Abstract
BackgroundA continued effort in neuroscience aims to understand the way brain circuits consisting of diverse neuronal types generate complex behavior following sensory input. A common feature of vertebrate visual systems is that lower-order and higher-order visual areas are reciprocally connected. Feedforward projections confer visual responsiveness to higher-order visual neurons while feedback projections likely serve to modulate responses of lower-order visual neurons in a context-dependent manner. Optic tectum is the largest first-order visual brain area in zebrafish and is reciprocally connected with the torus longitudinalis (TL), a second-order visual brain area that does not receive retinal input. A functional role for feedback projections from TL to tectum has not been identified. Here we aim to understand how this feedback contributes to visual processing.ResultsIn this study, we demonstrate that TL feedback projections to tectum drive binocular integration and spatial summation in a defined tectal circuit. We performed genetically targeted, cell type-specific functional imaging in tectal pyramidal neurons (PyrNs) and their two input neuron populations: retinal ganglion cells (RGCs) and neurons in TL. We find that PyrNs encode gradual changes in scene luminance using a complement of three distinct response classes that encode different light intensity ranges. Functional imaging of RGC inputs to tectum suggest that these response classes originate in the retina and RGC input specifies PyrN functional classes. In contrast, TL input serves to endow PyrNs with large, compound receptive fields that span both retinal hemifields.ConclusionsThese findings reveal a novel role for the zebrafish TL in driving binocular integration and spatial summation in tectal PyrNs. The neural circuit we describe generates a population of tectal neurons with large receptive fields tailored for detecting changes in the visual scene.
Highlights
A continued effort in neuroscience aims to understand the way brain circuits consisting of diverse neuronal types generate complex behavior following sensory input
Using the hspGGFF23c transgenic to drive GCaMP6s expression in stratum marginale (SM)-projecting torus longitudinalis (TL) neurons (SMTLs), we found that the majority of SMTL neurons had Receptive field (RF) that included regions of both retinal hemifields (56 of 59; Fig. 7G, H), additional evidence that binocular integration occurs in TL
Several lines of evidence suggest that spatial summation in this circuit is driven by neural convergence at the TL-pyramidal neurons (PyrNs) synapse
Summary
A continued effort in neuroscience aims to understand the way brain circuits consisting of diverse neuronal types generate complex behavior following sensory input. Optic tectum is the largest first-order visual brain area in zebrafish and is reciprocally connected with the torus longitudinalis (TL), a second-order visual brain area that does not receive retinal input. The zebrafish optic tectum is a powerful system to study how the brain processes visual inputs from the retina. In the adult zebrafish tectum single neuron recordings previously identified visually responsive neurons with large, compound receptive fields (RFs) [6]. These compound RFs consisted of multiple, noncontiguous regions of visual space. Do compound RFs in tectum arise via convergent feedback projections from a higher order visual area? Does tectum contain neurons that are Tesmer et al BMC Biology (2022) 20:24 innervated by RGCs with distant and nonoverlapping RFs? Alternatively, do compound RFs in tectum arise via convergent feedback projections from a higher order visual area?
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