Modern researchers have also learned to eavesdrop on the brain. Modern microelectrodes have tips so small that they can detect the electrical activity in a single neu­ron. Microelectrodes make possible some astonishingly precise find­ings; for example, we can now detect exactly where the information goes after a cat's whisker is stroked.

The electrical activity of the brain's approximately 100 billion neu­rons is orchestrated in regular waves that sweep across its surface. In the 1920s, Hans Berger, a young German psychiatrist, discovered this when he placed electrically sensitive disks on the head of his son, Klaus. When he attached the disks to a recording device, Berger ob­served that faint waves (changes in electrical activity in large portions of the brain) peaked about 10 times per second when Klaus was relaxed with his eyes closed. If Klaus opened his eyes and concentrated on a mental problem, the waves would speed up, indicating a more excited brain state. The electroencephalogram (EEG) is simply an amplified tracing of such waves by an instrument called an electroencephalo­graph. This method for measuring the gross activity of the whole brain is roughly like studying the activity of a car engine by listening to the hum of its motor.

It is now possible to detect by EEG the brain's response to a sound, a flash of light, or even a thought. By presenting a stimu­lus repeatedly and having a computer filter out electrical activity unre­lated to the stimulus, one can identify the electrical wave evoked by the stimulus. Observing abnormalities in such brain-wave responses is an easy, painless way to diagnose certain forms of brain damage.

Computers are also being used to analyze EEG data, providing information that was inconceivable 10 years ago. A high-tech instru­ment, called a brain mapper, neurometric analyzer, or brain-state ana­lyzer, analyzes the brain's electrical activity. The resulting "brain maps" provide valuable diagnostic information to psychiatrists, allow the effects of certain medications to be monitored, and increase the safety of brain surgery.

SENSORY REGISTERS

Consider what one intriguing memory experiment revealed about how sensory information first enters the memory system. As part of his doctoral research, George Sperling (1960) showed people three rows of three letters each for only 1/20th of a second. It was like trying to read by the flashes of a lightning storm. After the nine letters had disappeared from the screen, the subjects could recall only about half of them.

Why? Was it because they had insufficient time to see them? No, Sperling cleverly demonstrated that even at such lightning-flash speed, people actually can see and recall all the letters, but only mo­mentarily. Rather than ask subjects to recall all nine letters at once, Sperling instead would sound a high, medium, or low tone immedi­ately after the nine letters were flashed. This cue directed the subject to report only the letters of the top, middle, or bottom row, respectively. Now the subjects rarely missed a letter. Because they did not know in advance which row would be requested, all nine letters must have been momentarily available for recall.

Sperling's experiment revealed that we do have a fleeting photo­graphic memory called iconic memory. For a moment, the eyes register an exact representation of a scene, and can recall any part of it in amaz­ing detail. But only for a moment. If Sperling delayed the tone signal by as much as a second, the iconic memory was gone and the subjects once again recalled only about half of the letters. The visual screen clears quickly, as it must, lest new images be superimposed over old ones. For sound, the auditory sensory image, called echoic memory, disappears more slowly. The last few words spoken seem to linger for 3 or 4 seconds. Sometimes, just as you ask "What did you say?," you can hear in your mind the echo of what was said.

KNOWING WHAT WE KNOW

Sometimes we know more than we are aware of. Other times—perhaps when taking an exam—we discover that we do not know something as well as we thought we did. The difficulties of knowing what you know are strikingly evident in the amnesic patients who know how to do things without knowing that they know. The parallel to learning during infancy is intriguing: We recall nothing, yet what we learn reaches far into our future.

How accurate are we at assessing what we know? John Shaughnessy explored this question in an ex­periment with two groups of Hope College students. One group was (1) repeatedly shown dozens of factual statements, (2) asked to judge the likelihood that they would later remember each fact, and then (3) actually tested on their recall. Students in this group tended to feel fairly confident of their knowledge, even on the questions they later missed. Instead of constantly reading the statements, students in a second group also spent much of their time evaluating their knowledge by answering practice test questions. These students learned the facts just as well as did the mere-repetition group. What is more, the prac­tice-test group could better discriminate what they did and didn't know. Thus, self-testing not only encourages active rehearsal, it also can help you to know what you know—and thus to focus your study time on what you do not yet know. As the British statesman Benjamin Disraeli once said, "To be conscious that you are ignorant is a great step to knowledge."