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Title: Isolating effects of age with fair representation learning when assessing dementia

Paper content:
One of the most prevalent symptoms among the elderly population, dementia, can be detected by classifiers trained on linguistic features extracted from narrative transcripts.
However, these linguistic features are impacted in a similar but different fashion by the normal aging process.
Aging is therefore a confounding factor, whose effects have been hard for machine learning classifiers to isolate.
In this paper, we show that deep neural network (DNN) classifiers can infer ages from linguistic features, which is an entanglement that could lead to unfairness across age groups.
We show this problem is caused by undesired activations of v-structures in causality diagrams, and it could be addressed with fair representation learning.
We build neural network classifiers that learn low-dimensional representations reflecting the impacts of dementia yet discarding the effects of age.
To evaluate these classifiers, we specify a model-agnostic score $\Delta_{eo}^{(N)}$ measuring how classifier results are disentangled from age.
Our best models outperform baseline neural network classifiers in disentanglement, while compromising accuracy by as little as 2.56\% and 2.25\% on DementiaBank and the Famous People dataset respectively.
One in three seniors die of Alzheimer's and other types of dementia in the United States (Association, 2018) .
Although its causes are not yet fully understood, dementia impacts people's cognitive abilities in a detectable manner.
This includes different syntactic distributions in narrative descriptions BID28 , more pausing BID29 , higher levels of difficulty in recalling stories BID21 , and impaired memory generally BID20 .
Fortunately, linguistic features can be used to train classifiers to detect various cognitive impairments.
For example, BID8 detected primary progressive aphasia with up to 100% accuracy, and classified subtypes of primary progressive aphasia with up to 79% accuracy on a set of 40 participants using lexical-syntactic and acoustic features.
BID7 classified dementia from control participants with 82% accuracy on narrative speech.However, dementia is not the only factor causing such detectable changes in linguistic features of speech.
Aging also impairs cognitive abilities BID11 , but in subtly different ways from dementia.
For example, aging inhibits fluid cognitive abilities (e.g., cognitive processing speed) much more than the consolidated abilities (e.g., those related to cumulative skills and memories) BID4 .
In other words, the detected changes of linguistic features, including more pauses and decreased short-term memories, could attribute to just normal aging process instead of dementia.
Unfortunately, due to the high correlation between dementia and aging, it can be difficult to disentangle symptoms are caused by dementia or aging BID24 .
Age is therefore a confounding factor in detecting dementia.The effects of confounding factors are hard for traditional machine learning algorithms to isolate, and this is largely due to sampling biases in the data.
For example, some algorithms predict higher risk of criminal recidivism for people with darker skin colors BID15 , others identify images of smiling Asians as blinking BID19 , and GloVe word embeddings can project European-American names significantly closer to the words like 'pleasant' than African-American names BID3 .
It is preferable for classifiers to make decisions without biasing too heavily on demographic factors, and therefore to isolate the effects of confounding factors.
However, as we will show in Experiments, traditional neural network classifiers bias on age to infer dementia; this can lead to otherwise avoidable false positives and false negatives that are especially important to avoid in the medical domain.
Graphically, if both age A and dementia D cause changes in a feature X, the result is a v-structure BID17 A → X ← D which is activated upon observing X. In other words, the confounder A affects P (D|X) if we train the classifier in traditional ways, which is to collect data points {(X, D) (i) } and to learn an inference model P (D|X) approximating the affected P (D|X).Traditionally
, there are several ways to eliminate the effects of confounding factors A.Controlling A gives a posterior distribution P (D|X, A)P (A). This is unfortunately
unrealistic for small, imbalanced clinical datasets, in which sparsity may require stratification. However, the stratified
distributions P (D|X, A) can be far from a meaningful representation of the real world (as we will show, e.g., in FIG3 ). Moreover, a discrepancy
in the sizes of age groups can skew the age prior P (A), which would seriously inhibit the generalizability of a classifier.Controlling X Conducting a randomized control trial (RCT) on X removes all causal paths leading "towards" the variable X, which gives a de-confounded dataset P (D|do(X)) according to the notation in BID27 . However, RCTs on X are
even less practical because simultaneously controlling multiple features produces exponential number of scenarios, and doing this to more than 400 features require far more data points than any available dataset.Pre-adjusting X according to a pre-trained model X = f (A) per feature could also approximately generate the dataset P (D|do(X)). However, such a model
should consider participant differences, otherwise interpolating using a fixed age A would give exactly the same features for everybody. The participant differences
, however, are best characterized via X, which are the values you want to predict.To overcome the various problems with these methods, we let our classifiers be aware of cognitive impairments while actively filtering out any information related to aging. This is a fair representation
learning framework that protects age as a "sensitive attribute".Fair representation learning frameworks
can be used to train classifiers to equally consider the subjects with different sensitive attributes. A sensitive attribute (or "protected attribute
") can be race, age, or other variables whose impact should be ignored. In the framework proposed by BID32 , classifiers
were penalized for the differences in classification probabilities among different demographic groups. After training, the classifiers produced better
demographic similarities while compromising only a little overall accuracy. To push the fair representation learning idea further
, adversarial training can be incorporated. BID9 introduced generative adversarial networks, in
which a generator and a discriminator are iteratively optimized against each other. Incorporating adversarial training, BID22 proposed
a framework to learn a latent representation of data in order to limit its adversary's ability to classify based on the sensitive attributes.However, these approaches to fair representation learning only handle binary attributes. E.g., BID22 binarized age. To apply to cognitive impairments
detection, we want to represent
age on a continuous scale (with some granularity if necessary). We formulate a fairness metric for evaluating the ability of a classifier
to isolate a continuous-valued attribute. We also propose four models that compress high-dimensional feature vectors
into low-dimensional representations which encrypt age from an adversary. We show empirically that our models achieve better fairness metrics than baseline
deep neural network classifiers, while compromising accuracies by as little as 2.56% and 2.25% on our two empirical datasets, respectively.
We evaluate the performances of our four proposed neural networks against the DNN baseline.
As an additional ablation study, two variants of age-indep-entropy are also evaluated.
TAB1 : Evaluation results of our representation learning models.
The "age-indep" prefix are replaced with "*" in model names.
age-indep-simple and age-indep-autoencoder have better disentanglement scores, while the rest two models could have better accuracy.Accuracy The fair representation learning models compromise accuracy, in comparison to DNN baselines.
This confirms that part of the classification power of DNNs come from biasing with regards to age.
On DementiaBank, the age-indep-autoencoder reduces accuracy the least (only 2.56% in comparison to the DNN baseline).
On the Famous People data, age-indep-consensus and age-indep-entropy models compromise accuracies by only 2.25% and 2.75% respectively, which are not statistically different from the DNN baseline 7 .Disentanglement
In comparison to DNN baselines, our fair representation learning models improve disentanglement/fairness 8 , the improvements are mostly significant when measured by the two-group scores ∆eo . Also, the five-group
scores ∆eo are less stable for both datasets, and the scores in the Famous People have higher variances than in DementiaBank. Following is an explanation
. DementiaBank has ∼400 data
samples. In 5-fold cross validation
, each of the five age groups has only ∼16 samples during evaluation. Famous People data contains
∼250 samples, which increases the variance. When the number of groups,
N of ∆ (N ) eo , is kept small (e.g., ∼100 samples per label per group, as in DementiaBank N = 2), the fairness metrics are stable.
Here, we identify the problem of entangling age in the detection of cognitive impairments.
After explaining this problem with causality diagrams, we formulate it into a fair representation learning task, and propose a fairness score to measure the extent of disentanglement.
We put forward four fair representation learning models that learn low-dimensional representations of data samples containing as little age information as possible.
Our best model improves upon the DNN baseline in our fairness metrics, while compromising as little accuracy as 2.56% (on DementiaBank) and 2.25% (on the Famous People dataset).7 p = 0.20, 0.16 on 38-DoF one-tailed t-tests, respectively.
8 On DementiaBank, p = 0.01 and 0.03 for age-indep-simple and age-indep-entropy on ∆ (2) eo respectively; these are significant.
p = 0.08 and 0.09 on age-indep-autoencoder and age-indep-consensus-net on ∆ (2) eo respectively; these are marginally significant.
However, these differences are not as significant on ∆ Proof of Theorem For each of the age groups: |p a −p| + |n a −n| ≤ max{|p a − 0| + |n a − 0|, |p a − 0.5| + |n a − 0.5|} ≤ max{0.5, 1} = 1 Summing up the N a age groups results in our upper bound N a for non-trivial classifiers.

assistant: text: Show that age confounds cognitive impairment detection + solve with fair representation learning + propose metrics and models.