Crowder N A, Hietanen M A, Price N S C, Clifford C W G, Ibbotson M R
Visual Sciences, Research School of Biological Sciences, Australian National University, Canberra, ACT, 2061, Australia.
J Physiol. 2008 Sep 1;586(17):4107-19. doi: 10.1113/jphysiol.2008.156273. Epub 2008 Jul 3.
During normal vision, objects moving in the environment, our own body movements and our eye movements ensure that the receptive fields of visual neurons are being presented with continually changing contrasts. Thus, the visual input during normal behaviour differs from the type of stimuli traditionally used to study contrast coding, which are presented in a step-like manner with abrupt changes in contrast followed by prolonged exposure to a constant stimulus. The abrupt changes in contrast typically elicit brief periods of intense firing with low variability called onset transients. Onset transients provide the visual system with a powerful and reliable cue that the visual input has changed. In this paper we investigate visual processing in the primary visual cortex of cats in response to stimuli that change contrast dynamically. We show that 1-4 s presentations of dynamic increases and decreases in contrast can generate stronger contrast gain control than several minutes exposure to a stimulus of constant contrast. Thus, transient mechanisms of contrast coding are not only less variable than sustained responses but are also more rapid and flexible. Finally, we propose a quantitative model of contrast coding which accounts for changes in spike rate over time in response to dynamically changing image contrast.
在正常视觉过程中，环境中移动的物体、我们自身的身体运动以及眼球运动确保了视觉神经元的感受野不断接收到变化的对比度。因此，正常行为期间的视觉输入不同于传统上用于研究对比度编码的刺激类型，传统刺激以阶梯状方式呈现，对比度有突然变化，随后长时间暴露于恒定刺激。对比度的突然变化通常会引发短暂的强烈放电期，且变化较小，称为起始瞬变。起始瞬变给视觉系统提供了一个强大而可靠的线索，表明视觉输入发生了变化。在本文中，我们研究了猫初级视觉皮层对动态变化对比度刺激的视觉处理。我们发现，对比度动态增加和减少的1 - 4秒呈现所产生的对比度增益控制比持续几分钟暴露于恒定对比度刺激更强。因此，对比度编码的瞬态机制不仅比持续反应变化更小，而且更快且更灵活。最后，我们提出了一个对比度编码的定量模型，该模型解释了响应动态变化的图像对比度时，放电率随时间的变化。
