Long-Term Memory

Theories of Long-Term Memory

An early theory of LTM was the perceptual moment hypothesis. This theory proposed that people encode information in 100-millisecond bundles and store them into memory as an unbroken record, a continuous loop—10 snapshots per second, 600 per minute, and more than 500,000 per 16-hour day. This theory did not explain how or where these memories were stored or how they could be retrieved. Presumably most memories remained hidden from people except for unexpected remembrances or purposeful efforts to recall. Dreams, often comprising mindless yet vivid segments, might draw their substance from random samplings from this loop.

Subsequent research by the eminent neurologist Wilder Penfield lent credence to this theory. Penfield operated on patients who were afflicted with violent epileptic seizures. Removal of damaged regions of the brain often reduced the severity of the seizure. Before surgery, and with the patient fully conscious, Penfield applied a mild electrical current (which produces no pain) to exposed regions of the brain. How the patient responded to the current helped Penfield distinguish damaged tissue from healthy tissue. Many points of stimulation produced utterances and body movements. However, when regions of the temporal lobe were stimulated, vivid memories moving forward in real time were reported, as if this continuous “memory loop” had been engaged at some temporal location. One patient, a stenographer 10 years earlier, stated, “I could see the desks. I was there, and someone was calling to me, a man leaning on a desk with a pencil in his hand.” Another reported, “I hear music again,” and began to hum along, later noting, “There were instruments. It was as though it were being played by an orchestra. Definitely it was not as though I were imagining the tune to myself. I actually heard it.” Penfield, who operated on more than 1,000 patients, concluded that he had tapped into a “stream of consciousness” reflecting memories stored many years earlier.

However, not all patients reported re-activated memories, and other scientists could not replicate Penfield's findings. Other scientists simply were skeptical of the claims by Penfield, and this line of research largely disappeared.

Today, researchers know that some of Penfield's speculations, provocative as they were, must be incomplete if not wrong. For one, memories cannot be a veridical encoding of an event, like an unbiased camera recording of each day's activities. Rather, memories often contain embellishments, added at later points in time. Memories can even be created for an event that never occurred at all. In short, people store not only what they have experienced but also significant “add-ons” that can alter or distort the original memory. Second, it seems likely that people filter their experiences based on what engages their attention and emotions. What escapes their attention is likely not encoded at all and, therefore, is absent from memory.

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