Center for Magnetic Resonance Research (CMRR), University of Minnesota, MN 55455, United States; Department of Radiology, University of Minnesota, MN 55455, United States.
Center for Magnetic Resonance Research (CMRR), University of Minnesota, MN 55455, United States; Department of Neuroscience, University of Minnesota, MN 55455, United States.
Neuroimage. 2022 May 1;251:118978. doi: 10.1016/j.neuroimage.2022.118978. Epub 2022 Feb 7.
The mammalian neocortex exhibits a stereotypical laminar organization, with feedforward inputs arriving primarily into layer 4, local computations shaping response selectivity in layers 2/3, and outputs to other brain areas emanating via layers 2/3, 5 and 6. It cannot be assumed a priori that these signatures of laminar differences in neuronal circuitry are reflected in hemodynamic signals that form the basis of functional magnetic resonance imaging (fMRI). Indeed, optical imaging of single-vessel functional responses has highlighted the potential limits of using vascular signals as surrogates for mapping the selectivity of neural responses. Therefore, before fMRI can be employed as an effective tool for studying critical aspects of laminar processing, validation with single-vessel resolution is needed. The primary visual cortex (V1) in cats, with its precise neuronal functional micro-architecture, offers an ideal model system to examine laminar differences in stimulus selectivity across imaging modalities. Here we used cerebral blood volume weighted (wCBV) fMRI to examine if layer-specific orientation-selective responses could be detected in cat V1. We found orientation preference maps organized tangential to the cortical surface that typically extended across depth in a columnar fashion. We then examined arterial dilation and blood velocity responses to identical visual stimuli by using two- and three- photon optical imaging at single-vessel resolution-which provides a measure of the hemodynamic signals with the highest spatial resolution. Both fMRI and optical imaging revealed a consistent laminar response pattern in which orientation selectivity in cortical layer 4 was significantly lower compared to layer 2/3. This systematic change in selectivity across cortical layers has a clear underpinning in neural circuitry, particularly when comparing layer 4 to other cortical layers.
哺乳动物的新皮层表现出典型的分层组织，前馈输入主要到达第 4 层，局部计算在第 2/3 层塑造响应选择性，输出到其他脑区则通过第 2/3、5 和 6 层发出。不能先验地假设神经元电路分层差异的这些特征反映在形成功能磁共振成像 (fMRI) 基础的血流信号中。事实上，单血管功能反应的光学成像强调了使用血管信号作为映射神经反应选择性的替代物的潜在限制。因此，在 fMRI 可以作为研究分层处理关键方面的有效工具之前，需要进行单血管分辨率的验证。猫的初级视觉皮层 (V1) 具有精确的神经元功能微结构，是检查成像模式下刺激选择性的分层差异的理想模型系统。在这里，我们使用脑血流加权 (wCBV) fMRI 来检查是否可以在猫的 V1 中检测到特定于层的方位选择性反应。我们发现方位偏好图沿着皮质表面呈切线组织，通常以柱状方式延伸到深度。然后，我们使用双光子和三光子光学成像在单血管分辨率下检查了相同视觉刺激的动脉扩张和血流速度反应-这提供了具有最高空间分辨率的血流信号的测量。fMRI 和光学成像都揭示了一种一致的分层反应模式，其中皮质第 4 层的方位选择性明显低于第 2/3 层。这种在皮质层之间选择性的系统变化在神经回路中有明确的基础，特别是在将第 4 层与其他皮质层进行比较时。
