An early study on the role of the temporal cortex in memory seemed to provide evidence that forgotten memories are still there even though we cannot retrieve them. As part of a neurosurgical procedure, Penfield (1959) electrically stimulated portions of patients’ brains and asked them to report what they experienced (patients were conscious during the sur- gery, but the stimulation was painless). In this way, Penfield determined the functions of various portions of the brain. Stimulation of the temporal lobes led to reports of memories that patients were unable to report in normal recall, such as events from childhood. This seemed to provide evidence that much of what seems forgotten is still stored in memory. Unfortunately, it is hard to know whether the patients’ memory reports were accurate because there is no way to verify whether the reported events actually occurred. Therefore, although suggestive, the Penfield experiments are generally discounted by memory researchers.
A better experiment, conducted by Nelson (1971), also indicated that forgotten memories still exist. He had participants learn a list of 20 paired associates, each consisting of a number for which the participant had to recall a noun (e.g. 43-dog). The subjects studied the list and were tested on it until they could recall all the items without error. Participants returned for a retest 2 weeks later and were able to recall 75% percent of the associated nouns when cued with the numbers. However, the research question concerned the 25% that they could no longer recall—were these items really forgotten? Participants were given new learning trials on the 20 paired associates. The paired associ- ates they had missed were either kept the same or changed. For example, if a participant had learned 43-dog but failed to recall the response dog to 43, he or she might now be trained on either 43-dog (unchanged) or 43-house (changed). Participants were tested after studying the new list once. If the participants had lost all memory for the forgotten pairs, there should have been no difference between recall of changed and unchanged pairs. However, participants cor- rectly recalled 78% of the unchanged items formerly missed, but only 43% of the changed items. This large advantage for unchanged items indicates that participants had retained some memory of the original paired associates, even though they had been unable to recall them initially.
J. D. Johnson, McDuff, Rugg, and Norman (2009) report a brain-imaging study that also shows there are records of experiences in our brain that we can no longer remember. Participants saw a list of words and for each word they were asked to either imagine how an artist would draw the object denoted by the word or imagine functional uses for the object. The researchers trained a pattern classifier (a program for analyzing patterns of brain activity) to distinguish between words assigned to the artist task and words assigned to the uses task, based on dif- ferences in brain activity during the two tasks. Later, participants were shown the words again and the classifier was applied to their brain activation patterns. The classifier was able to recognize from these patterns what task the word had been assigned to with better than chance accuracy. It was successful at recogni- tion both for words that participants could recall studying and for words they could not remember, although the accuracy was somewhat lower for the words they could not remember. This indicates that even though we may have no con- scious memory of seeing something, aspects of how we experienced it will be retained in our brains.
These experiments do not prove that everything is remembered. They show only that appropriately sensitive tests can find evidence for remnants of some memories that appear to have been forgotten. In this chapter, we will discuss first how memories become less available with time, then some of the factors that determine our success in retrieving these memories.
■ Even when people appear to have forgotten memories, there is evi- dence that they still have some of these memories stored.
A better experiment, conducted by Nelson (1971), also indicated that forgotten memories still exist. He had participants learn a list of 20 paired associates, each consisting of a number for which the participant had to recall a noun (e.g. 43-dog). The subjects studied the list and were tested on it until they could recall all the items without error. Participants returned for a retest 2 weeks later and were able to recall 75% percent of the associated nouns when cued with the numbers. However, the research question concerned the 25% that they could no longer recall—were these items really forgotten? Participants were given new learning trials on the 20 paired associates. The paired associ- ates they had missed were either kept the same or changed. For example, if a participant had learned 43-dog but failed to recall the response dog to 43, he or she might now be trained on either 43-dog (unchanged) or 43-house (changed). Participants were tested after studying the new list once. If the participants had lost all memory for the forgotten pairs, there should have been no difference between recall of changed and unchanged pairs. However, participants cor- rectly recalled 78% of the unchanged items formerly missed, but only 43% of the changed items. This large advantage for unchanged items indicates that participants had retained some memory of the original paired associates, even though they had been unable to recall them initially.
J. D. Johnson, McDuff, Rugg, and Norman (2009) report a brain-imaging study that also shows there are records of experiences in our brain that we can no longer remember. Participants saw a list of words and for each word they were asked to either imagine how an artist would draw the object denoted by the word or imagine functional uses for the object. The researchers trained a pattern classifier (a program for analyzing patterns of brain activity) to distinguish between words assigned to the artist task and words assigned to the uses task, based on dif- ferences in brain activity during the two tasks. Later, participants were shown the words again and the classifier was applied to their brain activation patterns. The classifier was able to recognize from these patterns what task the word had been assigned to with better than chance accuracy. It was successful at recogni- tion both for words that participants could recall studying and for words they could not remember, although the accuracy was somewhat lower for the words they could not remember. This indicates that even though we may have no con- scious memory of seeing something, aspects of how we experienced it will be retained in our brains.
These experiments do not prove that everything is remembered. They show only that appropriately sensitive tests can find evidence for remnants of some memories that appear to have been forgotten. In this chapter, we will discuss first how memories become less available with time, then some of the factors that determine our success in retrieving these memories.
■ Even when people appear to have forgotten memories, there is evi- dence that they still have some of these memories stored.
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