Well to reply to this properly would be a very long article, maybe a book .. instead, seeing we are in this area, I'll just give you two illustrations of top-down effects in the brain.
Illustration 1: How does reading work? Here's a remarkable thing.
• Yu cn red this evn thogh words are mispelt,
• and this thuogh lwtters are wrong,
• And this though words missing.
How can it be we can make sense of garbled text in this way? One might think the brain would come to a grinding halt when confronted with such incomplete or grammatically incorrect text. But the brain does not work in a mechanistic way, first reading the letters, then assembling them into words, then assembling sentences. Instead our brains search for meaning all the time, predicting what should be seen and interpreting what we see based on our expectations in the current context.
Actually words by themselves may not make sense without their context. Consider:
• The horses ran across the plane,
• The plane landed rather fast,
• I used the plane to smooth the wood.
- what `plane' means differs in each case, and is understood from the context. Even the nature of a word (noun or verb) can depend on context:
• Her wound hurt as she wound the clock
This example shows you can't reliably tell from spelling how to pronouce words in English, because not only the meaning, but even pronunciation depends on context.
The underlying key point is that we are all driven by a search for meaning: this is one of the most fundamental aspects of human nature, as profoundly recorded by Viktor Frankl in his book Man's Search for Meaning. Understanding this helps us appreciate that reading is an ongoing holistic process: the brain predicts what should be seen, fills in what is missing, and interprets what is seen on the basis of what is already known and understood. And this is what happens when we learn to read, inspired by the search for understanding. One learns the rules of grammar and punctuation and spelling too of course; but such technical learning takes place as the process of meaning making unfolds. It is driven top down by our predictions on the basis of our understandings, based in meaning..
Illustration 2: Vision works the same way, as demonstrated by Dale Purves in his book "Brains: How They Seem to Work". The core of his argument is as follows (from the abstract of his article on visual illusions):
"The evolution of biological systems that generate behaviorally useful visual percepts has inevitably been guided by many demands. Among these are: 1) the limited resolution of photoreceptor mosaics (thus the input signal is inherently noisy); 2) the limited number of neurons available at higher processing levels (thus the information in retinal images must be abstracted in some way); and 3) the demands of metabolic efficiency (thus both wiring and signaling strategies are sharply constrained). The overarching obstacle in the evolution of vision, however, was recognized several centuries ago by George Berkeley, who pointed out that the information in images cannot be mapped unambiguously back onto real-world sources (Berkeley, 1975). In contemporary terms, information about the size, distance and orientation of objects in space are inevitably conflated in the retinal image. In consequence, the patterns of light in retinal stimuli cannot be related to their generative sources in the world by any logical operation on images as such. Nonetheless, to be successful, visually guided behavior must deal appropriately with the physical sources of light stimuli, a quandary referred to as the "inverse optics problem". "
The resolution is top-down shaping of vision by the cortex, based in prediction of what we ought to see. Visual illusions are evidence that this is the way the visual system solves this problem.
Intriguing, isn't it?
George
