Rainer G, Augath M, Trinath T, Logothetis N K
Max-Planck-Institute of Biological Cybernetics, Spemannstrasse 38, D-72076, Tübingen, Germany.
Curr Biol. 2001 Jun 5;11(11):846-54. doi: 10.1016/s0960-9822(01)00242-1.
The perceptual ability of humans and monkeys to identify objects in the presence of noise varies systematically and monotonically as a function of how much noise is introduced to the visual display. That is, it becomes more and more difficult to identify an object with increasing noise. Here we examine whether the blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) signal in anesthetized monkeys also shows such monotonic tuning. We employed parametric stimulus sets containing natural images and noise patterns matched for spatial frequency and intensity as well as intermediate images generated by interpolation between natural images and noise patterns. Anesthetized monkeys provide us with the unique opportunity to examine visual processing largely in the absence of top-down cognitive modulations and can thus provide an important baseline against which work with awake monkeys and humans can be compared.
We measured BOLD activity in occipital visual cortical areas as natural images and noise patterns, as well as intermediate interpolated patterns at three interpolation levels (25%, 50%, and 75%) were presented to anesthetized monkeys in a block paradigm. We observed reliable visual activity in occipital visual areas including V1, V2, V3, V3A, and V4 as well as the fundus and anterior bank of the superior temporal sulcus (STS). Natural images consistently elicited higher BOLD levels than noise patterns. For intermediate images, however, we did not observe monotonic tuning. Instead, we observed a characteristic V-shaped noise-tuning function in primary and extrastriate visual areas. BOLD signals initially decreased as noise was added to the stimulus but then increased again as the pure noise pattern was approached. We present a simple model based on the number of activated neurons and the strength of activation per neuron that can account for these results.
We show that, for our parametric stimulus set, BOLD activity varied nonmonotonically as a function of how much noise was added to the visual stimuli, unlike the perceptual ability of humans and monkeys to identify such stimuli. This raises important caveats for interpreting fMRI data and demonstrates the importance of assessing not only which neural populations are activated by contrasting conditions during an fMRI study, but also the strength of this activation. This becomes particularly important when using the BOLD signal to make inferences about the relationship between neural activity and behavior.
人类和猴子在存在噪声的情况下识别物体的感知能力会随着视觉显示中引入的噪声量而系统且单调地变化。也就是说，随着噪声增加，识别物体变得越来越困难。在此，我们研究麻醉猴子的血氧水平依赖性功能磁共振成像（BOLD fMRI）信号是否也显示出这种单调调谐。我们采用了包含自然图像和在空间频率和强度上匹配的噪声模式的参数化刺激集，以及通过自然图像和噪声模式之间的插值生成的中间图像。麻醉的猴子为我们提供了一个独特的机会，在很大程度上可以在没有自上而下认知调制的情况下检查视觉处理，从而可以提供一个重要的基线，与清醒猴子和人类的研究进行比较。
我们在枕叶视觉皮层区域测量了BOLD活动，将自然图像、噪声模式以及三种插值水平（25%、50%和75%）的中间插值模式以组块范式呈现给麻醉的猴子。我们在枕叶视觉区域观察到可靠的视觉活动，包括V1、V2、V3、V3A和V4以及颞上沟（STS）的底部和前缘。自然图像始终比噪声模式引发更高的BOLD水平。然而，对于中间图像，我们没有观察到单调调谐。相反，我们在初级和纹外视觉区域观察到一种特征性的V形噪声调谐函数。BOLD信号最初随着噪声添加到刺激中而降低，但随后随着接近纯噪声模式而再次增加。我们提出了一个基于激活神经元数量和每个神经元激活强度的简单模型，可以解释这些结果。
我们表明，对于我们的参数化刺激集，BOLD活动作为添加到视觉刺激中的噪声量的函数呈现非单调变化，这与人类和猴子识别此类刺激的感知能力不同。这为解释fMRI数据提出了重要的注意事项，并证明了不仅要评估在fMRI研究中通过对比条件激活了哪些神经群体，还要评估这种激活的强度的重要性。当使用BOLD信号推断神经活动与行为之间的关系时，这一点变得尤为重要。
