In linguistics, backchannels are listener responses at a point in time of a conversation where another person is the primary speaker. In other words, any utterances which are produced by a listening individual in a conversation. Backchannels can be both verbal and non-verbal in nature, and are frequently phatic expressions, primarily serving a social or meta-conversational purpose, rather than involving substantial two-way communication. For instance, some of the frequently used backchannel feedback would be "yeah", "uh-huh", "hm" or "right", expressing attention, understanding and agreement.
Definition and use
The term was coined by Victor Yngve in 1970, in the following passage: "In fact, both the person who has the turn and his partner are simultaneously engaged in both speaking and listening. This is because of the existence of what I call the back channel, over which the person who has the turn receives short messages such as 'yes' and 'uh-huh' without relinquishing the turn." Backchannel responses are a part of basic human interaction because to have a productive or meaningful person-person interaction humans must cooperate with one another when participating in a conversation. Meaning, when two people are involved in a conversation one person is generally the one talking and the other is listening and the listener can show the speaker they are cooperating through backchannel responses.
The term "backchannel" was designed to imply that there are two channels of communication operating simultaneously during a conversation. The predominant channel is that of the speaker who directs primary speech flow. The secondary channel of communication (or backchannel) is that of the listener which functions to provide continuers or assessments, defining a listener's comprehension and/or interest. In other words the term "backchannel" is used to differentiate between the roles of the people involved in a conversation. The person doing the speaking is thought to be communicating through the "front channel" while the person doing the listening is thought to be communicating through the "backchannel." The term "backchannel" does not necessarily define the listener's role in the conversation but helps us to understand how the person that is taking on the role of the listener responds to the person taking on the role of the speaker. Recent research, which can be seen below, has also suggested new terms for these two functions. They have proposed the term generic in place of continuers and specific in place of assessments.
Usually, the way backchannel is used would be a person telling a story or explaining something to one or more individuals, involved in a conversation, who would respond to him with short verbal messages or non-verbal body language. In order to indicate that they are listening and paying attention to the speaker, they might produce sounds as "right", "yeah", etc. or give a nod. Such acknowledgments or small gestures help the speaker understand that the listeners are interested and he should go on with his story. 
In recent years, scholars have challenged the mainstream definition by adding the "optionality" in the definition of "backchannel". The use of backchannel is never necessary and is always a supplement to a pre-existing conversation.
Classification of Backchannels
The Cooperative Principle was first introduced in 1975 by a British philosopher of language - Paul Grice. The principle implies both speaker and listener to contribute and cooperate with one another throughout a conversation, in order to have a mutual understanding. "Make your contribution such as is required, at the stage at which it occurs, by the accepted purpose or direction of the talk exchange in which you are engaged."  (Grice, 1975). The Cooperative Principle consists of four following maxims: maxim of quantity, quality, relations, and maxim of manner, respectively. The maxim of quantity stands for there being enough information in order to properly understand the context. The second maxim, the maxim of quality, requires a speaker have his speech backed up by evidences and facts in order to prevent any leak of misinformation. The maxim of relation makes sure that the person speaking is being relevant and sticks to the main point or topic of the conversation. The last and fourth maxim of manner, is responsible for getting straight to the point, being lucid, brief and having a clearly expressed or precise presentation. 
Backchannel responses can show that the listener understands, agrees, is surprised by, is angered by, and more by what the speaker is saying. Backchannel communication is present in all cultures and languages, though frequency and use may vary. For example, backchannel responses are not only a key component of spoken languages but they are also important in sign languages. Another example of the variability that occurs in backchannel responses in different languages is that Germans produce less backchannel responses and use back channel responses less frequently. Confusion or distraction can occur during an intercultural encounter if participants from both parties are not accustomed to the same backchannel norms. Studies have shown that when people learn a second language they learn or adapt to how people that are native speakers of that language use backchannel responses. This may occur in terms of the frequency that a person produces backchannel responses or what those responses sound like.
Types of backchannels
Due to research development in recent years, backchannel responses have been expanded to include sentence completions, requests for clarification, brief statements, and non-verbal responses and now fall into three categories: non-lexical, phrasal, and substantive.
A non-lexical backchannel is a vocalized sound that has little or no referential meaning but still verbalizes the listener's attention, and that frequently co-occurs with gestures. In English, sounds like uh-huh and hmm serve this role. Non-lexical backchannels generally come from a limited set of sounds not otherwise widely used in contentful conversational speech; as a result, they can be used to express support, surprise, or a need for clarification at the same time as someone else's conversational turn without causing confusion or interference.
English allows for the reduplication, or repetition, of syllables within a non-lexical backchannel, such as in responses like uh-huh, mm-hm, or um-hm, as well as for single-syllable backchanneling. In a study examining the use of two-syllable backchannels that focused on mm and mm-hm, Gardner found that the two tokens are generally not identical in function, with mm being used more productively as a continuer, a weak acknowledgment token, and a weak assessment marker. In contrast, mm-hm is generally used as a backchannel to signal that the speaker is yielding their conversational turn and allowing the other speaker to maintain control of the conversational floor.
Phrasal and substantive backchannels
Phrasal backchannels most commonly assess or acknowledge a speaker's communication with simple words or phrases (for example, "Really?" or "Wow!" in English). Substantive backchannels consist of more substantial turn-taking by the listener and usually manifest as asking for clarification or repetitions.
One of the conversational functions of phrasal backchannels is to assess or appraise a previous utterance. Goodwin argues that this is the case for the phrasal backchannel oh wow, where use of the backchannel requires a specific conversational context where something unexpected or surprising was said. Similarly, more substantive backchannels such as oh come on, are you serious? require a context where the speaker is responding to something exasperating or frustrating. In both of these cases, Goodwin argues that the backchannels focus only on addressing some aspect of the immediately proceeding utterance rather than the larger conversation itself. As a result, they have a broad conversational distribution, appearing both in the middle of extended talk as well as at the end of longer conversational turns.
Research in 2000 has pushed back on the notion of backchannels, in which the listener's role is merely to receive information provided by the speaker. Bavelas, Coates, and Johnson put forth evidence that listeners' responses help shape the content of the speaker. They grouped acknowledgment tokens into two categories: generic and specific. Generic responses could be considered back channels and would include mm hm and yeah, while specific responses would involve a reaction to the given context. Examples might include Oh! or a facial display of concern.
They transcribed students telling a fellow participant about a close call experience that they had had. With one group of participants, they had the listener perform another task to distract them from the story being told. The researchers asked independent reviewers to code the verbal and visual responses of the narration events as generic or specific. They also asked other independent reviewers to gauge the quality of the narration in each case.
They concluded that the responses from the distracted listeners included significantly fewer specific responses than from the undistracted listeners. In addition, they found that the quality of the narration was dramatically lower when the listener was distracted. Their basic contention was that listeners are co-narrators and help the storyteller in his or her narration. In other words, a storyteller tells a better story with an audience that is engaged than one that is not.
Tolins and Foxtree have also published research demonstrating how backchannel communication influences speakers. Their research was specifically looking at how speakers respond to generic responses compared to specific responses.
In 2017, Kyoto University's Graduate program of Informatics began developing a robot to assist individuals, more specifically the elderly, with mental health through the use of attentive listening. They utilized backchannel generation as a method for the robot to have some form of feedback to feel like a real conversation. Further research is being conducted to be more practical.
In 1997 there was a study on 205,000 general utterances that showed 19% of those constituted a "backchannel". This study was apart of a new method of "discourse detection" and "statistical modeling" that allowed them to have such a large sample size, giving the possibility of generalizing this data to larger communities.
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This article needs additional or more specific categories. (December 2018)