Einhäuser Wolfgang, Rutishauser Ueli, Koch Christof
Division of Biology, California Institute of Technology, Pasadena, CA, USA.
J Vis. 2008 Feb 15;8(2):2.1-19. doi: 10.1167/8.2.2.
In natural vision both stimulus features and task-demands affect an observer's attention. However, the relationship between sensory-driven ("bottom-up") and task-dependent ("top-down") factors remains controversial: Can task-demands counteract strong sensory signals fully, quickly, and irrespective of bottom-up features? To measure attention under naturalistic conditions, we recorded eye-movements in human observers, while they viewed photographs of outdoor scenes. In the first experiment, smooth modulations of contrast biased the stimuli's sensory-driven saliency towards one side. In free-viewing, observers' eye-positions were immediately biased toward the high-contrast, i.e., high-saliency, side. However, this sensory-driven bias disappeared entirely when observers searched for a bull's-eye target embedded with equal probability to either side of the stimulus. When the target always occurred in the low-contrast side, observers' eye-positions were immediately biased towards this low-saliency side, i.e., the sensory-driven bias reversed. Hence, task-demands do not only override sensory-driven saliency but also actively countermand it. In a second experiment, a 5-Hz flicker replaced the contrast gradient. Whereas the bias was less persistent in free viewing, the overriding and reversal took longer to deploy. Hence, insufficient sensory-driven saliency cannot account for the bias reversal. In a third experiment, subjects searched for a spot of locally increased contrast ("oddity") instead of the bull's-eye ("template"). In contrast to the other conditions, a slight sensory-driven free-viewing bias prevails in this condition. In a fourth experiment, we demonstrate that at known locations template targets are detected faster than oddity targets, suggesting that the former induce a stronger top-down drive when used as search targets. Taken together, task-demands can override sensory-driven saliency in complex visual stimuli almost immediately, and the extent of overriding depends on the search target and the overridden feature, but not on the latter's free-viewing saliency.
在自然视觉中，刺激特征和任务需求都会影响观察者的注意力。然而，感觉驱动（“自下而上”）和任务依赖（“自上而下”）因素之间的关系仍存在争议：任务需求能否完全、迅速且不受自下而上特征的影响地抵消强烈的感觉信号？为了测量自然条件下的注意力，我们记录了人类观察者观看户外场景照片时的眼动情况。在第一个实验中，对比度的平滑调制使刺激的感觉驱动显著性偏向一侧。在自由观看时，观察者的眼睛位置立即偏向高对比度，即高显著性的一侧。然而，当观察者搜索在刺激两侧以相等概率嵌入的靶心目标时，这种感觉驱动的偏向完全消失了。当目标总是出现在低对比度一侧时，观察者的眼睛位置立即偏向这个低显著性一侧，即感觉驱动的偏向发生了反转。因此，任务需求不仅会覆盖感觉驱动的显著性，还会积极地撤销它。在第二个实验中，用5赫兹的闪烁代替了对比度梯度。虽然在自由观看时偏向的持续性较差，但覆盖和反转的过程需要更长时间才能展开。因此，感觉驱动显著性不足并不能解释偏向反转。在第三个实验中，受试者搜索局部对比度增加的点（“奇异点”）而不是靶心（“模板”）。与其他条件不同，在这种情况下存在轻微的感觉驱动自由观看偏向。在第四个实验中，我们证明在已知位置，模板目标比奇异点目标被检测得更快，这表明当用作搜索目标时，前者会引发更强的自上而下驱动。综上所述，任务需求几乎可以立即覆盖复杂视觉刺激中的感觉驱动显著性，覆盖程度取决于搜索目标和被覆盖的特征，而不取决于后者的自由观看显著性。
