Large language models demonstrate emergent modularity, where functionally specialized components and circuits arise to handle specific tasks or task formats. If similar modules arise in models trained on more cognitively plausible datasets, it could inform debates surrounding what kinds of would be learnable given more human-like language learning signals. In this paper, we describe a multimodal vision-language model submitted to the BabyLM Challenge. Our model achieves similar performance to the best-performing architectures from last year, though visual information does not improve performance on text-only tasks over text-only models (in accordance with prior findings). To better understand how the model processes the evaluation tasks of the BabyLM Challenge, we leverage causal interpretability methods to locate the neurons that contribute to the model’s final decisions. We find that the models we train are highly modular: distinct components arise to process related tasks. Furthermore, on text-and-image tasks, adding or removing visual inputs causes the model to use distinct components to process the same textual inputs. This suggests that modal and task-specific specialization is efficiently learned, and that a high degree of functional specialization arises in even small-scale language models.

Transformers demonstrate impressive performance on a range of reasoning benchmarks. To evaluate the degree to which these abilities are a result of actual reasoning, existing work has focused on developing sophisticated benchmarks for behavioral studies. However, these studies do not provide insights into the internal mechanisms driving the observed capabilities. To improve our understanding of the internal mechanisms of transformers, we present a comprehensive mechanistic analysis of a transformer trained on a synthetic reasoning task. We identify a set of interpretable mechanisms the model uses to solve the task, and validate our findings using correlational and causal evidence. Our results suggest that it implements a depth-bounded recurrent mechanisms that operates in parallel and stores intermediate results in selected token positions. We anticipate that the motifs we identified in our synthetic setting can provide valuable insights into the broader operating principles of transformers and thus provide a basis for understanding more complex models.