Top-down and bottom-up selective mechanisms in attention: subcortical convergence in visual thalamus?

如果柠檬混入超市水果摊的柠檬中，很容易被发现。但是，潜伏在克莱门汀群中的一只无赖蜜柑不太容易被发现，一个新手饲养员可能需要先被指出来，然后才能看到它。这些是我们选择值得注意的物品的两种相反方式的简单例子，心理学家分别将其称为“自下而上”和“自上而下”。在第一个例子中，放错地方的石灰的明显质量被称为“凸起”。没有一种属性(例如‘绿色’或‘椭圆形’)本身就是显著的，这完全取决于上下文。在皮卡迪利广场，一切都被设计得很突出，没有什么真正占主导地位--给人一种视觉不堪重负的感觉，不知道首先看哪里。在这种情况下，一个“自上而下”的指令--一个指向提示，或者一个口头命令--可能会很有帮助。重定向凝视是视觉注意的典型表现，但不是自动的。当凝视处于静止状态时，注意力的焦点--大脑的眼睛--可能会四处游荡(就像在社交团体中一样，以避免眼神接触)。因此，注意力先于眼球运动，我们的目标是研究大脑在这个形成阶段到底发生了什么。我们建议通过记录丘脑的神经活动来做到这一点，丘脑是一个脑干器官，通常负责调节大脑皮层不同区域(大脑信息处理的顶层)之间的感觉信息流量。作为一种战略，这可以比作研究电视转播控制室的运作，因为它覆盖了比方说一级方程式大奖赛。控制室接收来自赛马场各地的多个馈送，电视导演选择最戏剧性的动作传输给观众。在这个类比中，丘脑(尤其是一个被称为‘枕骨’的部分)评估来自大脑皮层多个区域的信号--一些描述事件发生时，另一些预测这样的事件。然而，枕骨的“观察公众”只不过是提供其输入的一组大脑皮层区域。换句话说，枕骨是与大脑皮层进行双向通信的，它充当了一个装置，使皮质区域能够在它们之间投票决定哪个可见项目最值得关注。在麻醉状态下注意力不集中，所以我们计划记录一只警觉的非人类灵长类(NHP)的丘脑活动。动物面对一系列的物品，并被训练来选择突出的物品，或者是通过适当的视觉提示指出的非描述性物品。丘脑中的神经元，像其他视觉中心一样，有一个受限的‘接受野’(RF)--它仔细观察视野中的某个窗口。我们定位正在研究的神经元的RF，并安排它包含显著的或暗示的项目。下一次审判，目标物品可能在其他地方。我们寻求找到与RF内物品的行为意义相对应的神经活动，而不是与其特定的视觉特征无关。在实验中安排的自下而上和自上而下的情况下，活动的时间可能会有所不同，特别是在与自下而上和自上而下的皮质通路有不同连接的丘脑分支之间也是如此。这一战略还有另一个转折。我们调整任务的难度水平，并监控NHP的执行情况。可能需要更长的时间才能做出决定，或者开始犯错误。活动的变化是根据表现的准确性进行校准的，以指示行为对特定神经活动的依赖。最后，我们还可以对测试地点应用人工药物刺激或抑制，看看我们是否可以影响其RF中项目的选择--这些人工操作是否会改变行为，与神经记录提供的预测一致？

If a lime gets amongst the lemons at a supermarket fruit stall, it's easily spotted. But a rogue satsuma nestling amidst the clementines is less easily detected, and a novice stock-keeper might need it to be physically pointed out, before he can see it. These are simple examples of two converse ways in which we select an item deserving of attention, referred to by psychologists as 'bottom-up and 'top-down', respectively. In the first example the conspicuous quality of the misplaced lime is termed 'salience'. No property (e.g. 'green' or 'oval shaped') is inherently salient , it all depends on context. In Piccadilly Circus, where everything is designed to be salient, nothing really dominates - giving the sensation of being visually overwhelmed, and not knowing where to look first. In this situation, a 'top-down' instruction - a pointing cue, or a verbal command, can be rather helpful. Redirecting gaze is the typical expression of visual attention, but is not automatic. The focus of attention - the mind's eye - may rove about whilst gaze is frozen (as in social groupings, to avoid eye contact). Attention thus precedes an eye movement, and our aim is to study exactly what happens in the brain at this formative stage. We propose to do this by recording neural activity in the thalamus, a brainstem organ generally responsible for regulating the traffic of sensory information amongst different areas of the cerebral cortex (the brain's top level of information processing). As a strategy, this could be likened to studying the operation of a TV broadcast controlroom, as it covers, say, a Formula 1 Grand Prix. The controlroom receives multiple feeds from all over the racecourse, and the TV-director selects the most dramatic action for transmission to the viewing public. In the terms of this analogy, the thalamus (and in particular a component known as the 'pulvinar') assesses feeds from multiple areas of cerebral cortex - some describing events as they occur, others predicting such events. However, the pulvinar's 'viewing public' is nothing other than the set of cortical areas that provide its input. In other words, the pulvinar is in two-way communication with the cerebral cortex, and acts as a device enabling cortical areas to vote amongst themselves which visible item is the most attention-worthy. Attention is absent under anaesthesia, so we plan to record thalamic activity from an alert, nonhuman primate (NHP). The animal is confronted with an array of items and trained to select the salient item, or a non-descript one that has been pointed out by a suitable visual cue. A neuron in the thalamus, like other visual centres, has a restricted 'receptive field' (RF) - it scrutinises a certain window in the field of view. We locate the RF of a neuron under study, and arrange that it contains the salient, or cued item. Next trial, the target item may be elsewhere. We seek to find neural activity that corresponds to the behavioural significance of the item within the RF, independent of its particular visual features. The timing of activity may vary between the experimentally arranged bottom-up and top-down circumstances and especially, also, between thalamic subdivisions that are differentially connected to bottom-up and top-down cortical pathways. There is another twist to the strategy. We adjust the level of difficulty of the task and monitor how well the NHP performs. It may take longer to reach a decision, or begin to make mistakes. Changes in activity are calibrated against accuracy of performance, to index the dependency of behaviour on specific neural activity. Finally, we can also apply artificial pharmacological stimulation or inhibition to the test site to see if we can influence the selection of the item in its RF - will these artificial manipulations change behavior in line with predictions furnished by neural recordings?