Magazine article Phi Kappa Phi Forum

Capacity Limits of Information Processing in the Brain

Magazine article Phi Kappa Phi Forum

Capacity Limits of Information Processing in the Brain

Article excerpt

The human brain is heralded for its staggering complexity and processing capacity. Its hundred-billion neurons and several-hundred-trillion synaptic connections can process and exchange large amounts of information over a distributed network of brain tissue in a matter of milliseconds. Such massive parallel-processing capacity permits our brains to analyze complex images in one-tenth of a second, allowing us to visually experience the richness of the world. Likewise, the storage capacity of the human brain is nearly infinite. During our life-time, our brain will have amassed 10^sup 9^ to 10^sup 20^ bits of information, which is more than fifty-thousand times the amount of text contained in the U.S. Library of Congress, or more than five times the amount of the total printed material in the world!

Yet, for all our neuro-computational sophistication and processing power, we can barely attend to or hold in mind more than a few objects, and we can hardly perform more than one task at a time. We are routinely reminded of these severe shortcomings, for instance when attempting to read a complex passage while keeping an eye on some television program or when talking on the cell phone while driving. These anecdotal evidences have been rigorously documented in the laboratory; maintaining a cell phone conversation impairs simulated driving performance and can lead to 90 percent of observers failing to detect an unsuspected salient visual stimulus! The costs to society of our capacity limits in processing information are likely to be immense. For instance, driver inattention and other human errors have been estimated to account for nearly 40 percent of motor-vehicle accidents. How does such a sophisticated, multipurpose processing machine as the human brain exhibit such severe and, quite frankly, humbling limitations?

A rich history of scientific research has highlighted three major roadblocks during the flow of information from sensation to action. The first limitation concerns the time that it takes to consciously identify an object. We may have the impression that identification is instantaneous, but in fact this process can take more than half a second before our brain is free to identify a second object. A second, severely limited capacity is the number of objects that can be simultaneously maintained in short-term memory, estimated to be about four objects. It does not matter if more objects are shown to you; you will be able to remember or monitor only four of them. Finally, a third bottleneck arises when one must choose the proper course of action for an object or event. Suppose that while driving you see a road sign indicating your highway exit. While you are busy taking the proper action (for example, shifting lanes), you may be impaired at making other responses (such as answering a question from a passenger in your car).


Why does the sophisticated, multipurpose processing machine that is the human brain exhibit such severe limitations at multitasking? We can only speculate. The limited capacity of short-term memory is particularly puzzling, given that we can easily build computers that can rapidly store thousands of items. One possibility is that our brain did not need to be built to maintain a detailed representation of the visual world in our mind's eye because this detailed representation is just one look away. All we need to do is to open our eyes, and our visual system presents us with a detailed view of the world. It is also unlikely that during the evolution of the primate brain there was any strong pressure for our nervous system to fully identify visual objects or events in very rapid succession. Even under fight-or-flight situations, probably only one predator or rival needs to be identified at a time.

Similarly, it is unlikely that our ancestors had to make several split-second decisions at once. Action towards an object typically requires an orchestrated effort from our limbs; picking a fruit out of a tree may require using both hands (one for holding the branch and the other for plucking the fruit), fixing the eye on the fruit to optimize reaching accuracy, and positioning the legs in a particular way to ensure a stable posture. …

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