Large-scale visual neural datasets such as the Natural Scenes Dataset (NSD) are boosting NeuroAI research by enabling computational models of the brain with performances beyond what was possible just a decade ago. However, these datasets lack out-of-distribution (OOD) components, which are crucial for the development of more robust models. Here, we address this limitation by releasing NSD-synthetic, a dataset consisting of 7T fMRI responses from the eight NSD subjects for 284 carefully controlled synthetic images. We show that NSD-synthetic's fMRI responses reliably encode stimulus-related information and are OOD with respect to NSD. Furthermore, OOD generalization tests on NSD-synthetic reveal differences between models of the brain that are not detected with NSD - specifically, self-supervised deep neural networks better explain neural responses than their task-supervised counterparts. These results showcase how NSD-synthetic enables OOD generalization tests that facilitate the development of more robust models of visual processing, and the formulation of more accurate theories of human vision.