Kingdom Frederick A A, Prins Nicolaas, Hayes Anthony
McGill Vision Research Unit, Department of Ophthalmology, McGill University, Montreal, Quebec, Canada.
Vis Neurosci. 2003 Jan-Feb;20(1):65-76. doi: 10.1017/s0952523803201073.
The ability of the visual system to detect stimuli that vary along dimensions other than luminance or color--"second-order" stimuli--has been of considerable interest in recent years. An important unresolved issue is whether different types of second-order stimuli are detected by a single, all purpose, mechanism, or by mechanisms that are specific to stimulus type. Using a conventional psychophysical paradigm, we show that for a class of second-order stimuli--textures sinusoidally modulated in orientation (OM), spatial frequency (FM), and contrast (CM)--the human visual system employs mechanisms that are selective to stimulus type. Whereas the addition of a subthreshold mask to a test pattern of the same stimulus type was found to facilitate the detection of the test, no facilitation was observed when mask and test were of different types, suggesting mechanism independence for the different types of stimulus. This finding raises the important question of whether mechanism independence is compatible with the well-known filter-rectify-filter (FRF) model of second-order stimulus detection, since FRF mechanisms, in principle, do not discriminate between stimulus types. We show that for all mask/test combinations except those with CM masks, the FRF mechanism giving the largest response to the test modulation is largely unaffected by subthreshold levels of a different stimulus-type mask. For this reason, we cannot rule out the possibility that FRF mechanisms mediate the detection of our stimuli. For combinations involving CM masks, however, we propose that a process of contrast normalization renders the test stimulus insensitive to the mask stimulus.
近年来，视觉系统检测除亮度或颜色维度之外其他维度变化的刺激——“二阶”刺激的能力备受关注。一个重要的未解决问题是，不同类型的二阶刺激是由单一的通用机制检测，还是由特定于刺激类型的机制检测。使用传统的心理物理学范式，我们表明，对于一类二阶刺激——在方向（OM）、空间频率（FM）和对比度（CM）上进行正弦调制的纹理——人类视觉系统采用了对刺激类型具有选择性的机制。虽然发现向相同刺激类型的测试图案添加阈下掩蔽会促进测试的检测，但当掩蔽和测试为不同类型时，未观察到促进作用，这表明不同类型刺激的机制具有独立性。这一发现提出了一个重要问题，即机制独立性是否与二阶刺激检测的著名的滤波器 - 整流 - 滤波器（FRF）模型兼容，因为FRF机制原则上不区分刺激类型。我们表明，对于除了那些带有CM掩蔽的所有掩蔽/测试组合，对测试调制给出最大响应的FRF机制在很大程度上不受不同刺激类型掩蔽的阈下水平的影响。因此，我们不能排除FRF机制介导我们的刺激检测的可能性。然而，对于涉及CM掩蔽的组合，我们提出对比度归一化过程使测试刺激对掩蔽刺激不敏感。
