Semantic Representations, Lecture 9 Flashcards
Models of Semantic Memory
To date, models of semantic memory vary on four key dimensions: amodal vs. modality-specific (grounded), local vs. distributed, innate vs. experience-dependent, and stable vs. flexible.
Amodal: Not dependent on one or more perceptual systems
Grounded: Dependent on perceptual systems
Patient evidence:
DB: Alzheimer’s disease dementia – Deficit for living things but no difference in perceptual and functional features [Animate – Sensory (Impaired); Functional (Spared)]
IW: Semantic dementia – Deficit for non-living things, but impaired on Perceptual > Functional [Inanimate – Sensory (Spared); Functional (Impaired)]
A Hybrid Model: The Hub-and-Spoke Model
The hub-and-spoke model combines both amodal and grounded aspects of semantic memory.
Amodal ‘hub’: Representation of specific concepts (subordinate categories ‘robin’) irrespective of modality
Distributed, grounded ‘spokes’: Semantic representations grounded in sensory and bodily areas (‘spokes’)
Prediction of the hub-and-spoke model: If there is an amodal hub, focal damage to that region would lead to a global semantic impairment independent of modality of information.
Semantic Dementia (SD)
Semantic Dementia (SD) is a global semantic impairment which:
-> Is cross-modal
–> Shows strong typicality effects
—> Shows preservation of general relative to specific information
SD has relative sparing of other language and cognitive abilities.
Evidence for a Hierarchy of Representations in the Brain
Evidence for a hierarchy of representations in the brain comes from an fMRI study of naming and categorisation.
-> Processing at specific levels activates anterior temporal pole.
–> Hierarchical semantic models: patient evidence from Rogers et al., (2006) fMRI study.
Global Semantic Impairment in SD - TASKS:
-> Picture categorization: presented with a category label and a photo, indicate whether the label matches the photo. SD patients struggle to match specific labels.
-> Picture naming: SD patients gradually lose specific concepts and rely on superordinate category labels.
-> Colour and object recognition: SD patients are worse at identifying atypical features for a category.
-> Delayed-copy drawing: SD patients omit atypical features and add typical features, demonstrating a strong typicality effect.
Global Semantic Impairment in SD - EVIDENCE
Some part(s) of the brain is responsible for specific semantic concepts, regardless of task.
-> Fully grounded model: uniform but distributed damage to modality-specific areas could cause global deficits.
-> Hub and spoke model: damage to the amodal hub could affect generalization across categories and representation of atypical exemplars.
-> Imaging shows bilateral damage to anterior temporal lobe in SD patients.
Amodal Hub in Global Semantic Impairment
If SD’s global deficit is caused by damage to a circumscribed brain region, the amodal hub theory could explain the global deficit.
Other semantic impairments:
-> AD: semantic impairment due to spread of pathology.
-> HSVE: semantic impairment caused by viral infection.
-> Transcortical sensory aphasia: semantic impairment caused by stroke.
Evidence for the ATL as amodal hub: Alzheimer’s disease vs Semantic Dementia
Alzheimer’s disease (AD) and Semantic Dementia (SD) are two different types of neurodegenerative disorders that affect semantic memory.
AD is characterized by a gradual decline in memory and cognition, whereas SD is characterized by the progressive degradation of semantic knowledge.
In AD, brain damage is not limited to a specific brain region, but rather spreads across different areas of the brain, including the anterior temporal lobes (ATL).
In contrast, SD is characterized by a selective and progressive atrophy of the ATL, which results in a global semantic impairment.
This suggests that the ATL may play a crucial role in the integration and processing of semantic information across different modalities, and may serve as an amodal hub for semantic memory.
Evidence for the ATL as amodal hub: Herpes Simplex Viral Encephalitis vs Semantic Dementia
Herpes Simplex Viral Encephalitis (HSVE) is a viral infection that can cause damage to the brain, including the ATL.
The damage caused by HSVE is usually widespread and affects several brain regions, including the amygdala, hippocampus, peri-/entorhinal, parahippocampal and orbitofrontal cortex, insula and cingulate gyri.
The most common symptoms of HSVE are anterograde amnesia, but sometimes semantic deficits and/or executive function impairments can also occur.
When semantic deficits are present, the damage is often focused on the ATL, suggesting that this region plays a crucial role in the processing of semantic information and may serve as an amodal hub for semantic memory.
Evidence for the ATL as amodal hub: Transcortical Sensory Aphasia, caused by stroke
Transcortical Sensory Aphasia (TSA) is a language disorder caused by stroke, which affects the ability to access semantic representations.
In TSA, patients have intact fluency, but impaired comprehension and difficulty retrieving the appropriate word.
One interesting finding is that TSA patients can improve their semantic retrieval abilities with cumulative cueing, which suggests that the semantic representations themselves are intact, but there is a deficit in accessing them.
In contrast, in SD, patients have a degradation of semantic representations themselves, which leads to a global semantic impairment.
This suggests that the ATL may serve as an amodal hub for semantic memory, as damage to this region can result in deficits in accessing and/or degrading semantic representations across different modalities.
Syntax and its role in language production
Syntax refers to the rules that specify how words can be combined into sentences
Parsing is the process of assigning a syntactic structure to words
Speech without proper syntax is difficult to understand
Paul Broca localised ‘speech production’ to a dedicated ‘language centre’ (Broca’s area) through autopsy and associated it with the production of sentences
Broca’s aphasia and syntactic deficits
Broca’s aphasia is characterised by agrammatism (loss of grammar) in speech production
It involves deficits in both the production and comprehension of syntactically complex sentences
Broca’s area shows greater activity with increasing syntactic complexity in fMRI
Subdivisions of Broca’s area (BA44 and BA45) have different functions related to syntax processing
Syntax and semantics: Separable or not?
Semantic dementia patients show intact grammatical processing
Wernicke’s aphasia patients produce jargon-filled, grammatically accurate speech
There is a debate as to whether syntax and semantics are separate or not
Structure-driven vs discourse-driven theories of syntax processing
N400 and P600 ERP components
N400 is an ERP component that shows up in response to semantic anomalies regardless of sentence context
P600 is an ERP component that shows up in response to syntactic anomalies regardless of semantic content
These components provide evidence for the separability of syntax and semantics in language processing
fMRI of working memory vs. syntactic complexity shows anterior-posterior differences.
Functional magnetic resonance imaging (fMRI) studies have shown differences in brain activation patterns when working memory is compared to syntactic complexity.
The anterior regions of the brain are more active during working memory tasks, while the posterior regions are more active during syntactic complexity tasks.
