Gomez Daniel E P, Polimeni Jonathan R, Lewis Laura D
Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston, MA, United States.
Department of Radiology, Harvard Medical School, Boston, MA, United States.
Imaging Neurosci (Camb). 2024 Dec 16;2. doi: 10.1162/imag_a_00399. eCollection 2024.
The ability to detect fast responses with functional MRI depends on the speed of hemodynamic responses to neural activity, because hemodynamic responses act as a temporal low-pass filter which blurs rapid changes. However, the shape and timing of hemodynamic responses are highly variable across the brain and across stimuli. This heterogeneity of responses implies that the temporal specificity of functional MRI (fMRI) signals, or the ability of fMRI to preserve fast information, could also vary substantially across the cortex. In this work we investigated how local differences in hemodynamic response timing affect the temporal specificity of fMRI. We used ultra-high-field (7T) fMRI at high spatiotemporal resolution, studying the primary visual cortex (V1) as a model area for investigation. We used visual stimuli oscillating at slow and fast frequencies to probe the temporal specificity of individual voxels. As expected, we identified substantial variability in temporal specificity, with some voxels preserving their responses to fast neural activity more effectively than others. We investigated which voxels had the highest temporal specificity, and tested whether voxel timing was related to anatomical and vascular features. We found that low temporal specificity is only weakly explained by the presence of large veins or cerebral cortical depth. Notably, however, temporal specificity depended strongly on a voxel's position along the anterior-posterior anatomical axis of V1, with voxels within the calcarine sulcus being capable of preserving close to 25% of their amplitude as the frequency of stimulation increased from 0.05 Hz to 0.20 Hz, and voxels nearest to the occipital pole preserving less than 18%. These results indicate that detection biases in high-resolution fMRI will depend on the anatomical and vascular features of the area being imaged, and that these biases will differ depending on the timing of the underlying neuronal activity. While we attribute this variance primarily to hemodynamic effects, neuronal non-linearities may also influence response timing. Importantly, this spatial heterogeneity of temporal specificity suggests that it could be exploited to achieve higher specificity in some locations, and that tailored data analysis strategies may help improve the detection and interpretation of fast fMRI responses.
利用功能磁共振成像（fMRI）检测快速反应的能力取决于血液动力学对神经活动反应的速度，因为血液动力学反应起到了时间低通滤波器的作用，会模糊快速变化。然而，血液动力学反应的形状和时间在整个大脑和不同刺激下具有高度变异性。这种反应的异质性意味着功能磁共振成像（fMRI）信号的时间特异性，即fMRI保留快速信息的能力，在整个皮层中也可能有很大差异。在这项研究中，我们调查了血液动力学反应时间的局部差异如何影响fMRI的时间特异性。我们使用了高时空分辨率的超高场（7T）fMRI，将初级视觉皮层（V1）作为研究的模型区域。我们使用了以慢频率和快频率振荡的视觉刺激来探测各个体素的时间特异性。正如预期的那样，我们发现时间特异性存在很大差异，一些体素比其他体素更有效地保留了对快速神经活动的反应。我们研究了哪些体素具有最高的时间特异性，并测试了体素时间是否与解剖和血管特征相关。我们发现，大静脉的存在或大脑皮层深度对低时间特异性的解释作用较弱。然而，值得注意的是，时间特异性强烈依赖于体素在V1前后解剖轴上的位置，随着刺激频率从0.05Hz增加到0.20Hz，距状沟内的体素能够保留近25%的振幅，而最靠近枕极的体素保留的振幅不到18%。这些结果表明，高分辨率fMRI中的检测偏差将取决于成像区域的解剖和血管特征，并且这些偏差会因潜在神经元活动的时间不同而有所差异。虽然我们将这种差异主要归因于血液动力学效应，但神经元非线性也可能影响反应时间。重要的是，这种时间特异性的空间异质性表明，可以利用它在某些位置实现更高的特异性，并且量身定制的数据分析策略可能有助于改善对快速fMRI反应的检测和解释。
