arXiv subjects

Nils Leutenegger

Publications and source records attributed to Nils Leutenegger.

4 recordsLinked to original sources

arXiv2026

Evaluation Resolution Confounds Learning-Rule Comparisons in Model-Brain RSA of Early Visual Cortex

Representational similarity analysis (RSA) is increasingly used to ask which learning rules give convolutional networks brain-like representations. Because biologically plausible rules such as feedback alignment, predictive coding and STDP do not scale, studies that include them train small networks on small images (typically 32x32 CIFAR) and then compare them to brain responses recorded for naturalistic stimuli modeled at far higher resolution. We find that a common result here -- that untrained or locally trained networks rival or beat backpropagation at early visual cortex -- depends strongly on the resolution at which the network is evaluated. The V1 gap between an untrained and a backprop-trained network widens from -0.001 +/- 0.007 at the 32 px training resolution to +0.044 +/- 0.006 at 224 px, growing monotonically across six resolutions (n = 5 seeds). It holds in human fMRI and, directionally, in single-seed macaque electrophysiology, along the training trajectory, and for an ImageNet ResNet-50 and a Swin-Tiny transformer trained at 224 px. We test four candidate mechanisms and none accounts for it: train/eval resolution matching, low-level Gabor and pixel structure, the normalization state of the untrained baseline, and convergence of the pooled descriptor toward a global brightness statistic. A fifth experiment locates it: capping image detail at the training resolution while letting the pooled positions grow 12-fold removes about 90% of the effect, so the dependence lives on the image-content axis. One control result is worth stating separately: a single scalar luminance value per image reaches rho = 0.074 against V1, matching the untrained network's 0.075, bounding what this comparison can resolve at V1 here. The one learning effect that holds across resolution sits at LOC. Comparisons at early visual cortex must control, and report, the evaluation resolution.

q-bio.NC↗

arXiv2026

Supervised Training Rapidly Degrades Early Visual Cortex Alignment Across Biologically Plausible Learning Rules

CORRECTION (August 2026): the central finding of this paper is not supported. An evaluation-mode defect left the batch-normalisation layers of the predictive-coding and STDP conditions in training mode during feature extraction, producing their apparent preservation of V1 alignment. With the defect repaired, predictive coding degrades V1 alignment more than backpropagation does, not less. The finding that training degrades V1 alignment for every rule tested does survive. See the correction note on page 1; the original abstract below and the body are unchanged from v1. Corrected analysis: arXiv:2608.12408. Random, untrained neural networks consistently match or exceed trained networks in representational similarity to early visual cortex. This puzzling finding challenges the assumption that learning improves brain alignment. We investigate it by tracking representational similarity analysis (RSA) alignment to human fMRI data across training for four learning rules: backpropagation (BP), feedback alignment (FA), predictive coding (PC), and spike-timing-dependent plasticity (STDP). Using 720 object images from the THINGS database and fMRI data from three subjects across six visual ROIs, we measure Spearman correlations between model and brain representational dissimilarity matrices at eight training checkpoints (epochs 0-40). We find that (1) a single epoch of training reduces V1 alignment by 25-90%, depending on the learning rule; (2) backpropagation reduces V1 alignment most severely (delta r = -0.080), while predictive coding and STDP preserve substantially more (delta r ~ -0.04); and (3) a weaker, opposite tendency appears in object-selective cortex (LOC), where BP shows the largest increase in alignment during training, although the absolute change is small.

cs.LG↗

arXiv2026

Cross-Species RSA Reveals Conserved Early Visual Alignment but Divergent Higher-Area Rankings Across Human fMRI and Macaque Electrophysiology

CORRECTION (August 2026): an evaluation-mode defect in the shared feature-extraction pipeline affected the predictive-coding and STDP conditions. It applies to both sides of every comparison here: the human values are reprinted from the companion study and the macaque values use the same checkpoints. In a single-seed re-evaluation with repaired checkpoints, macaque STDP at V1 moves from 0.305 to 0.266 and PC from 0.210 to 0.178, level with the untrained baseline, so finding (2) holds for STDP but not for PC; the V4 and IT orderings also change, while V2 is unchanged. Findings (1), (3) and (4) are unaffected, and the other conditions change by at most 0.0034. The five-seed analysis, Kendall's tau, noise ceilings and stimulus control have not been re-computed. See the correction note on page 1; the original abstract below is unchanged from v1. Follow-up study: arXiv:2608.12408. Does the relationship between learning rules and brain alignment generalize across species? We test the same five learning rules against macaque electrophysiology. The macaque data come from MajajHong2015 (V4/IT, 3,200 presentations, 88/168 neurons) and FreemanZiemba2013 (V1/V2, 135 stimuli, 102/103 neurons). Using RSA with identical model weights from our human study, we find: (1) all models achieve higher alignment with macaque early visual cortex (rho = 0.15-0.30 at V1/V2) than with human fMRI (rho = 0.01-0.08); (2) STDP and PC produce the highest macaque V1/V2 alignment; (3) at IT, rankings show no detectable correlation across species (Kendall's tau = 0.00), though this null is expected given that n = 5 provides power only at tau = +/-1.0; (4) a pretrained ResNet-50 achieves rho = 0.25 at macaque IT, substantially above all custom CNN conditions (rho = 0.07-0.14), suggesting IT alignment is limited by model capacity rather than by the learning rule.

cs.LG↗

arXiv2026

Untrained CNNs Match Backpropagation at V1: A Systematic RSA Comparison of Four Learning Rules Against Human fMRI

CORRECTION (August 2026): an evaluation-mode defect affected the predictive-coding and STDP conditions of this study; those results should not be used pending re-computation. At V1 and 224px, predictive coding falls from rho = 0.056 to 0.016 and STDP from 0.064 to 0.037, so the claims that STDP leads among trained rules and that PC and STDP lead at V1/V2 are not supported. The random, backpropagation and feedback-alignment conditions, which carry the untrained-versus-trained claim, are unchanged to within 0.0013. The result is however strongly dependent on the evaluation resolution, held fixed at 224px here; see arXiv:2608.12408. See the correction note on page 1; the original abstract below and the body are unchanged from v1. A central question in computational neuroscience is whether the learning rule used to train a neural network determines how well its internal representations align with those of the human visual cortex. We present a systematic comparison of four learning rules (backpropagation (BP), feedback alignment (FA), predictive coding (PC), and spike-timing-dependent plasticity (STDP)) applied to identical convolutional architectures and evaluated against human fMRI data from the THINGS-fMRI dataset (720 stimuli, 3 subjects) using Representational Similarity Analysis (RSA). All models process stimuli at 224 x 224 resolution; results are averaged across 5 random seeds. Crucially, we include an untrained random-weights baseline that reveals the dominant role of architecture. At V1/V2, the untrained baseline exceeds backpropagation (rho = 0.076 vs. rho = 0.034; Delta-rho = +0.044, p < 0.001). At LOC, only BP reliably exceeds the random baseline (rho = 0.012 vs. -0.005, p < 0.001). At IT, all five conditions converge (rho = 0.008-0.014) with no significant pairwise differences among trained rules. Partial RSA confirms all effects survive pixel-similarity control.

cs.LG↗