# Fitting lapsing psychometric functions (with brms?)

I’d like to fit a psychometric function to some binomial data. In the past I’ve always used vanilla logistic regression, but if there are random errors this can bias the estimates of the slope and intercept of the curve (see for example Wichman & Hill, 2001). Thus I’d like to include the possibility that there’s a mixture of random/lapse responses (e.g. sampled from a single binomial error distribution) and on-task responses (as in normal logistic regression). The “classical” functional form of the model is that the response probability is a saturating logistic function with asymptotes not exactly equal to 0 or 1.

My question is whether I can express this in brms other than through the mechanism for specifying a “generic” non-linear function, and (if not) whether that comes with a big performance penalty. Of course I could write the stan code by hand, but I’d really like to be able to take advantage of brms’s ability to concisely specify random effects as well.

To be concrete, here’s an example dataset and three brms models: first is vanilla logistic regression, second is the manual nonlinear specification of logistic regression (which on my system is about 50% slower to evaluate, which I can bear), and the third is my first attempt at a lapsing logistic regression.

## Simulated data

``````library(tidyverse)
library(brms)

options(mc.cores = parallel::detectCores())

dat <- cross_df(list(x = seq(0, 10),
rep = seq(1, 5),
subj = seq(1, 10))) %>%
mutate(theta = 0.02 + 0.95*plogis(1 * (x-5)),
y = rbernoulli(n=length(theta), p=theta))

``````

## Vanilla logistic regression

``````
fit1 = brm(y ~ 1 + x, data=dat, family=bernoulli())

``````

## Nonlinear formula logistic regression

``````fit2 = brm(bf(y ~ 1 / (1+exp(-(eta))),
eta ~ 1+x,
nl=TRUE),
data=dat,
prior = set_prior("student_t(3,0,10)", class="b", nlpar="eta"))
``````

## Lapsing logistic regression

``````fit3 = brm(bf(y ~ lapselow + (1-lapsehigh-lapselow) * 1 / (1+exp(-(eta))),
eta ~ 1+x,
lapselow ~ 1,
lapsehigh ~ 1,
nl=TRUE),
data=dat,
inits="0",
prior = c(set_prior("student_t(3,0,10)", class="b", nlpar="eta"),
set_prior("uniform(0,0.1)", class="b", nlpar="lapselow"),
set_prior("uniform(0,0.1)", class="b", nlpar="lapsehigh")))
``````

As my attempt to increase `adapt_delta` might suggest, I’m getting a ton of divergent transitions with the last model (nearly 25%). So I’m afraid that it’s going to be hard to make this work without digging into the stan code (or doing some kind of hack to mimic stan’s transformation of bounded parameters)

### References

Wichmann, F. A., & Hill, N. J. (2001). The psychometric function: I. Fitting, sampling, and goodness of fit. Perception and Psychophysics, 63(8), 1293–1313.

Your third model runs very fast (less than 8 seconds on my computer) with no divergent transitions and good parameter recovery with a few small changes:

• Center x
``````set.seed(999)
guess <- .02
lapse <- .03
dat <- cross_df(list(x = seq(-5, 5),
rep = seq(1, 5),
subj = seq(1, 10))) %>%
mutate(theta = guess + (1-guess-lapse)*plogis(1 * (x)),
y = as.integer(rbernoulli(n=length(theta), p=theta)))
``````
• Assign priors as described here. (I think there’s also a typo in your model; the guess and lapse parameters are reversed from those discussed in Wichmann and Hill.) It would probably be best not to use flat priors with bounds but that seems to work for this example.
``````BF <- bf(
y ~ guess + (1-guess-lapse) * inv_logit(eta),
eta ~ 1 + x,
guess ~ 1,
lapse ~ 1,
nl = TRUE
)
p2 <- c(
prior(student_t(7, 0, 10), class = "b", nlpar = "eta"),
prior(beta(1, 1), nlpar = "lapse", lb = 0, ub = .1),
prior(beta(1, 1), nlpar = "guess", lb = 0, ub = .1)
)
fit_2 <- brm(
BF,
data = dat,
inits = 0,
prior = p2,
)
# ~8 seconds....
summary(fit_2)
Family: bernoulli
Formula: y ~ guess + (1 - guess - lapse) * inv_logit(eta)
eta ~ 1 + x
guess ~ 1
lapse ~ 1
Data: dat (Number of observations: 550)
Samples: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
total post-warmup samples = 4000

Population-Level Effects:
Estimate Est.Error l-95% CI u-95% CI Eff.Sample Rhat
eta_Intercept       0.03      0.21    -0.40     0.47       1972 1.00
eta_x               1.12      0.18     0.84     1.54       1312 1.01
guess_Intercept     0.03      0.02     0.00     0.08       1487 1.00
lapse_Intercept     0.04      0.02     0.00     0.08       1585 1.00

Samples were drawn using sampling(NUTS). For each parameter, Eff.Sample
is a crude measure of effective sample size, and Rhat is the potential
scale reduction factor on split chains (at convergence, Rhat = 1).
``````

I also tried this out with a probit link (you can use `Phi(eta)`) and it seems to be a little more accurate re the guess and lapse parameters.

Edit: I suppose the answer to your question is “No, you have to use the nonlinear syntax as other approaches (such as the link functions in `psyphy::logit.2asym()`) don’t work with brms. But the performance penalty is not bad.” Personally I think the nonlinear syntax is great and don’t really see the problem. Maybe with huge datasets the run time is too long?

2 Likes

Thanks for the help! I should have read the docs on the priors more closely but that looks like it does exactly what I wanted.

I don’t think it should be a problem, and if it is I’ll report back. I think the nonlinear formula thing is wonderful too! It eliminates one of the major pain points of Stan for me, the tradeoff between flexibility in specifying the model and easily generalizing to mixed effects designs.

1 Like

FWIW, I think most of the work is being done by setting the priors appropriately. Even with `x` not centered it runs fast and without divergent transitions.

1 Like

Would it not be possible to write this as a mixture of Bernoullis?

Something like this:

``````mix <- mixture(bernoulli("probit"), bernoulli("probit"))
fit_mix <- brm(bf(y ~ 1, mu1 ~ x, mu2 ~ 1),
data = dat,
family = mix)
``````

I haven’t really worked this out, purely based on intuition.

3 Likes

Oh that’s really clever! I didn’t realize brms supported mixture models but yes that’s essentially what this is. I take it `mu1` and `mu2` are automatically mapped to the means of the two mixtures?

yes, and `theta1` and `theta2` are the mixing proportions

so if this is correct, then using this model `theta2` should be equal to `lapse/(1-guess)`

1 Like

🙌 very cool

1 Like

really nice! Do you have a worked out example?

@awellis The mixture formulation of the lapsing model is great, but I find myself confused about why theta2 would be lapse/(1-guess). Are you using the terms lapse and guess in the same way as @matti above (following Wiechman and Hill, 2001), where guess determines the overall floor proportion and 1-lapse determines the ceiling proportion?

If the lapsing model is written as a mixture:

I interpret the parameters as follows:

• theta1: probability of not responding based on the stimulus
• inv_link(mu1): probability of successes when not responding based on the stimulus (‘bias’)
• theta2 = 1 - theta1: probability of responding based on stimulus
• inv_link(mu2): probability of successes when responding based on stimulus

I don’t see how theta1 is expected to be identical to lapse / (1-guess) where lapse and guess are interpreted as in Wiechman and Hill (2001). Rather, it would seem to me that

• theta1 = guess + lapse (since 1 - theta1 = 1 - guess - lapse)
• inv_link(mu1) = guess / (guess + lapse)

I’m probably missing something? Any help would be appreciated.

hi, I’ll get back to you on this one. It’s entirely possible that I got it wrong 😬

1 Like