Medical Research Council, Cognition and Brain Sciences Unit Cambridge, UK.
Front Psychol. 2013 Aug 12;4:493. doi: 10.3389/fpsyg.2013.00493. eCollection 2013.
The orientation of a large grating can be decoded from V1 functional magnetic resonance imaging (fMRI) data, even at low resolution (3-mm isotropic voxels). This finding has suggested that columnar-level neuronal information might be accessible to fMRI at 3T. However, orientation decodability might alternatively arise from global orientation-preference maps. Such global maps across V1 could result from bottom-up processing, if the preferences of V1 neurons were biased toward particular orientations (e.g., radial from fixation, or cardinal, i.e., vertical or horizontal). Global maps could also arise from local recurrent or top-down processing, reflecting pre-attentive perceptual grouping, attention spreading, or predictive coding of global form. Here we investigate whether fMRI orientation decoding with 2-mm voxels requires (a) globally coherent orientation stimuli and/or (b) global-scale patterns of V1 activity. We used opposite-orientation gratings (balanced about the cardinal orientations) and spirals (balanced about the radial orientation), along with novel patch-swapped variants of these stimuli. The two stimuli of a patch-swapped pair have opposite orientations everywhere (like their globally coherent parent stimuli). However, the two stimuli appear globally similar, a patchwork of opposite orientations. We find that all stimulus pairs are robustly decodable, demonstrating that fMRI orientation decoding does not require globally coherent orientation stimuli. Furthermore, decoding remained robust after spatial high-pass filtering for all stimuli, showing that fine-grained components of the fMRI patterns reflect visual orientations. Consistent with previous studies, we found evidence for global radial and vertical preference maps in V1. However, these were weak or absent for patch-swapped stimuli, suggesting that global preference maps depend on globally coherent orientations and might arise through recurrent or top-down processes related to the perception of global form.
大光栅的方向可以从 V1 功能磁共振成像 (fMRI) 数据中解码，即使分辨率较低（3 毫米各向同性体素）。这一发现表明，柱状水平的神经元信息可能可以通过 3T 的 fMRI 获得。然而，方向可解码性也可能源自全局方向偏好图。如果 V1 神经元的偏好偏向特定方向（例如，从注视点发出的放射状，或 cardinal，即垂直或水平），那么这种 across V1 的全局地图可能来自于自下而上的处理。全局地图也可能来自于局部递归或自上而下的处理，反映了非注意的知觉分组、注意力扩散或全局形状的预测编码。在这里，我们研究了 2 毫米体素的 fMRI 方向解码是否需要 (a) 全局一致的方向刺激和/或 (b) V1 活动的全局尺度模式。我们使用了相反方向的光栅（与 cardinal 方向平衡）和螺旋（与 radial 方向平衡），以及这些刺激的新颖的补丁交换变体。一对补丁交换刺激的两个刺激在每个位置都具有相反的方向（就像它们的全局一致的母体刺激一样）。然而，两个刺激看起来全局相似，是一个具有相反方向的补丁拼贴。我们发现所有刺激对都可以稳健地解码，这表明 fMRI 方向解码不需要全局一致的方向刺激。此外，对于所有刺激，在进行空间高通滤波后，解码仍然稳健，这表明 fMRI 模式的细粒度成分反映了视觉方向。与之前的研究一致，我们在 V1 中发现了全局径向和垂直偏好图的证据。然而，对于补丁交换刺激，这些图的强度很弱或不存在，这表明全局偏好图取决于全局一致的方向，并且可能通过与全局形状感知相关的递归或自上而下的过程产生。
