Chapter 8 Beyond a relay nucleus: neuroimaging views on the human LGN

Functional Localization of the Human Auditory and Visual Thalamus Using a Thalamic Localizer Functional Magnetic Resonance Imaging Task

Functional magnetic resonance imaging (fMRI) of the auditory and visual sensory systems of the human brain is an active area of investigation in the study of human health and disease. The medial geniculate nucleus (MGN) and lateral geniculate nucleus (LGN) are key thalamic nuclei involved in the processing and relay of auditory and visual information, respectively, and are the subject of blood-oxygen-level-dependent (BOLD) fMRI studies of neural activation and functional connectivity in human participants. However, localization of BOLD fMRI signal originating from neural activity in MGN and LGN remains a technical challenge, due in part to the poor definition of boundaries of these thalamic nuclei in standard T1-weighted and T2-weighted magnetic resonance imaging sequences. Here, we report the development and evaluation of an auditory and visual sensory thalamic localizer (TL) fMRI task that produces participant-specific functionally-defined regions of interest (fROIs) of both MGN and LGN, using 3 Tesla multiband fMRI and a clustered-sparse temporal acquisition sequence, in less than 16 minutes of scan time. We demonstrate the use of MGN and LGN fROIs obtained from the TL fMRI task in standard resting-state functional connectivity (RSFC) fMRI analyses in the same participants. In RSFC analyses, we validated the specificity of MGN and LGN fROIs for signals obtained from primary auditory and visual cortex, respectively, and benchmark their performance against alternative atlas- and segmentation-based localization methods. The TL fMRI task and analysis code (written in Presentation and MATLAB, respectively) have been made freely available to the wider research community.

The role of the thalamus in modular functional networks in temporal lobe epilepsy with cognitive impairment

OBJECTIVE

Cognitive deficit is common in patients with temporal lobe epilepsy (TLE). Here, we aimed to investigate the modular architecture of functional networks associated with distinct cognitive states in TLE patients together with the role of the thalamus in modular networks.

METHODS

Resting-state functional magnetic resonance imaging scans were acquired from 53 TLE patients and 37 matched healthy controls. All patients received the Montreal Cognitive Assessment test and accordingly were divided into TLE patients with normal cognition (TLE-CN, n = 35) and TLE patients with cognitive impairment (TLE-CI, n = 18) groups. The modular properties of functional networks were calculated and compared including global modularity Q, modular segregation index, intramodular connections, and intermodular connections. Thalamic subdivisions corresponding to the modular networks were generated by applying a 'winner-take-all' strategy before analyzing the modular properties (participation coefficient and within-module degree z-score) of each thalamic subdivision to assess the contribution of the thalamus to modular functional networks. Relationships between network properties and cognitive performance were then further explored.

RESULTS

Both TLE-CN and TLE-CI patients showed lower global modularity, as well as lower modular segregation index values for the ventral attention network and the default mode network. However, different patterns of intramodular and intermodular connections existed for different cognitive states. In addition, both TLE-CN and TLE-CI patients exhibited anomalous modular properties of functional thalamic subdivisions, with TLE-CI patients presenting a broader range of abnormalities. Cognitive performance in TLE-CI patients was not related to the modular properties of functional network but rather to the modular properties of functional thalamic subdivisions.

CONCLUSIONS

The thalamus plays a prominent role in modular networks and potentially represents a key neural mechanism underlying cognitive impairment in TLE.

Feature-Specific Salience Maps in Human Cortex

Priority map theory is a leading framework for understanding how various aspects of stimulus displays and task demands guide visual attention. Per this theory, the visual system computes a priority map, which is a representation of visual space indexing the relative importance, or priority, of locations in the environment. Priority is computed based on both salience, defined based on image-computable properties; and relevance, defined by an individual's current goals, and is used to direct attention to the highest-priority locations for further processing. Computational theories suggest that priority maps identify salient locations based on individual feature dimensions (e.g., color, motion), which are integrated into an aggregate priority map. While widely accepted, a core assumption of this framework, the existence of independent feature dimension maps in visual cortex, remains untested. Here, we tested the hypothesis that retinotopic regions selective for specific feature dimensions (color or motion) in human cortex act as neural feature dimension maps, indexing salient locations based on their preferred feature. We used fMRI activation patterns to reconstruct spatial maps while male and female human participants viewed stimuli with salient regions defined by relative color or motion direction. Activation in reconstructed spatial maps was localized to the salient stimulus position in the display. Moreover, the strength of the stimulus representation was strongest in the ROI selective for the salience-defining feature. Together, these results suggest that feature-selective extrastriate visual regions highlight salient locations based on local feature contrast within their preferred feature dimensions, supporting their role as neural feature dimension maps.SIGNIFICANCE STATEMENT Identifying salient information is important for navigating the world. For example, it is critical to detect a quickly approaching car when crossing the street. Leading models of computer vision and visual search rely on compartmentalized salience computations based on individual features; however, there has been no direct empirical demonstration identifying neural regions as responsible for performing these dissociable operations. Here, we provide evidence of a critical double dissociation that neural activation patterns from color-selective regions prioritize the location of color-defined salience while minimally representing motion-defined salience, whereas motion-selective regions show the complementary result. These findings reveal that specialized cortical regions act as neural "feature dimension maps" that are used to index salient locations based on specific features to guide attention.

Hi-Fi fMRI: High-resolution, fast-sampled and sub-second whole-brain functional MRI at 3T in humans

Functional magnetic resonance imaging (fMRI) is a methodological cornerstone of neuroscience. Most studies measure blood-oxygen-level-dependent (BOLD) signal using echo-planar imaging (EPI), Cartesian sampling, and image reconstruction with a one-to-one correspondence between the number of acquired volumes and reconstructed images. However, EPI schemes are subject to trade-offs between spatial and temporal resolutions. We overcome these limitations by measuring BOLD with a gradient recalled echo (GRE) with 3D radial-spiral phyllotaxis trajectory at a high sampling rate (28.24ms) on standard 3T field-strength. The framework enables the reconstruction of 3D signal time courses with whole-brain coverage at simultaneously higher spatial (1mm ³ ) and temporal (up to 250ms) resolutions, as compared to optimized EPI schemes. Additionally, artifacts are corrected before image reconstruction; the desired temporal resolution is chosen after scanning and without assumptions on the shape of the hemodynamic response. By showing activation in the calcarine sulcus of 20 participants performing an ON-OFF visual paradigm, we demonstrate the reliability of our method for cognitive neuroscience research.

Mnemonic representations in human lateral geniculate nucleus

There is a growing appreciation for the role of the thalamus in high-level cognition. Motivated by findings that internal cognitive state drives activity in feedback layers of primary visual cortex (V1) that target the lateral geniculate nucleus (LGN), we investigated the role of LGN in working memory (WM). Specifically, we leveraged model-based neuroimaging approaches to test the hypothesis that human LGN encodes information about spatial locations temporarily encoded in WM. First, we localized and derived a detailed topographic organization in LGN that accords well with previous findings in humans and non-human primates. Next, we used models constructed on the spatial preferences of LGN populations in order to reconstruct spatial locations stored in WM as subjects performed modified memory-guided saccade tasks. We found that population LGN activity faithfully encoded the spatial locations held in memory in all subjects. Importantly, our tasks and models allowed us to dissociate the locations of retinal stimulation and the motor metrics of memory-guided saccades from the maintained spatial locations, thus confirming that human LGN represents true WM information. These findings add LGN to the growing list of subcortical regions involved in WM, and suggest a key pathway by which memories may influence incoming processing at the earliest levels of the visual hierarchy.

Neuroplasticity of the Lateral Geniculate Nucleus in Response to Retinal Gene Therapy in a Group of Patients with RPE65 Mutations

Introduction

Previous works on experience-dependent brain plasticity have been limited to the cortical structures, overlooking subcortical visual structures such as the lateral geniculate nucleus (LGN). Animal studies have shown substantial experience dependent plasticity and using fMRI, human studies have demonstrated similar properties in patients with cataract surgery. However, in neither animal nor human studies LGN has not been directly assessed, mainly due to its small size, tissue heterogeneity, low contrast/noise ratio, and low spatial resolution.

Methods

Utilizing a new algorithm that markedly improves the LGN visibility, LGN was evaluated in a group of low vision patients before and after retinal intervention to reinstate vision and normal sighted matched controls.

Results

Between and within groups comparisons showed that patients had significantly smaller left (p< 0.0001) and right (p < 0.00002) LGN volumes at baseline as compared to the one-year follow-up volumes. The same baseline and one year comparison in controls was not significant. Significant positive correlations were observed between the incremental volume increase after gene therapy of the left LGN and the incremental increase in the right (r = 0.71, p < 0.02) and left (r = 0.72, p = 0.018) visual fields. Incremental volume increase of the right LGN also showed a similar positive slope but did not reach significance.

Discussion

These results show that despite significantly less volume at baseline, retinal gene therapy promotes robust expansion and increase in LGN volume. Reinstating vision may have facilitated the establishment of new connections between the retina and the LGN and/or unmasking of the dormant connections. The exact trajectory of the structural changes taking place in LGN is unclear but our data shows that even after years of low vision, the LGN in RPE65 patients has the potential for plasticity and expansion to a nearly normal volume one year after gene therapy administration.

Binocular Integration in the Primate Primary Visual Cortex

binocular vision, binocular fusion, binocular combination, LGN, V1.

Visual Tract Degradation in Bilateral Normal-Tension Glaucoma-Cortical Thickness Maps and Volumetric Study of Visual Pathway Areas

The aim of the study was to evaluate changes in the central visual pathways during the early and advanced stages of bilateral normal-tension glaucoma (NTG).

METHODS

The studied groups constituted patients with bilateral normal-tension glaucoma of the same stage (n = 45) and age-matched healthy volunteers (n = 17). All patients underwent ophthalmic examination and examination on a 1.5 Tesla Magnetic Resonance Scanner (Optima 360, GE Healthcare). Volume and cortical thickness analyses were performed using the open-source automated software package FreeSurfer.

RESULTS

There was a significant difference in lateral geniculate nuclei volume between the control and advanced glaucoma groups in the right hemisphere (p = 0.03) and in the left hemisphere between the early and advanced glaucoma patients (p = 0.026). The optic chiasm volume differed significantly between the control and advanced NTG groups (p = 0.0003) and between early and advanced glaucoma patients (p = 0.004). Mean cortical thickness analysis revealed a significant increase in values in the advanced glaucoma group in the right Brodmann area 17 (BA17) (p = 0.007) and right BA18 (p = 0.049) as compared to early NTG. In the left BA18 area, the mean thickness of the cortex in the early glaucoma group was significantly lower than in the control group (p = 0.03).

CONCLUSIONS

The increase in the grey matter thickness in the V1 region with more-advanced glaucoma stages may reflect compensatory hypertrophy. Additionally, the regions of the brain early affected during glaucoma with reduced thickness were the right lateral occipital gyrus and left lingual gyrus. The most prominent change during the course of glaucoma was the increase in grey matter thickness in the right cuneus.

Sustained attention induces altered effective connectivity of the ascending thalamo-cortical relay in obsessive-compulsive disorder

Abnormal function of the thalamo-cortical relay is considered a hallmark of obsessive-compulsive disorder (OCD) and aberrant network interactions may underpin many of the clinical and cognitive symptoms that characterize the disorder. Several statistical approaches have been applied to in vivo fMRI data to support the general loss of thalamo-cortical connectivity in OCD. However, (a) few studies have assessed the contextual constraints under which abnormal network interactions arise or (b) have used methods of effective connectivity to understand abnormal network interactions. Effective connectivity is a particularly valuable method as it describes the putative causal influences that brain regions exert over each other, as opposed to the largely statistical consistencies captured in functional connectivity techniques. Here, using dynamic causal modeling (DCM), we evaluated how attention demand induced inter-group differences (HC ≠ OCD) in effective connectivity within a motivated thalamo-cortical network. Of interest was whether these effects were observed on the ascending thalamo-cortical relay, essential for the sensory innervation of the cortex. fMRI time series data from sixty-two participants (OCD, 30; HC, 32) collected using an established sustained attention task were submitted to a space of 162 competing models. Across the space, models distinguished between competing hypotheses of thalamo-cortical interactions. Bayesian model selection (BMS) identified marginally differing likely generative model architectures in OCD and HC groups. Bayesian model averaging (BMA), was used to weight connectivity parameter estimates across all models, with each parameter weighted by each model's posterior probability, thus providing more stable estimates of effective connectivity. Inferential statistical analyses of estimated parameters revealed two principal results: (1) Significantly reduced intrinsic connectivity of the V1 → SPC pathway in OCD, suggested connective weakness in the early constituents of the dorsal visual pathway; (2) More pertinent with the discovery possibilities afforded by DCM, sustained attention in OCD patients induced significantly reduced contextual modulation of the ascending relay from the thalamus to the prefrontal cortex. These results form an important complement to our understanding of the contextual bases of thalamo-cortical network deficits in OCD, emphasizing vulnerability of the ascending relay.

Asymmetries around the visual field: From retina to cortex to behavior

Visual performance varies around the visual field. It is best near the fovea compared to the periphery, and at iso-eccentric locations it is best on the horizontal, intermediate on the lower, and poorest on the upper meridian. The fovea-to-periphery performance decline is linked to the decreases in cone density, retinal ganglion cell (RGC) density, and V1 cortical magnification factor (CMF) as eccentricity increases. The origins of polar angle asymmetries are not well understood. Optical quality and cone density vary across the retina, but recent computational modeling has shown that these factors can only account for a small percentage of behavior. Here, we investigate how visual processing beyond the cone photon absorptions contributes to polar angle asymmetries in performance. First, we quantify the extent of asymmetries in cone density, midget RGC density, and V1 CMF. We find that both polar angle asymmetries and eccentricity gradients increase from cones to mRGCs, and from mRGCs to cortex. Second, we extend our previously published computational observer model to quantify the contribution of phototransduction by the cones and spatial filtering by mRGCs to behavioral asymmetries. Starting with photons emitted by a visual display, the model simulates the effect of human optics, cone isomerizations, phototransduction, and mRGC spatial filtering. The model performs a forced choice orientation discrimination task on mRGC responses using a linear support vector machine classifier. The model shows that asymmetries in a decision maker's performance across polar angle are greater when assessing the photocurrents than when assessing isomerizations and are greater still when assessing mRGC signals. Nonetheless, the polar angle asymmetries of the mRGC outputs are still considerably smaller than those observed from human performance. We conclude that cone isomerizations, phototransduction, and the spatial filtering properties of mRGCs contribute to polar angle performance differences, but that a full account of these differences will entail additional contribution from cortical representations.

Improving the Quantification of the Lateral Geniculate Nucleus in Magnetic Resonance Imaging Using a Novel 3D-Edge Enhancement Technique

The lateral geniculate nucleus (LGN) is a small, inhomogeneous structure that relays major sensory inputs from the retina to the visual cortex. LGN morphology has been intensively studied due to various retinal diseases, as well as in the context of normal brain development. However, many of the methods used for LGN structural evaluations have not adequately addressed the challenges presented by the suboptimal routine MRI imaging of this structure. Here, we propose a novel method of edge enhancement that allows for high reliability and accuracy with regard to LGN morphometry, using routine 3D-MRI imaging protocols. This new algorithm is based on modeling a small brain structure as a polyhedron with its faces, edges, and vertices fitted with one plane, the intersection of two planes, and the intersection of three planes, respectively. This algorithm dramatically increases the contrast-to-noise ratio between the LGN and its surrounding structures as well as doubling the original spatial resolution. To show the algorithm efficacy, two raters (MA and ML) measured LGN volumes bilaterally in 19 subjects using the edge-enhanced LGN extracted areas from the 3D-T1 weighted images. The averages of the left and right LGN volumes from the two raters were 175 ± 8 and 174 ± 9 mm³, respectively. The intra-class correlations between raters were 0.74 for the left and 0.81 for the right LGN volumes. The high contrast edge-enhanced LGN images presented here, from a 7-min routine 3T-MRI acquisition, is qualitatively comparable to previously reported LGN images that were acquired using a proton density sequence with 30–40 averages and 1.5-h of acquisition time. The proposed edge-enhancement algorithm is not limited only to the LGN, but can significantly improve the contrast-to-noise ratio of any small deep-seated gray matter brain structure that is prone to high-levels of noise and partial volume effects, and can also increase their morphometric accuracy and reliability. An immensely useful feature of the proposed algorithm is that it can be used retrospectively on noisy and low contrast 3D brain images previously acquired as part of any routine clinical MRI visit.

Spatial attention modulates earliest visual processing: An electrical neuroimaging study

Several studies showed that shifting of visuospatial attention modulates sensory processing at multiple levels of the visual pathways and beyond, including the occipital striate cortices level. However, inconsistent findings have been reported thus leaving these issues still disputed. 21 participants took part to the present study (the EEG signals of 4 of them were discarded due to artifacts). We used ERPs and their neural sources to investigate whether shifting spatial attention in space across the horizontal meridian of the visual field affected striate cortices activation at the earliest latency. Time-series of scalp topographical maps indicated that, unlike ERPs to attentional-neutral central cues, ERPs to attention-directing local cues showed earliest polarity inversions as a function of stimulated field and processing latency range considered, at occipital-parietal electrodes. In between 60-75 ms, attentional shifting cues elicited a positivity for both visual fields, whereas at a later latency (75–90 ms) they elicited a positivity and a negativity for the upper and lower visual hemifields, respectively. Computed neural sources included striate, besides extrastriate, cortices for both visual hemifields and latency ranges. Conjointly, behavioral responses to targets were faster when they were preceded by local than by neutral cues, and when presented in the upper than the lower hemifield. Our findings support the hypothesis that attention shifts may affect early sensory processing in visual cortices.

Volume of Lateral Geniculate Nucleus in Patients with Glaucoma in 7Tesla MRI

The aim of the study was to assess the volume of the lateral geniculate nucleus (LGN) in patients with open-angle glaucoma in 7Tesla MRI and to evaluate its relation to RNFL thickness and VF indices. Material and methods. The studied group consisted of 20 open-angle glaucoma patients with bilaterally the same stage of glaucoma (11 with early glaucoma and nine with advanced glaucoma) and nine healthy volunteers from the Department of Diagnostics and Microsurgery of Glaucoma, Medical University of Lublin, Poland. Circumpapillary RNFL-thickness measurements were performed using OCT in all patients and visual fields were performed in the glaucoma group. A 7Tesla MRI was performed to assess the volume of both lateral geniculate bodies. Results. The LGN volume varied significantly between groups from 122.1 ± 14.4 mm³ (right LGN) and 101.6 ± 13.3 mm³ (left LGN) in the control group to 80.2 ± 17.7 mm³ (right LGN) and 71.8 ± 14.2 mm³ (left LGN) in the advanced glaucoma group (right LGN p = 0.003, left LGN p = 0.018). However, volume values from early glaucoma: right LGN = 120.2 ± 26.5 mm³ and left LGN = 103.2 ± 28.0 mm³ differed significantly only from values from the advanced group (right LGN p = 0.006, left LGN p = 0.012), but not from controls (right LGN p = 0.998, left LGN p = 0.986). There were no significant correlations between visual field indices (MD (mean deviation) and VFI (visual field index)) and LGN volumes in both glaucoma groups. Significant correlations between mean RNFL (retinal nerve fiber layers) thickness and corresponding and contralateral LGN were observed for the control group (corresponding LGN: p = 0.064; contralateral LGN: p = 0.031) and early glaucoma (corresponding LGN: p = 0.017; contralateral LGN: p = 0.008), but not advanced glaucoma (corresponding LGN: p = 0.496; contralateral LGN: p = 0.258). Conclusions. The LGN volume decreases in the course of glaucoma. These changes are correlated with RNFL thickness in early stages of glaucoma and are not correlated with visual field indices.

Escitalopram ameliorates differences in neural activity between healthy comparison and major depressive disorder groups on an fMRI Emotional conflict task: A CAN-BIND-1 study

BACKGROUND

Identifying objective biomarkers can assist in predicting remission/non-remission to treatment, improving remission rates, and reducing illness burden in major depressive disorder (MDD).

METHODS

Sixteen MDD 8-week remitters (MDD-8), twelve 16-week remitters (MDD-16), 14 non-remitters (MDD-NR) and 30 healthy comparison participants (HC) completed a functional magnetic resonance imaging emotional conflict task at baseline, prior to treatment with escitalopram, and 8 weeks after treatment initiation. Patients were followed 16 weeks to assess remitter status.

RESULTS

All groups demonstrated emotional Stroop in reaction time (RT) at baseline and Week 8. There were no baseline differences between HC and MDD-8, MDD-16, or MDD-NR in RT or accuracy. By Week 8, MDD-8 demonstrated poorer accuracy compared to HC. Compared to HC, the baseline blood-oxygen level dependent (BOLD) signal was decreased in MDD-8 in brain-stem and thalamus; in MDD-16 in lateral occipital cortex, middle temporal gyrus, and cuneal cortex; in MDD-NR in lingual and occipital fusiform gyri, thalamus, putamen, caudate, cingulate gyrus, insula, cuneal cortex, and middle temporal gyrus. By Week 8, there were no BOLD activity differences between MDD groups and HC.

LIMITATIONS

The Emotional Conflict Task lacks a neutral (non-emotional) condition, restricting interpretation of how mood may influence perception of non-emotionally valenced stimuli.

CONCLUSIONS

The Emotional Conflict Task is not an objective biomarker for remission trajectory in patients with MDD receiving escitalopram treatment. Escitalopram may have influenced emotion recognition in MDD groups in terms of augmented accuracy and BOLD signal in response to an Emotional Conflict Task, following 8 weeks of escitalopram treatment.

Reduced accuracy accompanied by reduced neural activity during the performance of an emotional conflict task by unmedicated patients with major depression: A CAN-BIND fMRI study

METHODS

We studied 48 MDD and 30 HC who performed an emotional conflict task in a functional magnetic resonance imaging (fMRI) scanner.

RESULTS

On the emotional conflict task, MDD and HC demonstrated a robust emotional Stroop effect in reaction time and accuracy. Overall, accuracy was lower in MDD compared to HC with no significant reaction time differences. The fMRI data indicated lower BOLD activation in MDD compared to HC on comparisons of all trials, congruent, incongruent, and incongruent > congruent trials in regions including right inferior temporal gyrus, lateral occipital cortex, and occipital fusiform gyrus. Behavioural and neuroimaging data indicated no group differences in fearful versus happy face processing.

LIMITATIONS

Inclusion of a neutral condition may have provided a valuable contrast to how MDD and HC process stimuli without emotional valence compared to stimuli with a strong emotional valence.

CONCLUSIONS

MDD and HC demonstrated a robust emotional Stroop effect. Compared to HC, MDD demonstrated an overall reduced accuracy on the emotional conflict task and reduced BOLD activity in regions important for face perception and emotion information processing, with no differences in responding to fearful versus happy faces. These findings provide support for the theory of emotion context insensitivity in individuals with depression.

Links Between the Amplitude Modulation of Low-Frequency Spontaneous Fluctuation Across Resting State Conditions and Thalamic Functional Connectivity

A comparison of the different types of resting state reveals some interesting characteristics of spontaneous brain activity that cannot be found in a single condition. Differences in the amplitude of low-frequency fluctuation (ALFF) between the eyes open (EO) and the eyes closed (EC) almost have a spatially distinct pattern with traditional EO-EC activation within sensory systems, suggesting the divergent functional roles of ALFF and activation. However, the underlying mechanism is far from clear. Since the thalamus plays an essential role in sensory processing, one critical step toward understanding the divergences is to depict the relationships between the thalamus and the ALFF modulation in sensory regions. In this preliminary study, we examined the association between the changes of ALFF and the changes of thalamic functional connectivity (FC) between EO and EC. We focused on two visual thalamic nuclei, the lateral geniculate nucleus (LGN) and the pulvinar (Pu). FC results showed that LGN had stronger synchronization with regions in lateral but not in medial visual networks, while Pu had a weaker synchronization with auditory and sensorimotor areas during EO compared with EC. Moreover, the patterns of FC modulation exhibited considerable overlaps with the ALFF modulation, and there were significant correlations between them across subjects. Our findings support the crucial role of the thalamus in amplitude modulation of low-frequency spontaneous activity in sensory systems, and may pave the way to elucidate the mechanisms governing distinction between evoked activation and modulation of low-frequency spontaneous brain activity.

Diagnostic utility of central damage determination in glaucoma by magnetic resonance imaging: An observational study

The aim of the present study was to investigate the utility of diffusion tensor imaging (DTI) parameters of the central visual pathway and the size of the lateral geniculate nucleus (LGN) in the diagnosis of primary open-angle glaucoma (POAG). To test this, 24 POAG patients and 24 age- and gender-matched controls underwent 3.0 Tesla magnetic resonance imaging examinations, including DTI and LGN structural imaging. Receiver operating characteristic (ROC) curves were generated and Spearman's correlation coefficients were also determined. The results indicated that regarding the discrimination of POAG patients from healthy controls, the fractional anisotropy (FA) values of the optic tract, at a cutoff of ≤0.412, with an area under the ROC curve of 0.931, exhibited the highest sensitivity (79.2%) and specificity (89.6%). The FA values of the optic tract and the optic radiation were significantly correlated with LGN size, while the mean diffusivity values were not. In conclusion, the FA value of the optic tract may be a sensitive and reliable biomarker for glaucoma evaluation.

Resting-state thalamic dysconnectivity in schizophrenia and relationships with symptoms

BACKGROUND

Schizophrenia (SZ) is a severe neuropsychiatric disorder associated with disrupted connectivity within the thalamic-cortico-cerebellar network. Resting-state functional connectivity studies have reported thalamic hypoconnectivity with the cerebellum and prefrontal cortex as well as thalamic hyperconnectivity with sensory cortical regions in SZ patients compared with healthy comparison participants (HCs). However, fundamental questions remain regarding the clinical significance of these connectivity abnormalities.

METHOD

Resting state seed-based functional connectivity was used to investigate thalamus to whole brain connectivity using multi-site data including 183 SZ patients and 178 matched HCs. Statistical significance was based on a voxel-level FWE-corrected height threshold of p < 0.001. The relationships between positive and negative symptoms of SZ and regions of the brain demonstrating group differences in thalamic connectivity were examined.

RESULTS

HC and SZ participants both demonstrated widespread positive connectivity between the thalamus and cortical regions. Compared with HCs, SZ patients had reduced thalamic connectivity with bilateral cerebellum and anterior cingulate cortex. In contrast, SZ patients had greater thalamic connectivity with multiple sensory-motor regions, including bilateral pre- and post-central gyrus, middle/inferior occipital gyrus, and middle/superior temporal gyrus. Thalamus to middle temporal gyrus connectivity was positively correlated with hallucinations and delusions, while thalamus to cerebellar connectivity was negatively correlated with delusions and bizarre behavior.

CONCLUSIONS

Thalamic hyperconnectivity with sensory regions and hypoconnectivity with cerebellar regions in combination with their relationship to clinical features of SZ suggest that thalamic dysconnectivity may be a core neurobiological feature of SZ that underpins positive symptoms.

Review of thalamocortical resting-state fMRI studies in schizophrenia

Brain circuitry underlying cognition, emotion, and perception is abnormal in schizophrenia. There is considerable evidence that the neuropathology of schizophrenia includes the thalamus, a key hub of cortical-subcortical circuitry and an important regulator of cortical activity. However, the thalamus is a heterogeneous structure composed of several nuclei with distinct inputs and cortical connections. Limitations of conventional neuroimaging methods and conflicting findings from post-mortem investigations have made it difficult to determine if thalamic pathology in schizophrenia is widespread or limited to specific thalamocortical circuits. Resting-state fMRI has proven invaluable for understanding the large-scale functional organization of the brain and investigating neural circuitry relevant to psychiatric disorders. This article summarizes resting-state fMRI investigations of thalamocortical functional connectivity in schizophrenia. Particular attention is paid to the course, diagnostic specificity, and clinical correlates of thalamocortical network dysfunction.

The thalamus as a relay station and gatekeeper: relevance to brain disorders

Here, we provide a review of behavioural, cognitive, and neural studies of the thalamus, including its role in attention, consciousness, sleep, and motor processes. We further discuss neuropsychological and brain disorders associated with thalamus function, including Parkinson’s disease, Alzheimer’s disease, Korsakoff’s syndrome, and sleep disorders. Importantly, we highlight how thalamus-related processes and disorders can be explained by the role of the thalamus as a relay station.

An evolving view of retinogeniculate transmission

The thalamocortical (TC) relay neuron of the dorsoLateral Geniculate Nucleus (dLGN) has borne its imprecise label for many decades in spite of strong evidence that its role in visual processing transcends the implied simplicity of the term "relay". The retinogeniculate synapse is the site of communication between a retinal ganglion cell and a TC neuron of the dLGN. Activation of retinal fibers in the optic tract causes reliable, rapid, and robust postsynaptic potentials that drive postsynaptics spikes in a TC neuron. Cortical and subcortical modulatory systems have been known for decades to regulate retinogeniculate transmission. The dynamic properties that the retinogeniculate synapse itself exhibits during and after developmental refinement further enrich the role of the dLGN in the transmission of the retinal signal. Here we consider the structural and functional substrates for retinogeniculate synaptic transmission and plasticity, and reflect on how the complexity of the retinogeniculate synapse imparts a novel dynamic and influential capacity to subcortical processing of visual information.

Reduced thalamocortical functional connectivity in temporal lobe epilepsy

OBJECTIVE

In temporal lobe epilepsy (TLE), the thalamus is well known for its role in the propagation and spread of epileptiform activity. However, the integrity of thalamocortical functional connectivity (FC) in TLE and its relation to specific seizure patterns have not yet been determined. We address these issues with resting-state functional magnetic resonance imaging (fMRI).

METHODS

Resting-state fMRI was performed on two groups of unilateral TLE patients: those with focal seizures only (16 left TLE, 16 right TLE) and those with additional generalized seizures (16 left TLE, 10 right TLE), and 16 matched controls. A thalamic parcellation based on FC between five nonoverlapping cortical seeds (prefrontal, motor, somatosensory, parietal-occipital, and temporal) and the ipsilateral thalamus was carried out to parcel each thalamus into five corresponding segments. FCs between each segment and its ipsilateral cortical seed were extracted and compared across groups using analyses of variance (ANOVAs).

RESULTS

Compared to healthy controls, patients with TLE displayed decreased thalamocortical FC in multiple posterior and ventromedial thalamic segments of both the ictal and nonictal hemispheres. Our parcellation analysis revealed that these thalamic regions were functionally connected to the parietal/occipital and temporal lobes. In patients with TLE with focal seizures these regional thalamocortical FC decreases were limited to the ictal hemisphere. In contrast, TLE patients with both focal and generalized epileptiform activity displayed FC decreases in both the ictal and nonictal thalamus involving the dorsolateral pulvinar, a region preferentially connected to the parietal and occipital lobes.

SIGNIFICANCE

Our data provide the first evidence of regional specific thalamocortical FC decreases in patients with unilateral TLE. Furthermore, our results demonstrate that patients with different seizure types present different thalamoparietal/occipital FC decrease patterns. While patients with focal seizures present thalamocortical FC decreases in the ictal hemisphere only, patients with additional generalized seizure activity also show thalamocortical FC decreases involving the thalamus in the nonictal hemisphere.

Attention alters orientation processing in the human lateral geniculate nucleus

Orientation selectivity is a cornerstone property of vision, commonly believed to emerge in the primary visual cortex. We found that reliable orientation information could be detected even earlier, in the human lateral geniculate nucleus, and that attentional feedback selectively altered these orientation responses. This attentional modulation may allow the visual system to modify incoming feature-specific signals at the earliest possible processing site.

Functional mapping of the magnocellular and parvocellular subdivisions of human LGN

The magnocellular (M) and parvocellular (P) subdivisions of primate LGN are known to process complementary types of visual stimulus information, but a method for noninvasively defining these subdivisions in humans has proven elusive. As a result, the functional roles of these subdivisions in humans have not been investigated physiologically. To functionally map the M and P subdivisions of human LGN, we used high-resolution fMRI at high field (7 T and 3 T) together with a combination of spatial, temporal, luminance, and chromatic stimulus manipulations. We found that stimulus factors that differentially drive magnocellular and parvocellular neurons in primate LGN also elicit differential BOLD fMRI responses in human LGN and that these responses exhibit a spatial organization consistent with the known anatomical organization of the M and P subdivisions. In test-retest studies, the relative responses of individual voxels to M-type and P-type stimuli were reliable across scanning sessions on separate days and across sessions at different field strengths. The ability to functionally identify magnocellular and parvocellular regions of human LGN with fMRI opens possibilities for investigating the functions of these subdivisions in human visual perception, in patient populations with suspected abnormalities in one of these subdivisions, and in visual cortical processing streams arising from parallel thalamocortical pathways.

The impact of the lateral geniculate nucleus and corticogeniculate interactions on efficient coding and higher-order visual object processing

Principles of efficient coding suggest that the peripheral units of any sensory processing system are designed for efficient coding. The function of the lateral geniculate nucleus (LGN) as an early stage in the visual system is not well understood. Some findings indicate that similar to the retina that decorrelates input signals spatially, the LGN tends to perform a temporal decorrelation. There is evidence suggesting that corticogeniculate connections may account for this decorrelation in the LGN. In this study, we propose a computational model based on biological evidence reported by Wang et al. (2006), who demonstrated that the influence pattern of V1 feedback is phase-reversed. The output of our model shows how corticogeniculate connections decorrelate LGN responses and make an efficient representation. We evaluated our model using criteria that have previously been tested on LGN neurons through cell recording experiments, including sparseness, entropy, power spectra, and information transfer. We also considered the role of the LGN in higher-order visual object processing, comparing the categorization performance of human subjects with a cortical object recognition model in the presence and absence of our LGN input-stage model. Our results show that the new model that considers the role of the LGN, more closely follows the categorization performance of human subjects.

Time perception impairs sensory-motor integration in Parkinson's disease

It is well known that perception and estimation of time are fundamental for the relationship between humans and their environment. However, this temporal information processing is inefficient in patients with Parkinson' disease (PD), resulting in temporal judgment deficits. In general, the pathophysiology of PD has been described as a dysfunction in the basal ganglia, which is a multisensory integration station. Thus, a deficit in the sensorimotor integration process could explain many of the Parkinson symptoms, such as changes in time perception. This physiological distortion may be better understood if we analyze the neurobiological model of interval timing, expressed within the conceptual framework of a traditional information-processing model called "Scalar Expectancy Theory". Therefore, in this review we discuss the pathophysiology and sensorimotor integration process in PD, the theories and neural basic mechanisms involved in temporal processing, and the main clinical findings about the impact of time perception in PD.

Lamination of the lateral geniculate nucleus of catarrhine primates

The lateral geniculate nucleus (LGN) of catarrhine primates - with the exception of gibbons - is typically described as a 6-layered structure, comprised of 2 ventral magnocellular layers, and 4 dorsal parvocellular layers. The parvocellular layers of the LGN are involved in color vision. Therefore, it is hypothesized that a 6-layered LGN is a shared-derived trait among catarrhines. This might suggest that in gibbons the lack of further subdivisions of the parvocellular layers is a recent change, and could be related to specializations of visual information processing in this taxon. To address these hypotheses, the lamination of the LGN was investigated in a range of catarrhine species, including several taxa not previously described, and the evolution of the LGN was reconstructed using phylogenetic information. The findings indicate that while all catarrhine species have 4 parvocellular leaflets, two main patterns of LGN parvocellular lamination occur: 2 undivided parvocellular layers in some species, and 4 parvocellular leaflets (with occasional subleaflets) in other species. LGN size was not found to be related to lamination pattern. Both patterns were found to occur in divergent clades, which is suggestive of homoplasy within the catarrhines in LGN morphology.

Performance limitations of relay neurons

Relay cells are prevalent throughout sensory systems and receive two types of inputs: driving and modulating. The driving input contains receptive field properties that must be transmitted while the modulating input alters the specifics of transmission. For example, the visual thalamus contains relay neurons that receive driving inputs from the retina that encode a visual image, and modulating inputs from reticular activating system and layer 6 of visual cortex that control what aspects of the image will be relayed back to visual cortex for perception. What gets relayed depends on several factors such as attentional demands and a subject's goals. In this paper, we analyze a biophysical based model of a relay cell and use systems theoretic tools to construct analytic bounds on how well the cell transmits a driving input as a function of the neuron's electrophysiological properties, the modulating input, and the driving signal parameters. We assume that the modulating input belongs to a class of sinusoidal signals and that the driving input is an irregular train of pulses with inter-pulse intervals obeying an exponential distribution. Our analysis applies to any [Formula: see text] order model as long as the neuron does not spike without a driving input pulse and exhibits a refractory period. Our bounds on relay reliability contain performance obtained through simulation of a second and third order model, and suggest, for instance, that if the frequency of the modulating input increases or the DC offset decreases, then relay increases. Our analysis also shows, for the first time, how the biophysical properties of the neuron (e.g. ion channel dynamics) define the oscillatory patterns needed in the modulating input for appropriately timed relay of sensory information. In our discussion, we describe how our bounds predict experimentally observed neural activity in the basal ganglia in (i) health, (ii) in Parkinson's disease (PD), and (iii) in PD during therapeutic deep brain stimulation. Our bounds also predict different rhythms that emerge in the lateral geniculate nucleus in the thalamus during different attentional states.

Consistent phosphenes generated by electrical microstimulation of the visual thalamus. An experimental approach for thalamic visual neuroprostheses

Most work on visual prostheses has centered on developing retinal or cortical devices. However, when retinal implants are not feasible, neuroprostheses could be implanted in the lateral geniculate nucleus (LGN) of the thalamus, the intermediate relay station of visual information from the retina to the visual cortex (V1). The objective of the present study was to determine the types of artificial stimuli that when delivered to the visual thalamus can generate reliable responses of the cortical neurons similar to those obtained when the eye perceives a visual image. Visual stimuli {S(i)} were presented to one eye of an experimental animal and both, the thalamic {RTh(i)} and cortical responses {RV1(i)} to such stimuli were recorded. Electrical patterns {RTh(i)*} resembling {RTh(i)} were then injected into the visual thalamus to obtain cortical responses {RV1(i)*} similar to {RV1(i)}. Visually- and electrically generated V1 responses were compared.

RESULTS

During the course of this work we: (i) characterized the response of V1 neurons to visual stimuli according to response magnitude, duration, spiking rate, and the distribution of interspike intervals; (ii) experimentally tested the dependence of V1 responses on stimulation parameters such as intensity, frequency, duration, etc., and determined the ranges of these parameters generating the desired cortical activity; (iii) identified similarities between responses of V1 useful to compare the naturally and artificially generated neuronal activity of V1; and (iv) by modifying the stimulation parameters, we generated artificial V1 responses similar to those elicited by visual stimuli. Generation of predictable and consistent phosphenes by means of artificial stimulation of the LGN is important for the feasibility of visual prostheses. Here we proved that electrical stimuli to the LGN can generate V1 neural responses that resemble those elicited by natural visual stimuli.

Imaging retinotopic maps in the human brain

A quarter-century ago visual neuroscientists had little information about the number and organization of retinotopic maps in human visual cortex. The advent of functional magnetic resonance imaging (MRI), a non-invasive, spatially-resolved technique for measuring brain activity, provided a wealth of data about human retinotopic maps. Just as there are differences amongst non-human primate maps, the human maps have their own unique properties. Many human maps can be measured reliably in individual subjects during experimental sessions lasting less than an hour. The efficiency of the measurements and the relatively large amplitude of functional MRI signals in visual cortex make it possible to develop quantitative models of functional responses within specific maps in individual subjects. During this last quarter-century, there has also been significant progress in measuring properties of the human brain at a range of length and time scales, including white matter pathways, macroscopic properties of gray and white matter, and cellular and molecular tissue properties. We hope the next 25years will see a great deal of work that aims to integrate these data by modeling the network of visual signals. We do not know what such theories will look like, but the characterization of human retinotopic maps from the last 25years is likely to be an important part of future ideas about visual computations.

Attention-driven auditory cortex short-term plasticity helps segregate relevant sounds from noise

How can we concentrate on relevant sounds in noisy environments? A "gain model" suggests that auditory attention simply amplifies relevant and suppresses irrelevant afferent inputs. However, it is unclear whether this suffices when attended and ignored features overlap to stimulate the same neuronal receptive fields. A "tuning model" suggests that, in addition to gain, attention modulates feature selectivity of auditory neurons. We recorded magnetoencephalography, EEG, and functional MRI (fMRI) while subjects attended to tones delivered to one ear and ignored opposite-ear inputs. The attended ear was switched every 30 s to quantify how quickly the effects evolve. To produce overlapping inputs, the tones were presented alone vs. during white-noise masking notch-filtered ±1/6 octaves around the tone center frequencies. Amplitude modulation (39 vs. 41 Hz in opposite ears) was applied for "frequency tagging" of attention effects on maskers. Noise masking reduced early (50-150 ms; N1) auditory responses to unattended tones. In support of the tuning model, selective attention canceled out this attenuating effect but did not modulate the gain of 50-150 ms activity to nonmasked tones or steady-state responses to the maskers themselves. These tuning effects originated at nonprimary auditory cortices, purportedly occupied by neurons that, without attention, have wider frequency tuning than ±1/6 octaves. The attentional tuning evolved rapidly, during the first few seconds after attention switching, and correlated with behavioral discrimination performance. In conclusion, a simple gain model alone cannot explain auditory selective attention. In nonprimary auditory cortices, attention-driven short-term plasticity retunes neurons to segregate relevant sounds from noise.

Feature-based attention modulates direction-selective hemodynamic activity within human MT

Attending to the spatial location or to nonspatial features of a stimulus modulates neural activity in cortical areas that process its perceptual attributes. The feature-based attentional selection of the direction of a moving stimulus is associated with increased firing of individual neurons tuned to the direction of the movement in area V5/MT, while responses of neurons tuned to opposite directions are suppressed. However, it is not known how these multiplicatively scaled responses of individual neurons tuned to different motion-directions are integrated at the population level, in order to facilitate the processing of stimuli that match the perceptual goals. Using functional magnetic resonance imaging (fMRI) the present study revealed that attending to the movement direction of a dot field enhances the response in a number of areas including the human MT region (hMT) as a function of the coherence of the stimulus. Attending the opposite direction, however, lead to a suppressed response in hMT that was inversely correlated with stimulus-coherence. These findings demonstrate that the multiplicative scaling of single-neuron responses by feature-based attention results in an enhanced direction-selective population response within those cortical modules that processes the physical attributes of the attended stimuli. Our results provide strong support for the validity of the "feature similarity gain model" on the integrated population response as quantified by parametric fMRI in humans.

Central mechanisms of odour object perception

The stimulus complexity of naturally occurring odours presents unique challenges for central nervous systems that are aiming to internalize the external olfactory landscape. One mechanism by which the brain encodes perceptual representations of behaviourally relevant smells is through the synthesis of different olfactory inputs into a unified perceptual experience--an odour object. Recent evidence indicates that the identification, categorization and discrimination of olfactory stimuli rely on the formation and modulation of odour objects in the piriform cortex. Convergent findings from human and rodent models suggest that distributed piriform ensemble patterns of olfactory qualities and categories are crucial for maintaining the perceptual constancy of ecologically inconstant stimuli.

The effect of orientation adaptation on responses of lateral geniculate nucleus neurons with high orientation bias in cats

Adaptation to stimulus orientation is assumed to have a cortical basis, but few studies have addressed whether it affects the activity of subcortical neurons. Using single-unit recording, we studied the effects of orientation adaptation on the responses of lateral geniculate nucleus (LGN) neurons with high orientation bias (OB) in anesthetized and paralyzed cats. Following adaptation to one stimulus orientation, the response at the adapting orientation was decreased, and the preferred orientation was shifted away from the adapting orientation. This phenomenon was similar to the effects observed for orientation adaptation in the primary visual cortex (V1), and was obvious when the adapting orientation was at an appropriate location relative to the original preferred orientation. Moreover, when the V1 was inactivated, the response at the adapting orientation was also decreased but the preferred orientation did not show a systematic shift after orientation adaptation in LGN. This result indicates that cortical feedback contributes to the effect of orientation adaptation on LGN neurons, which have a high OB. These data provide an example of how the corticothalamic loop modulates the processing of visual information, and suggest that the LGN is not only a simply passive relay but also a modulator of visual information.

Effects of antiepileptics on lateral geniculate nucleus-kindled seizures in rats

The present study was undertaken to clarify the characteristics of lateral geniculate nucleus (LGN) kindling in rats, especially the efficacies of antiepileptics, in comparison with those of amygdala (AMG) kindling. Daily electrical stimulation of the LGN led to the development of a generalized convulsion (kangaroo posture and falling back) in all subjects, similar to AMG kindling. The kindling response of the LGN differed from that of the AMG in a number of respects, that is, a high after-discharge (AD) threshold, a large number of stimulations for completion of kindling, and a different pattern of electroencephalogram (EEG) development. On the other hand, the oral administration of sodium valproate, carbamazepine, clobazam, or zonisamide caused dose-dependent inhibitions of both seizure stage and AD duration of LGN-kindled seizures, whereas ethosuximide had no significant effects. In addition, seizure stage was more potently inhibited than AD duration by these antiepileptics, particularly with clobazam. In conclusion, LGN kindling possesses characteristics that are different from AMG kindling. In addition, it was demonstrated that LGN kindling is a useful model, similar to other types of limbic system kindling, for the evaluation of antiepileptics.

Normal electrocortical facilitation but abnormal target identification during visual sustained attention in schizophrenia

Attentional deficits in schizophrenia have been investigated using target identification tasks which conflate the abilities to successfully (1) attend to possible target locations and (2) detect target events. Whether compromised attentional selectivity or abnormal target detection causes schizophrenia subjects' poor performance on visual attention tasks, therefore, is unknown. To address this issue, we measured the neural activity (using electroencephalography) of 17 schizophrenia and 17 healthy subjects during a target identification task. Participants viewed superimposed images (horizontal and vertical bars differing in color) and attended to one image to identify bar width changes in specific locations. Bars were frequency tagged so attention directed to unique parts of the images could be tracked. Steady-state visual evoked potentials (ssVEPs) were used to quantify attention-related neural activity to specific parts of the visual images. Behavioral performance and event-related potentials (ERPs) in response to the target events were used to quantify target detection abilities. For both schizophrenia and healthy subjects, attending to specific parts of the attended image enhanced brain activity related to attended bars and reduced activity evoked by unattended bars. Activity in relation to the spatially overlapping unattended image was unaffected. Schizophrenia patients, however, were impaired on target detection abilities on both behavioral and brain activity measures. Target-related behavioral and brain activity measures were highly correlated in both groups. These findings indicate that deficient target detection rather than compromised attentional selectivity accounts for previously reported visual attention deficits in schizophrenia.

Intrinsic functional relations between human cerebral cortex and thalamus

The brain is active even in the absence of explicit stimuli or overt responses. This activity is highly correlated within specific networks of the cerebral cortex when assessed with resting-state functional magnetic resonance imaging (fMRI) blood oxygen level-dependent (BOLD) imaging. The role of the thalamus in this intrinsic activity is unknown despite its critical role in the function of the cerebral cortex. Here we mapped correlations in resting-state activity between the human thalamus and the cerebral cortex in adult humans using fMRI BOLD imaging. Based on this functional measure of intrinsic brain activity we partitioned the thalamus into nuclear groups that correspond well with postmortem human histology and connectional anatomy inferred from nonhuman primates. This structure/function correspondence in resting-state activity was strongest between each cerebral hemisphere and its ipsilateral thalamus. However, each hemisphere was also strongly correlated with the contralateral thalamus, a pattern that is not attributable to known thalamocortical monosynaptic connections. These results extend our understanding of the intrinsic network organization of the human brain to the thalamus and highlight the potential of resting-state fMRI BOLD imaging to elucidate thalamocortical relationships.

Behavioral states, network states, and sensory response variability

We review data demonstrating that single-neuron sensory responses change with the states of the neural networks (indexed in terms of spectral properties of local field potentials) in which those neurons are embedded. We start with broad network changes--different levels of anesthesia and sleep--and then move to studies demonstrating that the sensory response plasticity associated with attention and experience can also be conceptualized as functions of network state changes. This leads naturally to the recent data that can be interpreted to suggest that even brief experience can change sensory responses via changes in network states and that trial-to-trial variability in sensory responses is a nonrandom function of network fluctuations, as well. We suggest that the CNS may have evolved specifically to deal with stimulus variability and that the coupling with network states may be central to sensory processing.

Dedicated and intrinsic models of time perception

Two general frameworks have been articulated to describe how the passage of time is perceived. One emphasizes that the judgment of the duration of a stimulus depends on the operation of dedicated neural mechanisms specialized for representing the temporal relationships between events. Alternatively, the representation of duration could be ubiquitous, arising from the intrinsic dynamics of nondedicated neural mechanisms. In such models, duration might be encoded directly through the amount of activation of sensory processes or as spatial patterns of activity in a network of neurons. Although intrinsic models are neurally plausible, we highlight several issues that must be addressed before we dispense with models of duration perception that are based on dedicated processes.

Induced visual sensitivity changes in chronic hemianopia

PURPOSE OF REVIEW

Postgeniculate lesions commonly result in visual field loss. These areas of blindness at the chronic stage were thought to be permanent and irreversible. Recent evidence demonstrating changes in visual sensitivity within the field defect following training is reviewed.

RECENT FINDINGS

It was previously demonstrated in animal models that visual deficits following brain injury are transient and less severe than those found in human patients. Previous attempts at visual rehabilitation, prompted by findings in the animal models, were successful in helping hemianopic patients to develop coping strategies. Nevertheless, these were criticized because some of the findings could be accounted for by imprecise or uncontrolled eye movements. More encouraging is recent research showing that repeated stimulation of the field defect, using a wide range of visual targets, may lead to increased visual sensitivity within the blind field. These findings are in accordance with our current understanding of visual pathways.

SUMMARY

Recent rehabilitation interventions, should they be proven effective in large-scale clinical trials using appropriate controls, could lead to an overhaul of the current therapeutic management of this condition.

Activation of superior colliculi in humans during visual exploration

Background

Visual, oculomotor, and – recently – cognitive functions of the superior colliculi (SC) have been documented in detail in non-human primates in the past. Evidence for corresponding functions of the SC in humans is still rare. We examined activity changes in the human tectum and the lateral geniculate nuclei (LGN) in a visual search task using functional magnetic resonance imaging (fMRI) and anatomically defined regions of interest (ROI). Healthy subjects conducted a free visual search task and two voluntary eye movement tasks with and without irrelevant visual distracters. Blood oxygen level dependent (BOLD) signals in the SC were compared to activity in the inferior colliculi (IC) and LGN.

Results

Neural activity increased during free exploration only in the SC in comparison to both control tasks. Saccade frequency did not exert a significant effect on BOLD signal changes. No corresponding differences between experimental tasks were found in the IC or the LGN. However, while the IC revealed no signal increase from the baseline, BOLD signal changes at the LGN were consistently positive in all experimental conditions.

Conclusion

Our data demonstrate the involvement of the SC in a visual search task. In contrast to the results of previous studies, signal changes could not be seen to be driven by either visual stimulation or oculomotor control on their own. Further, we can exclude the influence of any nearby neural structures (e.g. pulvinar, tegmentum) or of typical artefacts at the brainstem on the observed signal changes at the SC. Corresponding to findings in non-human primates, our data support a dependency of SC activity on functions beyond oculomotor control and visual processing.

Brain states: top-down influences in sensory processing

All cortical and thalamic levels of sensory processing are subject to powerful top-down influences, the shaping of lower-level processes by more complex information. New findings on the diversity of top-down interactions show that cortical areas function as adaptive processors, being subject to attention, expectation, and perceptual task. Brain states are determined by the interactions between multiple cortical areas and the modulation of intrinsic circuits by feedback connections. In perceptual learning, both the encoding and recall of learned information involves a selection of the appropriate inputs that convey information about the stimulus being discriminated. Disruption of this interaction may lead to behavioral disorders, including schizophrenia.