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= Comprehension of Metaphor in the Brain=

Metaphors are figures of speech used by a speaker to compare and create association between two seemingly unrelated objects or topics. In simple terms, metaphors compare two concrete objects or abstract topics without using the words "like" or "as". For example, the sentence “The rumor at the hospital is that the surgeon is a butcher,” contains a metaphor. In this example, the speaker associates the surgeon with the act of butchery. This is done in order to give the impression that the surgeon is incompetent.

Metaphors are not just simply literary devices. They are integral parts of life that involve a great deal of cognitive mechanisms. The subject of metaphor comprehension has gained a lot of interest among researchers studying figurative language processing. George Lakoff and Mark Johnson (philosopher) were the first to propose that the way humans understand and relate to the world is metaphorical. They theorized that metaphors reflect the way humans think and feel about certain objects or topics. Lakoff and Johnson’s idea that metaphors shape and are shaped by thoughts was widely influential in the field. Despite its popularity, the theory has also received its wide share of criticism from other researchers. For instance, some critics believe that Lakoff and Johnson's theory is not falsifiable, and thus not scientifically verifiable.

Searle’s classical account of metaphor comprehension is a well-known two-stage model. First, the listener must determine whether to look for a non-literation interpretation for the statement. Second, if the listener has decided to seek a metaphorical interpretation, then they must employee some set of principles or strategies for generating a lot of possible meanings. They must then identify which meaning is most likely to apply to the present statement. This complex process is influenced by many factors including context, prior knowledge of the object or topics, and the amount of cognitive effort allotted to processing the statement.

Researchers have studied and mapped the neural systems (system of neurons) underlying comprehension of metaphors using brain imaging tools such as functional magnetic resonance (fMRI),  positron emission tomography (PET), and  event-related potentials (ERPs). A number of brain structures have been shown to play a role in the processing of metaphors in the brain.

The Standard Model
Much like Searle’s classical account of metaphor comprehension, the standard model describes metaphor processing as occurring in two stages. It describes the fact that metaphorical meaning is accessed only after the literal meaning is rejected. Proponents of the model theorize that understanding metaphoric language takes longer than literal language. They also theorize that understanding metaphoric and literal language involves different cognitive processes.

Much of the research testing the predictions of the standard model have failed to find supporting evidence. Empirical studies have shown that literal processing of a statement containing a metaphor does not necessarily have to occur prior to figurative processing. Studies have also failed to find support for the prediction that metaphoric language takes longer to compute and process than literal language. Studies have shown that both literal and metaphoric languages are read and understood at the same rate.

The Parallel Hypothesis
The parallel hypothesis is based on the assumption that the literal and metaphorical meanings of a statement are accessed directly in parallel. Proponents of the parallel hypothesis predict that the literal meaning of a statement does not have to be rejected in order for a metaphorical one to be retrieved. Supporters of the hypothesis therefore expect that metaphoric and literal languages be processed using the same cognitive mechanisms.

The majority of brain imaging research to date has failed to find empirical support for the claims of the parallel hypothesis. The general trend in the literature has been to move away from the idea that literal and figurative meanings for language occur in parallel.

The Context Dependent Model
The context dependent model states that context is critical to uncovering the metaphoric meaning of a language. The idea behind the model is that metaphorical meaning is accessed based on the context of the statement containing the metaphor.

An example of a typical event-related potential (ERP) study testing the validity of this model placed metaphors in relevant (e.g., “Foxes are sly”) or irrelevant (e.g., “Foxes are ferocious”) contexts. Event-related potentials (ERPs) were recorded as participants processed these metaphors. Supporters of the context-dependent model would have predicted that metaphors with relevant and irrelevant contexts would modulate the N400 (neuroscience) brainwave response differently. The N400 is a part of a normal brain response to stimuli 400 milliseconds(ms) after its presentation. The study found that metaphors in irrelevant contexts elicited more negative event-related potentials (ERPs) in the N400 window (as well as the subsequent 600-100 ms interval) than metaphors in regular contexts do. In showing that metaphors with relevant context are easier to process, the results support the context-dependent hypothesis. As well, these results support the idea that context, overall, modulates metaphoric comprehension.

Overview of Hemispheric Activation
The left hemisphere of the brain is generally agreed-upon to be the dominant neural area for language processing. Despite this fact, the right hemisphere was initially theorized to be the dominant substrate involved in the comprehension of metaphor This hypothesis was based on the fact that greater right-hemisphere involvement was found to occur in higher-level, top-down, and more complicated sentence integration. These processes have been identified as being useful in the comprehension of metaphor. Examples of the processes used in metaphoric comprehension include generating inferences, conceptual blending, mapping and elaborating.

This right hemisphere hypothesis was based mainly on research conducted with brain-injured individuals. Early neuropsychological studies showed that subtle problems assessing metaphors can occur after right hemisphere damage. This hypothesis was somewhat supported by work with normal, non-injured persons. In the first such study with non-brain damaged individuals, Anaki and colleagues found that metaphoric meaning decayed rapidly in the left hemisphere while being held maintained in the right hemisphere. . Subsequent attempts to replicate the exact results reported by Anaki and colleagues’ work have been largely unsuccessful. Recent visual field priming experiments do not strongly support the claims that right hemisphere is the dominant substrate for metaphor comprehension. . The literature suggests that both hemispheres are useful in supporting metaphoric comprehension.

Although there is some evidence for greater right than left hemisphere involvement in metaphoric processing, the contribution of the each two hemispheres to metaphoric comprehension is necessary. Although not by identical mechanisms, metaphoric processing have been founded to be activated in both hemispheres across experiments. The exact involvement of the two hemispheres is still unclear to date. Future visual field experiments must examine hemispheric asymmetries in order to a gain a better understanding of the mechanisms by which the two hemispheres support metaphor comprehension.

Cognitive Neuroscience Research
A number of neuroanatomical structures are thought to play a role in the comprehension of metaphor. Neuroimaging methods such as functional magnetic resonance (fMRI), positron emission tomography (PET), and event-related potentials (ERPs) have been used to map these neuroanatomical structures.

A well-known positron emission tomography (PET) study conducted by Bottini and colleagues investigated the brain structures activated during comprehension of metaphor and literal language. Metaphoric and literal language was associated with similar activations in the left hemisphere. Comprehension of metaphor also activated the prefrontal cortex, the middle temporal gyrus, the precuneus, and the  posterior cingulate. . Other research showed that metaphoric and literal languages activate many of the same left hemisphere areas, with metaphoric language additionally activating right hemispheric areas. This research was interpreted as a strong support for the right hemisphere dominance theory.

Currently, most studies no longer report results in favor of a strong single hemisphere contribution for metaphoric language. It is commonly found in the literature that both hemispheres play a role in metaphoric comprehension. A more recent functional magnetic resonance imaging (fmRI) study showed reading metaphors in contrast to literal sentences signaled changes in the left lateral interior frontal, inferior temporal, and posterior middle/inferior temporal gyri. The strongest activation occurred in the anterior part of the left inferior frontal gyrus, an area equivalent to Brodmann area 45 and Brodmann area 47. These results indicated that the inferior frontal gyrus plays a key role in the process of understanding metaphors. Another recent study used event-related functional magnetic resonance imaging (ER-fMRI) to study the neural substrates involved in metaphoric comprehension. The results once again showed that metaphors activated the left inferior frontal gyrus. These results have since been replicated across a number of studies.



Other brain areas implicated in the comprehension of metaphor include the left superior frontal gyrus, an area equivalent to Brodmann area 8, as well as the left medial frontal gyrus, an area equivalent to Brodmann area 9.

The processing of metaphor likely recruits different brain areas depending on the cognitive effort a certain statement demands. It is important to remember that no two metaphoric expressions are the same. Neural resources recruited for processing a given metaphor depends on factors such as its familiarity, figurativeness, and difficulty. This idea was explored in a recent study, which compared neural activation using functional magnetic resonance imaging (fMRI) for literal sentences, familiar and easy to understand metaphors, unfamiliar and easy to understand metaphors, and unfamiliar and difficult metaphors. Unlike literal sentences, metaphors activated the right insula, left temporal pole, and right inferior frontal gyrus. Familiar metaphors recruited the right middle frontal gyrus. Easy metaphors recruited the left middle frontal gyrus, while difficult metaphors recruited the left inferior frontal gyrus. This study was important because it showed that different types of metaphors recruit different areas of the brain. Therefore, the evidence showing that comprehension of metaphors likely recruits different brain areas does not suggest evidence for modularity of the brain.

The graded stimulus hypothesis is a newly proposed theory of metaphoric language processing. . It is based on the idea that when a commonly used metaphor is conventionalized by a listener, its figurative meaning will be considered the more salient interpretation. According to this theory, preferential right hemisphere contribution should only be relevant for novel metaphors, but not frequently used metaphors. Most experiments do not differentiate between conventional and novel types of metaphors. The "graded stimulus hypothesis" attempts to account for the varying accounts between experiments by explaining the differences in terms of the salience of the stimuli.

A huge limitation in the study neural basis of metaphoric comprehension is that a wide variance exists in the tools and methods used across the studies. Experiments have been conducted in many languages including English, Mandarin ., German, Dutch and Japanese. The tasks performed vary from passive reading of metaphors to judgements of plausibility and understanding. Sometimes the metaphor language is presented visually and sometimes it is presented orally. All of these factors may potentially account for the large variance that exists in the brain imaging literature.

Future Work
The idea, first introduced by Lakoff and Johnson, that metaphors reflect the way humans think remains controversial. Many debates over the role of metaphoric thought continue within the scientific community today. On one side, some scientists criticize the theory of metaphoric thought because they view it as being unfalsifiable. Other scientists point to evidence they view strongly demonstrates that people conceptualize certain topics via metaphor. There is a great effort toward studying and understanding how cognition and metaphoric comprehension are interrelated.

A newly developing movement in the metaphor literature is looking at the role of embodied cognition in people’s use and understanding of metaphoric language. Embodied cognition refers to the idea that all constructs of the mind are shaped by aspects of the body such as the motor and perceptual systems. There is a strong view that metaphoric language and thought not only arise from, but are grounded within bodily experience. It is thought that the body is a source of information that influences the processing of metaphors. A study showed that people’s hunger somewhat predicts their use of metaphorical expressions for different forms of desire, such as “I hunger for fame”. . Another study showed that when people's facial expression was matched to metaphor content it increased  their ability to process the metaphor. . Scientists are currently seeking to understand how people's bodily interactions with their environment may mediate their comprehension of metaphor in the brain.

Scientists are seeking to build a more unified theory of metaphor comprehension in the brain. It is highly possible that humans do not process all types of metaphors in the same way. This theory will need be broad enough to deal with the many types of metaphoric language that people are normally exposed to and the factors that influence comprehension. One future challenge for the field will be to account for these different types of metaphors within a single theoretical framework. It will be an on-going challenge for the field. As research in the field of metaphoric language continues to progress, the goal of the field will be to clearly account for what happens sequentially during metaphoric processing in the brain.