Semantic networks for odors and colors in Alzheimer's disease

Odor identification errors reveal cognitive aspects of age-associated smell loss

Human olfaction can be extraordinarily sensitive, and its most common assessment method is odor identification (OID), where everyday odors are matched to word labels in a multiple-choice format. However, many older persons are unable to identify familiar odors, a deficit that is associated with the risk of future dementia and mortality. The underlying processes subserving OID in older adults are poorly understood. Here, we analyzed error patterns in OID to test whether errors could be explained by perceptual and/or semantic similarities among the response alternatives. We investigated the OID response patterns in a large, population-based sample of older adults in Sweden (n = 2479; age 60-100 years). Olfaction was assessed by a 'Sniffin ́ TOM OID test with 16 odors; each trial involved matching a target odor to a correct label among three distractors. We analyzed the pattern of misidentifications, and the results showed that some distractors were more frequently selected than others, suggesting cognitive or perceptual factors may be present. Relatedly, we conducted a large online survey of older adults (n = 959, age 60-90 years) who were asked to imagine and rate the perceptual similarity of the target odors and the three corresponding distractors (e.g. "How similar are these smells: apple and mint?"). We then used data from the Swedish web corpus and the Word2Vec neural network algorithm to quantify the semantic association strength between the labels of each target odor and its three distractors. These data sources were used to predict odor identification errors. We found that the error patterns were partly explained by both the semantic similarity between target-distractor pairs, and the imagined perceptual similarity of the target-distractor pair. Both factors had, however, a diminished prediction in older ages, as responses became gradually less systematic. In sum, our results suggest that OID tests not only reflect olfactory perception, but also likely involve the mental processing of odor-semantic associations. This may be the reason why these tests are useful in predicting dementia onset. Our insights into olfactory-language interactions could be harnessed to develop new olfactory tests that are tailored for specific clinical purposes.

International consensus statement on allergy and rhinology: Olfaction

BACKGROUND

The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O).

METHODS

Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus.

RESULTS

The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies.

CONCLUSION

This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

A model-based analysis of the impairment of semantic memory

We use cognitive models to evaluate three theories of the change in semantic memory caused by Alzheimer's disease. We use data from 14,096 clinical assessments of 3602 Alzheimer's patients and their caregivers. Each patient completed a semantic memory task involving the odd-one-out comparison of animal names. Each patient was also independently evaluated to determine their level of impairment. Our cognitive models assume a feature-based representation of the animals and odd-one-out choice probabilities based on common-feature similarities. We find no evidence for the restructured representation hypothesis, which claims that impairment causes changes in the features used to represent stimuli. We also find no evidence for the attention change hypothesis, which claims that impairment causes greater attention to be given to concrete features at the expense of more abstract features. We do find evidence for the noisy access hypothesis, which claims that odd-one-out choices become less determined by semantic similarity and more prone to the simple response strategy of choosing the last option. We conclude that the noisy access hypothesis provides a simple account of odd-one-out choice behavior throughout the progression of Alzheimer's disease. More elaborate theories involving changes to underlying mental representations and attention processes need to provide evidence they are superior to the noisy access account.

Development of the Spanish Version of Sniffin's Sticks Olfactory Identification Test: Normative Data and Validity of Parallel Measures

The Sniffin' Sticks Olfactory Identification Test is a tool for measurement of olfactory performance developed in Germany and validated in several countries. This research aims to develop the Spanish version of the Sniffin' Sticks Olfactory Identification Test and obtain normative values for the Spanish population. The parameters are free recall and subjective intensity of odorants are included. The influence of possible demographic covariates such as sex, age, smoking, or educational level are analyzed, and the items that best discriminate are studied. In addition, the internal structure validity of the blue and purple versions is studied as a parallel measure, and a cultural adaptation of the purple version is carried out. For this, three independent samples of normosmic healthy volunteers were studied. To obtain normative values, the sample was of 417 participants (18-89 years). For the internal structure validity study of both versions, the sample was 226 (18-70 years), and for familiarity of the purple version, the sample was 75 participants (21-79 years). Results indicated that men and women and smokers and non-smokers perform equally. However, differences were found as age progresses, being more pronounced after 60 years old in all three measurements of the identification test. This research also provides the items that best discriminate in the blue version and a cultural adaptation for the purple version. In conclusion, the Sniffin' Sticks Odor Identification Test is a suitable tool for olfactory assessment in the Spanish population. The instrument has been expanded with two new scores, and normative data as a function of age are provided. Its parallel version also seems appropriate for testing, as items have been culturally adapted and evidence of internal structure validity for both versions is reported.

Olfactory dysfunction in Alzheimer's disease Systematic review and meta-analysis

Alzheimer’s disease (AD), a neurodegenerative condition, is one of the most prevalent kinds of dementia, whose frequency doubles for every 5 years of age in elderly.

Estimating semantic networks of groups and individuals from fluency data

One popular and classic theory of how the mind encodes knowledge is an associative semantic network, where concepts and associations between concepts correspond to nodes and edges, respectively. A major issue in semantic network research is that there is no consensus among researchers as to the best method for estimating the network of an individual or group. We propose a novel method (U-INVITE) for estimating semantic networks from semantic fluency data (listing items from a category) based on a censored random walk model of memory retrieval. We compare this method to several other methods in the literature for estimating networks from semantic fluency data. In simulations, we find that U-INVITE can recover semantic networks with low error rates given only a moderate amount of data. U-INVITE is the only known method derived from a psychologically plausible process model of memory retrieval and one of two known methods that we found to be consistent estimators of this process: if semantic memory retrieval is consistent with this process, the procedure will eventually estimate the true network (given enough data). We conduct the first exploration of different methods for estimating psychologically-valid semantic networks by comparing people's similarity judgments of edges estimated by each network estimation method. To encourage best practices, we discuss the merits of each network estimation technique, provide a flow chart that assists with choosing an appropriate method, and supply code for others to employ these techniques on their own data.

The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases

Olfactory deficits are present in numerous neurodegenerative disorders and are accompanied by pathology in related brain regions. In several of these disorders, olfactory disturbances appear early and are considered as prodromal symptoms of the disease. In addition, pathological protein aggregates affect olfactory regions prior to other regions, suggesting that the olfactory system might be particularly vulnerable to neurodegenerative diseases. Exposed to the external environment, the olfactory epithelium and olfactory bulb allow pathogen and toxin penetration into the brain, a process that has been proposed to play a role in neurodegenerative diseases. Determining whether the olfactory bulb could be a starting point of pathology and of pathology spread is crucial to understanding how neurodegenerative diseases evolve. We argue that pathological changes following environmental insults contribute to the initiation of protein aggregation in the olfactory bulb, which then triggers the spread of the pathology within the brain by a templating mechanism in a prion-like manner. We review the evidence for the early involvement of olfactory structures in neurodegenerative diseases and the relationship between neuropathology and olfactory function. We discuss the vulnerability and putative underlying mechanisms by which pathology could be initiated in the olfactory bulb, from the entry of pathogens (promoted by increased permeability of the olfactory epithelium with aging or inflammation) to the sensitivity of the olfactory system to oxidative stress and inflammation. Finally, we review changes in protein expression and neural excitability triggered by pathogenic proteins that can promote pathogenesis in the olfactory bulb and beyond.

Assessment of Olfactory Memory in Olfactory Dysfunction

To assess all clinically relevant components of olfactory perception, examinations for olfactory sensitivity, discrimination, and identification are performed. Besides the standard perceptual test battery, episodic olfactory memory might offer additional information about olfactory abilities relative to these standard clinical tests. As both olfactory deficits and memory deficits are early symptoms in neurodegenerative disorders, olfactory memory may be of particular interest. However, to date little is known about episodic olfactory memory performance in patients with decreased olfactory function. This study includes the investigation of olfactory memory performance in 14 hyposmic patients (8 female, mean age 52.6 years) completing two episodic odor memory tests (Sniffin' Test of Odor Memory and Odor Memory Test). To control for a general impairment in memory function, a verbal and a figural memory test were carried out. A regression model with multiple predictors was calculated for both odor memory tests separately. Odor identification was identified as the only significant predictor for both odor memory tasks. From our results, we conclude that currently available olfactory memory tests are highly influenced by odor identification abilities, implying the need for the development and validation of additional tests in this field which could serve as additional olfactory perception variables for clinical assessment.

Use of Structural Assessment of Knowledge for Outcomes Assessment in the Neuroscience Classroom

Outcomes assessment of undergraduate neuroscience curricula should assess the ability to think integratively about basic neuroscience concepts based on two of the core competencies established by the Faculty for Undergraduate Neuroscience. The current study investigated whether the structural assessment of knowledge (SAK) approach, which evaluates the organization of an individual's knowledge structures, is effective for demonstrating learning of basic neuroscience concepts. Students in an introductory psychology course (n = 29), an introductory neuroscience course (n = 19), or an advanced behavioral neuroscience course (n = 15) completed SAK before and after learning gross brain anatomy and neuronal physiology. All students showed improvements in their SAK after short-term dissemination for gross brain anatomy, but not for neuronal physiology, concepts. Therefore, research is needed to determine whether the effectiveness of SAK in outcomes assessment depends on the content or teaching style. Additional research using SAK should also explore effectiveness for learning over longer time frames and correlations with student performance in the course. However, the results suggest SAK is a promising technique for outcomes assessment of undergraduate neuroscience curricula.

Long-term odor recognition memory in unipolar major depression and Alzheimer׳s disease

Major depression and Alzheimer׳s disease (AD) are often observed in the elderly. The identification of specific markers for these diseases could improve their screening. The aim of this study was to investigate long-term odor recognition memory in depressed and AD patients, with a view to identifying olfactory markers of these diseases. We included 20 patients with unipolar major depressive episodes (MDE), 20 patients with mild to moderate AD and 24 healthy subjects. We investigated the cognitive profile and olfactory memory capacities (ability to recognize familiar and unfamiliar odors) of these subjects. Olfactory memory test results showed that AD and depressed patients were characterized by significantly less correct responses and more wrong responses than healthy controls. Detection index did not differ significantly between patients with major depression and those with AD when the results were analyzed for all odors. However, MDE patients displayed an impairment of olfactory memory for both familiar and unfamiliar odors, whereas AD subjects were impaired only in the recognition of unfamiliar odors, with respect to healthy subjects. If preservation of olfactory memory for familiar stimuli in patients with mild to moderate AD is confirmed, this test could be used in clinical practice as a complementary tool for diagnosis.

Spared piriform cortical single-unit odor processing and odor discrimination in the Tg2576 mouse model of Alzheimer's disease

Alzheimer's disease is a neurodegenerative disorder that is the most common cause of dementia in the elderly today. One of the earliest reported signs of Alzheimer's disease is olfactory dysfunction, which may manifest in a variety of ways. The present study sought to address this issue by investigating odor coding in the anterior piriform cortex, the primary cortical region involved in higher order olfactory function, and how it relates to performance on olfactory behavioral tasks. An olfactory habituation task was performed on cohorts of transgenic and age-matched wild-type mice at 3, 6 and 12 months of age. These animals were then anesthetized and acute, single-unit electrophysiology was performed in the anterior piriform cortex. In addition, in a separate group of animals, a longitudinal odor discrimination task was conducted from 3-12 months of age. Results showed that while odor habituation was impaired at all ages, Tg2576 performed comparably to age-matched wild-type mice on the olfactory discrimination task. The behavioral data mirrored intact anterior piriform cortex single-unit odor responses and receptive fields in Tg2576, which were comparable to wild-type at all age groups. The present results suggest that odor processing in the olfactory cortex and basic odor discrimination is especially robust in the face of amyloid β precursor protein (AβPP) over-expression and advancing amyloid β (Aβ) pathology. Odor identification deficits known to emerge early in Alzheimer's disease progression, therefore, may reflect impairments in linking the odor percept to associated labels in cortical regions upstream of the primary olfactory pathway, rather than in the basic odor processing itself.