Patterns of sensory intermodality relationships in the cerebral cortex of the rat

Large-scale functional connectivity in associative learning: interrelations of the rat auditory, visual, and limbic systems

Large-scale functional connectivity in associative learning: interrelations of the rat auditory, visual, and limbic systems. J. Neurophysiol. 80: 3148-3162, 1998. Functional relations between specialized parts of the brain may be important determinants of learned behaviors. To study this, we examined the interrelations of the auditory system with several extraauditory structures in two groups of rats having different behavioral histories. Both groups were trained to associate a tone conditional stimulus (CS) with an aversive unconditional stimulus (US). For one group, a light presented with the tone predicted the absence of the US (group TL-). In the other group, the light was a neutral stimulus (group TL0). Fluorodeoxyglucose (FDG) incorporation was measured in the presence of the tone-light compound. Because the tone-light compound was physically identical for both groups, neural differences between groups reflected differences in the learned associative properties of the stimuli. Covariances of FDG uptake in the auditory system and extraauditory structures were examined using partial least squares. Three strong covariance or functional connectivity patterns were identified. The first pattern mainly reflected similarities between groups, with strong interrelations between the subcortical auditory system and the thalamocortical visual system, cerebellum, deep cerebellar nuclei, and midline thalamus. This pattern of interactions may represent part of a common circuit for relaying the associative value of the tone CS to the cerebellum and the midline thalamus. The external nucleus of the inferior colliculus and medial division of the medial geniculate nucleus were associated more strongly with this pattern for group TL-, which was interpreted as representing the change of the associative value of the tone by the light, mediated through extraauditory influences on these two regions. A second pattern involved midbrain auditory regions, superior colliculus, zona incerta, and subiculum and was stronger for group TL0. The relations between midbrain structures may represent the excitatory conditioned response (CR) evoked by the tone in this group. The final pattern was strongest in group TL- and involved interrelations of the thalamocortical auditory system with hippocampus, basolateral amygdala, and hypothalamus. This pattern may represent the learned inhibition of the CR to the tone in the presence of the light. These findings are consistent with behavioral studies suggesting that at least two types of associations are formed during associative learning. One is the sensory relation of the stimuli and another is the relation between the CS and the affective components of the US. These behavioral associations are mapped to the patterns of functional connectivity between auditory and extraauditory regions.

Cascade projections from somatosensory cortex to the rat basolateral amygdala via the parietal insular cortex

The pathways by which somatosensory information could be relayed from the cortex to the amygdaloid complex were investigated by using the anterograde axonal transport of biocytin following cortical microinjections. Injections of biocytin into head and limb areas of secondary somatosensory cortex (S2) produced heavy labeling of fibers and terminals in granular and dysgranular parietal insular cortex from bregma to 3.8 mm behind bregma but only extremely sparse labeling in the lateral and basolateral amygdaloid nuclei. Biocytin injections into granular parietal insular cortex produced a heavy labeling of the subjacent dysgranular parietal insular cortex, but only sparse labeling in the basolateral amygdala. Biocytin injections into dysgranular parietal insular cortex resulted in heavy labeling of the subjacent agranular parietal insular cortex and strong labeling of fibers and terminals in the dorsal part of lateral nucleus, with moderate labeling of fibers in the anterior and posterior basolateral nuclei, and the central nucleus. Injections into S2 labeled the ventroposterior medial, ventroposterior lateral and posterior thalamic nuclei; injections in rostral granular and dysgranular parietal insular cortex labeled the ventral posterior and parvicellular part of ventroposterior lateral thalamic nuclei; and injections in middle to caudal dysgranular parietal insular cortex labeled only the posterior nucleus. These results suggest that whereas somatosensory cortex projects only very sparsely to the amygdala, somatosensory-related inputs to the amygdala arise in the dysgranular parietal insular cortex. The association of dysgranular parietal insular cortex with the posterior thalamus suggests it may relay nociceptive information to the amygdala.

Cortico-cortical and cortico-amygdaloid projections of the rat occipital cortex: a Phaseolus vulgaris leucoagglutinin study

The efferent projections of the occipital cortex of the rat were investigated using the Phaseolus vulgaris leucoagglutinin anterograde tract tracing technique. Particular attention was focused on projections to the amygdala and amygdalopetal cortical areas. The primary visual cortex had projections to the medial and lateral portions of occipital area 2 and other cortical regions, but no projections to the amygdala or amygdalopetal cortical areas. The only occipital area that had direct projections to the amygdala was the most ventral portion of lateral occipital area 2, located just dorsal to temporal area 2. This occipitotemporal junction region, which received projections from secondary visual cortical areas but not from the primary visual cortex, had projections to the lateral nucleus, magnocellular basal nucleus, and lateral capsular subdivision of the central nucleus of the amygdala. Occipital area 2 had projections to seven amygdalopetal cortical regions: temporal area 2, temporal area 3, frontal area 2, ventrolateral orbitofrontal area, occipitotemporal junction region, lateral entorhinal area, and the perirhinal cortex. Projections to the perirhinal cortex targeted regions located adjacent to the parietal cortex and caudal temporal cortex, but not regions adjacent to the rostral temporal cortex. Other cortical regions receiving projections from medial and lateral portions of occipital area 2 included the presubiculum, retrosplenial areas, and caudal portions of the parietal cortical areas 1 and 2. The results of the present investigation, in conjunction with previous anatomical and neurobehavioral studies, support the concept that rodent cortical visual pathways, like those of primates, consist of a dorsal system involved with visuospatial functions and a ventral system involved with object recognition. As in primates, the ventral pathway projects to the temporal-perirhinal region in a cascading manner; only highly processed information from tertiary visual cortical areas reaches the amygdala. Unlike primates, however, cortical areas in the rat brain that receive highly processed visual information appear to be regions of multisensory convergence.

Evidence for a hypothalamothalamocortical circuit mediating pheromonal influences on eye and head movements

A method for simultaneous iontophoretic injections of the anterograde tracer Phaseolus vulgaris leukoagglutinin and the retrograde tracer fluorogold was used to characterize in the rat a hypothalamothalamocortical pathway ending in a region thought to regulate attentional mechanisms by way of eye and head movements. The relevant medial hypothalamic nuclei receive pheromonal information from the amygdala and project to specific parts of the thalamic nucleus reuniens and anteromedial nucleus, which then project to a specific lateral part of the retrosplenial area (or medial visual cortex). This cortical area receives a convergent input from the lateral posterior thalamic nucleus and projects to the superior colliculus. Bidirectional connections with the hippocampal formation suggest that activity in this circuit is modified by previous experience. Striking parallels with basal ganglia circuitry are noted.

A functional anatomical study of associative learning in humans

The purpose of the study was to map the functional neuroanatomy of simple associative learning in humans. Eyeblink conditioning was studied in eight normal volunteers using positron emission tomography and H215O. Regional cerebral blood flow was assessed during three sequential phases: (i) explicitly unpaired presentations of the unconditioned stimulus (air puff to the right eye) and conditioned stimulus (binaural tone), (ii) paired presentations of the two stimuli (associative learning), and (iii) presentation of the conditioned stimulus alone. During associative learning, relative to the unpaired phase, blood flow was significantly increased in primary auditory and left posterior cingulate cortices and significantly decreased in areas of the right cerebellar, right prefrontal, right parietal, and insular cortices and right neostriatum. The lateralization of the changes may relate to the functional organization of memory and learning processes in the brain. The activation in primary auditory cortex is an example, using a neuroimaging technique, of a learning-related change in primary sensory cortex in humans. The changes in areas such as the cerebellum, prefrontal cortex, and neostriatum provide support for their roles in associative learning as proposed by animal models. Moreover, these findings show that in humans, even simple classical conditioning involves distributed changes in multiple neural systems.

Polysensory evoked potentials in rat parietotemporal cortex: combined auditory and somatosensory responses

A 64 channel microelectrode array was used to map auditory evoked potentials (AEP), somatosensory evoked potentials (SEP) as well as combined auditory and somatosensory evoked potentials (ASEP) from a 7 x 7 mm2 area in rat parietotemporal neocortex. Cytochrome oxidase (CO) stained sections of layer IV were obtained in the same animals to provide anatomical information underlying epicortical field potentials. Epicortical responses evoked by click or vibrissa stimuli replicated earlier findings from our laboratory, and appeared as a family of waveforms centered on primary auditory (AI) or somatosensory (SI) cortical areas as determined from CO histology. Selective microinjections of HRP to AI and SI further confirmed their specific sensory relay nuclei in the thalamus. A small polysensory area between AI and SI, responded uniquely with an enhanced negative sharp wave to combined auditory and somatosensory stimuli. HRP retrograde labeling revealed that the thalamocortical projections to this area were from the posterior nuclear group (Po) and medial division of the medial geniculate (MGm). These data establish close relationships between epicortical AEP, SEP, and especially ASEP and corresponding cortical structures and thalamocortical projections. The neurogenesis of unimodal and polysensory evoked potentials is discussed in terms of specific and non-specific systems.

Corticoamygdaloid and corticocortical projections of the rat temporal cortex: a Phaseolus vulgaris leucoagglutinin study

The projections of the rat temporal cortex to the amygdala and cerebral cortex were studied using the sensitive anterograde tracer, Phaseolus vulgaris leucoagglutinin. These studies revealed that the core of temporal area 1 had no projections to the amygdala but did send efferents to several cortical fields that projected to the amygdala, including temporal area 2, temporal area 3, the lateral occipital area 2, and a cortical zone along the dorsal, rostral and caudal borders of temporal area 1 ("Tel fringe"). The temporal area 1 fringe cortex had light projections to the amygdala that were confined to the dorsolateral subdivision of the lateral amygdaloid nucleus. Temporal area 2 and the caudal portion of temporal area 3 had projections to both the dorsolateral and ventromedial subdivisions of the lateral nucleus; the projection from temporal area 2 targeted mainly the ventromedial subdivision, whereas the projection from the caudal portion of temporal area 3 terminated primarily in the dorsolateral subdivision. The rostral portion of temporal area 3 had projections to both subdivisions of the lateral nucleus and to the basal magnocellular nucleus. Temporal areas 2 and 3 also had light projections to the lateral capsular subdivision of the central amygdaloid nucleus. Temporal cortical areas exhibited extensive reciprocal connections with each other. Temporal areas with amygdaloid projections also had extensive projections to the perirhinal cortex. The results of the present investigation, in conjunction with other studies of temporal cortical connections, suggest that all temporal regions projecting to the amygdala are multimodal sensory areas. The core of temporal area 1, which is probably the primary auditory area, apparently has no direct projections to the amygdala. The differential projections of different temporal areas to the amygdala suggests the existence of several distinct multimodal pathways arranged in a parallel configuration.

Vibrissal stimulation affects glucose utilization in the trigeminal/somatosensory system of normal rats and rats prenatally exposed to ethanol

The effect of gestational ethanol exposure on stimulus-induced sensory activity in the trigeminal/somatosensory system was determined. The mature offspring of mothers fed an ethanol-containing diet (Et) or pair-fed a nutritionally matched control diet (Ct) were examined. The C-row mystacial whiskers were stimulated. Glucose utilization in the principal sensory nucleus of the trigeminal nerve (PSN), ventrobasal thalamus, and somatosensory cortex was determined with [14C]2-deoxyglucose autoradiography. In Ct- and Et-treated rats, whisker stimulation increased glucose utilization in C-row barrel(oid)s in the left PSN, the right ventrobasal thalamus, and the right somatosensory cortex. The rate of glucose utilization in the C-row barrel(oid)s and in nonstimulated regions was lower in the Et-treated rats than in controls. In the cortices of Ct-treated rats, the activity in the C-row barrels on the right side was greater than in the right nonbarrel somatosensory cortex. Et-treated rats also exhibited an increase in glucose utilization, albeit smaller than that in the Ct-treated rats. In contrast, the glucose utilization in the left B- and C-row barrels of Ct-treated rats was decreased. No such decrease was evident in the left cortices of Et-treated rats. Thus, stroking whiskers stimulates the activity of sites in the trigeminal/somatosensory system. In cortex, the definition of these sites is emphasized by depressed activity, i.e., "surround" inhibition, in sites connected via callosal or corticocortical projections. Prenatal exposure to ethanol depresses the metabolic activity regardless of the physiological state; however, the "surround" inhibition of cortical activity is eliminated by prenatal exposure to ethanol through an exuberant projection.

Visual and auditory cortical lesions following acquisition of an intensity discrimination in rats fail to disrupt cross-modal transfer

The effect of visual or auditory decortication on cross-modal transfer of an intensity discrimination was examined in rats. Twenty animals were first trained under either visual-auditory (V-A) or auditory-visual (A-V) cross-modal transfer (CMT) in a shuttlebox using a shock avoidance pardigm. Prior to the second training session, five of the A-V animals received auditory ablations and five V-A animals received visual ablations. The other 10 animals served as controls and received sham operations. The results reveal that CMT occurred in both experimental groups following cortical ablations. It is possible that information regarding stimulus intensity was transferred from a cortical region used during the original training session to the cortex used in the second or retraining session, prior to surgery. Alternatively, it may be that some subcortical structure (e.g. the amygdala, superior colliculus, or reticular formation) may be involved in CMT of intensity.

Cortical hierarchy reflected in the organization of intrinsic connections in macaque monkey visual cortex

Neuronal response properties vary markedly at increasing levels of the cortical hierarchy. At present it is unclear how these variations are reflected in the organization of the intrinsic cortical circuitry. Here we analyze patterns of intrinsic horizontal connections at different hierarchical levels in the visual cortex of the macaque monkey. The connections were studied in tangential sections of flattened cortices, which were injected with the anterograde tracer biocytin. We directly compared the organization of connections in four cortical areas representing four different levels in the cortical hierarchy. The areas were visual areas 1, 2, 4 and Brodman's area 7a (V1, V2, V4 and 7a, respectively). In all areas studied, injections labeled numerous horizontally coursing axons that formed dense halos around the injection sites. Further away, the fibers tended to form separate clusters. Many fibers could be traced along the way from the injection sites to the target clusters. At progressively higher order areas, there was a striking increase in the spread of intrinsic connections: from a measured distance of 2.1 mm in area V1 to 9.0 mm in area 7a. Average interpatch distance also increased from 0.61 mm in area V1 to 1.56 mm in area 7a. In contrast, patch size changed far less at higher order areas, from an average width of 230 micron(s) in area V1 to 310 micron(s) in area 7a. Analysis of synaptic bouton distribution along axons revealed that average interbouton distance remained constant at 6.4 micron(s) (median) in and out of the clusters and in the different cortical areas. Larger injections resulted in a marked increase in the number of labeled patches but only a minor increase in the spread of connections or in patch size. Thus, in line with the more global computational roles proposed for the higher order visual areas, the spread of intrinsic connections is increased with the hierarchy level. On the other hand, the clustered organization of the connections is preserved at higher order areas. These clusters may reflect the existence of cortical modules having blob-like dimensions throughout macaque monkey visual cortex.

Connections of somatosensory cortex in megachiropteran bats: the evolution of cortical fields in mammals

The cortical connections of the primary somatosensory area (SI or 3b), a caudal somatosensory field (area 1/2), the second somatosensory area (SII), the parietal ventral area (PV), the ventral somatosensory area (VS), and the lateral parietal area (LP) were investigated in grey headed flying foxes by injecting anatomical tracers into electrophysiologically identified locations in these fields. The receptive fields for clusters of neurons were mapped with sufficient density for injection sites to be related to the boundaries of fields, and to representations of specific body parts within the fields. In all cases, cortex was flattened and sectioned parallel to the cortical surface. Sections were stained for myelin and architectonic features of cortex were related to physiological mapping and connection patterns. We found patterns of topographic and nontopographic connections between 3b and adjacent anterior parietal fields 3a and 1/2, and fields caudolateral to 3b (SII and PV). Area 1/2 had both topographic and nontopographic connections with 3b, PP, and SII. Connections of SII and PV with areas 3b, 3a, and 1/2 were roughly topographic, although there was clear evidence for nontopographic connections between these fields. SII was most densely connected with area 1/2, while PV was most densely connected with 3b. SII had additional connections with fields in lateral parietal cortex and with subdivisions of motor cortex. Other connections of PV were with subdivisions of motor cortex and pyriform cortex. Laminar differences in connection patterns of SII and PV with surrounding cortex were also observed. Injections in the ventral somatosensory area revealed connections with SII, PV, area 1/2, auditory cortex, entorhinal cortex, and pyriform cortex. Finally, the lateral parietal field had very dense connections with posterior parietal cortex, caudal temporal cortex, and with subdivisions of motor cortex. Our results indicate that the 3b region is not homogeneous, but is composed of myelin dense and light regions, associated with 3b proper and invaginations of area 1/2, respectively. Connections of myelin dense 3b were different from invaginating portions of myelin light area 1/2. Our findings that 3b is densely interconnected with PV and moderately to lightly interconnected with SII supports the notion that SII and PV have been confused across mammals and across studies. Our connectional evidence provides further support for our hypothesis that area 1/2 is partially incorporated in 3b and has led to theories of the evolution of cortical fields in mammals.

Topographic organization of cortical and subcortical projections to posterior cingulate cortex in the cat: evidence for somatic, ocular, and complex subregions

The posterior cingulate area (CGp) of the cat consists of cortex on the exposed cingulate gyrus and in the adjacent ventral bank of the splenial sulcus. We have placed deposits of distinguishable fluorescent tracers at multiple restricted sites in CGp and have analyzed the distribution throughout the forebrain of neurons labeled by retrograde transport. Cortical projections to CGp arise (in approximately descending order of strength) from anterior cingulate cortex; prefrontal cortex and premotor areas including the frontal eye fields; visual areas including especially areas 7 and 20b; parahippocampal areas; insular cortex; somesthetic areas; and auditory areas. Corticocortical pathways are organized topographically with respect to the posterior-anterior axis in CGp. Projections from prefrontal cortex and other areas with complex (as opposed to sensory, motor, or limbic) functions are concentrated posteriorly; projections from visual and oculomotor areas are concentrated at an intermediate level; and projections from areas with somesthetic and somatomotor functions are concentrated anteriorly. Thalamic projections to CGp arise from the anterior nuclei (AD, AV, and AM), from restricted portions of the ventral complex (VAd, VAm, and VMP), from discrete sectors of the lateral complex (LD, LPs, and LPm), from the rostral crescent of intralaminar nuclei (CM, PC, and CL), and from the reuniens nucleus. Projections from AM, VAd, LD, and LPs are spatially ordered in the sense that more ventral thalamic neurons project to more anterior cortical sites. Projections from AV and AD are stronger at more posterior cortical sites but do not show other signs of topographic ordering. Projections from LPm, CM, PC, CL, and RE are diffuse. We conclude (1) that cortical afferents of CGp derive predominantly from neocortical areas including those with well established sensory and motor functions; (2) that limbic projections to CGp originate primarily in structures, including the hippocampus, which are associated with memory, as opposed to structures, including the amygdala, which are associated with emotional and instinctual behavior; and (3) that CGp contains subregions in which complex, ocular, or somatic afferents predominate.

Fluoro-Gold tracing of zinc-containing afferent connections in the mouse visual cortices

To identify zinc-containing projections to the visual areas, we injected Fluoro-Gold into the occipital cortex of the mouse. Five days later, the mice underwent an intravital selenium-labeling procedure to demonstrate the somata of neurons that give rise to zinc-containing boutons. Numerous double-labeled cells were seen in the ipsi- and contralateral primary (layers II/III and VI), and secondary visual cortices (layers II/III and VI). A few double-labeled cells were apparent in other cortical areas concerned with visual processing: the orbital cortex (layers II and III), the posterior portion of the medial agranular frontal cortex (layer V/VI border), and the temporal cortex (layer VI). The cingulate, retrosplenial, perirhinal, and lateral entorhinal cortices had lamina projecting to the visual cortex and separate lamina harboring zinc-containing cells. A spatial segregation of fluorescent and zinc-containing neurons was also seen in the claustrum. This integration or segregation of projecting and zinc-containing neurons may reflect the function of the cortical areas. N-methyl-D-aspartate receptor function is antagonized by physiological concentrations of zinc in vitro. It is proposed that zinc-positive projections from areas that perform basic visual functions are less likely to be modified by N-methyl-D-aspartate receptor-mediated processes than the zinc-negative connections from associational areas.