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The Anterior Temporal Lobe, often shortened to ATL, is located at the anterior (frontal) area of the temporal lobe. The ATL appears to have a critical role in semantic memory processing, with other functions relating to social cognition.

Structure
There has been some difficulty delineating the exact extent of the ATL. Its complex and diverse structure has made it difficult to find a functionally homogeneous region. Some definitions have limited the area that they define as the ATL to roughly Brodmann’s area 38, otherwise known as the temporal pole. The temporal pole is the most rostral area of the temporal lobe and is involved in many high-order cognitive functions. Other definitions have broadened the extent of the ATL to include the anterior half of the temporal gyri. The temporal gyri are the inferior temporal gyrus, the middle temporal gyrus, and the superior temporal gyrus. Further areas include anterior sections of the fusiform gyrus and the parahippocampal gyrus.

The ATL has connections with many other areas of the brain. It is connected to more posterior (rear) areas of the temporal lobe through the temporal gyri. The temporal gyri are important in processing sensory information. The inferior temporal gyrus, along with the fusiform gyrus exist along the ventral visual stream, which are involved in object recognition and form representation. The ventral visual stream starts from within the visual cortex in the occipital lobe and culminates in areas of the ATL. It has been proposed that there is posterior-to-anterior gradient, with more anterior regions involved in information of higher specificity. Other connections include the prefrontal cortex and the amygdala.

The ATL has historically been a difficult area in the brain to image using functional magnetic resonance imaging (fMRI) due to its location in the brain. Its location near the sinuses and air bone interfaces causes magnetic inhomogeneities meaning that attempts to use typical echo planar fMRI sequences resulted in image distortion, loss of signal and increased noise. More recent developments have used alternative echo planar imaging fMRI sequences as well as post-image acquisition corrections to mitigate these effects.

Semantic Memory
There have been many attempts to generalise the function of the ATL. The leading consensus is that it is primarily involved in the processing of semantic memory. Semantic memory is the knowledge we accumulate about the world, including facts and concepts. Language, object recognition and social cognition are some examples of semantic information. The earliest evidence for the ATL’s role in the processing of semantic memory is through the study of semantic dementia patients. Semantic dementia is sometimes seen as a variant of the frontotemporal dementia, in which areas of the frontal lobe and the temporal lobe atrophy. It is characterised by deficits notably in semantic memory processing, while leaving other cognitive functions relatively intact. Magnetic resonance imaging (MRI) studies of the brains of those with semantic dementia have revealed tissue loss in the ATL. In semantic representation tasks, the ventral and inferolateral ATL, made up of the anterior inferior temporal gyrus, the anterior fusiform gyrus, and the anterior superior temporal sulcus, have been found to be highly activated. fMRI results suggest that semantic representation is highly lateralised to the left hemisphere. In contrast, using repetitive transcranial magnetic stimulation (rTMS) over both the left and right hemisphere ATLs elicited a reduction in performance in synonym judgment tasks. To explain the differences in lateralisation, it has been suggested that the semantic judgment tasks require more language processing which is more lateralised towards the left hemisphere. In concordance with this finding, semantic dementia patients with higher atrophy in the left hemisphere have been found to have more difficulty with naming. rTMS has also shown the hemispheric nature of language processing with use over the left hemisphere of the ATL causing an inability to form irregular past tense forms of verbs. There are also differences in the processing of concrete versus abstract words. The anterior superior temporal gyrus and sulcus have been found to activate more strongly to abstract over concrete words.

Semantic Hub Theories
One view of ATL function is that it operates as a hub for semantic memory. It has been theorised that it is an ‘amodal’ or domain-general hub in that it will process all kinds of semantic information. As a hub, it is where all semantic information regardless of modality can be brought together for representation. The large array of deficits in semantic processing in patients with semantic dementia has provided evidence that the ATL processes a large amount, if not all kinds of semantic information. The ATL as an amodal or domain-general hub has been contended, with counter theories stating that instead it and its constituent parts respond to particular forms of semantic information, more in line with domain-specificity. Research has helped distinguish at least 34 functionally distinct parcels (brain divisions) within the ATL.

One such theory of domain-specificity, the social knowledge hypothesis, suggests that the anterior temporal lobe is much more receptive to social conceptual information, rather than general conceptual information. Evidence for this comes from frontotemporal dementia patients with atrophy in the right anterior superior ATL were found to have much more impairment understanding social concepts compared to other non-social abstract concepts. This social knowledge hypothesis argues that three separate areas are involved in social cognition: the dorsal, ventral and superior temporal pole as well as hemispheric differences in function. The left ATL is associated with proper name retrieval, the right with feelings of familiarity. The dorsal area is associated with identifying voices and the ventral area is associated with identifying faces.

An alternative domain-general theory, the graded semantic hub theory, proposes the idea that the ATL semantically processes different types of semantic information in a graded configuration. A central amodal area exists in the form of the ventrolateral ATL. The further you move away from this central area, the more specialised the form of semantic processing becomes. Research attempting to evidence this used theory of mind tasks and non-social semantic tasks to show that for both tasks the ventrolateral ATL was active. The activity of the ventrolateral ATL suggested its role as a central amodal area, involved in processing multiple modalities of semantic information.

Social Cognition and Face Recognition
Social cognition has also been identified as key information processed by the ATL. Studies have also shown that frontotemporal dementias with only temporal lobe damage, excluding the frontal lobe, have led to increases in socially eccentric behaviour as well as sociopathic behaviours characterised by decreased affect and empathy. The changes in social behaviours were mostly observed in those with right anterior temporal damage. Right hemisphere damage to the ATL has also been associated with difficulty processing emotions or processing famous faces.

The anterior inferior temporal gyrus in the ATL has been hypothesised to be important in semantic control as well as social cognition. In a meta-analysis of fMRI studies, the left anterior inferior temporal gyrus was found to be active in four different social abilities: theory of mind, trait inference, empathy, and moral reasoning. Further studies have implicated that the left ventrolateral ATL consisting of the inferior and middle temporal gyri, is active in theory of mind tasks. Right ATL dysfunction due to semantic dementia has also seen theory of mind impairments. The ATL has also been implicated in moral cognition, the idea that the brain uses emotion and reasoning to make moral decisions.

Most research into face recognition have recognised two different involved regions, the fusiform face area (FFA) and the occipital face area (OFA). An area of the ATL has been identified which is interconnected with both the FFA and OFA. An anterior temporal face area has been proposed as a link between face recognition and facial memory. In certain forms of prosopagnosia, it has been found that there was normal activity in the face areas as well as normal connectivity to the amygdala, which processes emotions. However, it was also found that there was reduced activity in the ATL, as well a decrease in connectivity between the ATL to the face areas and the amygdala.

Anterior Temporal Lobectomy
An anterior temporal lobectomy is a form of treatment for drug resistant temporal lobe epilepsy (TLE). The procedure involves removing sections of the ATL. Although the procedure is relatively effective at treating TLE, it can leave the patient with certain semantic deficits, especially in naming tasks. Comparing left-versus-right hemisphere lobectomies have shown differences in these deficits. Left-hemisphere anterior temporal lobectomies are highly associated with impairment in language-dependent cognitive tasks, compared with right-hemisphere lobectomies. Difficulty in discrimination between semantically related objects also occurs.