Brain Research Unit and MEG Core, O.V. Lounasmaa Laboratory, School of Science, Aalto University, FI-00076 Aalto, Espoo, Finland.
J Neurosci. 2013 May 1;33(18):7691-9. doi: 10.1523/JNEUROSCI.3905-12.2013.
Current knowledge about the precise timing of visual input to the cortex relies largely on spike timings in monkeys and evoked-response latencies in humans. However, quantifying the activation onset does not unambiguously describe the timing of stimulus-feature-specific information processing. Here, we investigated the information content of the early human visual cortical activity by decoding low-level visual features from single-trial magnetoencephalographic (MEG) responses. MEG was measured from nine healthy subjects as they viewed annular sinusoidal gratings (spanning the visual field from 2 to 10° for a duration of 1 s), characterized by spatial frequency (0.33 cycles/degree or 1.33 cycles/degree) and orientation (45° or 135°); gratings were either static or rotated clockwise or anticlockwise from 0 to 180°. Time-resolved classifiers using a 20 ms moving window exceeded chance level at 51 ms (the later edge of the window) for spatial frequency, 65 ms for orientation, and 98 ms for rotation direction. Decoding accuracies of spatial frequency and orientation peaked at 70 and 90 ms, respectively, coinciding with the peaks of the onset evoked responses. Within-subject time-insensitive pattern classifiers decoded spatial frequency and orientation simultaneously (mean accuracy 64%, chance 25%) and rotation direction (mean 82%, chance 50%). Classifiers trained on data from other subjects decoded the spatial frequency (73%), but not the orientation, nor the rotation direction. Our results indicate that unaveraged brain responses contain decodable information about low-level visual features already at the time of the earliest cortical evoked responses, and that representations of spatial frequency are highly robust across individuals.
目前，关于视觉输入到大脑皮层的确切时间的知识在很大程度上依赖于猴子的尖峰时间和人类的诱发反应潜伏期。然而，量化激活起始时间并不能明确描述刺激特征特定信息处理的时间。在这里，我们通过从单次脑磁图（MEG）响应中解码低级视觉特征来研究早期人类视觉皮层活动的信息含量。我们在九名健康受试者观看环形正弦光栅（在视觉场中从 2 度到 10 度持续 1 秒）时测量了 MEG，光栅的空间频率（0.33 个周期/度或 1.33 个周期/度）和方向（45°或 135°）；光栅要么是静止的，要么从 0 度顺时针或逆时针旋转到 180 度。使用 20 毫秒移动窗口的时间分辨分类器在 51 毫秒（窗口的较晚边缘）处的空间频率，65 毫秒处的方向和 98 毫秒处的旋转方向超过了机会水平。空间频率和方向的解码精度分别在 70 和 90 毫秒达到峰值，与起始诱发反应的峰值相对应。在不受时间影响的个体内模式分类器同时解码空间频率和方向（平均准确率为 64%，机会为 25%）和旋转方向（平均准确率为 82%，机会为 50%）。从其他受试者的数据中训练的分类器可以解码空间频率（73%），但不能解码方向，也不能解码旋转方向。我们的结果表明，未平均的大脑反应已经在最早的皮层诱发反应时包含了可解码的低级视觉特征信息，并且空间频率的表示在个体之间具有高度的稳健性。
