Slide Slam B4
The role of the medial parietal cortex in referring to common objects
Elliot Murphy1,2, Kiefer J Forseth1,2, Oscar Woolnough1,2, Patrick S Rollo1, Zachary Roccaforte1, Nitin Tandon1,2,3; 1University of Texas Health Science Center, Houston, Texas, 2Texas Institute for Restorative Neurotechnologies, Houston, Texas, 3Memorial Hermann Hospital, Houston, Texas
The ability to establish referential relations is a hallmark of language. While formal processes of semantic computation have received detailed theoretical treatment, the neural architecture for language composition remains less well understood. Here, we isolate the mapping of sentence representations onto successful versus unsuccessful referents. We used intracranial electrocorticography to address this issue, providing the most detailed, high spatiotemporal resolution neurobiological analyses to date of linguistic reference. Patients (n=56) implanted with either penetrating depth or surface grid electrodes (n=13,298) for the evaluation of medically refractory epilepsy were asked to quickly and accurately articulate the name of common objects in response to orthographic descriptions (average: 6.5 words, range: 3-12 words; average response time: 1385ms), presented in rapid serial visual presentation (500ms per word). The final word in each sentence was either referential to a lexical item, or non-referential (“A person at the circus who makes you [laugh/commute]”) (84 trials per condition). We analyzed broadband gamma (70–150Hz) activity at each electrode to index cognitive engagement. We found robust responses in medial parietal cortex, para-hippocampal cortex and lateral prefrontal cortex for the encoding of semantically coherent referential expressions. We identified greater gamma activity at the onset of the final word in referential trials across left inferior frontal regions (150ms post-onset), medial parietal cortex (250ms), para-hippocampal gyrus (400ms) and ventromedial prefrontal cortex (730ms). We also found greater gamma power for non-referential trials in pSTG, marking the violation effect. Medial parietal regions mediate action encoding, which possibly explains these effects, since non-referential items often involved impossible/failed actions (e.g. “Something that grows on your [face/name]”). These regions are also part of the default network, which shows activity increases during endogenous attentional tasks, with representational search likely being greater for referential relations. We also contrasted non-referential trials that were semantically coherent and contained only weak violations of lexico-semantic or syntactic rules (e.g., “Something you beat with grass”, which does not refer to a lexical item but which is still semantically coherent) with non-referential trials that contained strong violations (e.g., “A glass made of Wednesday”). We found increased gamma activity in anterior inferior frontal gyrus and posterior middle temporal gyrus for semantically coherent non-referential trials. This points to a role for these regions in in constructing semantically legal structures, which do not require ongoing lexical search but purely the evaluation of the read-out of semantic composition operations. Our intracranial analyses in a large patient cohort represent the first direct recordings of a central component of natural language semantics: reference to lexical concepts. Insight into this fronto-parietal referential network will contribute to rehabilitative solutions and neuro-prosthetic designs for individuals with reading deficits, and in particular patients with anomic aphasia and alexia.