Contradictory Behavioral Biases Result from the Influence of Past Stimuli on Perception

Biases such as the preference of a particular response for no obvious reason, are an integral part of psychophysics. Such biases have been reported in the common two-alternative forced choice (2AFC) experiments, where participants are instructed to compare two consecutively presented stimuli. However, the principles underlying these biases are largely unknown and previous studies have typically used ad-hoc explanations to account for them. Here we consider human performance in the 2AFC tone frequency discrimination task, utilizing two standard protocols. In both protocols, each trial contains a reference stimulus. In one (Reference-Lower protocol), the frequency of the reference stimulus is always lower than that of the comparison stimulus, whereas in the other (Reference protocol), the frequency of the reference stimulus is either lower or higher than that of the comparison stimulus. We find substantial interval biases. Namely, participants perform better when the reference is in a specific interval. Surprisingly, the biases in the two experiments are opposite: performance is better when the reference is in the first interval in the Reference protocol, but is better when the reference is second in the Reference-Lower protocol. This inconsistency refutes previous accounts of the interval bias, and is resolved when experiments statistics is considered. Viewing perception as incorporation of sensory input with prior knowledge accumulated during the experiment accounts for the seemingly contradictory biases both qualitatively and quantitatively. The success of this account implies that even simple discriminations reflect a combination of sensory limitations, memory limitations, and the ability to utilize stimuli statistics.无明显诱因下偏向特定反应的这类偏差，是心理物理学中不可或缺的组成部分。这类偏差已在常规二择一迫选（two-alternative forced choice, 2AFC）实验中被报道，此类实验要求被试对连续呈现的两种刺激进行比较。然而，这类偏差背后的核心原理迄今仍未明确，既往研究通常仅采用特设解释来予以说明。本研究聚焦二择一迫选音调频率辨别任务中的人类行为表现，采用两种标准实验范式。两种范式下，每个试次均包含一个参考刺激。其中一种为参考-低范式（Reference-Lower protocol），其参考刺激的频率始终低于比较刺激；另一种为参考范式（Reference protocol），其参考刺激的频率可高于或低于比较刺激。我们观测到了显著的间隔偏差：当参考刺激处于某一特定时段时，被试的任务表现更优。令人意外的是，两项实验中的偏差方向完全相反：在参考范式中，参考刺激处于第一个时段时被试表现更优；而在参考-低范式中，参考刺激处于第二个时段时表现更优。这一矛盾现象驳斥了既往关于间隔偏差的解释，但通过考量实验的统计特性，该矛盾可得到解决。将知觉视为实验过程中累积的先验知识与感官输入的整合过程，这一框架能够在定性与定量层面同时解释上述看似矛盾的偏差。该框架的成功表明，即便是简单的辨别任务，其行为表现也同时受到感官局限、记忆局限以及利用刺激统计特性的能力的共同影响。