Archive for 11/04/2009

Visual memory

In a recent ScienceDaily item (here) there is a report on a paper, Short-Term Memory for Figure-Ground Organization in the Visual Cortex, by P. O’Herron and R. von der Heydt. The problem they looked at was the stability of vision.

“As we look at the world around us, images flicker into our brains like so many disparate pixels on a computer screen that change every time our eyes move, which is several times a second. Yet we don’t perceive the world as a constantly flashing computer display.”
“…Recent studies have hotly debated whether the visual system uses a buffer to store image information and if so, the duration of that storage,…We found that the answer is ‘yes,’ the brain in fact stores the last image seen for up to two seconds.”
“The image that the brain grabs and holds onto momentarily is not detailed; it’s more like a rough sketch of the layout of objects in the scene. This may elucidate, at least in part, how the brain creates for us a stable visual world when the information coming in through our eyes changes at a rapid-fire pace: up to four times in a single second.”
“The study was based on recordings of activity in nerve cells in the V2 region of the brains of macaques, whose visual systems closely resemble that of humans…. discovering memory in this region was quite a surprise because the usual understanding is that neurons in the visual cortex simply respond to visual stimulation, but do not have a memory of their own.”

This fits with the idea that there are two levels of conscious images: a very detailed image of two or three aspects of the scene which changes often and a much, much less detailed image of the whole visual field that changes far less often.  In combining these two levels, we have the feeling (illusion) of seeing the whole visual field in detail all at once. What happens for vision is probably also happening in the other senses.