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@EItanm1999 EItanm1999 commented Sep 20, 2026 •

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Register ddm_uniform_st as a supported model

Adds the registry entry for the DDM with uniform trial-to-trial
variability in non-decision time (t_trial ~ Uniform(t - st, t + st); st
is the half-width, SD = st/sqrt(3)). approx_differentiable only, LAN via
ddm_uniform_st.onnx; bounds are the network's training box - in particular
t >= 0.25 and st in [1e-3, 0.25].

Requires the ddm_uniform_st LAN to be published to the franklab/HSSM hub as
ddm_uniform_st.onnx (90 KB; conversion and parity check scripted separately).
The simulator side is the same config that released ssm-simulators
(0.14.0) ships as ddm_st; the explicit name ddm_uniform_st arrives with
our ssm-simulators PR "Add ddm_normal_st and register ddm_uniform_st as
the explicit name of ddm_st" (branch feat/ddm-normal-st-model-config,
stacked on #359, not yet opened). Until that alias ships,
hssm.simulate_data("ddm_uniform_st") needs the ssms name ddm_st.

Correct inference for this model also needs the st-aware admissibility
floor (fix/ensure-positive-ndt-st-aware): without it the stock guard
floors real density in [t - st, t] on ~0.14% of trials and biases t.

  • Records that the bounds are the network's training box rather than modelling
    choices, and that st means a uniform half-width in ddm_uniform_st but a Normal SD in
    ddm_normal_st, so equal st values are not equal dispersions.
  • Lists the model where users look for it, and adds a changelog entry.

Stacked on #1292 (first commit is #1292's; review the top commit). Model string is ddm_uniform_st; the same config exists upstream in ssm-simulators as ddm_st, and an ssm-simulators PR registers ddm_uniform_st as its explicit alias so hssm.simulate_data accepts the new name. The network ddm_uniform_st.onnx is not yet on franklab/HSSM; until it is, constructing the model needs loglik=<local path>.

Tests: the branch's own test files pass on current main (fefed57) in an environment with ssm-simulators 0.14.0 and bambi 0.21; see the commit for the added cases. Fork CI has not been approved for this fork, so no check-runs appear here.

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Summary by CodeRabbit

  • New Features

    • Added support for the ddm_uniform_st model with uniformly distributed non-decision-time variability.
    • Enabled its approximate-differentiable likelihood, parameter bounds, and predictive sampling support.
  • Bug Fixes

    • Updated response-time constraints for applicable trial-varying non-decision-time models to use t - st, preserving valid density across the variable-time range.
  • Documentation

    • Updated likelihood tutorials and changelog documentation to describe ddm_uniform_st, its likelihood, and parameter semantics.

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📝 Walkthrough

Walkthrough

The pull request adds ddm_uniform_st support, including its configuration, simulator mapping, types, documentation, and tests. It also updates ensure_positive_ndt to use t - st as the admissibility edge when st is present.

Changes

Uniform non-decision-time support

Layer / File(s) Summary
Model configuration and documentation
src/hssm/_types.py, src/hssm/config.py, src/hssm/modelconfig/ddm_uniform_st_config.py, tests/test_modelconfig.py, docs/tutorials/likelihoods.ipynb, docs/changelog.md
ddm_uniform_st is added to the supported model types. Its configuration defines parameters, bounds, likelihood metadata, and the ddm_st simulator mapping. Documentation and tests cover the new model.
st-aware response-time validation
src/hssm/distribution_utils/dist.py, tests/distribution_utils/test_distribution_utils.py
ensure_positive_ndt uses t - st when st is present. Tests cover fixed t, moving st, and sz or sv cases.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature · Severity of issue fixed: Low

Suggested reviewers: alexanderfengler

Merge Risk: 🟡 Moderate · up to ef9f9

The new model may fail through normal construction without a local likelihood file, while a public helper path uses the wrong simulator name; these paths should be corrected or explicitly deferred before merge.

🚥 Pre-merge checks | ✅ 5
✅ Passed checks (5 passed)
Check name Status Explanation
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title identifies the main change: registering a DDM model with trial-varying non-decision time. It is concise and related to the changes, although it abbreviates the exact model name `ddm_uniform_…
Docstring Coverage ✅ Passed Docstring coverage is 91.67% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 6 files.
Linked Issues check ✅ Passed Check skipped because no linked issues were found for this pull request.
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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@src/hssm/modelconfig/ddm_uniform_st_config.py`:
- Around line 26-34: Set the model configuration’s simulator alias to “ddm_st”
by adding the rv entry near the existing response and list_params settings,
while preserving “ddm_uniform_st.onnx” as the likelihood artifact.

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Comment thread src/hssm/modelconfig/ddm_uniform_st_config.py

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Actionable comments posted: 3


  • 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@docs/changelog.md`:
- Line 7: Update the changelog entry for ddm_uniform_st to replace the `#XXXX`
placeholder with the actual issue or pull request number, or remove the
reference if no number is available.

In `@tests/distribution_utils/test_distribution_utils.py`:
- Line 328: Update the parametrized test’s rt values to include expected_edge
explicitly, covering the exact boundary for both fixed-t and st cases and
preserving the requirement that LOGP_LB is selected at the inclusive threshold.

In `@tests/test_modelconfig.py`:
- Line 300: Remove the negative assertion for "ddm_uniform_st" from the test,
while retaining the assertions that lk_approx_differentiable["rv"] equals
"ddm_st" and that this value exists in ssms_model_config.

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@EItanm1999

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Both networks are public at hf.co/Eitanm/ddm-st-lans for anyone who wants to test now.

@EItanm1999
EItanm1999 force-pushed the feat/register-ddm-st branch 2 times, most recently from bf537d8 to fbfa6fd Compare September 22, 2026 19:00
EItanm1999 and others added 2 commits September 22, 2026 15:13
ensure_positive_ndt floors the log-likelihood wherever rt - t <= 1e-15. That is
the correct support edge for a fixed non-decision time, but not for the LAN
`*_st` models, which follow the half-width st convention of ssms: there t is
drawn per trial from Uniform(t - st, t + st), so the fastest admissible response
time is t - st and the band [t - st, t] carries real density. The guard replaced
that entire band with LOGP_LB regardless of what the likelihood returned.
Applied unconditionally at both call sites, affecting every LAN `*_st` model.

Measured on ddm_st: compiling HSSM's observed-RV logp for a model whose
likelihood IS the exact ddm_st quadrature and comparing against the same
quadrature called directly gave differences of -5.3 to -12586.8 nats, varying
with theta. With the guard skipped the two agree to exactly 0.0000 on 7 of 8
parameter vectors (the 8th had a subject z outside its bound). So this guard
accounts for the entire discrepancy while parameters stay in bounds.

p_outlier partially masks it: the floored value is wrapped in the lapse mixture,
so affected trials emerge at log(0.05/20) = -5.99 rather than at LOGP_LB, which
is why this presented as bad geometry rather than an obvious -inf.

The t - st edge is stated as correct under the half-width convention of
ssms/cssm, which every LAN *_st model follows, rather than as a universal law.
full_ddm is the one bundled model on the other convention: its only likelihood
is the blackbox wrapper around hddm_wfpt, which reads st as a full width and
puts its own edge at t - st/2. No separate factor is needed, because that edge
sits above t - st and hddm_wfpt already returns zero density across
[t - st, t - st/2), which the wrapper maps to the same lower bound. Verified:
across a 0.28-0.46 RT scan the raw likelihood's highest floored rt is exactly
t - st/2, and the guard changes no value at any rt (max |delta| 0.0).

Only st moves the response-time support edge. sz (starting point) and sv (drift)
do not, and are deliberately not consulted; a regression test pins that down.

Tests are one parametrized case per convention (fixed t, st moves the edge,
sz/sv leave it) over an explicit RT vector that straddles all three edges, so
the in-band assertion no longer depends on an unseeded draw. They compare
against the imported LOGP_LB rather than a -66.1 literal: under
PYTENSOR_FLAGS=floatX=float32 the bound is -66.0999984741211, and wrapping it in
np.array defeats NumPy's weak promotion, so the literal form failed there.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Adds the registry entry for the DDM with uniform trial-to-trial
variability in non-decision time (t_trial ~ Uniform(t - st, t + st); st
is the half-width, SD = st/sqrt(3)). approx_differentiable only, LAN via
ddm_uniform_st.onnx; bounds are the network's training box - in particular
t >= 0.25 and st in [1e-3, 0.25].

Requires the ddm_uniform_st LAN to be published to the franklab/HSSM hub as
ddm_uniform_st.onnx (90 KB; conversion and parity check scripted separately).
Released ssm-simulators (0.14.0) ships this simulator as 'ddm_st', so the
config sets rv: "ddm_st" inside the likelihood block; 'ddm_uniform_st'
becomes an ssms alias in lnccbrown/ssm-simulators#361.

Correct inference for this model also needs the st-aware admissibility
floor (fix/ensure-positive-ndt-st-aware): without it the stock guard
floors real density in [t - st, t] on ~0.14% of trials and biases t.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

- Records that the bounds are the network's training box rather than modelling
  choices, and that st means a uniform half-width in ddm_uniform_st but a Normal SD in
  ddm_normal_st, so equal st values are not equal dispersions.
- Lists the model where users look for it, and adds a changelog entry.
…n_model_matrix_matches_defaults passes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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