The use of prior assumptions in human visual processing.

当我们看世界时，进入我们眼睛的光线的复杂模式被用来恢复我们环境的三维结构。为了快速有效地做到这一点，我们的视觉系统做出了一定的假设。例如，在没有其他信息的情况下，我们假设物体是从上面被照亮的（“光从上面先验”）。想象一个鸡蛋放在阳光下。阴影使鸡蛋的顶部变亮，底部变暗。在看不到光源的情况下，我们假设光源在头顶，并利用这个假设来帮助确定鸡蛋的形状，即凸形。相反，如果你拿起一个打开的蛋盒，每个凹陷的顶部都是暗的，底部则是亮的，这有助于你看到它是凹的。“来自上方的光”先验帮助我们从物体的阴影模式中估计物体的形状：这个过程称为“形状-阴影”（SFS）。我们感兴趣的是，视觉假设（“先验”）是天生的，还是在与环境的互动中习得的。鸡总是把阴影模式理解为好像光源在头顶上，即使在光线总是从下面来的环境中饲养。相比之下，在光源总是偏向一侧的环境中训练的人类则会相应地学习解释物体。一系列的实验将调查人类视觉系统的适应性有多强。观察者将通过“机械臂”来观察和触摸虚拟物体，这使我们能够操纵物体的凹凸感。通过这种方法，我们将训练人们相信物体是从侧面或下面被照亮的。我们将测量这种训练如何影响对其他新对象的解释，以及效果持续多久。因此，我们可以揭示人类视觉系统如何以及在什么时间尺度上适应环境的变化。基于阴影模式的差异，在许多凸对象（例如按钮）中很容易找到凹对象（例如酒窝）。这种凸/凹视觉搜索任务是最简单的，如果物体从上面被照亮，与“从上面的光”一致。这提供了一种有用的方法来研究“从上面的光”先验，以及它如何与其他视觉场景信息相互作用。实现“上面的光”先验的机制可能非常简单，也可能相当复杂。例如，关于“来自上方的光线”是否先于头部移动的证据是混杂的，因此，如果你是倒立的，你会认为物体是从下方被照亮的。我们可以通过让观察者用他们的头或身体在不同的方向上执行视觉搜索任务来研究这个问题。我们还可以问SFS是否受到上下文的影响。例如，如果周围的物体从侧面明显被照亮，那么视觉系统也可能将中心物体解释为从侧面被照亮。当观察者主动移动光源时，视觉搜索性能也会发生变化。视觉搜索行为将与形状判断（凸与凹）以及对亮或暗物体出现的判断进行比较。所有这些任务都使用“自上而下的光”先验，但可能受到上下文操作的不同影响。例如，头部位置可能会影响形状判断，但不会影响视觉搜索。任务之间的差异将使我们了解涉及形状和阴影的各种判断背后的机制分歧。这项工作与对视觉系统使用的假设感兴趣的科学家有很大的关系；很少有研究调查先验是如何被学习的，它们有多普遍，以及它们是如何受到上下文信息的影响的。更广泛地说，任何使用视觉或触摸信息的远程呈现（例如虚拟现实，远程手术）的人都应该意识到观察者用来解释显示的假设。例如，显示方向和以前的经验可能会极大地影响3D感知，特别是在线索减少的情况下。

The complex pattern of light entering our eyes as we view the world is used to recover the three-dimensional structure of our environment. To do this quickly and efficiently our visual system makes certain assumptions. For example, in the absence of other information, we assume that objects are illuminated from above ('light-from-above prior'). Imagine an egg sitting in the sun. Shading makes the egg bright on the top and darker underneath. Without seeing the light-source we assume that it is overhead and use this assumption to help determine the shape of the egg, i.e. convex. In contrast, if you held up an open egg-box, each depression would be dark at the top and brighter at the bottom, helping you to see it as concave. The 'light-from-above' prior helps us to estimate the shape of an object from its shading pattern: a process called 'shape-from-shading' (SFS). We are interested in whether visual assumptions ('priors') are innate or learnt in response to interactions with the environment. Chickens always interpret shading patterns as if the light source was overhead, even if raised in an environment where light always comes from below. In contrast, humans trained in an environment where the light-source is always to one side learn to interpret objects accordingly. A series of experiments will investigate just how adaptable the human visual system is. Observers will view and touch virtual objects via a 'robot arm' that allows us to manipulate whether the objects feel convex or concave. With this method we will train people to believe that objects are lit from the side or from below. We will measure how this training affects the interpretation of other, novel objects, and how long the effects last. We can thus uncover how and over what time-scale the human visual system adapts to changes in its environment.A concave object (e.g. dimple) is easy to find amongst many convex objects (e.g. buttons), based on differences in the shading patterns. This convex / concave visual search task is easiest if the objects are lit from above, consistent with the 'light-from-above' prior. This provides a useful method to investigate the 'light-from-above' prior and how it interacts with other visual scene information.The mechanisms for implementing the 'light-from-above' prior could be very simple or quite complex. For example, evidence is mixed on whether the 'light-from-above' prior moves with the head, such that if you were upside-down you would assume that objects were lit from below. We can investigate this by asking observers to perform visual search tasks with their head or body at different orientations. We can also ask whether SFS is affected by context. For example, if surrounding objects are clearly lit from the side, then the visual system might also interpret central objects as if they are lit from the side. Visual search performance may also change when observers actively move the light source.Visual search behaviour will be compared to shape judgements (convex vs. concave) and judgements about how light or dark objects appear. All of these tasks use the 'light-from-above' prior but may be differentially affected by contextual manipulations. For example, head position might affect shape judgements but not visual search. Differences between the tasks will inform us about the divergence of mechanisms behind various judgements involving shape and shading.This work is of great relevance to scientists interested in the assumptions that the visual system uses; little research has investigated how priors are learnt, how general they are and how they are affected by contextual information. More broadly, anyone using remote presentation of visual or touch information (e.g. virtual reality, remote surgery) should be aware of the assumptions that observers use to interpret displays. For example, display orientation and previous experience could dramatically affect 3D perception, especially under diminished cue conditions.