Visual cortical responses to the input from the amblyopic eye are suppressed during binocular viewing.

Amblyopia is a visual disorder caused by an anomalous early visual experience. It has been suggested that suppression of the visual input from the weaker eye might be a primary underlying mechanism of the amblyopic syndrome. However, it is still an unresolved question to what extent neural responses to the visual information coming from the amblyopic eye are suppressed during binocular viewing. To address this question we measured event-related potentials (ERP) to foveal face stimuli in amblyopic patients, both in monocular and binocular viewing conditions. The results revealed no difference in the amplitude and latency of early components of the ERP responses between the binocular and fellow eye stimulation. On the other hand, early ERP components were reduced and delayed in the case of monocular stimulation of the amblyopic eye as compared to the fellow eye stimulation or to binocular viewing. The magnitude of the amblyopic effect measured on the ERP amplitudes was comparable to that found on the fMRI responses in the fusiform face area using the same face stimuli and task conditions. Our findings showing that the amblyopic effects present on the early ERP components in the case of monocular stimulation are not manifested in the ERP responses during binocular viewing suggest that input from the amblyopic eye is completely suppressed already at the earliest stages of visual cortical processing when stimuli are viewed by both eyes.

Using diffusion MRI for measuring the temperature of cerebrospinal fluid within the lateral ventricles.

AIM: Hypothermia is often induced to reduce brain injury in newborns, following perinatal hypoxic-ischaemic events, and in adults following traumatic brain injury, stroke or cardiac arrest. We aimed to devise a method, based on diffusion-weighted MRI, to measure non-invasively the temperature of the cerebrospinal fluid in the lateral ventricles. METHODS: The well-known temperature dependence of the water diffusion constant was used for the estimation of temperature. We carried out diffusion MRI measurements on a 3T Philips Achieva Scanner involving phantoms (filled with water or artificial cerebrospinal fluid while slowly cooling from 41 to 32 degrees C) and healthy adult volunteers. RESULTS: The estimated temperature of water phantoms followed that measured using a mercury thermometer, but the estimates for artificial cerebrospinal fluid were 1.04 degrees C lower. After correcting for this systematic difference, the estimated temperature within the lateral ventricles of volunteers was 39.9 degrees C. Using diffusion directions less sensitive to cerebrospinal fluid flow, it was 37.7 degrees C, which was in agreement with the literature. CONCLUSION: Although further improvements are needed, measuring the temperature within the lateral ventricles using diffusion MRI is a viable method that may be useful for clinical applications. We introduced the method, identified sources of error and offered remedies for each.

Modulation of backward pattern masking by focal visual attention.

The effect of focal visual attention on backward pattern masking was investigated using an orientation discrimination task. The results show that attention reduces primarily the effect of interruption masking, the later component of pattern masking, which occurs when the delay between the target and mask onset is about 50-150 ms. The strongest spatial cueing effect, i.e. the strongest reduction of the orientation discrimination threshold due to focal attention, was observed at intermediate (approximately 100 ms) target-to-mask stimulus onset asynchrony (SOA). There was a weak effect of cueing at shorter SOAs, and no or a very weak attentional effect was present at longer target-to-mask SOAs, where the pattern masking effect is absent. The dynamics of attentional modulation of backward pattern masking correlates closely with the dynamics of the attentional modulation of neuronal responses in the early visual cortex.

Contextual modulation of orientation discrimination is independent of stimulus processing time.

Contextual inhibition of neural activity in the primary visual cortex begins immediately and is most pronounced in the early transient response component. Using backward masking to control available processing time, we investigated whether the interaction between perceptual contextual modulation and processing time reflects the neural dynamics of contextual inhibition. We found that the threshold elevation due to contextual inhibition in an orientation-discrimination task is essentially independent of the available processing time and that it is closely related to contextual inhibition of the early transient response component of orientation-selective neurons in the primary visual cortex.

Visual exploration of form and position with identical stimuli: functional anatomy with PET.

Visual form and position perception in primates is thought to engage two different sets of cortical visual areas. However, the original concept of two functionally different and anatomically segregated pathways has been challenged by recent investigations. Using identical stimuli in the centre of the visual field with no external cues, we examined whether discrimination of form aspects and position aspects would indeed activate occipito-temporal and occipito-parietal areas, respectively. We measured and localised regional cerebral blood flow (rCBF) changes in the brain with positron emission tomography (PET) and 15O-butanol while the subjects performed four visual tasks: position discrimination (PD), form discrimination (FD), joint form and position discrimination (FPD), and a control task. Discrimination of form contrasted with discrimination of position resulted in rCBF increases in the lateral occipital and fusiform gyri. Discrimination of position contrasted with discrimination of form yielded rCBF increases in the left frontal eye field and middle frontal gyrus. No extra activations were seen when the joint form and position discrimination task was contrasted with either the individual form and position discrimination tasks. When the individual form and position discrimination tasks were contrasted with the control task, form discrimination resulted in activations in both occipito-temporal and occipito-parietal visual cortical regions, as well as in the right middle-frontal gyrus. Position discrimination resulted in activation in occipito-parietal visual cortical regions, the left frontal eye field and the left middle frontal gyrus. These findings are consistent with the view that the processing of visual position information activates occipito-parietal visual regions. On the other hand, the processing of 2D visual form information, in addition to the activation of occipito-temporal neuronal populations, also involves the parietal cortex. Form and position discrimination activated different nonsymmetrical prefrontal fields. Although the visual stimuli were identical, the network of activated cortical fields depended on whether the task was a form discrimination task or a position discrimination task, indicating a strong task dependence of cortical networks underlying form and position discrimination in the human brain. In contrast to former studies, however, these task-dependent macronetworks are overlapping in the posterior parietal cortex, but differentially engage the occipito-temporal and the prefrontal cortex.

Light and electron microscopic demonstration of mGluR5 metabotropic glutamate receptor immunoreactive neuronal elements in the rat cerebellar cortex.

The cellular and subcellular localization of the mGluR5 metabotropic glutamate receptor subtype was studied in the rat cerebellar cortex, by using the preembedding immunoperoxidase and immunogold techniques. Light microscopic observations revealed an abundant, intense labeling of neurons in the granular layer as well as in the molecular layer. Lugaro and Golgi cells exhibited an intense mGluR5 immunoreactivity, while only a fraction of the neurons in the molecular layer were found to be mGluR5 immunopositive. In addition to a dense plexus of immunoreactive dendrites in the molecular layer of the cerebellar cortex, the mGluR5 immunopositive Golgi cell dendrites resembling axons at the light microscopic level were also labeled in the granular layer. At the ultrastructural level, mGluR5 immunoreactivity was present in neuronal elements postsynaptic to axon terminals of different morphology. By using a pre-embedding immunogold method, it was found that mGluR5 immunoreactivity is accumulated at the plasma membranes extrasynaptically as well as at the periphery of the postsynaptic specializations, mainly of the parallel fiber synaptic contacts. These findings provide morphological evidence that mGluR5 is expressed by a population of neurons in the cerebellar cortex and can synaptically be activated via the parallel fiber system.

Immunocytochemical visualization of the mGluR1a metabotropic glutamate receptor at synapses of corticothalamic terminals originating from area 17 of the rat.

Pre-embedding immunogold histochemistry was combined with Phaseolus vulgaris leucoagglutinin anterograde tract tracing in order to analyse the relationship between the subcellular localization of the GluR1a metabotropic glutamate receptors and the distribution of corticothalamic synapses in the dorsal lateral geniculate nucleus (dLGN) and the lateral posterior nucleus (LP) of the rat. The injection of the tracer into area 17 labelled two types of corticothalamic terminals: (i) the small boutons constituting the majority of the labelled fibres which form asymmetrical synapses both in the dLGN and LP; and (ii) the giant terminals typically participating in glomerulus-like synaptic arrangements and found exclusively in the lateral posterior nucleus. The small corticothalamic terminals often established synapses with mGluR1a-immunopositive dendrites, with immunometal particles concentrated at the periphery of their postsynaptic membranes. In contrast, the synapses formed by giant boutons in the lateral posterior nucleus were always mGluR1a-immunonegative. We conclude that the corticothalamic fibres forming the small synaptic terminals are the most likely candidates for the postulated mGluR-mediated modulation of visual information flow by corticothalamic feedback mechanisms.

Light and electron microscopic analysis of synaptic input from cortical area 17 to the lateral posterior nucleus in cats.

The morphology and synaptic organization of the corticothalamic (CT) fibres from area 17 were studied in the lateral posterior nucleus (LP) of the thalamus in cats. Injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHAL) into primary visual cortex labelled a band of CT fibres in the LP with terminal field confined to its lateral division "LP1". PHAL-labelled CT axons in the LP1 gave rise to both en passant and terminal boutons. They usually established several synaptic contacts--often in complex glomerulus-like synaptic arrangements--with dendritic shafts of large diameter and presynaptic dendrites containing pleomorphic vesicles. Postsynaptic targets of the PHAL-labelled CT boutons were characterized by postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. It appeared that, in the LP1 of the cat, almost half (44.5%) of the postsynaptic dendrites to CT boutons from area 17 belonged to the GABA-immunopositive interneurons and the majority (41%) of these GABA-immunopositive dendrites were F2 terminals. These results indicate that the CT axons from the striate cortex in the LP of the cat, in addition to a direct excitatory action, exert a powerful feed-forward inhibition on the thalamic principal cells.

Cellular and subcellular localization of the mGluR5a metabotropic glutamate receptor in rat spinal cord.

The cellular and subcellular distribution of the mGluR5a metabotropic glutamate receptor was studied in the spinal cord of the rat using an antibody raised against a mGluR5a-specific carboxy-terminal peptide. Strong mGluR5a-immunoreactivity (mGluR5a-ir) was found in the laminae I-II of the dorsal horn, which gradually decreased towards the deeper layers. At the electron microscopical level, mGluR5a-ir was present exclusively in neuronal somata and dendrites. Immunometal labelling revealed that mGluR5a-ir is concentrated at the periphery of postsynaptic densities of asymmetrical synapses or localized extrasynaptically at dendritic and somatic membranes. The mGluR5a-immunoreactive dendritic profiles were often targeted by synaptic boutons with the morphological characteristics of C-fibre terminals. These observations provide evidence for mGluR5a being involved in the nociceptive transmission at the dorsal horn.

Quantitative electron microscopic analysis of synaptic input from cortical areas 17 and 18 to the dorsal lateral geniculate nucleus in cats.

Cortical feedback is the largest extraretinal projection to the lateral geniculate nucleus. This input is thought to modulate the transfer of visual information in a state-dependent manner. The quantitative distribution and synaptology of axon terminals arising from different cortical areas is still an unsolved question. To address this problem, the synaptic termination pattern of corticogeniculate axons from cortical areas 17 and 18 entering the lateral geniculate nucleus of the cat was examined. The Phaseolus vulgaris leucoagglutinin anterograde tract tracing method was used for the labeling of corticogeniculate terminals. Postsynaptic targets were characterized by postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. In both laminae A and A1, labeled corticogeniculate axons from area 17 established synaptic contacts with GABA-immunopositive, interneuronal dendritic profiles more frequently (17.5% of all axons) than did labeled axon terminals from area 18 (7% of axons). Conversely, 76% of labeled corticogeniculate axons from area 17, as opposed to 87% of labeled axons from area 18, terminated on GABA-immunonegative relay cell dendrites. Furthermore, the mean diameter of GABA-negative relay cell dendrites postsynaptic to labeled axons from area 17 was significantly smaller than the diameter of relay cell dendrites synapsing with labeled terminals from area 18. These results indicate that the corticogeniculate axons from cortical areas 17 and 18 exhibit different synaptic termination patterns in the dorsal lateral geniculate nucleus of the cat, suggesting that these two projections may subserve different functions in visual information processing.

Diazepam binding inhibitor fragment 33-50 (octadecaneuropeptide) immunoreactivity in the cerebellar cortex is restricted to glial cells.

The distribution of octadecaneuropeptide (ODN)-like immunoreactivity (LI) and its relationship to gamma-aminobutyric acid (GABA)-LI were investigated in the cerebellar cortex of adult rats with electron microscopy. At the electron microscopic level, ODN-LI was found exclusively in glial cells. In addition to Bergmann glia and its processes, cerebellar astrocytes were also labelled, encapsulating unlabelled neuronal elements of the cerebellum. These ODN-LI glial processes were observed in close apposition to synaptic junctions, but immunoreactivity could not be found in the synaptic cleft or in association with neuronal membranes. Since GABA-LI is always associated with neuronal elements, the colocalization of GABA- and ODN-LI could not be confirmed in the cerebellar cortex. Our results do not support the assumption that ODN is a neuron-specific processing product of diazepam binding inhibitor.