An interhemispheric neural circuit allowing binocular integration in the optic tectum

Binocular stereopsis requires the convergence of visual information from corresponding points in visual space seen by two different lines of sight. This may be achieved by superposition of retinal input from each eye onto the same downstream neurons via ipsi- and contralaterally projecting optic nerve fibers. Zebrafish larvae can perceive binocular cues during prey hunting but have exclusively contralateral retinotectal projections. Here we report brain activity in the tectal neuropil ipsilateral to the visually stimulated eye, despite the absence of ipsilateral retinotectal projections. This activity colocalizes with arbors of commissural neurons, termed intertectal neurons (ITNs), that connect the tectal hemispheres. ITNs are GABAergic, establish tectal synapses bilaterally and respond to small moving stimuli. ITN-ablation impairs capture swim initiation when prey is positioned in the binocular strike zone. We propose an intertectal circuit that controls execution of the prey-capture motor program following binocular localization of prey, without requiring ipsilateral retinotectal projections.

The Influence of “Blind” Distractors on Eye Movement Trajectories in Visual Hemifield Defects

There is evidence that some visual information in blind regions may still be processed in patients with hemifield defects after cerebral lesions (“blindsight”). We tested the hypothesis that, in the absence of retinogeniculostriate processing, residual retinotectal processing may still be detected as modifications of saccades to seen targets by irrelevant distractors in the blind hemifield. Six patients were presented with distractors in the blind and intact portions of their visual field and participants were instructed to make eye movements to targets in the intact field. Eye movements were recorded to determine if blind-field distractors caused deviation in saccadic trajectories. No deviation was found in one patient with an optic chiasm lesion, which affect both retinotectal and retinogeniculostriate pathways. In five patients with lesions of the optic radiations or the striate cortex, the results were mixed, with two of the five patients showing significant deviations of saccadic trajectory away from the “blind” distractor. In a second experiment, two of the five patients were tested with the target and the distractor more closely aligned. Both patients showed a “global effect,” in that saccades deviated toward the distractor, but the effect was stronger in the patient who also showed significant trajectory deviation in the first experiment. Although our study confirms that distractor effects on saccadic trajectory can occur in patients with damage to the retinogeniculostriate visual pathway but preserved retinotectal projections, there remain questions regarding what additional factors are required for these effects to manifest themselves in a given patient.

Metabolic activity in optic tectum during regeneration of retina in adult goldfish

Retinal and visual function returns following retinal destruction by ouabain in adult goldfish (Carassius auratus). Although the precise cellular mechanisms are unclear, the ability to regenerate CNS neurons and connections that subsequently sustain visual behavior is remarkable, especially for an adult vertebrate. In this paper, we ask whether visual stimulation via new retinal cells can activate existing cells in the optic tectum, which normally receives the largest retinal projection in this species. The right eyes of adult goldfish were injected with ouabain. After 1–18 weeks the conscious, freely moving fish were exposed to spatially and temporally varying visual stimuli and the resulting tectal metabolic activity was determined with the autoradiographic deoxyglucose method. In normal controls without lesions, visual stimulation produced equally strong metabolic activity in both tectal hemispheres, peaking in the layer where most retinotectal projections terminate (N = 6). One week after ouabain injection, metabolic activity in the contralateral, deprived tectum was dramatically reduced (N = 5), closely resembling the effect of unilateral ocular enucleation (N = 5). However, 9–18 weeks after ouabain injection, metabolic activity in the deprived tectum recovered to a level that was statistically indistinguishable from normal controls (N = 6). These findings suggest that, after a comprehensive cytotoxic lesion of the retina, regenerated ganglion cells not only establish new connections with the preexisting optic tectum, but also effectively transmit visual information they receive from newly generated photoreceptors to the “old” tectum.

The influence of viewing eye on pseudoneglect magnitude

Various factors influence the degree of leftward error (pseudoneglect) that typifies the performance of normal individuals in line bisection tasks. This experiment reveals that the eye through which stimuli are viewed also exerts a modulating influence on spatial attention, as indexed by significant alterations in the magnitude of pseudoneglect. Using a forced-choice tachistoscopic line bisection protocol, 24 participants (12 male; 12 female) bisected horizontally oriented lines (22.6° w × 0.39° h) presented to central vision in 3 conditions: left uniocular viewing (L), right uniocular viewing (R), and binocular viewing (B). Perceived line midpoint, a measure of bisection accuracy, deviated significantly leftward of veridical ( p < .05) in all viewing conditions, confirming a tonic asymmetry of visuospatial attention in normal young observers. In addition, a significant influence of viewing condition was found (p < .05) where pseudoneglect was greatest in the L condition, followed by the B and R conditions, respectively. Analysis of the slopes of the psychometric functions revealed significantly greater bisection precision in the binocular versus uniocular viewing conditions (p < .05). The results are interpreted to suggest that phasic effects on spatial attention can be produced by uniocular viewing via asymmetric retinotectal projections. The results are consistent with activation–orientation theories of attentional asymmetry. (JINS, 2001, 7, 391–395.)

Electrophysiological evidence for transient topographic organization of retinotectal projections during optic nerve regeneration in the lizard, Ctenophorus ornatus

In the lizard, Ctenophorus ornatus, anatomical studies have revealed that optic axons regenerate to visual centers within 2 months of nerve crush but that, from the outset, the regenerated projections lack topographic order (Beazley et al., 1997; Dunlop et al., 1997b). Here we assess the functional topography of the regenerated retinotectal projections by electrophysiological recording of extracellular multiunit responses to visual stimulation and by observing the lizards' ability to capture live prey. At the completion of the electrophysiology, DiI was applied locally to the retina and the topography of the tectal projection later assessed. Electrophysiology revealed that, at 2–4.2 months, responses were weak and habituated readily; no retinotopic order was detected. Between 4.5–6 months, responses were more reliable and the majority of lizards displayed a crude retinotopic order, especially in the ventro-temporal to dorso-nasal retinal axis. Although responses were variable between 6–9 months, they tended to be more reliable again thereafter. However, from 6–18 months, the projection consistently lacked topography with many retinal regions projecting to each tectal locus. Lizards, including those with electrophysiological evidence of crude retinotopy, were consistently unable to capture live prey using the experimental eye. Labelling with DiI confirmed the absence of anatomical retinotopy throughout. Taken together, the electrophysiological and anatomical data indicate that retinotopically appropriate axon terminals (or parts thereof) are transiently active whilst inappropriately located ones are silent. Presumably in lizard map-making cues fade with time and/or the mechanisms are lacking to stabilize and refine the ephemeral map. Moreover, the transient retinotopy is insufficient for useful visual function.