Ramachandran V S, Cobb S
Brain and Perception Laboratory, University of California at San Diego, La Jolla 92093-0109.
Nature. 1995 Jan 5;373(6509):66-8. doi: 10.1038/373066a0.
How does the human visual system 'bind' different fragments in the visual scene to create enduring representations of objects? A visual illusion known as 'metacontrast' or backward masking provides compelling evidence that perception is not instantaneous and that it occurs sequentially in distinct stages. If a solid white target square is displayed for 50 ms in a tachistoscope, switched off, and followed by a 50 ms display of two flanking mask squares, remarkably, subjects report seeing only the two flanking squares: the first square is simply not 'seen'. By plotting the magnitude of masking as a function of the delay between the target and mask (the stimulus onset asynchrony), one can obtain a characteristic 'U'-shaped function with optimum masking occurring at about 50 ms, and no masking with synchronous target and mask presentations or at delays higher than 300 ms. The illusion is also highly sensitive to elementary stimulus dimensions such as colour, orientation and spatial frequency, and it has been suggested that it is based on 'low level' autonomous visual mechanisms rather than cognitive processes. Here we describe a novel visual stimulus that demonstrates that metacontrast can be strongly modulated by 'top down' influence such as voluntary visual attention.
人类视觉系统是如何将视觉场景中的不同片段“绑定”在一起，以创建物体的持久表征的？一种被称为“元对比”或逆向掩蔽的视觉错觉提供了令人信服的证据，表明感知并非瞬间发生，而是在不同阶段依次进行。如果在速示器中显示一个实心白色目标方块50毫秒，然后关闭，接着显示两个侧翼掩蔽方块50毫秒，那么值得注意的是，受试者报告只看到了两个侧翼方块：第一个方块根本没有“被看到”。通过绘制掩蔽强度作为目标与掩蔽之间延迟（刺激开始异步）的函数，可以得到一个特征性的“U”形函数，最佳掩蔽出现在大约50毫秒时，当目标与掩蔽同步呈现或延迟高于300毫秒时则没有掩蔽。这种错觉对颜色、方向和空间频率等基本刺激维度也高度敏感，并且有人认为它基于“低级”自主视觉机制而非认知过程。在这里，我们描述了一种新颖的视觉刺激，它表明元对比可以受到诸如自愿视觉注意等“自上而下”影响的强烈调制。
