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Machines Like Us

What is forgetting?

Wednesday, 26 September 2012
by Jee Hyun Kim

If memory can be defined as “a past that becomes a part of me," can forgetting be defined as “a past that is no longer a part of me”?

Smokers who have abstained for years may not consciously be able to recall the sensation brought forth by smoking, but can suddenly feel craving upon seeing a smoking-related cue – often a cigarette brand logo – and relapse into smoking again. Tobacco companies know this only too well.

This illustrates a past that may have been forgotten but is not gone. It’s consistent with what’s known as the cue-dependent theory of forgetting, which states difficulty in recollection when the stimuli present during memory encoding are absent. Upon presentation of such stimuli, recollection becomes easy.

When a memory exists but cannot be recalled, such forgetting represents a retrieval failure. When a memory cannot be recovered in any way it represents a storage failure.

I have experienced such storage failure repeatedly throughout my education. Information I have studied before an exam is usually remembered during the exam.

As soon as the exam finishes, the information seems to immediately fade, mostly fading beyond recovery. It’s always made me feel like a faker.

This type of forgetting is consistent with what’s known as the trace decay theory of forgetting. It states that without rehearsal, memory will gradually decay over time, to disappear for ever.

Unfortunately, both psychological theories outlined above are very limited in terms of explaining forgetting.

Cue-dependent theory is criticised because memories can generalise over time, their elements becoming less specific. Trace decay theory fails to explain different fading speeds of different memories.

Infantile amnsesia

Neuroscience comes to the rescue at this point. What happens to the brain when something is forgotten? Although there are not many studies, some insight on the neural basis of forgetting has been provided by infantile amnesia research.

Infantile amnesia commonly refers to the general inability to remember experiences that happened early in life, before three to five years of age. This is a pervasive phenomenon displayed by all humans.

Even Dr Sheldon Cooper of TV’s The Big Bang Theory – who has photographic, or eidetic, memory – cannot recall events prior to “that drizzly Tuesday” when his mother stopped breastfeeding him.

Importantly, infantile amnesia is not due to an inability to form episodic memories before that age. Children younger than three years have been shown to be able to encode specific episodic memories and even remember them for two years.

Instead, these memories do not persist into later childhood and adulthood, indicating children forget at a more rapid rate than adults.

Amazingly, infantile amnesia is ubiquitous. It is believed to occur in all altricial species (species that, like us, require parental care after birth), and has been observed even in worms, goldfish, chickens and rats. These species require care-givers to survive into adulthood.

This is in contrast to precocial species (species that don’t require parental care) such as guinea pigs, that do not require care-giving to survive.