A fundamental question about human memory is why some
experiences are remembered whereas others are forgotten.
Episodic memory is often described as the conscious memory
for personal everyday experiences. Episodic encoding,
therefore, refers to the processes by which an experience
is transformed into an enduring memory trace.
Psychological studies have shown that the memorability
of an experience is influenced greatly by the cognitive
operations engaged during initial encoding of that experience.
For example, semantic processing leads to superior memorability
relative to nonsemantic processing. Lesion studies have
shown that damage in the prefrontal cortex results in
modest episodic deficits, specifically in terms of executive
control of cognitive functions. Typical studies of brain-injured
amnesic patients, however, cannot clearly distinguish
between the effects of brain damage on the encoding of
memories and their retrieval from storage.
An additional line of evidence for the role of prefrontal
cortex in episodic encoding, therefore, comes from functional
neuroimaging studies. These studies have implicated feft
prefrontal cortex in verbal encoding. It has been shown
that left prefrontal activation is greater during semantic
encoding relative to nonsemantic encoding. Additionally,
left prefrontal participation decreases and memorization
is impaired when semantic encoding operations are disrupted.
Typically, these studies have relied on blocked experimental
designs, where trials from each encoding condition are
presented sequentially, inseparable from each other during
the functional scan. While blocked designs allow comparison
between encoding conditions that yield, on average, higher
or lower levels of subsequent recollection, they do not
allow a direct trial-by-trial comparison between specific
encoding trials that lead to subsequent remembering and
those that lead to subsequent forgetting.
Results from event-related potential (ERP) studies,
which allow for trial-by-trial analysis, suggest that
the neural signature during verbal encoding differs for
subsequently remembered and subsequently forgotten experiences,
with remembered experiences being associated with a greater
positive-going response over frontal and parietal regions.
However, ERP studies are characterized by limited spatial
resolution. Thus, the precise functional neuroanatomic
encoding differences that predict whether a particular
verbal experience will be remembered or forgotten are
currently unknown.
To address these issues, the neural correlates of incidental
word encoding were examined in two whole-brain functional
magnetic resonance imaging (FMRI) studies. One experiment
used blocked-design procedures to investigate how systematic
manipulation of the encoding task affects prefrontal activation,
whereas the other used newly developed event-related procedures
that allow direct comparison between specific encoding
trials that result in subsequent remembering and forgetting.
Results revealed that what makes a verbal experience
memorable partially depends on the extent to which left
prefrontal regions are engaged during the experience.
Verbal experiences may be more memorable when semantic
and phonological attributes of the experience are extensively
processed via participation of left prefrontal regions.
These regions may serve to organize these attributes in
working memory.
A specific experience may elicit the recruitment of these
processes to a greater or lesser extent because of variable
task demands, shifts in subjects' strategies, characteristics
of target items, or attentional modulations. Regardless
of the source of this variability, greater recruitment
of left prefrontal processes will tend to produce more
memorable verbal experiences.
