oko meduzy mutations affect neuronal patterning in the zebrafish retina and reveal cell-cell interactions of the retinal neuroepithelial sheet. (1/1271)

Mutations of the oko meduzy (ome) locus cause drastic neuronal patterning defect in the zebrafish retina. The precise, stratified appearance of the wild-type retina is absent in the mutants. Despite the lack of lamination, at least seven retinal cell types differentiate in oko meduzy. The ome phenotype is already expressed in the retinal neuroepithelium affecting morphology of the neuroepithelial cells. Our experiments indicate that previously unknown cell-cell interactions are involved in development of the retinal neuroepithelial sheet. In genetically mosaic animals, cell-cell interactions are sufficient to rescue the phenotype of oko meduzy retinal neuroepithelial cells. These cell-cell interactions may play a critical role in the patterning events that lead to differentiation of distinct neuronal laminae in the vertebrate retina.  (+info)

Retinal input induces three firing patterns in neurons of the superficial superior colliculus of neonatal rats. (2/1271)

By using an in vitro isolated brain stem preparation, we recorded extracellular responses to electrical stimulation of the optic tract (OT) from 71 neurons in the superficial superior colliculus (SC) of neonatal rats (P1-13). At postnatal day 1 (P1), all tested neurons (n = 10) already received excitatory input from the retina. Sixty-nine (97%) superficial SC neurons of neonatal rats showed three response patterns to OT stimulation, which depended on stimulus intensity. A weak stimulus evoked only one spike that was caused by activation of non-N-methyl-D-aspartate (NMDA) glutamate receptors. A moderate stimulus elicited a short train (<250 ms) of spikes, which was induced by activation of both NMDA and non-NMDA receptors. A strong stimulus gave rise to a long train (>300 ms) of spikes, which was associated with additional activation of L-type high-threshold calcium channels. The long train firing pattern could also be induced either by temporal summation of retinal inputs or by blocking gamma-aminobutyric acid-A receptors. Because retinal ganglion cells show synchronous bursting activity before eye opening at P14, the retinotectal inputs appear to be sufficient to activate L-type calcium channels in the absence of pattern vision. Therefore activation of L-type calcium channels is likely to be an important source for calcium influx into SC neurons in neonatal rats.  (+info)

Local excitatory circuits in the intermediate gray layer of the superior colliculus. (3/1271)

We have used photostimulation and whole cell patch-clamp recording techniques to examine local synaptic interactions in slices from the superior colliculus of the tree shrew. Uncaging glutamate 10-75 microm from the somata of neurons in the intermediate gray layer elicited a long-lasting inward current, due to direct activation of glutamate receptors on these neurons, and brief inward currents caused by activation of presynaptic neurons. The synaptic responses occurred as individual currents or as clusters that lasted up to several hundred milliseconds. Excitatory synaptic responses, which reversed at membrane potentials near 0 mV, could be evoked by uncaging glutamate anywhere within 75 microm of an intermediate layer neuron. Our results indicate the presence of extensive local excitatory circuits in the intermediate layer of the superior colliculus and support the hypothesis that such intrinsic circuitry contributes to the development of presaccadic command bursts.  (+info)

Role of primate superior colliculus in preparation and execution of anti-saccades and pro-saccades. (4/1271)

We investigated how the brain switches between the preparation of a movement where a stimulus is the target of the movement, and a movement where a stimulus serves as a landmark for an instructed movement elsewhere. Monkeys were trained on a pro-/anti-saccade paradigm in which they either had to generate a pro-saccade toward a visual stimulus or an anti-saccade away from the stimulus to its mirror position, depending on the color of an initial fixation point. Neural activity was recorded in the superior colliculus (SC), a structure that is known to be involved in the generation of fast saccades, to determine whether it was also involved in the generation of anti-saccades. On anti-saccade trials, fixation during the instruction period was associated with an increased activity of collicular fixation-related neurons and a decreased activity of saccade-related neurons. Stimulus-related and saccade-related activity was reduced on anti-saccade trials. Our results demonstrate that the anti-saccade task involves (and may require) the attenuation of preparatory and stimulus-related activity in the SC to avoid unwanted pro-saccades. Because the attenuated pre-saccade activity that we found in the SC may be insufficient by itself to elicit correct anti-saccades, additional movement signals from other brain areas are presumably required.  (+info)

Correlation of primate superior colliculus and reticular formation discharge with proximal limb muscle activity. (5/1271)

We studied the discharge of neurons from both the superior colliculus (SC) and the underlying mesencephalic reticular formation (MRF) and its relation to the simultaneously recorded activity of 11 arm muscles. The 242 neurons tested with a center-out reach task yielded 2,586 pairs of neuron/muscle cross-correlations (normalized, such that perfect correlations are +/-1.0). Of these, 43% had peaks with magnitude as large as 0.15, a value that corresponds to the 5% level of significance, and 16% were as large as 0.25. The great majority of peaks in this latter group was positive. The median lag time within this group was 52 ms, indicating that the neuronal discharge tended to precede the correlated muscle activity. We found a small but significantly higher proportion of cells with these relatively strong correlations in the MRF than in the SC. For both areas, these occurred most frequently with muscles of the shoulder girdle and became less frequent for axial as well as for increasingly distal arm musculature. The results support a role for the SC and MRF in guiding the arm during reach movements via the control of proximal limb musculature.  (+info)

Spatial characteristics of visual-auditory summation in human saccades. (6/1271)

Bimodal (auditory + visual) stimuli reduce saccade latencies in human observers to a degree that exceeds levels predictable by probabilistic summation between parallel, independent unimodal pathways. These interactions have been interpreted in terms of converging visual and auditory afferents within the oculomotor pathways, specifically within the superior colliculus (SC). The present work describes the spatial tuning of auditory-visual summation in human saccades, using diagnostics derived from stochastic models of information processing. Consistent with expectations based on the electrophysiology of the SC, the magnitude of facilitation varied with the degree of spatial correspondence, and the spatial tuning was quite coarse.  (+info)

Patterns of synchronization in the superior colliculus of anesthetized cats. (7/1271)

Sensorimotor transformations in the mammalian superior colliculus (SC) are mediated by large sets of distributed neurons. For such distributed coding systems, stimulus superposition poses problems attributable to the merging of neural populations coding for different stimuli. Such superposition problems could be overcome by synchronization of neuronal discharges, because it allows the selection of a subset of distributed responses for further joint processing. To assess the putative role of such a temporal binding mechanism in the SC, we have applied correlation analysis to visually evoked collicular activity. We performed recordings of single-unit and multiunit activity in the SC of anesthetized and paralyzed cats with multiple electrodes. Autocorrelation analysis revealed that collicular neurons often discharged in broad (20-100 msec) bursts or with an oscillatory patterning in the alpha- and beta-frequency range. Significantly modulated cross-correlograms were observed in 50% (128 of 258) of the collicular multiunit recording pairs, and for these pairs significant correlations occurred in 44% of the stimulation epochs. For the single-unit pairs, significant interactions were observed in 14 of 48 cases studied (29%). Collicular cross-correlograms were often oscillatory, and these oscillations covered a broad frequency range of up to 100 Hz, with a predominance of oscillation frequencies in the alpha- and beta-range. In the majority of the significant correlograms (64%) the phase lag of the center peak was <5 msec. The probability of collicular synchronization increased with the overlap of the receptive fields and the proximity of the recording sites. Correlations were also observed between cells in the superficial and deep SC layers. Collicular synchronization required activation of the respective cells with a single coherent stimulus and broke down when the neurons were activated with two different stimuli. These data are consistent with the notion that collicular synchrony could define assemblies of functionally related cells.  (+info)

Modulation of EphA receptor function by coexpressed ephrinA ligands on retinal ganglion cell axons. (8/1271)

The Eph family is thought to exert its function through the complementary expression of receptors and ligands. Here, we show that EphA receptors colocalize on retinal ganglion cell (RGC) axons with EphA ligands, which are expressed in a high-nasal-to-low-temporal pattern. In the stripe assay, only temporal axons are normally sensitive for repellent axon guidance cues of the caudal tectum. However, overexpression of ephrinA ligands on temporal axons abolishes this sensitivity, whereas treatment with PI-PLC both removes ephrinA ligands from retinal axons and induces a striped outgrowth of formerly insensitive nasal axons. In vivo, retinal overexpression of ephrinA2 leads to topographic targeting errors of temporal axons. These data suggest that differential ligand expression on retinal axons is a major determinant of topographic targeting in the retinotectal projection.  (+info)