Investigating Single-Block Recurrence in Vision Transformers for Image Recognition

Vision Transformers (ViTs) implement depth by stacking independently parameterized blocks, but it remains unclear how much of this parameterization is necessary and how much can be replaced by recurrent reuse. We study this question with bViT, a single-block recurrent ViT that repeatedly applies the same transformer block while preserving the iterative computation of a deep model. On ImageNet-1K, bViT-B reaches 0.779 validation accuracy compared with 0.789 for ViT-B under the same training recipe and computational budget, while using 8.6M rather than 86.6M parameters. This correspondence becomes stronger with model width, while narrow recurrent models exhibit a substantial performance gap. Beyond classification, the single-block formulation provides a controlled testbed for studying how transformer computation evolves with depth, since the same heads, neurons, and weight matrices can be tracked across recurrent steps. Analyses of attention, activation patterns, and step-conditioned spectral pruning reveal temporally organized behavior and step-dependent utilization of the shared parameters. bViT also transfers competitively to downstream tasks while enabling highly parameter-efficient adaptation. Our work shows that much of the performance associated with independently parameterized ViT depth can be recovered through recurrent reuse of a single sufficiently wide transformer block.