Testing Neuronal Accounts of Anisotropic Motion Perception with Computational Modelling

There is an over-representation of neurons in early visual cortical areas that respond most strongly to cardinal (horizontal and vertical) orientations and directions of visual stimuli, and cardinal- and oblique-preferring neurons are reported to have different tuning curves. Collectively, these neuronal anisotropies can explain two commonly-reported phenomena of motion perception – the oblique effect and reference repulsion – but it remains unclear whether neuronal anisotropies can simultaneously account for both perceptual effects. We show in psychophysical experiments that reference repulsion and the oblique effect do not depend on the duration of a moving stimulus, and that brief adaptation to a single direction simultaneously causes a reference repulsion in the orientation domain, and the inverse of the oblique effect in the direction domain. We attempted to link these results to underlying neuronal anisotropies by implementing a large family of neuronal decoding models with parametrically varied levels of anisotropy in neuronal direction-tuning preferences, tuning bandwidths and spiking rates. Surprisingly, no model instantiation was able to satisfactorily explain our perceptual data. We argue that the oblique effect arises from the anisotropic distribution of preferred directions evident in V1 and MT, but that reference repulsion occurs separately, perhaps reflecting a process of categorisation occurring in higher-order cortical areas.早期视觉皮层区域内，对主方向（水平与垂直）视觉刺激的朝向和方向产生最强响应的神经元存在过度表征现象；且偏好主朝向与斜向的神经元具有不同的调谐曲线（tuning curves）。总体而言，这些神经元各向异性（neuronal anisotropies）可以解释两种常见的运动知觉现象——倾斜效应（oblique effect）与参考排斥效应（reference repulsion），但目前仍不清楚神经元各向异性能否同时解释这两种知觉效应。我们通过心理物理实验（psychophysical experiments）发现，参考排斥效应与倾斜效应均不依赖于运动刺激的呈现时长；且对单一方向的短暂视觉适应，可同时在朝向域引发参考排斥效应，并在方向域引发倾斜效应的反向效应。为了将上述实验结果与潜在的神经元各向异性机制建立关联，我们构建了一大类神经元解码模型（neuronal decoding models），这些模型的神经元方向调谐偏好、调谐带宽（tuning bandwidths）与锋电位发放率（spiking rates）的各向异性水平均通过参数化方式调控。令人意外的是，没有任何一种模型参数配置能够令人满意地解释我们得到的知觉实验数据。我们认为，倾斜效应源于初级视皮层（V1，Primary Visual Cortex）与中颞叶视觉区（MT，Middle Temporal Area）内偏好方向的各向异性分布，而参考排斥效应则由独立的机制产生，这或许反映了高阶皮层区域内存在的分类加工过程。