The brain and external visual objects

Scientific American's March 2010 issue carries this downright New Agey cover featuring the article, "The Brain's Dark Energy" by Marcus Raichle of the Washington University School of Medicine.

Raichle's article concerns what he calls "the brain's default mode network (DMN)", the "brain's dark energy" of the title. As he explains, recent neurological studies have indicated that the brain at rest is performing far more work than had previously been known. A sidebar expresses the development in our understanding of this activity:

OLD VIEW: Brain scans originally seemed to suggest that most neurons were quiet until needed for some activity, such as reading, at which point the brain fired up and expended energy on the signaling needed for the task.

NEW VIEW: In recent years additional neuroimaging experiments have shown that the brain maintains a high level of activity even when nominally “at rest.” In fact, reading or other routine tasks require minimal additional energy, no more than a 5 percent increment, over what is already being consumed when in this highly active baseline state.

In explaining that this DMN activity plays an important part in filling in the blanks in information our brains receive from the outside world. And, in a passage that adds some important perspective on philosophical notions of perception, he explains that our visual perception of external objects delivers only a fraction of the visual information it receives to the brain:

Visual information ... degrades significantly as it passes from the eye to the visual cortex.

Of the virtually unlimited information available in the world around us, the equivalent of 10 billion bits per second arrives on the retina at the back of the eye. Because the optic nerve attached to the retina has only a million output connections, just six million bits per second can leave the retina, and only 10,000 bits per second make it to the visual cortex.

After further processing, visual information feeds into the brain regions responsible for forming our conscious perception. Surprisingly, the amount of information constituting that conscious perception is less than 100 bits per second. Such a thin stream of data probably could not produce a perception if that were all the brain took into account; the intrinsic activity must play a role.

Yet another indication of the brain’s intrinsic processing power comes from counting the number of synapses, the contact points between neurons. In the visual cortex, the number of synapses devoted to incoming visual information is less than 10 percent of those present. Thus, the vast majority must represent internal connections among neurons in that brain region. [my emphasis]

.Tags: human brain, marcus raichle