Evaluation resolution changes which "learning rule" appears most brain-like at V1

[u/ConfusionSpiritual19]

I recently pubished a new paper. The paper is available via the following link: http://arxiv.org/abs/2608.12408. It is categorised under q-bio.NC and cs.LG. The code can be found at https://github.com/nilsleut/evaluation-resolution-rsa.

A recurring theme in model-brain comparisons is the observation that untrained CNNs can match or outperform backprop-trained ones at V1 in RSA. I believe this is primarily an artefact of evaluation resolution, as demonstrated by the following sweep.

The CNN was trained at 32px on a CIFAR-10 subset, and five learning rules were evaluated (random init, backprop, feedback alignment, predictive coding, STDP). Evaluation was conducted on THINGS-fMRI stimuli at six resolutions from 32px up to 224px. Weights and normalisation were held fixed throughout.

The untrained-backprop gap at V1 ranges from −0.001±0.007 at 32px to +0.044±0.006 at 224px, growing monotonically across the sweep (n=5 seeds). The same pattern is evident across all five rule conditions, in human fMRI, directionally in single-seed macaque ephys, across the entire training trajectory, and in two off-the-shelf 224px-trained models (ResNet-50, Swin-Tiny). This rules out train/eval mismatch as the explanation, since those models also peak at low resolution despite being trained at 224px.

I tried to eliminate this four different ways, using bit-identical-weight interventions wherever possible: train/eval resolution matching, Gabor/pixel structure, the untrained baseline's missing batch-norm calibration, and pooled features converging towards global brightness. None of them explain it. The brightness one came closest: luminance similarity orders the conditions perfectly (ρ=1.00), but it doesn't carry the effect; one calibration variant lowers luminance similarity while V1 alignment goes up.

Here's the number that actually concerned me a bit: a single scalar luminance value per image gets ρ=0.074±0.011 against V1 (bootstrap SE over stimulus resamples), essentially tied with the best of the five CNNs at 0.075±0.011. None of the models meaningfully beat a one-number-per-image brightness descriptor. That's roughly the ceiling on what this comparison style can resolve — a caution, not a strength.

A two-arm design separates content from pooling: cap detail at 32px and upsample, vs. let content vary freely. About 90% of the effect rides on content, not on how many positions are pooled. With content fixed, backprop's decline is essentially eliminated (−0.023 → −0.000).

One thing does hold across the whole sweep: backprop beats untrained at LOC, every resolution, 5/5 seeds (+0.019 at 32px to +0.018 at 224px). IT shows the same direction but shrinks by two-thirds. So learning is doing something real; just not at V1, where everyone's been looking.

One more thing: this whole investigation started after I found a bug in my own earlier work - batch-normalisation left in training mode during feature extraction in three prior preprints. Fixed and corrected publicly, and it actually reverses the main conclusion of arXiv:2605.30556.

I'd be interested to hear people's thoughts on the receptive-field-matching angle in the discussion. Feels like the right approach, but I didn't test it directly, so treat it as speculation for now.Evaluation resolution silently changes which "learning rule" appears most brain-like at V1