  Reconstructing visual stimuli from human brain activities provides a
promising opportunity to advance our understanding of the brain's visual system
and its connection with computer vision models. Although deep generative models
have been employed for this task, the challenge of generating high-quality
images with accurate semantics persists due to the intricate underlying
representations of brain signals and the limited availability of parallel data.
In this paper, we propose a two-phase framework named Contrast and Diffuse
(CnD) to decode realistic images from functional magnetic resonance imaging
(fMRI) recordings. In the first phase, we acquire representations of fMRI data
through self-supervised contrastive learning. In the second phase, the encoded
fMRI representations condition the diffusion model to reconstruct visual
stimulus through our proposed concept-aware conditioning method. Experimental
results show that CnD reconstructs highly plausible images on challenging
benchmarks. We also provide a quantitative interpretation of the connection
between the latent diffusion model (LDM) components and the human brain's
visual system. In summary, we present an effective approach for reconstructing
visual stimuli based on human brain activity and offer a novel framework to
understand the relationship between the diffusion model and the human brain
visual system.
