Image motion with color contrast suffices to elicit an optokinetic reflex in Xenopus laevis tadpoles

Asymmetric retinal direction tuning predicts optokinetic eye movements across stimulus conditions

Across species, the optokinetic reflex (OKR) stabilizes vision during self-motion. OKR occurs when ON direction-selective retinal ganglion cells (oDSGCs) detect slow, global image motion on the retina. How oDSGC activity is integrated centrally to generate behavior remains unknown. Here, we discover mechanisms that contribute to motion encoding in vertically tuned oDSGCs and leverage these findings to empirically define signal transformation between retinal output and vertical OKR behavior. We demonstrate that motion encoding in vertically tuned oDSGCs is contrast-sensitive and asymmetric for oDSGC types that prefer opposite directions. These phenomena arise from the interplay between spike threshold nonlinearities and differences in synaptic input weights, including shifts in the balance of excitation and inhibition. In behaving mice, these neurophysiological observations, along with a central subtraction of oDSGC outputs, accurately predict the trajectories of vertical OKR across stimulus conditions. Thus, asymmetric tuning across competing sensory channels can critically shape behavior.

Homeostatic plasticity of eye movement performance in Xenopus tadpoles following prolonged visual image motion stimulation

Visual image motion-driven ocular motor behaviors such as the optokinetic reflex (OKR) provide sensory feedback for optimizing gaze stability during head/body motion. The performance of this visuo-motor reflex is subject to plastic alterations depending on requirements imposed by specific eco-physiological or developmental circumstances. While visuo-motor plasticity can be experimentally induced by various combinations of motion-related stimuli, the extent to which such evoked behavioral alterations contribute to the behavioral demands of an environment remains often obscure. Here, we used isolated preparations of Xenopus laevis tadpoles to assess the extent and ontogenetic dependency of visuo-motor plasticity during prolonged visual image motion. While a reliable attenuation of large OKR amplitudes can be induced already in young larvae, a robust response magnitude-dependent bidirectional plasticity is present only at older developmental stages. The possibility of older larvae to faithfully enhance small OKR amplitudes coincides with the developmental maturation of inferior olivary-Purkinje cell signal integration. This conclusion was supported by the loss of behavioral plasticity following transection of the climbing fiber pathway and by the immunohistochemical demonstration of a considerable volumetric extension of the Purkinje cell dendritic area between the two tested stages. The bidirectional behavioral alterations with different developmental onsets might functionally serve to standardize the motor output, comparable to the known differential adaptability of vestibulo-ocular reflexes in these animals. This homeostatic plasticity potentially equilibrates the working range of ocular motor behaviors during altered visuo-vestibular conditions or prolonged head/body motion to fine-tune resultant eye movements.