http://cercor.oxfordjournals.org Cerebral Cortex - RSS feed of current issue 1460-2199 December 2009 Cerebral Cortex 1047-3211 http://cercor.oxfordjournals.org/cgi/content/short/19/12/NP?rss=1 Fri, 06 Nov 2009 17:37:06 PST info:doi/10.1093/cercor/bhp256 Oxford University Press 12 19 NP 2009-12-01 NP Cover http://cercor.oxfordjournals.org/cgi/content/short/19/12/NP-a?rss=1 Fri, 06 Nov 2009 17:37:06 PST info:doi/10.1093/cercor/bhp257 Oxford University Press 12 19 NP 2009-12-01 NP Editorial Board http://cercor.oxfordjournals.org/cgi/content/short/19/12/NP-b?rss=1 Fri, 06 Nov 2009 17:37:06 PST info:doi/10.1093/cercor/bhp258 Oxford University Press 12 19 NP 2009-12-01 NP Subscriptions http://cercor.oxfordjournals.org/cgi/content/short/19/12/NP-c?rss=1 Fri, 06 Nov 2009 17:37:06 PST info:doi/10.1093/cercor/bhp259 Oxford University Press 12 19 NP 2009-12-01 NP Table of Contents http://cercor.oxfordjournals.org/cgi/content/short/19/12/2767?rss=1 Semantic memory refers to knowledge about people, objects, actions, relations, self, and culture acquired through experience. The neural systems that store and retrieve this information have been studied for many years, but a consensus regarding their identity has not been reached. Using strict inclusion criteria, we analyzed 120 functional neuroimaging studies focusing on semantic processing. Reliable areas of activation in these studies were identified using the activation likelihood estimate (ALE) technique. These activations formed a distinct, left-lateralized network comprised of 7 regions: posterior inferior parietal lobe, middle temporal gyrus, fusiform and parahippocampal gyri, dorsomedial prefrontal cortex, inferior frontal gyrus, ventromedial prefrontal cortex, and posterior cingulate gyrus. Secondary analyses showed specific subregions of this network associated with knowledge of actions, manipulable artifacts, abstract concepts, and concrete concepts. The cortical regions involved in semantic processing can be grouped into 3 broad categories: posterior multimodal and heteromodal association cortex, heteromodal prefrontal cortex, and medial limbic regions. The expansion of these regions in the human relative to the nonhuman primate brain may explain uniquely human capacities to use language productively, plan, solve problems, and create cultural and technological artifacts, all of which depend on the fluid and efficient retrieval and manipulation of semantic knowledge.

]]> Fri, 06 Nov 2009 17:37:06 PST info:doi/10.1093/cercor/bhp055 Oxford University Press 12 19 2796 2009-12-01 2767 Feature Article http://cercor.oxfordjournals.org/cgi/content/short/19/12/2797?rss=1 We investigated sex differences in the resting-state neural correlates of Openness to Experience, a universal personality trait defined by cognitive flexibility, attention to feelings, creativity, and preference for novelty. Using resting-state positron-emission tomography from 100 older individuals (>55 years of age), we identified associations between Openness and resting-state regional cerebral blood flow that replicated across 2 assessments of the same sample, approximately 2 years apart. Openness correlated positively with prefrontal activity in women, anterior cingulate activity in men, and orbitofrontal activity in both sexes, which suggests that areas linked to cognitive flexibility (women), monitoring processes (men), and reward and emotional processing (both) underlie individual differences in Openness. The results challenge the implicit assumption that the same trait will rely on the same neural mechanisms across all who express it.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp066 Oxford University Press 12 19 2802 2009-12-01 2797 Feature Article http://cercor.oxfordjournals.org/cgi/content/short/19/12/2803?rss=1 When paired with aversive events, visual conditioned stimuli (CS) provoke increased activations in visual cortex. It is unclear however whether these changes reflect cognitive processes such as expectancy of the aversive unconditioned stimulus (US), or implicit associative learning of the contingencies outside awareness. Here, we used the "gambler's fallacy" phenomenon to parametrically and inversely manipulate the expectancy of an US and the number of conditioning trials: Increasing the number of CS–US pairings was associated with participants expecting the US to be less likely and vice versa. Magnetocortical activity evoked by the CS in occipital and supplementary motor areas was linearly related to the associative strength (number of CS–US pairings), but decreased as a function of expectancy. These results suggest that the cortical facilitation of fear cue processing is determined by associative strength and previous exposure to learning contingencies rather than by the cognitive anticipation for the US.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp052 Oxford University Press 12 19 2809 2009-12-01 2803 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2810?rss=1 Intrinsic properties, morphology, and local network circuitry of identified layer 5 and layer 6 auditory corticothalamic neurons were compared. We injected fluorescent microspheres into the mouse auditory thalamus to prelabel corticothalamic neurons, then recorded and filled labeled layer 5 or layer 6 auditory cortical neurons in vitro. We observed low-threshold bursting in adult, but not juvenile, layer 5 corticothalamic neurons that was voltage and time dependent with nonlinear input–output properties, whereas adult layer 6 corticothalamic neurons demonstrated a regular spiking. Layer 5 corticothalamic neurons had larger somata, thicker apical dendrites and were more likely to have a layer 1 apical dendrite than layer 6 neurons. Using laser photostimulation, identified layer 5 corticothalamic neurons received excitatory input from a wide area of layers 2/3, 4, and 5 with widespread -aminobutyric acidergic input from layer 2/3 and lower layer 5, whereas layer 6 corticothalamic neurons from the same cortical column received circumscribed excitatory input and discrete patches of inhibition derived from layer 6 of adjacent columns. These data demonstrate that layer 5 and layer 6 corticothalamic neurons receive unique sets of inputs and process them in different manners, supporting the hypothesis that layer-specific corticothalamic projections play distinct roles in information processing.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp050 Oxford University Press 12 19 2826 2009-12-01 2810 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2827?rss=1 Spatial memory formation is enabled through synaptic information processing, in the form of persistent strengthening and weakening of synapses, within the hippocampus. It is, however, unclear how relevant spatial information is selected for encoding, in preference to less pertinent information. As the noradrenergic locus coeruleus (LC) becomes active in response to novel experiences, we hypothesized that the LC may provide the saliency signal required to promote hippocampal encoding of relevant information through changes in synaptic strength. Test pulse stimulation evoked stable basal synaptic transmission at Schaffer collateral (SC)–CA1 stratum radiatum synapses in freely behaving adult rats. Coupling of these test pulses with electrical stimulation of the LC induced long-term depression (LTD) at SC–CA1 synapses and induced a transient suppression of theta-frequency oscillations. Effects were N-methyl-D-aspartate and β-adrenergic receptor dependent. Activation of the LC also increased CA1 noradrenalin levels and facilitated the encoding of spatial memory for a single episode via a β-adrenoceptor–dependent mechanism. Our results demonstrate that the LC plays a key role in the induction of hippocampal LTD and in promoting the encoding of spatial information. This LC–hippocampal interaction may reflect a means by which salient information is distinguished for subsequent synaptic processing.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp065 Oxford University Press 12 19 2837 2009-12-01 2827 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2838?rss=1 The impact of developmental ablation of Pax6 function on morphology and functional connectivity of the adult cerebrum was studied in cortex-specific Pax6 knockout mice (Pax6cKO) using structural magnetic resonance imaging (MRI), manganese-enhanced MRI, and diffusion tensor MRI in conjunction with fiber tractography. Mutants presented with decreased volumes of total brain and olfactory bulb, reduced cortical thickness, and altered layering of the piriform cortex. Tracking of major neuronal fiber bundles revealed a disorganization of callosal fibers with an almost complete lack of interhemispheric connectivity. In Pax6cKO mice intrahemispheric callosal fibers as well as intracortical fibers were predominantly directed along a rostrocaudal orientation instead of a left–right and dorsoventral orientation found in controls. Fiber disorganization also involved the septohippocampal connection targeting mostly the lateral septal nucleus. The hippocampus was rostrally extended and its volume was increased relative to that of the forebrain and midbrain. Manganese-induced MRI signal enhancement in the CA3 region suggested a normal function of hippocampal pyramidal cells. Noteworthy, several morphologic disturbances in gray and white matter of Pax6cKO mice were similar to observations in human aniridia patients. The present findings indicate an important role of Pax6 in the development of both the cortex and cerebral fiber connectivity.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp057 Oxford University Press 12 19 2847 2009-12-01 2838 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2848?rss=1 The parietal mechanisms for the control of hand movement trajectory were studied by recording cell activity in area 5 of monkeys making direct reaches to visual targets and online corrections of movement trajectory, after change of target location in space. The activity of hand-related cells was fitted with a linear model including hand position, movement direction, and speed. The neural activity modulation mostly led, but also followed, hand movement. When a change of hand trajectory occurred, the pattern of activity associated with the movement to the first target evolved into that typical of the movement to the second one, thus following the corresponding variations of the hand kinematics. The visual signal concerning target location in space did not influence the firing activity associated with the direction of hand movement within the first 150 ms after target presentation. This might be the time necessary for the visuo-motor transformation underlying reaching. We conclude that online control of hand trajectory not only resides in the relationships between neural activity and kinematics, but, under specific circumstances, also on the coexistence of signals about ongoing and future hand movement direction.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp058 Oxford University Press 12 19 2864 2009-12-01 2848 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2865?rss=1 To understand the relationship between the structure and function of primate neocortical areas at a molecular level, we have been screening for genes differentially expressed across macaque neocortical areas by restriction landmark cDNA scanning (RLCS). Here, we report enriched expression of the paraneoplastic antigen-like 5 gene (PNMA5) in association areas but not in primary sensory areas, with the lowest expression level in primary visual cortex. In situ hybridization in the primary sensory areas revealed PNMA5 mRNA expression restricted to layer II. Along the ventral visual pathway, the expression gradually increased in the excitatory neurons from the primary to higher visual areas. This differential expression pattern was very similar to that of retinol-binding protein (RBP) mRNA, another association-area-enriched gene that we reported previously. Additional expression analysis for comparison of other genes in the PNMA gene family, PNMA1, PNMA2, PNMA3, and MOAP1 (PNMA4), showed that they were widely expressed across areas and layers but without the differentiated pattern of PNMA5. In mouse brains, PNMA1 was only faintly expressed and PNMA5 was not detected. Sequence analysis showed divergence of PNMA5 sequences among mammals. These findings suggest that PNMA5 acquired a certain specialized role in the association areas of the neocortex during primate evolution.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp062 Oxford University Press 12 19 2879 2009-12-01 2865 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2880?rss=1 Whether normal word reading includes a stage of visual processing selectively dedicated to word or letter recognition is highly debated. Characterizing pure alexia, a seemingly selective disorder of reading, has been central to this debate. Two main theories claim either that 1) Pure alexia is caused by damage to a reading specific brain region in the left fusiform gyrus or 2) Pure alexia results from a general visual impairment that may particularly affect simultaneous processing of multiple items. We tested these competing theories in 4 patients with pure alexia using sensitive psychophysical measures and mathematical modeling. Recognition of single letters and digits in the central visual field was impaired in all patients. Visual apprehension span was also reduced for both letters and digits in all patients. The only cortical region lesioned across all 4 patients was the left fusiform gyrus, indicating that this region subserves a function broader than letter or word identification. We suggest that a seemingly pure disorder of reading can arise due to a general reduction of visual speed and span, and explain why this has a disproportionate impact on word reading while recognition of other visual stimuli are less obviously affected.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp059 Oxford University Press 12 19 2890 2009-12-01 2880 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2891?rss=1 Preterm birth results in significant neurodevelopmental disability. A neonatal rodent model of chronic sublethal hypoxia (CSH), which mimics effects of preterm birth, was used to characterize neurodevelopmental consequences of prolonged exposure to hypoxia using tissue anisotropy measurements from diffusion tensor imaging. Corpus callosum, cingulum, and fimbria of the hippocampus revealed subtle, yet significant, hypoxia-induced modifications during maturation (P15–P51). Anisotropy differences between control and CSH mice were greatest at older ages (>P40) in these regions. Neither somatosensory cortex nor caudate putamen revealed significant differences between control and CSH mice at any age. We assessed control and CSH mice using tests of general activity and cognition for behavioral correlates of morphological changes. Open-field task revealed greater locomotor activity in CSH mice early in maturation (P16–P18), whereas by adolescence (P40–P45) differences between control and CSH mice were insignificant. These results may be associated with lack of cortical and subcortical anisotropy differences between control and CSH mice. Spatial-delayed alternation and free-swim tasks in adulthood revealed lasting impairments for CSH mice in spatial memory and behavioral laterality. These differences may correlate with anisotropy decreases in hippocampal and callosal connectivities of CSH mice. Thus, CSH mice revealed developmental and behavioral deficits that are similar to those observed in low birth weight preterm infants.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp068 Oxford University Press 12 19 2901 2009-12-01 2891 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2902?rss=1 Corticofugal projections to the thalamus reveal 2 axonal morphologies, each associated with specific physiological attributes. These determine the functional characteristics of thalamic neurons. It is not clear, however, whether such features characterize the corticofugal projections that mediate multisensory integration in superior colliculus (SC) neurons. The cortico-collicular projections from cat anterior ectosylvian sulcus (AES) are derived from its visual, auditory, and somatosensory representations and are critical for multisensory integration. Following tracer injections into each subdivision, 2 types of cortico-collicular axons were observed. Most were categorized as type I and consisted of small-caliber axons traversing long distances without branching, bearing mainly small boutons. The less frequent type II had thicker axons, more complex branching patterns, larger boutons, and more complex terminal boutons. Following combinatorial injections of 2 different fluorescent tracers into defined AES subdivisions, fibers from each were seen converging onto individual SC neurons and indicate that such convergence, like that in the corticothalamic system, is mediated by 2 distinct morphological types of axon terminals. Nevertheless, and despite the conservation of axonal morphologies across different subcortical systems, it is not yet clear if the concomitant physiological attributes described in the thalamus are directly applicable to multisensory integration.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp060 Oxford University Press 12 19 2915 2009-12-01 2902 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2916?rss=1 The morphology of axonal and dendritic arbors in the immature cerebral cortex influences the degree of anisotropy in water diffusion. This enables cortical maturation to be monitored by the noninvasive technique of diffusion tensor magnetic resonance imaging (DTI). Herein, we utilized DTI of postmortem ferret brain to quantify regional and temporal patterns in cortical maturation. We found that diffusion anisotropy within the isocortex decreases over the first month of life, coinciding closely in time with expansion of axonal and dendritic cellular processes of pyramidal neurons. Regional patterns consist of differences between allocortex and isocortex, a regional anisotropy gradient that closely parallels the transverse neurogenetic gradient, and differences between primary and nonprimary isocortical areas. By combining the temporal and regional factors, the isocortical developmental gradient magnitude corresponds to a 5-day difference in maturity between relatively developed rostral/caudal isocortex at the gradient source and less mature isocortex at the occipital pole. Additionally, the developmental trajectory of primary areas precedes nonprimary areas by 2.7 days. These quantitative estimates coincide with previous histological studies of ferret development. Similarities in cerebral cortical diffusion anisotropy observed between ferret and other species suggest the framework developed here is of general potential relevance.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp061 Oxford University Press 12 19 2929 2009-12-01 2916 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2930?rss=1 Although the inferior parietal cortex (IPC) has been consistently implicated in mathematical cognition, the functional roles of its subdivisions are poorly understood. We address this problem using probabilistic cytoarchitectonic maps of IPC subdivisions intraparietal sulcus (IPS), angular gyrus (AG), and supramarginal gyrus. We quantified IPC responses relative to task difficulty and individual differences in task proficiency during mental arithmetic (MA) tasks performed with Arabic (MA-A) and Roman (MA-R) numerals. The 2 tasks showed similar levels of activation in 3 distinct IPS areas, hIP1, hIP2, and hIP3, suggesting their obligatory role in MA. Both AG areas, PGa and PGp, were strongly deactivated in both tasks, with stronger deactivations in posterior area PGp. Compared with the more difficult MA-R task, the MA-A task showed greater responses in both AG areas, but this effect was driven by less deactivation in the MA-A task. AG deactivations showed prominent overlap with lateral parietal nodes of the default mode network, suggesting a nonspecific role in MA. In both tasks, greater bilateral AG deactivation was associated with poorer performance. Our findings suggest a close link between IPC structure and function and they provide new evidence for behaviorally salient functional heterogeneity within the IPC during mathematical cognition.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp063 Oxford University Press 12 19 2945 2009-12-01 2930 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2946?rss=1 We used functional magnetic resonance imaging (fMRI) to investigate the brain basis of overt and covert forms of attention during search, while employing stringent control of both eye movements and attentional shifts. A factorial design compared overt and covert forms of goal-directed serial search versus stimulus-driven tracking. To match ocular changes and the number and magnitude of attention shifts across cells in the design, stimulus-driven tracking involved trial-specific "replay" of previous goal-directed eye movements. We found that, in terms of cortical activations, engagement of the dorsal fronto-parietal network by goal-directed attention did not depend on oculomotor requirements, being found similarly for covert attention, in accord with other work. However, analyses of effective connectivity (or "functional coupling") revealed that information flow within this network changed significantly as a function of both the task (goal-directed or stimulus-driven) and the overt versus covert form of attention. Additionally, we observed a distinct set of subcortical regions (pulvinar and caudate nucleus) engaged primarily during the covert form of goal-directed search. We conclude that dynamics within the dorsal fronto-parietal attentional system flexibly reorganize to integrate task demands and oculomotor requirements.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp064 Oxford University Press 12 19 2958 2009-12-01 2946 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2959?rss=1 Spike timing–dependent plasticity (STDP) is a strong candidate for an N-methyl-D-aspartate (NMDA) receptor-dependent form of synaptic plasticity that could underlie the development of receptive field properties in sensory neocortices. Whilst induction of timing-dependent long-term potentiation (t-LTP) requires postsynaptic NMDA receptors, timing-dependent long-term depression (t-LTD) requires the activation of presynaptic NMDA receptors at layer 4-to-layer 2/3 synapses in barrel cortex. Here we investigated the developmental profile of t-LTD at layer 4-to-layer 2/3 synapses of mouse barrel cortex and studied their NMDA receptor subunit dependence. Timing-dependent LTD emerged in the first postnatal week, was present during the second week and disappeared in the adult, whereas t-LTP persisted in adulthood. An antagonist at GluN2C/D subunit–containing NMDA receptors blocked t-LTD but not t-LTP. Conversely, a GluN2A subunit–preferring antagonist blocked t-LTP but not t-LTD. The GluN2C/D subunit requirement for t-LTD appears to be synapse specific, as GluN2C/D antagonists did not block t-LTD at horizontal cross-columnar layer 2/3-to-layer 2/3 synapses, which was blocked by a GluN2B antagonist instead. These data demonstrate an NMDA receptor subunit-dependent double dissociation of t-LTD and t-LTP mechanisms at layer 4-to-layer 2/3 synapses, and suggest that t-LTD is mediated by distinct molecular mechanisms at different synapses on the same postsynaptic neuron.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp067 Oxford University Press 12 19 2969 2009-12-01 2959 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2970?rss=1 Previous studies have proposed a variety of mechanisms by which attention influences neuronal activity. Here we investigated the mechanisms of attention in the striate cortex of monkeys performing a spatial or an object-based attention task at various stimulus contrasts and compared neuronal contrast response functions with and without attention. Our data are best described by an "additive" interaction: The influence of attention on the neuronal response is relatively independent of the stimulus contrast, at least when the stimulus has enough contrast to become visible. This shows that attention adds to the neuronal responses in a largely contrast invariant manner. These data support recent functional magnetic resonance imaging studies and suggest that feedback from higher areas exerts a constant attentional drive that is mostly task not stimulus driven.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp070 Oxford University Press 12 19 2981 2009-12-01 2970 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2982?rss=1 Retinal lesions induce a topographic reorganization in the corresponding lesion projection zone (LPZ) in the visual cortex of adult cats. To gain a better insight into the reactivation dynamics, we investigated the alterations in cortical activity throughout area 17. We implemented in situ hybridization and real-time polymerase chain reaction to analyze the spatiotemporal expression patterns of the activity marker genes zif268 and c-fos. The immediate early gene (IEG) data confirmed a strong and permanent activity decrease in the center of the LPZ as previously described by electrophysiology. A recovery of IEG expression was clearly measured in the border of the LPZ. We were able to register reorganization over 2.5–6 mm. We also present evidence that the central retinal lesions concomitantly influence the activity in far peripheral parts of area 17. Its IEG expression levels appeared dependent of time and distance from the LPZ. We therefore propose that coupled changes in activity occur inside and outside the LPZ. In conclusion, alterations in activity reporter gene expression throughout area 17 contribute to the lesion-induced functional reorganization.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp069 Oxford University Press 12 19 2992 2009-12-01 2982 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/2993?rss=1 By virtue of its widespread afferent projections, perirhinal cortex is thought to bind polymodal information into abstract object-level representations. Consistent with this proposal, deficits in cross-modal integration have been reported after perirhinal lesions in nonhuman primates. It is therefore surprising that imaging studies of humans have not observed perirhinal activation during visual–tactile object matching. Critically, however, these studies did not differentiate between congruent and incongruent trials. This is important because successful integration can only occur when polymodal information indicates a single object (congruent) rather than different objects (incongruent). We scanned neurologically intact individuals using functional magnetic resonance imaging (fMRI) while they matched shapes. We found higher perirhinal activation bilaterally for cross-modal (visual–tactile) than unimodal (visual–visual or tactile–tactile) matching, but only when visual and tactile attributes were congruent. Our results demonstrate that the human perirhinal cortex is involved in cross-modal, visual–tactile, integration and, thus, indicate a functional homology between human and monkey perirhinal cortices.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp073 Oxford University Press 12 19 3000 2009-12-01 2993 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/3001?rss=1 Neural connectivity at the cellular and mesoscopic level appears very specific and is presumed to arise from highly specific developmental mechanisms. However, there are general shared features of connectivity in systems as different as the networks formed by individual neurons in Caenorhabditis elegans or in rat visual cortex and the mesoscopic circuitry of cortical areas in the mouse, macaque, and human brain. In all these systems, connection length distributions have very similar shapes, with an initial large peak and a long flat tail representing the admixture of long-distance connections to mostly short-distance connections. Furthermore, not all potentially possible synapses are formed, and only a fraction of axons (called filling fraction) establish synapses with spatially neighboring neurons. We explored what aspects of these connectivity patterns can be explained simply by random axonal outgrowth. We found that random axonal growth away from the soma can already reproduce the known distance distribution of connections. We also observed that experimentally observed filling fractions can be generated by competition for available space at the target neurons—a model markedly different from previous explanations. These findings may serve as a baseline model for the development of connectivity that can be further refined by more specific mechanisms.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp071 Oxford University Press 12 19 3010 2009-12-01 3001 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/3011?rss=1 -Aminobutyric acid (GABA)–releasing inhibitory interneurons, a critical component of cortical circuitry, are involved in myriad known functional roles. However, information regarding the cytoarchitectural, physiological, and molecular properties of interneurons in posterior piriform cortex (PPC) is sparse. Taking advantage of the glutamic acid decarboxylase (GAD)67–enhanced green fluorescent protein (EGFP) mouse, we used in vitro whole-cell patch-clamp techniques to record from GABAergic interneurons across all 3 layers of PPC and, subsequently, to reconstruct their morphology. For the first time, 5 groups of interneurons are identified, whose firing types are defined based on those described within neocortex. Interestingly, each interneuron group with a distinct firing type also exhibits unique morphological properties, laminar distributions, and excitatory synaptic properties. The dendritic and axonal processes demonstrate subtype-specific orientations and a differential expression of spines and boutons, respectively. In addition, the active and passive electrophysiological properties of these cells show marked intergroup differences. Immunohistochemical techniques revealed a laminar-specific distribution of calcium-binding proteins and vasoactive intestinal peptide (VIP) expression. Surprisingly, excitatory synaptic properties in several groups lack target-specific differences seen in neocortical circuits, reflecting a circuit arranged with less complexity. These data aid in the identification of PPC interneurons and allow us to make well-supported postulations about their functional properties.

]]> Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp072 Oxford University Press 12 19 3029 2009-12-01 3011 Articles http://cercor.oxfordjournals.org/cgi/content/short/19/12/3030?rss=1 Fri, 06 Nov 2009 17:37:07 PST info:doi/10.1093/cercor/bhp204 Oxford University Press 12 19 3030 2009-12-01 3030 Articles