Our goal was to determine parvalbumin and calbindin distributions in the different subdivisions of mouse auditory thalamus and auditory cortex. Following fixation, FVB mouse brains (postnatal days 38-80) were sectioned along coronal and horizontal planes, then processed for parvalbumin and calbindin immunohistochemistry (antibodies: parvalbumin pa-235, calbindin-d-28k cl-300). Strong complementary differences in calcium binding protein distributions were found in mouse auditory thalamus. The ventral division of the medial geniculate, which is the principal relay to primary auditory cortex, exhibited dense parvalbumin but weak calbindin immunoreactivity. In contrast, most of the 'secondary' auditory thalamic regions surrounding the ventral division showed strong calbindin and lighter parvalbumin levels. Thus, the mouse auditory thalamus is composed of a parvalbumin positive 'core' surrounded by a calbindin positive 'shell'. Parvalbumin immunoreactivity was also more prominent in the primary auditory cortex than in the secondary belt auditory cortex. Calbindin immunoreactivity in auditory cortex was less clearly divided along primary/secondary lines, especially in supragranular layers. However, within infragranular layers, there was heavier staining in belt areas than in primary auditory cortex. In auditory thalamus, parvalbumin labeling was largely confined to the neuropil, whereas calbindin labeling involved somata and neuropil. In auditory cortex, somata and neuropil were positive for both proteins.In summary, the calcium binding proteins parvalbumin and calbindin were found to be differentially distributed within the primary and non-primary regions of mouse auditory forebrain. These differences in protein distribution may contribute to the distinct types of physiological responses that occur in the primary vs. non-primary areas.

Functional organization of auditory cortex in the Mongolian gerbil (Meriones unguiculatus). IV. Connections with anatomically characterized subcortical structures

The subcortical connections of the four tonotopically organized fields of the auditory cortex of the Mongolian gerbil, namely the primary (AI), the anterior (AAF), the dorsoposterior (DP) and the ventroposterior field (VP), were studied predominantly by anterograde transport of biocytin injected into these fields. In order to allow the localization of connections with respect to subdivisions of subcortical auditory structures, their cyto-, fibre- and chemoarchitecture was characterized using staining methods for cell bodies, myelin and the calcium-binding protein parvalbumin. Each injected auditory cortical field has substantial and reciprocal connections with each of the three subdivision of the medial geniculate body (MGB), namely the ventral (MGv), dorsal (MGd) and medial division (MGm). However, the relative strengths of these connections vary: AI is predominantly connected with MGv, AAF with MGm and MGv, and DP and VP with MGd and MGv. The connections of at least AI and MGv are topographic: injections into caudal low-frequency AI label laterorostral portions of MGv, whereas injections into rostral high-frequency AI label mediocaudal portions of MGv. All investigated auditory fields send axons to the suprageniculate, posterior limitans, laterodorsal and lateral posterior thalamic nuclei, with strongest projections from DP and VP, as well as to the reticular and subgeniculate thalamic nuclei. AI, AAF, DP and VP project to all three subdivisions of the inferior colliculus, namely the dorsal cortex, external cortex and central nucleus ipsilaterally and to the dorsal and external cortex contralaterally. They also project to the deep and intermediate layers of the ipsilateral superior colliculus, with strongest projections from DP and VP to the lateral and basolateral amygdaloid nuclei, the caudate putamen, globus pallidus and the pontine nuclei. In addition, AAF and particularly DP and VP project to paralemniscal regions around the dorsal nucleus of the lateral lemniscus (DNLL), to the DNLL itself and to the rostroventral aspect of the superior olivary complex. Moreover, DP and VP send axons to the dorsal lateral geniculate nucleus. The differences with respect to the existence and/or relative strengths of subcortical connections of the examined auditory cortical fields suggest a somewhat different function of each of these fields in auditory processing.