Implied indirectly, without being directly expressed.
In order for information to be encoded into memory, we must first pay attention to it. When a person pays attention to a particular piece of information, this process is called attentional capture. By paying attention to particular information (and not other information), a person creates memories that could be (and probably are) different from someone else in the same situation. This is why two people can see the same situation but create different memories about it—each person performs attentional capture differently. There are two main types of attentional capture: explicit and implicit.
Explicit Attentional Capture
Explicit attentional capture is when a stimulus that a person has not been attending to becomes salient enough that the person begins to attend to it and becomes cognizant of its existence. Very simply, it's when something new catches your focus and you become aware of and focused on that new stimulus. This is what happens when you are working on your homework and someone calls your name, drawing your complete attention.
Implicit Attentional Capture
Implicit attentional capture is when a stimulus that a person has not been attending to has an impact on the person's behavior, whether or not they're cognizant of that impact or the stimulus. If you are working on your homework and there is quiet but annoying music in the background, you may not be aware of it, but your overall focus and performance on your homework might be affected. Implicit attentional capture is important to understand when driving, because while you might not be aware of the effect a stimulus like loud music or an uncomfortable temperature is having on your driving, your performance will nevertheless be affected.
Working memory is the part of the memory that actively holds many pieces of information for short amounts of time and manipulates them. The working memory has sub-systems that manipulate visual and verbal information, and it has limited capacity. We take in thousands of pieces of information every second; this is stored in our working memory. The working memory decides (based on past experiences, current thoughts, or information in long-term memory) if any particular piece of information is important or relevant. In other words, if the information is not used or deemed important, it will be forgotten. Otherwise, it is moved from the short-term memory and committed to long-term memory.
One famous example of attentional capture is the cocktail party effect, which is the phenomenon of being able to focus one's auditory attention on a particular stimulus while filtering out a range of other stimuli, much the same way that a partygoer can focus on a single conversation in a noisy room. This effect is what allows most people to tune into a single voice and tune out all others.
Research suggests a close link between working memory and attentional capture, or the process of paying attention to particular information. A person pays attention to a given stimulus, either consciously (explicitly, with awareness) or unconsciously. This stimulus is then encoded into working memory, at which point the memory is manipulated either to associate it with another familiar concept or with another stimulus within the current situation. If the information is deemed important enough to store indefinitely, the experience will be encoded into long-term memory. If not, it will be forgotten with other unimportant information. There are several theories to explain how certain information is selected to be encoded while other information is discarded.
The Filter Model
The formerly accepted filter model proposes that this filtering of information from sensory to working memory is based on specific physical properties of stimuli. For every frequency there exists a distinct nerve pathway; our attention selects which pathway is active and can thereby control which information is passed to the working memory. This way it is possible to follow the words of one person with a certain vocal frequency even though there are many other sounds in the surrounding area.
The filter model is not fully adequate. Attenuation theory, a revision of the filter model, proposes that we attenuate (i.e., reduce) information that is less relevant but do not filter it out completely. According to this theory, information with ignored frequencies can still be analyzed, but not as efficiently as information with relevant frequencies.
Attenuation theory differs from late-selection theory, which proposes that all information is analyzed first and judged important or unimportant later; however, this theory is less supported by research.