Damage to the ventromedial prefrontal cortex (vmPFC) in humans disrupts planning abilities in naturalistic settings. However, it is unknown which components of planning are affected in these patients, including selecting the relevant information, simulating future states, or evaluating between these states. To address this question, we leveraged computational paradigms to investigate the role of vmPFC in planning, using the board game task "Four-in-a-Row" (18 lesion patients, 9 female
30 healthy control participants, 16 female) and the simpler "Two-Step" task measuring model-based reasoning (49 lesion patients, 27 female
20 healthy control participants, 13 female). Damage to vmPFC disrupted performance in Four-in-a-Row compared with both control lesion patients and healthy age-matched controls. We leveraged a computational framework to assess different component processes of planning in Four-in-a-Row and found that impairments following vmPFC damage included shallower planning depth and a tendency to overlook game-relevant features. In the "Two-Step" task, which involves binary choices across a short future horizon, we found little evidence of planning in all groups and no behavioral differences between groups. Complex yet computationally tractable tasks such as "Four-in-a-Row" offer novel opportunities for characterizing neuropsychological planning impairments, which in vmPFC patients we find are associated with oversights and reduced planning depth.