From letters to composed concepts: A magnetoencephalography study of reading

Abstract

Language comprehension requires the recognition of individual words and the combination of their meanings to yield complex concepts or interpretations. This combinatory process often requires the insertion of unstated semantic material between words, based on thematic or feature knowledge. For example, the phrase horse barn is not interpreted as a blend of a horse and a barn, but specifically a barn where horses are kept. Previous neuroscientific evidence suggests that left posterior and anterior temporal cortex underpin thematic and feature-based concept knowledge, respectively, but much remains unclear about how these areas contribute to combinatory language processing. Using magnetoencephalography, we contrasted source-localized responses to modifier-noun phrases involving thematic relations versus feature modifications, while also examining how lower-level orthographic processing fed composition. Participants completed three procedures examining responses to letter-strings, adjective-noun phrases, and noun-noun combinations that varied the semantic relations between words. We found that sections of the left anterior temporal lobe, posterior temporal lobe, and cortex surrounding the angular gyrus were all engaged in the minimal composition of adjective-noun phrases, a more distributed network than in most prior studies of minimal composition. Of these regions, only the left posterior temporal lobe was additionally sensitive to implicit thematic relations between composing words, suggesting that it houses a specialized relational processing component in a wider composition network. We additionally identified a left occipitotemporal progression from orthographic to lexical processing, feeding ventral anterior areas engaged in the combination of word meanings. Finally, by examining source signal leakage, we characterized the degree to which these responses could be distinguished from one another using source estimation.

Keywords: angular gyrus; anterior temporal lobe; composition; magnetoencephalography; posterior temporal lobe; semantics.

FIGURE 1

Trial structures for each experimental procedure. Left: Experiment 1 replicated the procedure of Bemis and Pylkkänen (2011) and contained two types of blocks. In Composition Blocks, participants read two‐word phrases (red boat) or one‐word phrases, wherein the modifier was replaced with an unpronounceable consonant string (xqk boat). Their task was to indicate if a subsequent picture matched all of the words they read on that trial. In List Blocks (bottom), participants read lists of two nouns (cup, boat) or one‐word lists (xqk, boat), and then indicated if a subsequent picture matched any of the words on that trial. Top‐right: In Experiment 2, designed to parallel Experiment 1, participants read three types of phrasal stimuli: Contents Compounds (modifier specifies the function/contents of the head noun, considered a thematic relationship; trophy cabinet), Material Compounds (modifier specifies the material of the head noun, considered a nonthematic relationship; metal cabinet), or adjective‐noun phrases in which the adjective denoted a color (green cabinet; nonthematic). They were then asked to judge whether a following picture matched the meaning of the phrase they read on each trial. Bottom‐right: Participants also completed a letter‐string response localizer task (see Gwilliams et al.,  for full details), which involved passive viewing of letter‐ and symbol‐string stimuli. See the main text for complete details on all three procedures

FIGURE 2

Letter‐string response localizer. (a) Participants viewed one‐letter items embedded in high and low visual noise, four‐letter words embedded in high and low visual noise, and one‐ and four‐unit length symbol strings, in low visual noise. (b) A dissociation between high and low visual noise stimuli was observed in left occipital cortext between 100 and 180 ms post stimulus onset (p <.0001). (c) In the left posterior fusiform gyrus, high and low noise letter‐string stimuli elicited opposite responses between 105 and 175 ms (p <.0001). (d) Responses in the left anterior fusiform and inferior temporal gyrus showed an initial dissociation in responses to letter‐string stimuli and symbol stimuli (across both one‐ and four‐unit lengths) between 130 and 165 ms after stimulus onset (p <.0001). (e) A second dissociation between String Type was found slightly later, between 260 and 300 ms (p <.0001) in a cluster that was also found in the anterior fusiform, but located slightly more posterior than the initial String Type dissociation. (f) The center of mass for each cluster is shown on the inflated cortical surface

FIGURE 3

Experiment 1. Adjective‐noun composition. Responses were examined in three regions of interest (ROIs), top left. Significant composition effects, defined as magnitude increases in response to two‐word phrase (e.g., red boat) relative to two‐word lists (cup, boat) and one‐word baseline materials (xkq boat), were found in all three ROIs. In each time series, 0 ms indicates the onset of the second word on each trial (i.e., the phrasal head). Shown here are representative clusters from each region. Stars and lines indicate significant composition effects. In the ATL (bottom‐left), significant effects were found in both the 150–300 ms and 450–600 ms windows, while the AG (top‐right) showed the effects in the 300–450 ms, and 450–600 ms windows. The left PTL (bottom‐right) showed composition effects in all three analysis time windows. Although the one‐word list condition is not shown here for ease of visualization, all composition effects, by test definition, also showed a dissociation from this fourth condition

FIGURE 4

Experiment 1. Analysis of the functional ROI responses defined from the letter‐string response localizer. In the occipital and posterior fusiform ROIs, shown to be sensitive to the level of visual noise in letter‐ and symbol‐string stimuli, all conditions elicited markedly consistent responses. Significant composition effects were found in the posterior fusiform, Noise Level II ROI, but only relatively late in time (300 ms onward). The more anterior, String Type I, ROI displayed a transient dissociation between phrases and the remaining conditions at approximately 100 ms after the onset of the phrasal head, and no subsequent dissociations. This effect was found outside of the a priori analysis windows. In the slightly more posterior, String Type II, ROI, significant composition effects were found in all three analysis windows, spanning 150–600 ms. In each timeseries, 0 ms indicates the onset of the second word on each trial (i.e., the phrasal head). Stars and lines indicate significant effects in the primary analysis windows

FIGURE 5

Experiment 2. Top: Contents Compounds, involving a spatial or functional relation between the combined nouns, elicited increased response magnitudes in the left posterior temporal ROI relative to both the Material Compounds and Color‐Noun Phrases. This pattern appeared to begin in response to the phrasal modifiers and re‐emerged for the first 100–200 ms after the onset of the phrasal head. Bottom: No significant differences between the Experiment 2 phrasal stimuli were found in the series of ventral surface ROIs defined from the letter‐string response localizer

FIGURE 6

Left: Peak localization error (PLE) was calculated as the Euclidean distance between the position of each constituent source and the absolute maximum in that source's PSF. PLE for an ROI was calculated as the average PLE of all constituent sources. Right: Average CTFs for the sources within each region that showed similarly timed adjective‐noun composition effects, with half maximum absolute amplitude applied as a mask. The CTFs for the AG and PTL regions showed clear overlap, suggesting that responses localized to each may be conflated. The CTF for the more ventral String Type ROI does not appear to overlap with the more lateral sites when this half amplitude is applied, however, a small degree of overlap with the ATL adjective‐noun cluster can be seen in the continuous valued CTFs in Figure S4. String Type indicates the String Type II ROI; Adj‐Noun indicates clusters identified in the Experiment 1 contrast of adjective‐noun composition; AG, angular gyrus; ATL, anterior temporal lobe; PTL, posterior temporal lobe

FIGURE 7

Summary of the primary findings. (1) The results of the letter‐string response localizer suggest a visual to lexical transformation along the ventral surface of the left hemisphere in support of visual word recognition. White circles indicate the approximate location of each relevant cluster, which showed similar localizations and morphology to those identified by Gwilliams et al. (2016). The grey arrow indicates the inferred flow of information processing, from posterior to anterior. (2) Composition in color + noun phrases elicited the engagement of left perisylvian regions of interest, including the anterior temporal lobe, posterior temporal lobe, and angular gyrus, along with anterior sections of the left fusiform gyrus. Although a large patch of cortex is highlighted, only smaller portions of this may contain the true generators of this effect (see Figures 2, 3, 4, 5). (3) Of those areas engaged by color + noun combinations, only the left posterior temporal lobe ROI was further modulated by the need to retrieve or specify thematic relations between composing words in noun–noun combinations