Pulvinar and other subcortical connections of dorsolateral visual cortex in monkeys

Effective connectivity in subcortical visual structures in de novo Patients with Parkinson's Disease

BACKGROUND

Parkinson's disease (PD) manifests with the appearance of non-motor symptoms before motor symptoms onset. Among these, dysfunctioning visual structures have recently been reported to occur at early disease stages.

OBJECTIVE

This study addresses effective connectivity in the visual network of PD patients.

METHODS

Using functional MRI and dynamic causal modeling analysis, we evaluated the connectivity between the superior colliculus, the lateral geniculate nucleus and the primary visual area V1 in de novo untreated PD patients (n = 22). A subset of the PD patients (n = 8) was longitudinally assessed two times at two months and at six months after starting dopaminergic treatment. Results were compared to those of age-matched healthy controls (n = 22).

RESULTS

Our results indicate that the superior colliculus drives cerebral activity for luminance contrast processing both in healthy controls and untreated PD patients. The same effective connectivity was observed with neuromodulatory differences in terms of neuronal dynamic interactions. Our main findings were that the modulation induced by luminance contrast changes of the superior colliculus connectivity (self-connectivity and connectivity to the lateral geniculate nucleus) was inhibited in PD patients (effect of contrast: p = 0.79 and p = 0.77 respectively). The introduction of dopaminergic medication in a subset (n = 8) of the PD patients failed to restore the effective connectivity modulation observed in the healthy controls.

INTERPRETATION

The deficits in luminance contrast processing in PD was associated with a deficiency in connectivity adjustment from the superior colliculus to the lateral geniculate nucleus and to V1. No differences in cerebral blood flow were observed between controls and PD patients suggesting that the deficiency was at the neuronal level. Administration of a dopaminergic treatment over six months was not able to normalize the observed alterations in inter-regional coupling. These findings highlight the presence of early dysfunctions in primary visual areas, which might be used as early markers of the disease.

An MRI method for parcellating the human striatum into matrix and striosome compartments in vivo

The mammalian striatum is comprised of intermingled tissue compartments, matrix and striosome. Though indistinguishable by routine histological techniques, matrix and striosome have distinct embryologic origins, afferent/efferent connections, surface protein expression, intra-striatal location, susceptibilities to injury, and functional roles in a range of animal behaviors. Distinguishing the compartments previously required post-mortem tissue and/or genetic manipulation; we aimed to identify matrix/striosome non-invasively in living humans. We used diffusion MRI (probabilistic tractography) to identify human striatal voxels with connectivity biased towards matrix-favoring or striosome-favoring regions (determined by prior animal tract-tracing studies). Segmented striatal compartments replicated the topological segregation and somatotopic organization identified in animal matrix/striosome studies. Of brain regions mapped in prior studies, our human brain data confirmed 93% of the compartment-selective structural connectivity demonstrated in animals. Test-retest assessment on repeat scans found a voxel classification error rate of 0.14%. Fractional anisotropy was significantly higher in matrix-like voxels, while mean diffusivity did not differ between the compartments. As mapped by the Talairach human brain atlas, 460 regions were significantly biased towards either matrix or striosome. Our method allows the study of striatal compartments in human health and disease, in vivo, for the first time.

Cellular-resolution gene expression profiling in the neonatal marmoset brain reveals dynamic species- and region-specific differences

Precise spatiotemporal control of gene expression in the developing brain is critical for neural circuit formation, and comprehensive expression mapping in the developing primate brain is crucial to understand brain function in health and disease. Here, we developed an unbiased, automated, large-scale, cellular-resolution in situ hybridization (ISH)-based gene expression profiling system (GePS) and companion analysis to reveal gene expression patterns in the neonatal New World marmoset cortex, thalamus, and striatum that are distinct from those in mice. Gene-ontology analysis of marmoset-specific genes revealed associations with catalytic activity in the visual cortex and neuropsychiatric disorders in the thalamus. Cortically expressed genes with clear area boundaries were used in a three-dimensional cortical surface mapping algorithm to delineate higher-order cortical areas not evident in two-dimensional ISH data. GePS provides a powerful platform to elucidate the molecular mechanisms underlying primate neurobiology and developmental psychiatric and neurological disorders.

Integrative and Network-Specific Connectivity of the Basal Ganglia and Thalamus Defined in Individuals

The basal ganglia, thalamus, and cerebral cortex form an interconnected network implicated in many neurological and psychiatric illnesses. A better understanding of cortico-subcortical circuits in individuals will aid in development of personalized treatments. Using precision functional mapping-individual-specific analysis of highly sampled human participants-we investigated individual-specific functional connectivity between subcortical structures and cortical functional networks. This approach revealed distinct subcortical zones of network specificity and multi-network integration. Integration zones were systematic, with convergence of cingulo-opercular control and somatomotor networks in the ventral intermediate thalamus (motor integration zones), dorsal attention and visual networks in the pulvinar, and default mode and multiple control networks in the caudate nucleus. The motor integration zones were present in every individual and correspond to consistently successful sites of deep brain stimulation (DBS; essential tremor). Individually variable subcortical zones correspond to DBS sites with less consistent treatment effects, highlighting the importance of PFM for neurosurgery, neurology, and psychiatry.

The mediodorsal pulvinar coordinates the macaque fronto-parietal network during rhythmic spatial attention

Spatial attention is discontinuous, sampling behaviorally relevant locations in theta-rhythmic cycles (3-6 Hz). Underlying this rhythmic sampling are intrinsic theta oscillations in frontal and parietal cortices that provide a clocking mechanism for two alternating attentional states that are associated with either engagement at the presently attended location (and enhanced perceptual sensitivity) or disengagement (and diminished perceptual sensitivity). It has remained unclear, however, how these theta-dependent states are coordinated across the large-scale network that directs spatial attention. The pulvinar is a candidate for such coordination, having been previously shown to regulate cortical activity. Here, we examined pulvino-cortical interactions during theta-rhythmic sampling by simultaneously recording from macaque frontal eye fields (FEF), lateral intraparietal area (LIP), and pulvinar. Neural activity propagated from pulvinar to cortex during periods of engagement, and from cortex to pulvinar during periods of disengagement. A rhythmic reweighting of pulvino-cortical interactions thus defines functional dissociations in the attention network.

The Computational Anatomy of Visual Neglect

Visual neglect is a debilitating neuropsychological phenomenon that has many clinical implications and-in cognitive neuroscience-offers an important lesion deficit model. In this article, we describe a computational model of visual neglect based upon active inference. Our objective is to establish a computational and neurophysiological process theory that can be used to disambiguate among the various causes of this important syndrome; namely, a computational neuropsychology of visual neglect. We introduce a Bayes optimal model based upon Markov decision processes that reproduces the visual searches induced by the line cancellation task (used to characterize visual neglect at the bedside). We then consider 3 distinct ways in which the model could be lesioned to reproduce neuropsychological (visual search) deficits. Crucially, these 3 levels of pathology map nicely onto the neuroanatomy of saccadic eye movements and the systems implicated in visual neglect.

Active Inference, Novelty and Neglect

In this chapter, we provide an overview of the principles of active inference. We illustrate how different forms of short-term memory are expressed formally (mathematically) through appealing to beliefs about the causes of our sensations and about the actions we pursue. This is used to motivate an approach to active vision that depends upon inferences about the causes of 'what I have seen' and learning about 'what I would see if I were to look there'. The former could manifest as persistent 'delay-period' activity - of the sort associated with working memory, while the latter is better suited to changes in synaptic efficacy - of the sort that underlies short-term learning and adaptation. We review formulations of these ideas in terms of active inference, their role in directing visual exploration and the consequences - for active vision - of their failures. To illustrate the latter, we draw upon some of our recent work on the computational anatomy of visual neglect.

Visual contrast sensitivity deficits in 'normal' visual field of patients with homonymous visual field defects due to stroke: a pilot study

BACKGROUND

Homonymous visual field defects (VFD) are common following stroke, and often recover, partially or fully, by unknown mechanisms. In clinical practice, visual field recovered on perimetry is often considered perceptually normal. However, studies have shown contrast sensitivity (CS) deficits in patients with stroke and homonymous VFD. This study investigated the origin of visual CS loss in patients with VFD due to stroke. We hypothesised that CS deficits would be found in visual field areas appearing normal on perimetry, in patients with ischaemic stroke affecting the retrochiasmal visual system, and that the spatiotemporal properties of this CS loss would be consistent with those of 'blindsight', perhaps suggesting similar underlying mechanisms.

METHODS

CS measurements were made in 20 healthy participants, and in 7 patients with stroke causing homonymous VFD sparing foveal vision, measured using Humphrey static perimetry (SITA-Fast 24-2 procedure). Importantly, patients with concomitant visuospatial neglect were excluded. CS measurements were made using a modification of the method of increasing contrast, corrected for reaction time. Three spatial stimuli were used, at several spatial frequencies: (1) large sinusoidal gratings; (2) foveal Gabor patches; and (3) Gabor patches presenting in the putatively recovered visual field, near VFD. Stimuli with different temporal profiles were used to selectively stimulate transient and sustained visual channels, to provide insight into mechanisms of visual loss and/or recovery. Analysis of variance (ANOVA) was used in the analysis of the measurements, allowing for correction for age and stimulus eccentricity.

RESULTS

ANOVA for sustained grating stimuli showed orientation-selective (horizontal) CS loss (p = 0.025); no such loss was apparent in the central visual field (foveal Gabor stimuli). Localised CS close to VFD was reduced in stroke-affected hemifields compared with unaffected hemifields (p ≤ 0.005), though these areas appeared normal on perimetry. In these areas, CS was relatively preserved for transient compared with sustained stimuli (Wilcoxon signed rank tests).

CONCLUSIONS

The finding of specific CS deficits in the normal-appearing visual field of patients with homonymous VFD due to stroke suggests that static perimetry provides an inadequate assessment of visual function in these patients, with clear implications for testing of vision in clinical practice. The results are consistent with relative sparing of the transient/magnocellular visual channel. These findings demand further investigation. If confirmed in larger, longitudinal studies, this will have important implications for the mechanisms of recovery, and may provide a target for visual rehabilitation - for example, using repeated detection practice ('perceptual learning').

Processing of the S-cone signals in the early visual cortex of primates

The short-wavelength-sensitive (S) cones play an important role in color vision of primates, and may also contribute to the coding of other visual features, such as luminance and motion. The color signals carried by the S cones and other cone types are largely separated in the subcortical visual pathway. Studies on nonhuman primates or humans have suggested that these signals are combined in the striate cortex (V1) following a substantial amplification of the S-cone signals in the same area. In addition to reviewing these studies, this review describes the circuitry in V1 that may underlie the processing of the S-cone signals and the dynamics of this processing. It also relates the interaction between various cone signals in V1 to the results of some psychophysical and physiological studies on color perception, which leads to a discussion of a previous model, in which color perception is produced by a multistage processing of the cone signals. Finally, I discuss the processing of the S-cone signals in the extrastriate area V2.

Comparative anatomy of marmoset and mouse cortex from genomic expression

Advances in mouse neural circuit genetics, brain atlases, and behavioral assays provide a powerful system for modeling the genetic basis of cognition and psychiatric disease. However, a critical limitation of this approach is how to achieve concordance of mouse neurobiology with the ultimate goal of understanding the human brain. Previously, the common marmoset has shown promise as a genetic model system toward the linking of mouse and human studies. However, the advent of marmoset transgenic approaches will require an understanding of developmental principles in marmoset compared to mouse. In this study, we used gene expression analysis in marmoset brain to pose a series of fundamental questions on cortical development and evolution for direct comparison to existing mouse brain atlas expression data. Most genes showed reliable conservation of expression between marmoset and mouse. However, certain markers had strikingly divergent expression patterns. The lateral geniculate nucleus and pulvinar in the thalamus showed diversification of genetic organization between marmoset and mouse, suggesting they share some similarity. In contrast, gene expression patterns in early visual cortical areas showed marmoset-specific expression. In prefrontal cortex, some markers labeled architectonic areas and layers distinct between mouse and marmoset. Core hippocampus was conserved, while afferent areas showed divergence. Together, these results indicate that existing cortical areas are genetically conserved between marmoset and mouse, while differences in areal parcellation, afferent diversification, and layer complexity are associated with specific genes. Collectively, we propose that gene expression patterns in marmoset brain reveal important clues to the principles underlying the molecular evolution of cortical and cognitive expansion.

Thalamocortical connections of functional zones in posterior parietal cortex and frontal cortex motor regions in New World monkeys

Posterior parietal cortex (PPC) links primate visual and motor systems and is central to visually guided action. Relating the anatomical connections of PPC to its neurophysiological functions may elucidate the organization of the parietal-frontal network. In owl and squirrel monkeys, long-duration electrical stimulation distinguished several functional zones within the PPC and motor/premotor cortex (M1/PM). Multijoint forelimb movements reminiscent of reach, defense, and grasp behaviors characterized each functional zone. In PPC, functional zones were organized parallel to the lateral sulcus. Thalamocortical connections of PPC and M1/PM zones were investigated with retrograde tracers. After several days of tracer transport, brains were processed, and labeled cells in thalamic nuclei were plotted. All PPC zones received dense inputs from the lateral posterior nucleus and the anterior pulvinar. PPC zones received additional projections from ventral lateral (VL) divisions of motor thalamus, which were also the primary source of input to M1/PM. Projections to PPC from rostral motor thalamus were sparse. Dense projections from ventral posterior (VP) nucleus of somatosensory thalamus distinguished the rostrolateral grasp zone from the other PPC zones. PPC connections with VL and VP provide links to cerebellar nuclei and the somatosensory system, respectively, that may integrate PPC functions with M1/PM.

The connectivity of the human pulvinar: a diffusion tensor imaging tractography study

Previous studies in nonhuman primates and cats have shown that the pulvinar receives input from various cortical and subcortical areas involved in vision. Although the contribution of the pulvinar to human vision remains to be established, anatomical tracer and electrophysiological animal studies on cortico-pulvinar circuits suggest an important role of this structure in visual spatial attention, visual integration, and higher-order visual processing. Because methodological constraints limit investigations of the human pulvinar's function, its role could, up to now, only be inferred from animal studies. In the present study, we used an innovative imaging technique, Diffusion Tensor Imaging (DTI) tractography, to determine cortical and subcortical connections of the human pulvinar. We were able to reconstruct pulvinar fiber tracts and compare variability across subjects in vivo. Here we demonstrate that the human pulvinar is interconnected with subcortical structures (superior colliculus, thalamus, and caudate nucleus) as well as with cortical regions (primary visual areas (area 17), secondary visual areas (area 18, 19), visual inferotemporal areas (area 20), posterior parietal association areas (area 7), frontal eye fields and prefrontal areas). These results are consistent with the connectivity reported in animal anatomical studies.

Cortical connections of the visual pulvinar complex in prosimian galagos (Otolemur garnetti)

The pulvinar complex of prosimian primates is not as architectonically differentiated as that of anthropoid primates. Thus, the functional subdivisions of the complex have been more difficult to determine. In the present study, we related patterns of connections of cortical visual areas (primary visual area, V1; secondary visual area, V2; and middle temporal visual area, MT) as well as the superior colliculus of the visual midbrain, with subdivisions of the pulvinar complex of prosimian galagos (Otolemur garnetti) that were revealed in brain sections processed for cell bodies (Nissl), cytochrome oxidase, or myelin. As in other primates, the architectonic methods allowed us to distinguish the lateral pulvinar (PL) and inferior pulvinar (PI) as major divisions of the visual pulvinar. The connection patterns further allowed us to divide PI into a large central nucleus (PIc), a medial nucleus (PIm), and a posterior nucleus (PIp). Both PL and PIc have separate topographic patterns of connections with V1 and V2. A third, posterior division of PI, PIp, does not appear to project to V1 and V2 and is further distinguished by receiving inputs from the superior colliculus. All these subdivisions of PI project to MT. The evidence suggests that PL of galagos contains a single, large nucleus, as in monkeys, and that PI may have only three subdivisions, rather than the four subdivisions of monkeys. In addition, the cortical projections of PI nuclei are more widespread than those in monkeys. Thus, the pulvinar nuclei in prosimian primates and anthropoid primates have evolved along somewhat different paths.

The thalamocortical projection systems in primate: an anatomical support for multisensory and sensorimotor interplay

Multisensory and sensorimotor integrations are usually considered to occur in superior colliculus and cerebral cortex, but few studies proposed the thalamus as being involved in these integrative processes. We investigated whether the organization of the thalamocortical (TC) systems for different modalities partly overlap, representing an anatomical support for multisensory and sensorimotor interplay in thalamus. In 2 macaque monkeys, 6 neuroanatomical tracers were injected in the rostral and caudal auditory cortex, posterior parietal cortex (PE/PEa in area 5), and dorsal and ventral premotor cortical areas (PMd, PMv), demonstrating the existence of overlapping territories of thalamic projections to areas of different modalities (sensory and motor). TC projections, distinct from the ones arising from specific unimodal sensory nuclei, were observed from motor thalamus to PE/PEa or auditory cortex and from sensory thalamus to PMd/PMv. The central lateral nucleus and the mediodorsal nucleus project to all injected areas, but the most significant overlap across modalities was found in the medial pulvinar nucleus. The present results demonstrate the presence of thalamic territories integrating different sensory modalities with motor attributes. Based on the divergent/convergent pattern of TC and corticothalamic projections, 4 distinct mechanisms of multisensory and sensorimotor interplay are proposed.

First order connections of the visual sector of the thalamic reticular nucleus in marmoset monkeys (Callithrix jacchus)

The thalamic reticular nucleus (TRN) supplies an important inhibitory input to the dorsal thalamus. Previous studies in non-primate mammals have suggested that the visual sector of the TRN has a lateral division, which has connections with first-order (primary) sensory thalamic and cortical areas, and a medial division, which has connections with higher-order (association) thalamic and cortical areas. However, the question whether the primate TRN is segregated in the same manner is controversial. Here, we investigated the connections of the TRN in a New World primate, the marmoset (Callithrix jacchus). The topography of labeled cells and terminals was analyzed following iontophoretic injections of tracers into the primary visual cortex (V1) or the dorsal lateral geniculate nucleus (LGNd). The results show that rostroventral TRN, adjacent to the LGNd, is primarily connected with primary visual areas, while the most caudal parts of the TRN are associated with higher order visual thalamic areas. A small region of the TRN near the caudal pole of the LGNd (foveal representation) contains connections where first (lateral TRN) and higher order visual areas (medial TRN) overlap. Reciprocal connections between LGNd and TRN are topographically organized, so that a series of rostrocaudal injections within the LGNd labeled cells and terminals in the TRN in a pattern shaped like rostrocaudal overlapping "fish scales." We propose that the dorsal areas of the TRN, adjacent to the top of the LGNd, represent the lower visual field (connected with medial LGNd), and the more ventral parts of the TRN contain a map representing the upper visual field (connected with lateral LGNd).

Pulvinar contributions to the dorsal and ventral streams of visual processing in primates

The visual pulvinar is part of the dorsal thalamus, and in primates it is especially well developed. Recently, our understanding of how the visual pulvinar is subdivided into nuclei has greatly improved as a number of histological procedures have revealed marked architectonic differences within the pulvinar complex. At the same time, there have been unparalleled advances in understanding of how visual cortex of primates is subdivided into areas and how these areas interconnect. In addition, considerable evidence supports the view that the hierarchy of interconnected visual areas is divided into two major processing streams, a ventral stream for object vision and a dorsal stream for visually guided actions. In this review, we present evidence that a subset of medial nuclei in the inferior pulvinar function predominantly as a subcortical component of the dorsal stream while the most lateral nucleus of the inferior pulvinar and the adjoining ventrolateral nucleus of the lateral pulvinar are more devoted to the ventral stream of cortical processing. These nuclei provide cortico-pulvinar-cortical interactions that spread information across areas within streams, as well as information relayed from the superior colliculus via inferior pulvinar nuclei to largely dorsal stream areas.

Enhanced performance with brain stimulation: attentional shift or visual cue?

The premotor theory of visual spatial attention proposes that the same brain activity that prepares for saccades to one part of the visual field also facilitates visual processing at that same region of the visual field. Strong support comes from improvements in performance by electrical stimulation of presaccadic areas, including the frontal eye field and superior colliculus (SC). Interpretations of these stimulation experiments are hampered by the possibility that stimulation might be producing an internal visual flash or phosphene that attracts attention as a real flash would. We tested this phosphene hypothesis in the SC by comparing the effect of interchanging real visual stimuli and electrical stimulation. We first presented a veridical visual cue at the time SC stimulation improved performance; if a phosphene improved performance at this time, a real cue should do so in the same manner, but it did not. We then changed the time of SC visual-motor stimulation to when we ordinarily presented the veridical visual cue, and failed to improve performance. Last, we shifted the site of SC stimulation from the visual-motor neurons of the SC intermediate layers to the visual neurons of the superficial layers to determine whether stimulating visual neurons produced a larger improvement in performance, but it did not. Our experiments provide evidence that a phosphene is not responsible for the shift of attention that follows SC stimulation. This added evidence of a direct shift of attention is consistent with a key role of the SC in the premotor theory of attention.

CLARIFYING HOMOLOGIES IN THE MAMMALIAN CEREBRAL CORTEX: THE CASE OF THE THIRD VISUAL AREA (V3)

1. Experiments in mammalian models are the main source of information on the neural architecture underlying human visual perception, establishing scientific boundaries for the interpretation of experiments using non-invasive techniques in humans and for the realistic modelling of visual processes. Thus, it is important to define the homology between visual areas in different species. 2. To date, relatively few visual areas can be defined with certainty across mammalian Orders. Here, we review the evidence pointing to the fact that the third visual area (V3; or area 19) is a crucial node of a system involved in shape recognition that exists in most, if not all, eutherian mammals. 3. The size and shape of area V3 are variable, even between species that belong to the same Order. Although some features of the visuotopic organization of V3 are constant (including the relative location of the representations of the upper and lower quadrant and correspondence between the anterior border and the representation of the vertical meridian of the visual field), others are variable between species and even individuals. A complex pattern of representation, involving topological discontinuities, can exist. 4. In addition to its location in relation to the first (V1) and second (V2) visual areas, the identification of V3 homologues can be aided by certain other features, including low myelination, weak cytochrome oxidase reactivity, response properties that are indicative in the processing of stimulus shape, relationship to clusters of neurons forming interhemispheric connections and projections from the koniocellular (W-cell-like) components of the lateral geniculate nucleus. 5. Recent research in primates has clarified the organization of the V3 homologue in members of this Order. Regions of cortex that were formerly thought to belong to V3 (including a densely myelinated region near the dorsal midline) are better considered as part of a separate dorsomedial area, involved in motion analysis and visuomotor integration. The redefined V3, which includes the 'ventral posterior area' and parts of the dorsolateral complex proposed by earlier studies, is very similar to V3 (area 19) of other species in terms of structure and function.

Corticocortical and thalamocortical information flow in the primate visual system

Visual cortex in primates contains a mosaic of several dozen visual areas that collectively occupy a large fraction of cerebral cortex (approximately 50% in the macaque; approximately 25% in humans). These areas are richly interconnected by hundreds of reciprocal corticocortical pathways that underlie an anatomically based hierarchy containing multiple processing streams. In addition, there is a complex pattern of reciprocal connections with the pulvinar, which itself contains about 10 architectonically distinct subdivisions. Information flow through these corticocortical and corticothalamic circuits is regulated very dynamically by top-down as well as bottom-up processes, including directed visual attention. This chapter evaluates current hypotheses and evidence relating to the interaction between thalamocortical and corticocortical circuitry in the dynamic regulation of information flow.

Neurochemical organization of chimpanzee inferior pulvinar complex

The pulvinar of primates, which connects with all visual areas, has been implicated in visual attention and in control of eye movements. Recently, five separate neurochemical subdivisions of a region termed the inferior pulvinar complex have been identified in monkeys (Gray et al. [1999] J Comp Neurol 409:452-468; Gutierrez et al. [1995] J Comp Neurol 363:545-562), and comparable subdivisions have been mapped in humans (Cola et al. [1999] NeuroReport 10:3733-3738). In the present study, we investigated the inferior pulvinar of the chimpanzee (Pan troglodytes), the closest evolutionary relative of humans, using cytochrome oxidase (CO) and acetylcholinesterase (AChE) histochemistry, and immunocytochemistry for calbindin. Each staining method demarcated five histochemical zones corresponding, from medial to lateral, to the posterior (PI(P)), medial (PI(M)), central PI(C)), lateral (PI(L)), and the lateral-shell (PI(L-S)) divisions in monkeys. The PI(P) division stained darkly for calbindin and lightly for CO and AChE. The PI(M) division was characterized by less neuropil staining for calbindin, and by distinct, intensely stained patches of CO and AChE. PI(C) appeared lighter than adjacent divisions with CO and AChE histochemistry and was moderately stained with calbindin. PI(L) was moderately to darkly stained with each method and was adjoined by a lighter staining shell, PI(L-S). Thus, in the aspects of organization we examined, the inferior pulvinar of chimpanzees closely resembles that of humans and monkeys. This investigation provides a foundation for more detailed studies of the thalamic relationships of extrastriate cortex in apes and humans.

Cortical neurophysiology of anticipatory anxiety: an investigation utilizing steady state probe topography (SSPT)

The precise role of the cortex in human anxiety is not well characterised. Previous imaging research among healthy controls has reported alterations in regional cerebral blood flow (rCBF) within the prefrontal and temporal cortices during periods of anxious anticipation; however, the temporal dynamics of this activity has yet to be examined in detail. The present study examined cortical Steady State Probe Topography (SSPT) changes associated with anticipatory anxiety (AA), allowing examination of the temporal continuity and the excitatory or inhibitory nature of AA activations. We recorded Steady State Visually Evoked Potentials (SSVEPs) at 64 scalp locations, skin conductance, and self reported anxiety among 26 right-handed males while relaxed and during the anticipation of an electric shock. Relative to the baseline condition, the AA condition was associated with significantly higher levels of self-reported anxiety and increased phasic skin conductance levels. Across the seven second imaging window, AA was associated with increased SSVEP latency within medial anterior frontal, left dorsolateral prefrontal and bilateral temporal regions. In contrast, increased SSVEP amplitude and decreased SSVEP latency were observed within occipital regions. The observed SSVEP latency increases within frontal and temporal cortical regions are suggestive of increased localised inhibitory processes within regions reciprocally connected to subcortical limbic structures. Occipital SSVEP latency decreases are suggestive of increased excitatory activity. SSVEP amplitude increases within occipital regions may be associated with an attentional shift from external to internal environment. The current findings provide further support for the involvement of frontal, anterior temporal, and occipital cortical regions during anticipatory anxiety, and suggest that both excitatory and inhibitory processes are associated with AA alterations.

The visual pulvinar in tree shrews I. Multiple subdivisions revealed through acetylcholinesterase and Cat-301 chemoarchitecture

Tree shrews are highly visual mammals closely related to primates. They have a large visual pulvinar complex, but its organization and relation to visual cortex is only partly known. We processed brain sections through the pulvinar with seven different procedures in an effort to reveal histologically distinct compartments. The results revealed three major subdivisions. A dorsal subdivision, Pd, stains darkly for acetylcholinesterase (AChE) and occupies the dorsoposterior one-third of the pulvinar complex. A ventral subdivision, Pv, stains darkly when processed with the Cat-301 antibody and occupies the ventroanterior fifth of the pulvinar complex along the brachium of the superior colliculus. Unexpectedly, part of Pv is ventral to the brachium. A large central subdivision, Pc, stains moderately dark for AChE and cytochrome oxidase (CO), and very light for Cat-301. Pc includes about half of the pulvinar complex, with parts on both sides of the brachium of the superior colliculus. These architectonic results demonstrate that the pulvinar complex of tree shrews is larger and has more subdivisions than previously described. The complex resembles the pulvinar of primates by having a portion ventral to the brachium and by having histochemically distinct nuclei; the number of nuclei is less than in primates, however.

Cortical and subcortical contributions to the representation of temporal information

Converging evidence suggests that temporal representations of brief durations are derived subcortically. We tested split-brain patient JW in order to investigate whether these representations project bilaterally or unilaterally to cortex. Using visual stimuli to signal time intervals, JW was asked to compare the duration of a pair of standard stimuli that were presented bilaterally with a comparison stimulus that was presented to either the left or right visual field. Assuming the hand of response is controlled by the contralateral cerebral hemisphere, a hand by visual field interaction was predicted if the representation of stimulus duration was restricted to the cerebral hemisphere receiving the lateralized stimulus. However, we failed to observe this interaction for two different ranges of stimulus durations, both in the hundred (Experiment 2) to hundreds (Experiment 1) of milliseconds range. Instead, there was a consistent right hemisphere advantage in task performance. When the task then required a discrimination based on the physical size of the stimuli rather than their duration, an interaction between response hand and visual field was obtained (Experiment 3). Taken together, these results suggest that (1) even though the comparison stimulus was presented unilaterally, the representation of its duration was available to both cerebral hemispheres, and (2) a right hemisphere advantage in psychophysical tasks requiring the comparison of successive stimuli is observed for temporal and non-temporal judgments.