Adaptive feature detection from differential processing in parallel retinal pathways

To transmit information efficiently in a changing environment, the retina adapts to visual contrast by adjusting its gain, latency and mean response. Additionally, the temporal frequency selectivity, or bandwidth changes to encode the absolute intensity when the stimulus environment is noisy, and intensity differences when noise is low. We show that the On pathway of On-Off retinal amacrine and ganglion cells is required to change temporal bandwidth but not other adaptive properties. This remarkably specific adaptive mechanism arises from differential effects of contrast on the On and Off pathways. We analyzed a biophysical model fit only to a cell’s membrane potential, and verified pharmacologically that it accurately revealed the two pathways. We conclude that changes in bandwidth arise mostly from differences in synaptic threshold in the two pathways, rather than synaptic release dynamics as has previously been proposed to underlie contrast adaptation. Different efficient codes are selected by different thresholds in two independently adapting neural pathways.为在动态变化的环境中高效传递信息，视网膜会通过调整自身增益、潜伏期与平均响应水平，适配视觉对比度的变化。此外，当刺激环境存在噪声时，其时间频率选择性（即带宽）会调整以编码绝对光强；而当噪声水平较低时，则转而编码光强差异。我们证实，开-关型视网膜无长突细胞与神经节细胞的给光通路（On pathway）是调控时间带宽变化的必要结构，但并不参与其他自适应特性的调节。这种极具特异性的自适应机制，源于对比度对给光通路与撤光通路（Off pathway）的差异化影响。我们采用了仅基于单个细胞膜电位拟合得到的生物物理模型，并通过药理学实验验证了该模型能够准确区分两条视觉通路。我们的结论是，时间带宽的变化主要源于两条通路间的突触阈值差异，而非此前被提出作为对比度自适应基础的突触释放动力学机制。两条独立自适应的神经通路通过各自不同的阈值，选择了适配不同场景的高效编码方式。