Danilova Marina V, Bondarko Valeria M
Laboratory of Visual Physiology, I. P. Pavlov Institute of Physiology RAS, St. Petersburg, Russia.
J Vis. 2007 Nov 27;7(2):25.1-18. doi: 10.1167/7.2.25.
We describe several experiments on contour interactions and crowding effects at the resolution limit of the visual system. As test stimuli we used characters that are often employed in optometric practice for testing visual acuity: Landolt C's, Snellen E's, and rectangular gratings. We tested several hypotheses that have been put forward to explain contour interaction and crowding effects. In Experiment 1 and Experiment 2, Landolt C's were the test stimuli, and bars, or Landolt C's, or gratings served as distractors. In Experiment 1, we showed that neither scale invariance nor spatial frequency selectivity is a characteristic of foveal crowding effects. These results allowed us to conclude that mechanisms other than lateral masking contribute to observers' performance in 'crowded' tasks. R. F. Hess, S. C. Dakin, and N. Kappor (2000) suggested that the spatial frequency band most appropriate for target recognition is shifted by the surrounding bars to higher spatial frequencies that cannot be resolved by observers. Our Experiment 2 rejects this hypothesis as the experimental data do not follow theoretical predictions. In Experiment 3, we employed Snellen E's, both as test stimuli and as distractors. The masking functions were similar to those measured in Experiment 1 when the test Landolt C was surrounded by Landolt C's. In Experiment 4, we extended the range of test stimuli to rectangular gratings; same-frequency or high-frequency gratings were distractors. In this case, if the distracting gratings had random orientation from trial to trial, the critical spacing was twice larger than in the first three experiments. If the orientation of the distractors was fixed during the whole experiment, the critical spacing was similar to that measured in the first three experiments. We suggest that the visual system can use different mechanisms for the discrimination of different test stimuli in the presence of particular surround. Different receptive fields with different spatial characteristics can be employed. To explain why crowding effects at the resolution limit of the visual system are not scale invariant, we suggest that a range of stimuli, slightly varying in size, may all be processed by the same neural channel--the channel with the smallest receptive fields of the visual system.
我们描述了在视觉系统分辨率极限下进行的几项关于轮廓相互作用和拥挤效应的实验。作为测试刺激，我们使用了验光实践中常用于测试视力的字符：兰道环（Landolt C）、斯内伦视标（Snellen E）和矩形光栅。我们测试了为解释轮廓相互作用和拥挤效应而提出的几种假设。在实验1和实验2中，兰道环作为测试刺激，而条形、兰道环或光栅用作干扰物。在实验1中，我们表明尺度不变性和空间频率选择性都不是中央凹拥挤效应的特征。这些结果使我们能够得出结论，除了侧向掩蔽之外的其他机制也会影响观察者在“拥挤”任务中的表现。R. F. 赫斯（R. F. Hess）、S. C. 达金（S. C. Dakin）和N. 卡波尔（N. Kappor）（2000年）提出，最适合目标识别的空间频率带会被周围的条形移至观察者无法分辨的更高空间频率。我们的实验2否定了这一假设，因为实验数据并不符合理论预测。在实验3中，我们使用斯内伦视标作为测试刺激和干扰物。当测试兰道环被兰道环包围时，掩蔽函数与实验1中测得的相似。在实验4中，我们将测试刺激的范围扩展到矩形光栅；同频率或高频光栅用作干扰物。在这种情况下，如果每次试验中干扰光栅的方向是随机的，临界间距比前三个实验中的大两倍。如果在整个实验过程中干扰物的方向是固定的，临界间距与前三个实验中测得的相似。我们认为，在存在特定背景的情况下，视觉系统可以使用不同的机制来区分不同的测试刺激。可以采用具有不同空间特征的不同感受野。为了解释为什么在视觉系统分辨率极限下的拥挤效应不是尺度不变的，我们提出，一系列大小略有不同的刺激可能都由同一神经通道处理——即视觉系统中具有最小感受野的通道。
