How much do we see? In fact, we see very little, just a few things our attention happens to focus on. Every time we look at something we just pick up a few features and 'recognize' the whole picture from our past experiences and memories. For example, when we enter a familiar room, we do not have to examine every item there to recognize it. We just know what is there and where everything is located. A quick glance is enough. So do we actually see the environment or do we just know 'what is there'? In fact, our perceptual reconstruction (or 'what we think we see') comes from two opposite directions - from outside (environmental stimuli) and inside (mental images we have stored in the brain). The more familiar the environment or situation, the less we actually perceive it. The brain does not need to process all the stimuli; it just 'fills in the gaps' and 'predicts' the final picture.
There is much evidence that one of the problems many autistic people experience is their inability to distinguish between foreground and background stimuli. They often are unable to discriminate between relevant and irrelevant stimuli. What is background to others may be equally foreground to them. They perceive everything without filtration or selection. As Donna Williams describes it, they seem to have no sieve in their brain to select the information that is worth being attended. This results in a paradoxical phenomenon: sensory information is received in infinite detail and holistically at the same time. It can be described as 'gestalt perception', i.e. perception of the whole scene as a single entity with all the details perceived (not processed!) simultaneously. They may be aware of the information others miss, but the processing of 'holistic situations' can be overwhelming. As there is too much information coming in, it is hard to know which stimuli to attend. It is often difficult for the autistic person to 'break' the whole picture into meaningful entities, to 'draw the boundaries' around plenty of tiny sensory pieces to make them meaningful items.
In contrast to our guessing 'what is there' from our experience and memory instead of actually seeing it, autistic children seem to be unable to filter the incoming information and tend to perceive all the stimuli around them. Instead of 'inventing' the world as we do, they actually perceive it. Such 'acute-perception' brings overwhelming information the brain cannot cope with.
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