Recursive hierarchical embedding in vision is impaired by posterior middle temporal gyrus lesions.

The generation of hierarchical structures is central to language, music and complex action. Understanding this capacity and its potential impairments requires mapping its underlying cognitive processes to the respective neuronal underpinnings. In language, left inferior frontal gyrus and left posterior temporal cortex (superior temporal sulcus/middle temporal gyrus) are considered hubs for syntactic processing. However, it is unclear whether these regions support computations specific to language or more generally support analyses of hierarchical structure. Here, we address this issue by investigating hierarchical processing in a non-linguistic task. We test the ability to represent recursive hierarchical embedding in the visual domain by contrasting a recursion task with an iteration task. The recursion task requires participants to correctly identify continuations of a hierarchy generating procedure, while the iteration task applies a serial procedure that does not generate new hierarchical levels. In a lesion-based approach, we asked 44 patients with left hemispheric chronic brain lesion to perform recursion and iteration tasks. We modelled accuracies and response times with a drift diffusion model and for each participant obtained parametric estimates for the velocity of information accumulation (drift rates) and for the amount of information accumulated before a decision (boundary separation). We then used these estimates in lesion-behaviour analyses to investigate how brain lesions affect specific aspects of recursive hierarchical embedding. We found that lesions in the posterior temporal cortex decreased drift rate in recursive hierarchical embedding, suggesting an impaired process of rule extraction from recursive structures. Moreover, lesions in inferior temporal gyrus decreased boundary separation. The latter finding does not survive conservative correction but suggests a shift in the decision criterion. As patients also participated in a grammar comprehension experiment, we performed explorative correlation-analyses and found that visual and linguistic recursive hierarchical embedding accuracies are correlated when the latter is instantiated as sentences with two nested embedding levels. While the roles of the inferior temporal gyrus and posterior temporal cortex in linguistic processes are well established, here we show that posterior temporal cortex lesions slow information accumulation (drift rate) in the visual domain. This suggests that posterior temporal cortex is essential to acquire the (knowledge) representations necessary to parse recursive hierarchical embedding in visual structures, a finding mimicking language acquisition in young children. On the contrary, inferior frontal gyrus lesions seem to affect recursive hierarchical embedding processing by interfering with more general cognitive control (boundary separation). This interesting separation of roles, rooted on a domain-general taxonomy, raises the question of whether such cognitive framing is also applicable to other domains.

Recursion in action: An fMRI study on the generation of new hierarchical levels in motor sequences.

Generation of hierarchical structures, such as the embedding of subordinate elements into larger structures, is a core feature of human cognition. Processing of hierarchies is thought to rely on lateral prefrontal cortex (PFC). However, the neural underpinnings supporting active generation of new hierarchical levels remain poorly understood. Here, we created a new motor paradigm to isolate this active generative process by means of fMRI. Participants planned and executed identical movement sequences by using different rules: a Recursive hierarchical embedding rule, generating new hierarchical levels; an Iterative rule linearly adding items to existing hierarchical levels, without generating new levels; and a Repetition condition tapping into short term memory, without a transformation rule. We found that planning involving generation of new hierarchical levels (Recursive condition vs. both Iterative and Repetition) activated a bilateral motor imagery network, including cortical and subcortical structures. No evidence was found for lateral PFC involvement in the generation of new hierarchical levels. Activity in basal ganglia persisted through execution of the motor sequences in the contrast Recursive versus Iteration, but also Repetition versus Iteration, suggesting a role of these structures in motor short term memory. These results showed that the motor network is involved in the generation of new hierarchical levels during motor sequence planning, while lateral PFC activity was neither robust nor specific. We hypothesize that lateral PFC might be important to parse hierarchical sequences in a multi-domain fashion but not to generate new hierarchical levels.

Recursive music elucidates neural mechanisms supporting the generation and detection of melodic hierarchies.

The ability to generate complex hierarchical structures is a crucial component of human cognition which can be expressed in the musical domain in the form of hierarchical melodic relations. The neural underpinnings of this ability have been investigated by comparing the perception of well-formed melodies with unexpected sequences of tones. However, these contrasts do not target specifically the representation of rules generating hierarchical structure. Here, we present a novel paradigm in which identical melodic sequences are generated in four steps, according to three different rules: The Recursive rule, generating new hierarchical levels at each step; The Iterative rule, adding tones within a fixed hierarchical level without generating new levels; and a control rule that simply repeats the third step. Using fMRI, we compared brain activity across these rules when participants are imagining the fourth step after listening to the third (generation phase), and when participants listened to a fourth step (test sound phase), either well-formed or a violation. We found that, in comparison with Repetition and Iteration, imagining the fourth step using the Recursive rule activated the superior temporal gyrus (STG). During the test sound phase, we found fronto-temporo-parietal activity and hippocampal de-activation when processing violations, but no differences between rules. STG activation during the generation phase suggests that generating new hierarchical levels from previous steps might rely on retrieving appropriate melodic hierarchy schemas. Previous findings highlighting the role of hippocampus and inferior frontal gyrus may reflect processing of unexpected melodic sequences, rather than hierarchy generation per se.

Self-similarity and recursion as default modes in human cognition.

Humans generate recursive hierarchies in a variety of domains, including linguistic, social and visuo-spatial modalities. The ability to represent recursive structures has been hypothesized to increase the efficiency of hierarchical processing. Theoretical work together with recent empirical findings suggests that the ability to represent the self-similar structure of hierarchical recursive stimuli may be supported by internal neural representations that compress raw external information and increase efficiency. In order to explicitly test whether the representation of recursive hierarchies depends on internalized rules we compared the processing of visual hierarchies represented either as recursive or non-recursive, using task-free resting-state fMRI data. We aimed to evaluate the relationship between task-evoked functional networks induced by cognitive representations with the corresponding resting-state architecture. We observed increased connectivity within Default Mode Network (DMN) related brain areas during the representation of recursion, while non-recursive representations yielded increased connectivity within the Fronto-Parietal Control-Network. Our results suggest that human hierarchical information processing using recursion is supported by the DMN. In particular, the representation of recursion seems to constitute an internally-biased mode of information-processing that is mediated by both the core and dorsal-medial subsystems of the DMN. Compressed internal rule representations mediated by the DMN may help humans to represent and process hierarchical structures in complex environments by considerably reducing information processing load.