Evidence for the contribution of a threshold retrieval process to semantic memory

It is widely held that episodic retrieval can recruit two processes: a threshold context retrieval process (recollection) and a continuous signal strength process (familiarity). Conversely the processes recruited during semantic retrieval are less well specified. We developed a semantic task analogous to single-item episodic recognition to interrogate semantic recognition receiver-operating characteristics (ROCs) for a marker of a threshold retrieval process. We fitted observed ROC points to three signal detection models: two models typically used in episodic recognition (unequal variance and dual-process signal detection models) and a novel dual-process recollect-to-reject (DP-RR) signal detection model that allows a threshold recollection process to aid both target identification and lure rejection. Given the nature of most semantic questions, we anticipated the DP-RR model would best fit the semantic task data. Experiment 1 (506 participants) provided evidence for a threshold retrieval process in semantic memory, with overall best fits to the DP-RR model. Experiment 2 (316 participants) found within-subjects estimates of episodic and semantic threshold retrieval to be uncorrelated. Our findings add weight to the proposal that semantic and episodic memory are served by similar dual-process retrieval systems, though the relationship between the two threshold processes needs to be more fully elucidated.     

Semantic similarity of labels and inductive generalization: Taking a second look

Prior research suggests that preschoolers can generalize object properties based on category information conveyed by semantically-similar labels. However, previous research did not control for co-occurrence probability of labels in natural speech. The current studies re-assessed children’s generalization with semantically-similar labels. Experiment 1 indicated that adults made category-based inferences regardless of co-occurrence probability; however, 4-year-olds generalized with semantically-similar labels that co-occurred in child-directed speech (e.g., bunny–rabbit) but not with non-co-occurring labels (e.g., crocodile–alligator). Experiment 2 indicated that generalization with semantically-similar labels increased gradually between 4- and 6-years of age. These results are discussed in relation to theories of early learning.     

Dissociation and association of the embodied representation of tool-use verbs and hand verbs: An fMRI study

Embodied semantic theories suppose that representation of word meaning and actual sensory-motor processing are implemented in overlapping systems. According to this view, association and dissociation of different word meaning should correspond to dissociation and association of the described sensory-motor processing. Previous studies demonstrate that although tool-use actions and hand actions have overlapping neural substrates, tool-use actions show greater activations in frontal–parietal–temporal regions that are responsible for motor control and tool knowledge processing. In the present study, we examined the association and the dissociation of the semantic representation of tool-use verbs and hand action verbs. Chinese verbs describing tool-use or hand actions without tools were included, and a passive reading task was employed. All verb conditions showed common activations in areas of left middle frontal gyrus, left inferior frontal gyrus (BA 44/45) and left inferior parietal lobule relative to rest, and all conditions showed significant effects in premotor areas within the mask of hand motion effects. Contrasts between tool-use verbs and hand verbs demonstrated that tool verbs elicited stronger activity in left superior parietal lobule, left middle frontal gyrus and left posterior middle temporal gyrus. Additionally, psychophysiological interaction analyses demonstrated that tool verbs indicated greater connectivity among these regions. These results suggest that the brain regions involved in tool-use action processing also play more important roles in tool-use verb processing and that similar systems may be responsible for word meaning representation and actual sensory-motor processing.     

The N400 effect in children: relationships with comprehension, vocabulary and decoding

Using event-related potentials (ERPs), we investigated the N400 (an ERP component that occurs in response to meaningful stimuli) in children aged 8-10 years old and examined relationships between the N400 and individual differences in listening comprehension, word recognition and non-word decoding. Moreover, we tested the claim that the N400 effect provides a valuable indicator of behavioural vocabulary knowledge. Eighteen children were presented with picture-word pairs that were either ‘congruent’ (the picture depicted the spoken word) or ‘incongruent’ (they were unrelated). Three peaks were observed in the ERP waveform triggered to the onset of the picture-word stimuli: an N100 in fronto-central channels, an N200 in central-parietal channels and an N400 in frontal, central and parietal channels. In contrast to the N100 peak, the N200 and N400 peaks were sensitive to semantic incongruency with greater peak amplitudes for incongruent than congruent conditions. The incongruency effects for each peak correlated positively with listening comprehension but when the peak amplitudes were averaged across congruent/incongruent conditions they correlated positively with non-word decoding. These findings provide neurophysiological support for the position that sensitivity to semantic context (reflected in the N400 effect) is crucial for comprehension whereas phonological decoding skill relates to more general processing differences reflected in the ERP waveform. There were no correlations between ERP and behavioural measures of expressive or receptive vocabulary knowledge for the same items, suggesting that the N400 effect may not be a reliable estimate of vocabulary knowledge in children aged 8-10 years.     

Differential prefrontal–temporal neural correlates of semantic processing in children

This study used functional magnetic resonance imaging (fMRI) to examine brain–behavior correlations in a group of 16 children (9- to 12-year-olds). Activation was measured during a semantic judgment task presented in either the visual or auditory modality that required the individual to determine whether a final word was related in meaning to one of two previous words (e.g., found–tank–lost). The main finding was that higher performers (i.e., accuracy) were associated with more activation in posterior representational systems including the inferior and middle temporal gyri, whereas lower performers were associated with more activation in anterior regions including the inferior and middle frontal gyri. This pattern of results was interpreted as reflecting an elaborated semantic representational system in temporal areas for the high accuracy performers that allowed them to efficiently and accurately make meaning based judgments. The low accuracy performers may have an inaccurate or weakly interconnected semantic system that results in greater use of frontal areas in a feature selection process.     

Hemispheric asymmetries in the split-fovea model of semantic processing

We report a series of neural network models of semantic processing of single English words in the left and the right hemispheres of the brain. We implement the foveal splitting of the visual field and assess the influence of this splitting on a mapping from orthography to semantic representations in single word reading. The models were trained on English four-letter words, presented according to their frequency in all positions encountered during normal reading. The architecture of the model interacted with the training set to produce processing asymmetries comparable to those found in behavioral studies. First, the cueing effects of dominant and subordinate meanings of ambiguous words were different for words presented to the left or to the right of the input layer. Second, priming effects of groups of related words were stronger in the left input than the right input of the model. These effects were caused by coarser-coding in the right half compared with the left half of the model, an emergent effect of the split model interacting with informational asymmetries in the left and right parts of words in the lexicon of English. Some or all of the behavioral data for reading single words in English may have a similar origin.     

What do graded effects of semantic transparency reveal about morphological processing?

We examined the influence of semantic transparency on morphological facilitation in English in three lexical decision experiments. Decision latencies to visual targets (e.g., CASUALNESS) were faster after semantically transparent (e.g., CASUALLY) than semantically opaque (e.g., CASUALTY) primes whether primes were auditory and presented immediately before onset of the target (Experiment 1a) or visual with an stimulus onset asynchrony (SOA) of 250 ms (Experiment 1b). Latencies did not differ at an SOA of 48 ms (Experiment 2) or with a forward mask at an SOA of 83 ms (Experiment 3). Generally, effects of semantic transparency among morphological relatives were evident at long but not at short SOAs with visual targets, regardless of prime modality. Moreover, the difference in facilitation after opaque and transparent primes was graded and increased with family size of the base morpheme.     

Semantic category effect and emotional valence in female university students

The semantic category effect represents a category dissociation between biological and nonbiological objects in picture naming. The aim of this preliminary study was to further examine this phenomenon, and to explore the possible association between the effect and subjective emotional valence for the named objects. Using a speeded picture naming task, vocal reaction times for 45 items were divided into four categories based on emotional valence rating and semantic category, and examined in 36 female university students. Analyses of the data indicated an "animate/inanimate" category dissociation favouring animate objects, in tandem with a potential relationship between subjective emotional valence and semantic processing underlying picture naming.