Department of Neurology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115, and
Jamaica Plain Veterans Administration Hospital, Boston, Massachusetts, 02130.
J Neurosci. 2019 Feb 27;39(9):1671-1687. doi: 10.1523/JNEUROSCI.2012-18.2019. Epub 2019 Jan 15.
Neurons in primary visual cortex are strongly modulated both by stimulus contrast and by fluctuations of internal inputs. An important question is whether the population code is preserved under these conditions. Changes in stimulus contrast are thought to leave the population code invariant, whereas the effect of internal gain modulations remains unknown. To address these questions we studied how the direction-of-motion of oriented gratings is encoded in layer 2/3 primary visual cortex of mouse (with C57BL/6 background, of either sex). We found that, because contrast gain responses across cells are heterogeneous, a change in contrast alters the information distribution profile across cells leading to a violation of contrast invariance. Remarkably, internal input fluctuations that cause commensurate firing rate modulations at the single-cell level result in more homogeneous gain responses, respecting population code invariance. These observations argue that the brain strives to maintain the stability of the neural code in the face of fluctuating internal inputs. Neuronal responses are modulated both by stimulus contrast and by the spontaneous fluctuation of internal inputs. It is not well understood how these different types of input impact the population code. Specifically, it is important to understand whether the neural code stays invariant in the face of significant internal input modulations. Here, we show that changes in stimulus contrast lead to different optimal population codes, whereas spontaneous internal input fluctuations leave the population code invariant. This is because spontaneous internal input fluctuations modulate the gain of neuronal responses more homogeneously across cells compared to changes in stimulus contrast.
初级视觉皮层中的神经元受到刺激对比度和内部输入波动的强烈调节。一个重要的问题是，在这些条件下，群体编码是否得到保留。刺激对比度的变化被认为不会改变群体编码，而内部增益调制的影响仍不清楚。为了解决这些问题，我们研究了在具有 C57BL/6 背景（无论性别）的小鼠初级视觉皮层 2/3 层中，定向光栅的运动方向是如何编码的。我们发现，由于细胞间对比度增益反应的异质性，对比度的变化改变了细胞间的信息分布轮廓，导致对比度不变性的破坏。值得注意的是，导致单细胞水平上放电率调制相当的内部输入波动会导致更均匀的增益反应，从而尊重群体编码不变性。这些观察结果表明，大脑在面对波动的内部输入时，努力保持神经编码的稳定性。神经元的反应受到刺激对比度和内部输入自发波动的调节。不同类型的输入如何影响群体编码还不太清楚。具体来说，了解在内部输入发生重大调制时，神经编码是否保持不变是很重要的。在这里，我们表明刺激对比度的变化导致不同的最优群体编码，而自发的内部输入波动使群体编码保持不变。这是因为与刺激对比度的变化相比，自发的内部输入波动更均匀地调节神经元反应的增益。
