Department of Neuroscience, Baylor College of Medicine, Houston, Texas 77030, USA.
J Neurosci. 2012 Apr 4;32(14):4923-34. doi: 10.1523/JNEUROSCI.5777-11.2012.
Neurons in a variety of species, both vertebrate and invertebrate, encode the kinematics of objects approaching on a collision course through a time-varying firing rate profile that initially increases, then peaks, and eventually decays as collision becomes imminent. In this temporal profile, the peak firing rate signals when the approaching object's subtended size reaches an angular threshold, an event which has been related to the timing of escape behaviors. In a locust neuron called the lobula giant motion detector (LGMD), the biophysical basis of this angular threshold computation relies on a multiplicative combination of the object's angular size and speed, achieved through a logarithmic-exponential transform. To understand how this transform is implemented, we modeled the encoding of angular velocity along the pathway leading to the LGMD based on the experimentally determined activation pattern of its presynaptic neurons. These simulations show that the logarithmic transform of angular speed occurs between the synaptic conductances activated by the approaching object onto the LGMD's dendritic tree and its membrane potential at the spike initiation zone. Thus, we demonstrate an example of how a single neuron's dendritic tree implements a mathematical step in a neural computation important for natural behavior.
各种物种的神经元，包括脊椎动物和无脊椎动物，通过时变的放电率模式来编码接近碰撞物体的运动学，该模式最初增加，然后达到峰值，最后随着碰撞的临近而衰减。在这个时间模式中，峰值放电率信号表示接近物体的遮挡大小达到一个角度阈值，这个事件与逃避行为的时间有关。在一种叫做叶状巨动作探测器（LGMD）的蝗虫神经元中，这种角度阈值计算的生物物理基础依赖于物体的角度大小和速度的乘法组合，这是通过对数-指数变换实现的。为了了解这种变换是如何实现的，我们根据其前突触神经元的实验确定的激活模式，对 LGMD 通向路径上的角速度编码进行了建模。这些模拟表明，角速度的对数变换发生在接近物体激活的突触电导和 LGMD 的树突棘膜电位之间。因此，我们展示了一个例子，说明单个神经元的树突棘如何实现对自然行为很重要的神经计算中的一个数学步骤。
