Cerebral Cortex - current issue

Information Processing Streams in Rodent Barrel Cortex: The Differential Functions of Barrel and Septal Circuits

Alloway, K. D. — 2008-04-10

Rodent somatosensory cortex contains an isomorphic map of the mystacial whiskers in which each whisker is represented by neuronal populations, or barrels, that are separated from each other by intervening septa. Separate afferent pathways convey somatosensory information to the barrels and septa that represent the input stages for 2 partially segregated circuits that extend throughout the other layers of barrel cortex. Whereas the barrel-related circuits process spatiotemporal information generated by whisker contact with external objects, the septa-related circuits encode the frequency and other kinetic features of active whisker movements. The projection patterns from barrel cortex indicate that information processed by the septa-related circuits is used both separately and in combination with information from the barrel-related circuits to mediate specific functions. According to this theory, outputs from the septal processing stream modulate the brain regions that regulate whisking behavior, whereas both processing streams cooperate with each other to identify external stimuli encountered by passive or active whisker movements. This theoretical view prompts several testable hypotheses about the coordination of neuronal activity during whisking behavior. Foremost among these, motor brain regions that control whisker movements are more strongly coordinated with the septa-related circuits than with the barrel-related circuits.

Evidence for Impaired Long-Term Potentiation in Schizophrenia and Its Relationship to Motor Skill Leaning

2008-04-10

Several lines of evidence suggest that schizophrenia (SCZ) is associated with disrupted plasticity in the cortex. However, there is little direct neurophysiological evidence of aberrant long-term potentiation (LTP)–like plasticity in SCZ and little human evidence to establish a link between LTP to learning and memory. LTP was evaluated using a neurophysiological paradigm referred to as paired associative stimulation (PAS). PAS involves pairing of median nerve electric stimulation with transcranial magnetic stimulation (TMS) over the contralateral motor cortex (for abductor pollicis brevis muscle activation) delivered at 25-ms interstimulus interval. This pairing was delivered at a frequency of 0.1 Hz for 30 min. LTP was reflected by the change in motor evoked potentials (MEPs) before and after PAS. In addition, motor skill learning was assessed using the rotary pursuit task. Compared with healthy subjects, patients with SCZ demonstrated significant MEP facilitation deficits following PAS and impaired rotary-pursuit motor learning. Across all subjects there was a significant association between LTP and motor skill learning. These data provide evidence for disrupted LTP in SCZ, whereas the association between LTP with motor skill learning suggests that the deficits in learning and memory in SCZ may be mediated through disordered LTP.

Spatio temporal Dynamics of Face Recognition

2008-04-10

To better understand face recognition, it is necessary to identify not only which brain structures are implicated but also the dynamics of the neuronal activity in these structures. Latencies can then be compared to unravel the temporal dynamics of information processing at the distributed network level. To achieve high spatial and temporal resolution, we used intracerebral recordings in epileptic subjects while they performed a famous/unfamiliar face recognition task. The first components peaked at 110 ms in the fusiform gyrus (FG) and simultaneously in the inferior frontal gyrus, suggesting the early establishment of a large-scale network. This was followed by components peaking at 160 ms in 2 areas along the FG. Important stages of distributed parallel processes ensued at 240 and 360 ms involving up to 6 regions along the ventral visual pathway. The final components peaked at 480 ms in the hippocampus. These stages largely overlapped. Importantly, event-related potentials to famous faces differed from unfamiliar faces and control stimuli in all medial temporal lobe structures. The network was bilateral but more right sided. Thus, recognition of famous faces takes place through the establishment of a complex set of local and distributed processes that interact dynamically and may be an emergent property of these interactions.

Cognitive Control, Goal Maintenance, and Prefrontal Function in Healthy Aging

Paxton, J. L., Barch, D. M., Racine, C. A., Braver, T. S. — 2008-04-10

Cognitive control impairments in healthy older adults may partly reflect disturbances in the ability to actively maintain goal-relevant information, a function that depends on the engagement of lateral prefrontal cortex (PFC). In 2 functional magnetic resonance imaging studies, healthy young and older adults performed versions of a task in which contextual cues provide goal-relevant information used to bias processing of subsequent ambiguous probes. In Study 1, a blocked design and manipulation of the cue–probe delay interval revealed a generalized pattern of enhanced task-related brain activity in older adults but combined with a specific delay-related reduction of activity in lateral PFC regions. In Study 2, a combined blocked/event-related design revealed enhanced sustained (i.e., across-trial) activity but a reduction in transient trial-related activation in lateral PFC among older adults. Further analyses of within-trial activity dynamics indicated that, within these and other lateral PFC regions, older adults showed reduced activation during the cue and delay period but increased activation at the time of the probe, particularly on high-interference trials. These results are consistent with the hypothesis that age-related impairments in goal maintenance abilities cause a compensatory shift in older adults from a proactive (seen in young adults) to a reactive cognitive control strategy.

Critical Spatial Frequencies for Illusory Contour Processing in Early Visual Cortex

Zhan, C. A., Baker, C. L. — 2008-04-10

Single neurons in primate V2 and cat A18 exhibit identical orientation tuning for sinewave grating and illusory contour stimuli. This cue invariance is also manifested in similar orientation maps to these stimuli, but in V1/A17 the illusory contour maps appear reversed. We hypothesized that this map reversal depends upon the spatial frequencies of the inducers in the illusory contours, relative to the spatial selectivities of these brain areas. We employed intrinsic signal optical imaging to measure orientation maps in cat A17/18 to illusory contours with inducers at spatial frequencies from 0.15 to 1.6 cpd. A17 illusory contour maps were indeed reversed compared with grating-driven maps for inducer spatial frequencies <1.3 cpd, whereas A18 maps were invariant. Simulations based on known neurophysiology demonstrated that map reversal can arise from linear filtering, and map invariance can be explained by a nonlinear (filter-rectify-filter) mechanism. The simulation also correctly predicted that A17 could show invariant maps when the inducer spatial frequency is sufficiently high (1.6 cpd), and that A18 maps could reverse at lower inducer frequencies (0.18 cpd). Thus, the map reversal or invariance to illusory contours depends critically on the relationship of the inducer spatial frequencies to the spatial filtering properties of neurons in each brain area.

Involvement of the Cerebellar Dorsal Vermis in Vergence Eye Movements in Monkeys

Nitta, T., Akao, T., Kurkin, S., Fukushima, K. — 2008-04-10

Frontal-eyed primates use both smooth pursuit in frontoparallel planes (frontal pursuit) and pursuit-in-depth (vergence pursuit) to track objects moving slowly in 3-dimensional (3D) space. To understand how 3D-pursuit signals represented in frontal eye fields are processed further by downstream pathways, monkeys were trained to pursue a spot moving in 3D virtual space. We characterized pursuit signals in Purkinje (P) cells in the cerebellar dorsal vermis and their discharge during vergence pursuit. In 41% of pursuit P-cells, 3D-pursuit signals were observed. However, the majority of vermal-pursuit P-cells (59%) discharged either for vergence pursuit (43%) or for frontal pursuit (16%). Moreover, the majority (74%) of vergence-related P-cells carried convergence signals, displaying both vergence eye position and velocity sensitivity during sinusoidal and step vergence eye movements. Preferred frontal-pursuit directions of vergence + frontal-pursuit P-cells were distributed in all directions. Most pursuit P-cells (73%) discharged before the onset of vergence eye movements; the median lead time was 16 ms. Muscimol infusion into the sites where convergence P-cells were recorded resulted in a reduction of peak convergence eye velocity, of initial convergence eye acceleration, and of frontal-pursuit eye velocity. These results suggest involvement of the dorsal vermis in conversion of 3D-pursuit signals and in convergence eye movements.

Lack of Orientation and Direction Selectivity in a Subgroup of Fast-Spiking Inhibitory Interneurons: Cellular and Synaptic Mechanisms and Comparison with Other Electrophysiological Cell Types

Nowak, L. G., Sanchez-Vives, M. V., McCormick, D. A. — 2008-04-10

Neurons in cat area 17 can be grouped in 4 different electrophysiological cell classes (regular spiking, intrinsically bursting, chattering, and fast spiking [FS]). However, little is known of the functional properties of these different cell classes. Here we compared orientation and direction selectivity between these cell classes in cat area 17 and found that a subset of FS inhibitory neurons, usually with complex receptive fields, exhibited little selectivity in comparison with other cell types. Differences in occurrence and amplitude of gamma-range membrane fluctuations, as well as in numbers of action potentials in response to optimal visual stimuli, did not parallel differences observed for orientation and direction selectivity. Instead, differences in selectivity resulted mostly from differences in tuning of the membrane potential responses, although variations in spike threshold also contributed: weakly selective FS neurons exhibited both a lower spike threshold and more broadly tuned membrane potential responses in comparison with the other cell classes. Our results are consistent with the hypothesis that a subgroup of FS neurons receives connections and possesses intrinsic properties allowing the generation of weakly selective responses. The existence of weakly selective inhibitory neurons is consistent with orientation selectivity models that rely on broadly tuned inhibition.

Rhesus Monkey and Human Share a Similar Topography of the Corpus Callosum as Revealed by Diffusion Tensor MRI In Vivo

Hofer, S., Merboldt, K.-D., Tammer, R., Frahm, J. — 2008-04-10

A recent study of the corpus callosum (CC) in humans revealed a new topographical arrangement of the cortical connectivity pattern. To explore the CC topography in nonhuman primates, we applied magnetic resonance diffusion tensor imaging and tract tracing techniques in individual rhesus monkeys in vivo. The results demonstrate that the CC topography of primates and humans is surprisingly similar. In particular, the relatively large representation and caudal extension of commissural frontal fibers in the CC is observed in both the monkey and human brain. If evolutionary changes in relative brain volumes are reflected in the arrangement of related fibers crossing the CC, the current study is in line with the fact that the relative volume of the frontal lobe did not significantly increase after the split of the hominid line from other primates.

Enhanced Response of Neurons in Rat Somatosensory Cortex to Stimuli Containing Temporal Noise

Lak, A., Arabzadeh, E., Diamond, M. E. — 2008-04-10

Sensory stimuli under natural conditions often consist of a temporally irregular sequence of events, contrasting with the periodic sequences commonly used as stimuli in the laboratory. These experiments compared the responses of neurons in rat barrel cortex with trains of whisker movements with different frequencies; each train possessed either a periodic or an irregular, "noisy" temporal structure. Periodic stimulus trains were composed of a sequence of 21 whisker deflections separated by 20 equal interdeflection intervals (IDIs). Noisy trains were matched for mean IDI but included intervals shorter and longer than the mean IDI. Cortical responses were equivalent for periodic and noisy stimuli for frequencies up to 10 Hz. Above 10 Hz, temporal noise led to a larger response magnitude, and this effect was amplified as deflection frequency increased. Noise also caused a sharpening of the temporal precision of response to the individual deflections of the stimulus train. Cortical neurons thus appear to be "tuned" to respond in a different way to stimuli characterized by temporal unpredictability. As a consequence, perceptual judgments that depend on somatosensory cortical firing rate may be affected by the presence of temporal noise.

Cortical Connections of the Macaque Anterior Intraparietal (AIP) Area

2008-04-10

We traced the cortical connections of the anterior intraparietal (AIP) area, which is known to play a crucial role in visuomotor transformations for grasping. AIP displayed major connections with 1) areas of the inferior parietal lobule convexity, the rostral part of the lateral intraparietal area and the SII region; 2) ventral visual stream areas of the lower bank of the superior temporal sulcus and the middle temporal gyrus; and 3) the premotor area F5 and prefrontal areas 46 and 12. Additional connections were observed with the caudal intraparietal area and the ventral part of the frontal eye field. This study suggests that visuomotor transformations for object-oriented actions, processed in AIP, rely not only on dorsal visual stream information related to the object's physical properties but also on ventral visual stream information related to object identity. The identification of direct anatomical connections with the inferotemporal cortex suggests that AIP also has a unique role in linking the parietofrontal network of areas involved in sensorimotor transformations for grasping with areas involved in object recognition. Thus, AIP could represent a crucial node in a cortical circuit in which hand-related sensory and motor signals gain access to representations of object identity for tactile object recognition.

Tracking the Temporal Dynamics of Updating Cognitive Control: An Examination of Error Processing

West, R., Travers, S. — 2008-04-10

In 2 experiments, we used event-related brain potentials (ERPs) to examine the temporal dynamics of neural processes related to adjustments of cognitive control following errors in the counting Stroop task. The ERPs elicited by errors revealed the error-related negativity (ERN) and error positivity consistent with a large body of literature. In addition, errors were associated with a frontal slow wave between 200 and 2000 ms after the response that was consistent with the activity of neural generators in the lateral frontal cortex. The ERN and frontal slow wave were correlated with posterror slowing of response time and positive affect (i.e., happiness and calmness) during task performance. These data are consistent with the idea that interactions between anterior cingulate cortex and lateral frontal cortex support adjustments of cognitive control and that this neural network is sensitive to the influence of affect experienced during task performance.

Unusual Patch-Matrix Organization in the Retrosplenial Cortex of the reeler Mouse and Shaking Rat Kawasaki

2008-04-10

The rat granular retrosplenial cortex (GRS) is a simplified cortex, with distinct stratification and, in the uppermost layers, distinct modularity. Thalamic and cortical inputs are segregated by layers and in layer 1 colocalize, respectively, with apical dendritic bundles originating from neurons in layers 2 or 5. To further investigate this organization, we turned to reelin-deficient reeler mouse and Shaking rat Kawasaki. We found that the disrupted lamination, evident in Nissl stains in these rodents, is in fact a patch–matrix mosaic of segregated afferents and dendrites. Patches consist of thalamocortical connections, visualized by vesicular glutamate transporter 2 (VGluT2) or AChE. The surrounding matrix consists of corticocortical terminations, visualized by VGluT1 or zinc. Dendrites concentrate in the matrix or patches, depending on whether they are OCAM positive (matrix) or negative (patches). In wild-type rodents and, presumably, mutants, OCAM⁺ structures originate from layer 5 neurons. By double labeling for dendrites (filled by Lucifer yellow in fixed slice) and OCAM immunofluorescence, we ascertained 2 populations in reeler: dendritic branches either preferred (putative layer 5 neurons) or avoided (putative supragranular neurons) the OCAM⁺ matrix. We conclude that input–target relationships are largely preserved in the mutant GRS and that dendrite–dendrite interactions involving OCAM influence the formation of the mosaic configuration.

A Frontoparietal Network for Spatial Attention Reorienting in the Auditory Domain: A Human fMRI/MEG Study of Functional and Temporal Dynamics

2008-04-10

Several studies have identified a supramodal network critical to the reorienting of attention toward stimuli at novel locations and which involves the right temporoparietal junction and the inferior frontal areas. The present functional magnetic resonance imaging (fMRI)\magnetoencephalography (MEG) study investigates: 1) the cerebral circuit underlying attentional reorienting to spatially varying sound locations; 2) the circuit related to the regular change of sound location in the same hemifield, the change of sound location across hemifields, or sounds presented randomly at different locations on the azimuth plane; 3) functional temporal dynamics of the observed cortical areas exploiting the complementary characteristics of the fMRI and MEG paradigms. fMRI results suggest 3 distinct roles: the supratemporal plane appears modulated by variations of sound location; the inferior parietal lobule is modulated by the cross-meridian effect; and the inferior frontal cortex is engaged by the inhibition of a motor response. MEG data help to elucidate the temporal dynamics of this network by providing high-resolution time series with which to measure latency of neural activation manipulated by the reorienting of attention.

Decomposing the Neural Correlates of Antisaccade Eye Movements Using Event-Related fMRI

2008-04-10

The antisaccade task is a model of the conflict between an unwanted reflexive response (which must be inhibited) and a complex volitional response (which must be generated). The present experiment aimed to investigate separately the neural correlates of these cognitive components using a delayed saccade paradigm to dissociate saccade inhibition from generation. Seventeen healthy volunteers completed event-related functional magnetic resonance imaging at 1.5 T during saccades to and away from a peripheral visual target (prosaccades and antisaccades, respectively). Saccades were requested in response to an auditory go signal on average 12 s after peripheral target appearance. It was found that the right supramarginal gyrus showed significantly greater activation during the inhibition phase than the generation phase of the paradigm for both antisaccade and prosaccade trials, suggesting a role in saccade inhibition or stimulus detection. On the other hand, the right lateral frontal eye field and bilateral intraparietal sulcus showed evidence of selective involvement in antisaccade generation. Ventrolateral and dorsolateral prefrontal cortices showed comparable levels of activation in both phases of the task. These areas likely fulfill a more general supervisory role in the volitional control of eye movements, such as stimulus appraisal, task set, and decision making.

Action Outcomes Are Represented in Human Inferior Frontoparietal Cortex

de C. Hamilton, A. F., Grafton, S. T. — 2008-04-10

The simple action of pressing a switch has many possible interpretations—the actor could be turning on a light, deleting critical files from a computer, or even turning off a life-support system. In each of these cases, the motor parameters of the action are the same but the physical outcome differs. We report evidence of suppressed responses in right inferior parietal and right inferior frontal cortex when participants saw repeated movies showing the same action outcome, but these regions did not distinguish the kinematic parameters by which the action was accomplished. Thus, these brain areas encode the physical outcomes of human actions in the world. These results are compatible with a hierarchical model of human action understanding in which a cascade of specialized processes from occipital to parietal and frontal regions allow humans to understand the physical consequences of actions in the world and the intentions underlying those actions.

Shared Neural Resources between Music and Language Indicate Semantic Processing of Musical Tension-Resolution Patterns

Steinbeis, N., Koelsch, S. — 2008-04-10

Harmonic tension-resolution patterns have long been hypothesized to be meaningful to listeners familiar with Western music. Even though it has been shown that specifically chosen musical pieces can prime meaningful concepts, the empirical evidence in favor of such a highly specific semantic pathway has been lacking. Here we show that 2 event-related potentials in response to harmonic expectancy violations, the early right anterior negativity (ERAN) and the N500, could be systematically modulated by simultaneously presented language material containing either a syntactic or a semantic violation. Whereas the ERAN was reduced only when presented concurrently with a syntactic language violation and not with a semantic language violation, this pattern was reversed for the N500. This is the first piece of evidence showing that tension- resolution patterns represent a route to meaning in music.

Rhythmic Spontaneous Activity in the Piriform Cortex

2008-04-10

Slow spontaneous rhythmic activity is generated and propagates in neocortical slices when bathed in an artificial cerebrospinal fluid with ionic concentrations similar to the ones in vivo. This activity is extraordinarily similar to the activation of the cortex in physiological conditions (e.g., slow-wave sleep), thus representing a unique in vitro model to understand how cortical networks maintain and control ongoing activity. Here we have characterized the activity generated in the olfactory or piriform cortex and endopiriform nucleus (piriform network). Because these structures are prone to generate epileptic discharges, it seems critical to understand how they generate and regulate their physiological rhythmic activity. The piriform network gave rise to rhythmic spontaneous activity consisting of a succession of up and down states at an average frequency of 1.8 Hz, qualitatively similar to the corresponding neocortical activity. This activity originated in the deep layers of the piriform network, which displayed higher excitability and denser connectivity. A remarkable difference with neocortical activity was the speed of horizontal propagation (114 mm/s), one order of magnitude faster in the piriform network. Properties of the piriform cortex subserving fast horizontal propagation may underlie the higher vulnerability of this area to epileptic seizures.

Cortical Steady-State Responses to Central and Peripheral Auditory Beats

Draganova, R., Ross, B., Wollbrink, A., Pantev, C. — 2008-04-10

Different types of generation mechanisms of 40-Hz auditory steady-state response (ASSR) were investigated using diotic and dichotic stimulation with 500- and 540-Hz pure tones of 1.0-s duration and 2.0-s stimulus onset asynchrony. When the sum of both tones was presented to both ears simultaneously, they interacted at cochlear level and resulted in perception of a 40-Hz beat termed "peripheral beat." Dichotic presentation of the 500-Hz tone to one ear and the 540-Hz tone to the other one resulted in beat perception as the effect of central interaction, most likely in the superior olivary nuclei and was termed "central beat." ASSR and transient N1m responses were found in the averaged 151-channel whole-head magnetoencephalographic recordings under both stimulus conditions and were modeled with single spatiotemporal equivalent current dipoles in both hemispheres. The ASSR sources in both conditions were more anterior, more inferior, and more medial compared with N1m sources. Right hemispheric lateralization of the magnetic field strength was found for the ASSR in both stimulus conditions. Although the central and peripheral beat interacted at different levels of the auditory system, the initial responses were projected along the afferent auditory pathway and activated common cortical sources.

Que PASA? The Posterior-Anterior Shift in Aging

Davis, S. W., Dennis, N. A., Daselaar, S. M., Fleck, M. S., Cabeza, R. — 2008-04-10

A consistent finding from functional neuroimaging studies of cognitive aging is an age-related reduction in occipital activity coupled with increased frontal activity. This posterior–anterior shift in aging (PASA) has been typically attributed to functional compensation. The present functional magnetic resonance imaging sought to 1) confirm that PASA reflects the effects of aging rather than differences in task difficulty; 2) test the compensation hypothesis; and 3) investigate whether PASA generalizes to deactivations. Young and older participants were scanned during episodic retrieval and visual perceptual tasks, and age-related changes in brain activity common to both tasks were identified. The study yielded 3 main findings. First, inconsistent with a difficulty account, the PASA pattern was found across task and confidence levels when matching performance among groups. Second, supporting the compensatory hypothesis, age-related increases in frontal activity were positively correlated with performance and negatively correlated with the age-related occipital decreases. Age-related increases and correlations with parietal activity were also found. Finally, supporting the generalizability of the PASA pattern to deactivations, aging reduced deactivations in posterior midline cortex but increased deactivations in medial frontal cortex. Taken together, these findings demonstrate the validity, function, and generalizability of PASA, as well as its importance for the cognitive neuroscience of aging.

Attention and Executive Systems Abnormalities in Adults with Childhood ADHD: A DT-MRI Study of Connections

2008-04-10

Attention-deficit/hyperactivity disorder (ADHD) is hypothesized to be due, in part, to structural defects in brain networks influencing cognitive, affective, and motor behaviors. Although the current literature on fiber tracts is limited in ADHD, gray matter abnormalities suggest that white matter (WM) connections may be altered selectively in neural systems. A prior study (Ashtari et al. 2005), using diffusion tensor magnetic resonance imaging (DT-MRI), showed alterations within the frontal and cerebellar WM in children and adolescents with ADHD. In this study of adults with childhood ADHD, we hypothesized that fiber pathways subserving attention and executive functions (EFs) would be altered. To this end, the cingulum bundle (CB) and superior longitudinal fascicle II (SLF II) were investigated in vivo in 12 adults with childhood ADHD and 17 demographically comparable unaffected controls using DT-MRI. Relative to controls, the fractional anisotropy (FA) values were significantly smaller in both regions of interest in the right hemisphere, in contrast to a control region (the fornix), indicating an alteration of anatomical connections within the attention and EF cerebral systems in adults with childhood ADHD. The demonstration of FA abnormalities in the CB and SLF II in adults with childhood ADHD provides further support for persistent structural abnormalities into adulthood.

Influence of Binocular Competition on the Expression Profiles of CRMP2, CRMP4, Dyn I, and Syt I in Developing Cat Visual Cortex

Cnops, L., Hu, T.-T., Burnat, K., Arckens, L. — 2008-04-10

The visual cortex is vulnerable to changes in visual input, especially during the critical period when numerous molecules drive the refinement of the circuitry. From a list of potential actors identified in a recent proteomics study, we selected 2 collapsin response mediator proteins (CRMP2/CRMP4) and 2 synaptic proteins, Dynamin I (Dyn I) and Synaptotagmin I (Syt I), for in-depth analysis of their developmental expression profile in cat visual cortex. CRMP2 and CRMP4 levels were high early in life and clearly declined toward adulthood. In contrast, Dyn I expression levels progressively augmented during maturation. Syt I showed low levels at eye opening and in adults, high levels around the peak of the critical period, and maximal levels at juvenile age. We further determined a role for each molecule in ocular dominance plasticity. CRMP2 and Syt I levels decreased in area 17 upon monocular deprivation, whereas CRMP4 and Dyn I levels remained unaffected. In contrast, binocular removal of pattern vision had no influence on CRMP2 and Syt I expression in kitten area 17. This study illustrates that not the loss of quality of vision through visual deprivation, but disruption of normal binocular visual experience is crucial to induce the observed molecular changes.