Role of the Left Anterior Temporal Pole in Semantic Integration

One of the most rewarding aspects of studying neuroscience, for me, is realizing just how good I have it. Through the study of the myriad disorders and deficits that we have so far been exposed to, the subtle processes bubbling just under the surface of our awareness come into stark relief. In Dr. Hurley's talk, we were ushered into the world of aphasic research. Aphasias, and their associated anomias, as we have learned result from damage within the left hemisphere's language network. Hurley brought to our attention the incredible diversity of this disorder, both in its pathology and symptom onset, but focused on a subset of aphasias known as Primary Progressive Aphasia (PPA). PPA is a syndrome in which there is a progressive degeneration of neural tissue due to a neurodegenerative condition. PPA offers an opportunity for researchers to study the gradual and concentrated deterioration of the left hemisphere's language network, which is a common target of neurodegenerative disease. The syndrome can be so selective in some cases that it can produce observable subsets, each with its own focal atrophy and associated symptoms. Of particular interest to Hurley and his team, as well as other researchers, is the subset of PPA known as semantic PPA, or simply PPA-S. Individuals with PPA-S have a unique impairment in their word comprehension along with the other typical anomias. Imaging studies performed on these patients revealed that, unlike other PPA patients who showed atrophy typically in the classic left-hemisphere language regions, PPA-S patients showed peak atrophy in the anterior part of the left temporal lobe, referred to as the temporal pole. This evidence suggests that the left temporal pole may have some function in semantic word memory.

The idea that the left temporal pole has a key role in semantic memory storage has been a highly contested topic in the literature on the brain's language networks. The question is whether or not this area is involved in the storage of semantic memory for words and should thus be included in a revised language network model. The two schools of thought that are generally in contention are ones that propose highly distributed models for semantic memory, meaning that semantic memory would be achieved through a variety of brain regions working in tandem, versus the researchers that would argue for the temporal pole acting as a "semantic hub" integrating information from other brain regions involved in language processing.

Summary diagrams comparing (A) the large-scale semantic network of the human brain and (B) a probable homologous network in the macaque monkey brain, comprised of posterior inferior parietal cortex (PG/7a), STS, parahippocampal cortex (TF, TH), dorsolateral prefrontal cortex, posterior cingulate and retrosplenial cortex, lateral orbital frontal cortex, and VMPFC. Source: J.R. Binder, R.H. Desai, W.W. Graves, L.L. Conant Where is the semantic system? A critical review and meta-analysis of 120 functional neuroimaging studiesCereb. Cortex, 19 (2009), pp. 2767–2796

 A team of researchers at Harvard University Medical School lend their weight to the latter option. Their study, published last October in the journal NeuroImage, summarizes the debate between the two competing models and gives meta-analyses that seem to support both camps. One meta-analysis, published by Binder et al. in 2009, gathered the results from 120 neuro-imaging studies that were focused on finding the "semantic system". The studies chosen for this meta-analysis were carefully selected for their differentiation of semantic tasks from other language and cognitive tasks. Binder et al. identified seven regions that are consistently activated during semantic processing in these studies: posterior inferior parietal lobule (angular gyrus), middle temporal gyrus, fusiform and parahippocampal gyri, dorsomedial prefrontal cortex, inferior frontal gyrus, ventromedial prefrontal cortex, and posterior cingulate gyrus. These data seem to support the idea that a multitude of regions are involved in semantic processing. The second meta-analysis reviewed by the team at Harvard was performed by Visser et al. in 2010 and it looked at data from 164 fMRI and PET studies that were focused on the temporal pole. They concluded from this analysis that the temporal pole was engaged in semantic processing regardless of the type of task or stimulus used. This lead the researchers from the second meta-analysis to the temporal pole as a hub for semantic processing. The researchers also emphasized the careful study of the temporal pole in imaging, a point that Hurley alluded to as well, as it is often excluded from the field of view or obscured by the distortion artifact and so activation in the area can often be missed. So with these two extensive meta-analyses in hand, the team at Harvard sought to add fuel to the flame in the debate.


The Harvard team, who I will henceforth refer to as Brickhouse et al. regardless of which of the researchers name it is was actually published under simply for my own amusement (and because everyone's a Commodores fan), studied a population of patients with the early stages of Alzheimer's disease. Along with the cognitive decline usually associated with Alzheimer's disease, it is often the case that patients also have a deficit in semantic memory. So Brickhouse and his team hypothesized that they could correlate atrophy in these Alzheimer's patients with one of the two semantic processing models. In other words, they believed that if semantic memory is facilitated by the network of brain regions proposed by Binder, then they would observe greater atrophy in these regions and an accompanying high degree of semantic impairment. However, if they observed this high degree of semantic impairment in patients with the most atrophy localized in the temporal pole, then they could support the semantic hub model.

Results of Brickhouse et al. study. Peak atrophy is shown in anterior temporal pole and is correlated with the highest degree of semantic impairment in Alzheimer's patients.

Brickhouse and his team found that deficits in cued naming, a measure of semantic impairment, was correlated with the cortical thinning of the left temporal pole. But what Brickhouse makes clear is that these results in no way mark an end to the debate. The temporal pole's role in semantic processing is still highly contested and includes variations on the two theories previously mentioned. Many researchers have come to agree that the task of semantic processing is achieved through a distributed network of regions, like any cognitive process really. A more accurate description of the debate offered by the Harvard team is whether or not this distributed activity is modulated by an amodal hub, that evidence seems to suggest is located in the left temporal pole. As articulated best by the Harvard researchers,

 "Such a hub would facilitate efficient, accurate access to semantic knowledge, regardless of task modality (e.g., picture naming, spontaneous speech, reading or writing). Although we have focused on the distributed versus hub models in the present study, there are additional theories regarding the role of the ATL in the brain systems subserving semantic memory." 

So as imaging methods are refined and neuropsychological tests perfected, hopefully we will be able to shed more light on the role of the temporal lobe and whether or not it is playing as crucial of a role as researchers like Hurley and his team at Northwestern have proposed for it. What excites me the most as a student in the field is knowing that some of the most important discoveries have yet to be made. When I used to think back on the field, I pictured the earliest researchers in language, people like Broca and Wernicke, and thought how I or anyone else would never achieve such recognition. But now, put at rest by the knowledge that incredible things are being learned each year by contemporaries that I may someday have the opportunity to work with, I am filled with the hope and desire that maybe one day I, or anyone of us, can be immortalized in the textbooks of young neuroscientists who hate us solely for being another name and study to remember.  


Sources: Naming impairment in Alzheimer's disease is associated with left anterior temporal lobe atrophy

Hurley, Robert S. "Neural Mechanisms of Object Naming and Word Comprehension in Primary Progressive Aphasia." Journal of Neuroscience. 32.14 (2012): 4848 – 4855.