# Recursion beyond language: Lexical and arithmetic interference in visual hierarchical embedding

## TL;DR

The study explores how humans process visual hierarchies, finding that recursion is more resilient than iteration when cognitive resources are taxed.

## Contribution

The paper introduces a dual-task paradigm to show that visual recursion relies on symbolic resources but can flexibly compensate under interference.

## Key findings

- Lexical retrieval and arithmetic impaired both visual recursion and iteration tasks.
- Drift diffusion modeling revealed symbolic interference reduced evidence accumulation and decision thresholds.
- Visual recursion outperformed iteration in accuracy under interference, suggesting compensatory strategies.

## Abstract

The capacity to represent recursive hierarchical embedding (RHE) is considered a hallmark of human cognition. Yet, it remains debated whether non-linguistic recursion depends on language-specific, domain-general, or independent visuospatial mechanisms. In two experiments, we tested this question using a dual-task paradigm. Participants performed either a visual recursion task (REC) or a matched visual iteration task (ITE) while concurrently engaging in lexical retrieval (LEX), serial arithmetic (MATH), visual delayed match-to-sample (VIS), or no interference (NONE). Both LEX and MATH—tasks that require serial, symbolic, generative processing—reliably reduced accuracy in REC and ITE, with interference being stronger for ITE. Drift diffusion modeling confirmed that LEX and MATH slowed evidence accumulation and reduced decision thresholds, indicating less efficient and more impulsive responding. Interestingly, while LEX and MATH lengthened response times in ITE, they shortened them in REC, suggesting compensatory processing strategies. By contrast, VIS did not impair accuracy; instead, it increased drift and boundary separation, consistent with more cautious evidence accumulation. Cognitive covariates further revealed that verbal working memory predicted REC more strongly than ITE under baseline conditions; however, this advantage did not account for the greater interference observed in ITE. Taken together, these findings suggest that visual hierarchical processing involves serial symbolic resources, but recursion is more resilient than iteration, possibly due to the availability of parallel or configural strategies. This supports a hybrid architecture in which linguistic-like symbolic operations contribute to building visual hierarchies, but alternative mechanisms can buffer recursion when these resources are taxed.

The online version contains supplementary material available at 10.1007/s00426-026-02252-2.

Recursive structure is fundamental to human cognition, yet its cognitive basis outside language is debated. Using an interference paradigm, we show that visual hierarchy building depends on symbolic resources tapped by lexical retrieval and serial arithmetic, while purely visual load does not impair accuracy. Iteration is more vulnerable than recursion, suggesting it is less able to recruit compensatory configural-parallel mechanisms. These findings refine theories of hierarchical cognition and offer a general framework for testing recursion across domains beyond language.

The online version contains supplementary material available at 10.1007/s00426-026-02252-2.

Developed a dual-task paradigm to assess the cognitive bases of visual hierarchical processing.

Lexical retrieval and serial arithmetic impaired both visual recursion and iteration, whereas visual interference did not.

Drift diffusion modeling showed that symbolic interference reduced evidence accumulation and lowered decision thresholds.

Under interference, visual recursion shortened response times while outperforming iteration in accuracy.

Findings suggest visual recursion primarily relies on serial symbolic generative systems but flexibly compensates when taxed.

The online version contains supplementary material available at 10.1007/s00426-026-02252-2.