Mullen Kathy T, Beaudot William H A, Ivanov Iliya V
McGill Vision Research, Department of Ophthalmology, McGill University, Montréal, Canada.
J Vis. 2011 Mar 8;11(3):6. doi: 10.1167/11.3.6.
In studies of shape processing, a crucial distinction is made between the global stages, which integrate across features to define shape, and earlier stages that encode individual components. We investigate whether shape discrimination thresholds for radial frequency (RF) patterns are limited at this global stage or whether the information in individual components supports threshold. We use achromatic and chromatic (L/M- and S-cone opponent) radial frequency (RF) patterns of different contour thicknesses (0.75-6 cpd). First, we show using sections of an RF4 that shape discrimination thresholds are invariant with cycle number from 1 to the complete pattern. Performance for a single cycle displayed alone is as good as for the whole RF, indicating that information within a single RF cycle is sufficient to support the whole shape discrimination threshold, arguing against an influence of global processing. Second, we find similar thresholds for the discrimination of RF patterns and modulated line stimuli, also arguing against global effects. Third, we calculate a metric for the intrinsic orientation variation in a stimulus cycle at threshold and show that this potentially accounts for the improvement in shape and line discrimination thresholds with modulation frequency from RF1 to RF6. Higher threshold discrimination for chromatic compared to achromatic patterns may reflect the poorer orientation discrimination of color vision, rather than a deficit for global processing. We propose that the global stages of shape processing are not revealed at threshold but are enabled only for well-defined shapes at suprathreshold modulations.
在形状加工的研究中，全局阶段（整合各种特征以定义形状）和编码单个成分的早期阶段之间存在关键区别。我们研究径向频率（RF）图案的形状辨别阈值是在这个全局阶段受到限制，还是单个成分中的信息支持该阈值。我们使用了不同轮廓厚度（0.75 - 6周/度）的消色差和色差（L/M-和S-视锥细胞对立型）径向频率（RF）图案。首先，我们通过RF4的片段表明，形状辨别阈值对于从1个周期到完整图案的周期数是不变的。单独显示的单个周期的表现与整个RF的表现一样好，这表明单个RF周期内的信息足以支持整个形状辨别阈值，这与全局加工的影响相悖。其次，我们发现RF图案和调制线条刺激辨别的阈值相似，这也与全局效应相悖。第三，我们计算了阈值下刺激周期内固有方向变化的指标，并表明这可能解释了从RF1到RF6随着调制频率形状和线条辨别阈值的提高。与消色差图案相比，色差图案的阈值辨别更高可能反映了颜色视觉的方向辨别较差，而不是全局加工的缺陷。我们提出，形状加工的全局阶段在阈值时并未显现，而是仅在阈上调制时对于明确界定的形状才得以启用。
