Correlations between the fMRI BOLD signal and visual perception

Using fMRI and a psychophysical task involving letter identification, Kleinschmidt et al. (2002) (this issue of Neuron) delineate two patterns of neural activation, which manifest in different cortical regions: a transient activation, correlated with the change of a percept, and a longer-term hysteresis, correlated with the maintenance of the percept. These findings are provocative and suggest that neural hysteresis is mediated by visual structures that interact with higher-order regions to support longer-term maintenance of a percept.     

Neural network model of the primary visual cortex: From functional architecture to lateral connectivity and back

The role of intrinsic cortical dynamics is a debatable issue. A recent optical imaging study (Kenet et al., 2003) found that activity patterns similar to orientation maps (OMs), emerge in the primary visual cortex (V1) even in the absence of sensory input, suggesting an intrinsic mechanism of OM activation. To better understand these results and shed light on the intrinsic V1 processing, we suggest a neural network model in which OMs are encoded by the intrinsic lateral connections. The proposed connectivity pattern depends on the preferred orientation and, unlike previous models, on the degree of orientation selectivity of the interconnected neurons. We prove that the network has a ring attractor composed of an approximated version of the OMs. Consequently, OMs emerge spontaneously when the network is presented with an unstructured noisy input. Simulations show that the model can be applied to experimental data and generate realistic OMs. We study a variation of the model with spatially restricted connections, and show that it gives rise to states composed of several OMs. We hypothesize that these states can represent local properties of the visual scene. Action Editor: Jonathan D. Victor     

Auditory cortex mapmaking: principles, projections, and plasticity

Maps of sensory receptor epithelia and computed features of the sensory environment are common elements of auditory, visual, and somatic sensory representations from the periphery to the cerebral cortex. Maps enhance the understanding of normal neural organization and its modification by pathology and experience. They underlie the derivation of the computational principles that govern sensory processing and the generation of perception. Despite their intuitive explanatory power, the functions of and rules for organizing maps and their plasticity are not well understood. Some puzzles of auditory cortical map organization are that few complete receptor maps are available and that even fewer computational maps are known beyond primary cortical areas. Neuroanatomical evidence suggests equally organized connectional patterns throughout the cortical hierarchy that might underlie map stability. Here, we consider the implications of auditory cortical map organization and its plasticity and evaluate the complementary role of maps in representation and computation from an auditory perspective.     

Do cortical maps depend on the timing of sensory input? Experimental evidence and computational model

Fast adaptations in the functional organization of primary sensory cortex are generally assumed to result from changes of network connectivity. However, the effects of intrinsic neuronal excitability alterations due to the activation of neighboring cortical representational zones, which might as well account for the changes of cortical representative maps, have been paid little attention to. In a recent experiment (Braun et al. 2000b) we showed by neuromagnetic source imaging that random or fixed sequence stimulation of three digits of both hands led to stimulation-timing-induced changes in primary somatosensory (SI) cortical maps. The distance between the cortical representation of thumb and middle finger became significantly shorter during the fixed sequence stimulation. The analysis on the time course of the cortical map changes revealed that these reorganizations occurred within minutes and were fully reversible. The previously reported results were interpreted as the involvement of a superordinate center responsible for detecting and activating the appropriate maps. Here we present an alternative parsimonious explanation that is supported by a computational model. Based on the experimental evidence, we developed a simple model that took intrinsic neuronal excitability together with subthreshold activation into account and assumed partial cortical overlap of the representational zones of neighboring digits. Furthermore, in the model the neuronal excitability decayed slowly with respect to the stimulation frequency. The observed cortical map changes in the experiment could be reproduced by the two-layer feed-forward computational network. Our model thus suggests that the dynamic shifts of cortical maps can be explained by the state and time course of intrinsic neuronal excitability and subthreshold activation, without involving changes in network connectivity.     

There's more than one way to scale a synapse

TNFalpha has been proposed to underlie synaptic scaling, but the mechanism and functional significance of this remain unclear. In this issue of Neuron, Cingolani et al. demonstrate that TNFalpha can mediate scaling through the regulation of beta3 integrins. Kaneko et al. show that TNFalpha-dependent synaptic scaling plays an important role in visual cortical plasticity.     

Pairing-induced changes of orientation maps in cat visual cortex.

We have studied the precise temporal requirements for plasticity of orientation preference maps in kitten visual cortex. Pairing a brief visual stimulus with electrical stimulation in the cortex, we found that the relative timing determines the direction of plasticity: a shift in orientation preference toward the paired orientation occurs if the cortex is activated first visually and then electrically; the cortical response to the paired orientation is diminished if the sequence of visual and electrical activation is reversed. We furthermore show that pinwheel centers are less affected by the pairing than the pinwheel surround. Thus, plasticity is not uniformly distributed across the cortex, and, most importantly, the same spike time-dependent learning rules that have been found in single-cell in vitro studies are also valid on the level of cortical maps.     

In the eye of the beholder: visual experience and categories in the human brain

How does experience change representations of visual objects in the brain? Do cortical object representations reflect category membership? In this issue of Neuron, Jiang et al. show that category training leads to sharpening of neural responses in high-level visual cortex; in contrast, category boundaries may be represented only in prefrontal cortex.     

Time to change: retina sends a messenger to promote plasticity in visual cortex

Maturation of GABA inhibitory circuitry in primary visual cortex activates the critical period of plasticity, but the underlying mechanisms are not well understood. In the August 8th issue of Cell, Sugiyama et al. demonstrate that visual experience promotes the passage of a retina-derived homeoprotein along the visual pathway, which nurtures subclasses of cortical interneurons implicated in regulating critical period plasticity.     

Local axon guidance in cerebral cortex and thalamus: are we there yet?

Normal brain function requires the development of precise connections between thalamus and cerebral cortex. In this issue of Neuron, Cang et al. and Tori and Levitt argue that EphA/ephrin-A signaling in the target tissue guides sensory thalamic axons to the correct cortical area, and sensory cortical axons to precise thalamic targets. Although EphA/ephrin-A signaling organizes sensory maps within areas, and thalamocortical axons in the internal capsule, both papers argue that each developmental event is dissociable from the others.     

Superimposed hemifields in primary visual cortex of achiasmic individuals

In the rare condition of achiasma, the visual cortex in each hemisphere receives information from both halves of the visual field. How is this "doubling" of information accommodated in V1? In this issue of Neuron, Hoffmann et?al. (2012) investigate the cortical consequences of this anomaly.     

Vision and cortical map development

Functional maps arise in developing visual cortex as response selectivities become organized into columnar patterns of population activity. Recent studies of developing orientation and direction maps indicate that both are sensitive to visual experience, but not to the same degree or duration. Direction maps have a greater dependence on early vision, while orientation maps remain sensitive to experience for a longer period of cortical maturation. There is also a darker side to experience: abnormal vision through closed lids produces severe impairments in neuronal selectivity, rendering these maps nearly undetectable. Thus, the rules that govern their formation and the construction of the underlying neural circuits are modulated-for better or worse-by early vision. Direction maps, and possibly maps of other properties that are dependent upon precise conjunctions of spatial and temporal signals, are most susceptible to the potential benefits and maladaptive consequences of early sensory experience.     

Otx2's incredible journey

A surprising new mechanism that regulates the plasticity of postnatal neurons is reported in this issue by Sugiyama et al. (2008). These authors show in mice that visual experience triggers cell-to-cell transfer of the homeoprotein Otx2 to cortical interneurons, where it promotes maturation of inhibitory neural circuitry and opens the critical period for plasticity in the visual cortex.     

Early retinal activity and visual circuit development

Does spontaneous retinal activity prior to vision play a role in the establishment of visual maps? In this issue of Neuron, two separate papers by Huberman et al. and Hooks and Chen demonstrate a role for early spontaneous retinal activity in the establishment of ocular dominance columns and synaptic refinement at retinogeniculate synapses.     

Cortical bone maintenance and geometry of the tibia in prehistoric children from Nubia's Batn el Hajar

The relationship between advancing age in adults and patterns of cortical bone maintenance has been extensively documented for archaeological populations (Dewey, et al., 1969; Van Gerven et al., 1969; Perzigian, 1973). Most recently, this research has been expanded to include a more thorough consideration of the geometric properties of bone in relationship to adult age changes (Martin and Atkinsin, 1977; Ruff and Hayes, 1983). To date, however, few studies have documented subadult patterns of cortical bone maintenance in archaeological populations and none have incorporated the relationship between patterns of cortical bone loss and gain and the changing geometric properties of growing bone. Using a sample of 172 tibias from children excavated from the Medieval Christian site of Kulubnarti, located in Nubia's Batn el Hajar, the present research examines the relationship between percent cortical area, bone mineral content, and cross-sectional moments of inertia. Among these children, bone mineral content increases steadily from birth in spite of a reduction in percent cortical area during early and late childhood. It appears, therefore, that tissue quality of the bone is not adversely affected by the reduction. Furthermore, the reduction in percent cortical area in later childhood corresponds to a dramatic increase in bending strength measured by cross-sectional moments of inertia. Thus, whether this cortical reduction is due entirely or in part to either normal modeling or nutritional stress, the tissue and organ quality of the bone is not adversely affected.     

Modeling Magnification and Anisotropy in the Primate Foveal Confluence

A basic organizational principle of the primate visual system is that it maps the visual environment repeatedly and retinotopically onto cortex. Simple algebraic models can be used to describe the projection from visual space to cortical space not only for V1, but also for the complex of areas V1, V2 and V3. Typically a conformal (angle-preserving) projection ensuring local isotropy is regarded as ideal and primate visual cortex is often regarded as an approximation of this ideal. However, empirical data show systematic deviations from this ideal that are especially relevant in the foveal projection. The aims of this study were to map the nature of anisotropy predicted by existing models, to investigate the optimization targets faced by different types of retino-cortical maps, and finally to propose a novel map that better models empirical data than other candidates. The retino-cortical map can be optimized towards a space-conserving homogenous representation or a quasi-conformal mapping. The latter would require a significantly enlarged representation of specific parts of the cortical maps. In particular it would require significant enlargement of parafoveal V2 and V3 which is not supported by empirical data. Further, the recently published principal layout of the foveal singularity cannot be explained by existing models. We suggest a new model that accurately describes foveal data, minimizing cortical surface area in the periphery but suggesting that local isotropy dominates the most foveal part at the expense of additional cortical surface. The foveal confluence is an important example of the detailed trade-offs between the compromises required for the mapping of environmental space to a complex of neighboring cortical areas. Our models demonstrate that the organization follows clear morphogenetic principles that are essential for our understanding of foveal vision in daily life.     

An incremental Hebbian learning model of the primary visual cortex with lateral plasticity and real input patterns

We present a simplified binocular neural network model of the primary visual cortex with separate ON/OFF-pathways and modifiable afferent as well as intracortical synaptic couplings. Random as well as natural image stimuli drive the weight adaptation which follows Hebbian learning rules stabilized with constant norm and constant sum constraints. The simulations consider the development of orientation and ocular dominance maps under different conditions concerning stimulus patterns and lateral couplings. With random input patterns realistic orientation maps with +/- 1/2-vortices mostly develop and plastic lateral couplings self-organize into mexican hat type structures on average. Using natural greyscale images as input patterns, realistic orientation maps develop as well and the lateral coupling profiles of the cortical neurons represent the two point correlations of the input image used.     

The functional logic of cortico-pulvinar connections

The pulvinar is an 'associative' thalamic nucleus, meaning that most of its input and output relationships are formed with the cerebral cortex. The function of this circuitry is little understood and its anatomy, though much investigated, is notably recondite. This is because pulvinar connection patterns disrespect the architectural subunits (anterior, medial, lateral and inferior pulvinar nuclei) that have been the traditional reference system. This article presents a simplified, global model of the organization of cortico-pulvinar connections so as to pursue their structure-function relationships. Connections between the cortex and pulvinar are topographically organized, and as a result the pulvinar contains a 'map' of the cortical sheet. However, the topography is very blurred. Hence the pulvinar connection zones of nearby cortical areas overlap, allowing indirect transcortical communication via the pulvinar. A general observation is that indirect cortico-pulvino-cortical circuits tend to mimic direct cortico-cortical pathways: this is termed 'the replication principle'. It is equally apt for certain pairs (or groups) of nearby cortical areas that happen not to connect with each other. The 'replication' of this non-connection is achieved by discontinuities and dislocations of the cortical topography within the pulvinar, such that the associated pair of connection zones do not overlap. Certain of these deformations can be used to divide the global cortical topography into specific sub-domains, which form the natural units of a connectional subdivision of the pulvinar. A substantial part of the pulvinar also expresses visual topography, reflecting visual maps in occipital cortex. There are just two well-ordered visual maps in the pulvinar, that both receive projections from area V1, and several other occipital areas; the resulting duplication of cortical topography means that each visual map also acts as a separate connection domain. In summary, the model identifies four topographically ordered connection domains, and reconciles the coexistence of visual and cortical maps in two of them. The replication principle operates at and below the level of domain structure. It is argued that cortico-pulvinar circuitry replicates the pattern of cortical circuitry but not its function, playing a more regulatory role instead. Thalamic neurons differ from cortical neurons in their inherent rhythmicity, and the pattern of cortico-thalamic connections must govern the formation of specific resonant circuits. The broad implication is that the pulvinar acts to coordinate cortical information processing by facilitating and sustaining the formation of synchronized trans-areal assemblies; a more pointed suggestion is that, owing to the considerable blurring of cortical topography in the pulvinar, rival cortical assemblies may be in competition to recruit thalamic elements in order to outlast each other in activity.     

Retinotopic organization of striate and extrastriate visual cortex in the golden hamster (Mesocricetus auratus).

The visual topography within striate and lateral extrastriate visual cortices was studied in adult hamsters. The cortical areas 17 and 18a in the left hemisphere were electrophysiologically mapped upon stimulation of the right eye, correlating receptive field positions in the visual field with cortical recording sites. Reference lesions were placed at selected cortical sites. Like in rats and other mammals, the lateral extrastriate cortex contained multiple representations of the visual field. Rostral area 18a contained the rostrolateral maps, with medial and lateral divisions. More caudally and sharing a common border with V1, maps in lateromedial, posterolateral and posterior areas were found. More laterally and forming a "third tier" of visual maps, anterolateral, laterolateral-anterior, laterolateral and laterolateral-posterior areas were found. There was also an indication of a possible pararhinal map. The plan so defined is virtually identical to that of rats. The results may be useful to understand a basic mammalian plan in the organization of the visual cortex.     

Dynamic statistical parametric mapping: combining fMRI and MEG for high-resolution imaging of cortical activity

Functional magnetic resonance imaging (fMRI) can provide maps of brain activation with millimeter spatial resolution but is limited in its temporal resolution to the order of seconds. Here, we describe a technique that combines structural and functional MRI with magnetoencephalography (MEG) to obtain spatiotemporal maps of human brain activity with millisecond temporal resolution. This new technique was used to obtain dynamic statistical parametric maps of cortical activity during semantic processing of visually presented words. An initial wave of activity was found to spread rapidly from occipital visual cortex to temporal, parietal, and frontal areas within 185 ms, with a high degree of temporal overlap between different areas. Repetition effects were observed in many of the same areas following this initial wave of activation, providing evidence for the involvement of feedback mechanisms in repetition priming.     

Tactile shape processing

Neuroimaging techniques may aid in the identification of areas of the human brain that are involved in tactile shape perception. Bodeg?rd et al. (2001) relate differences in the properties of tactile stimuli to differences in areas of cortical activation to infer tactile processing in the somatosensory network.