I found some fMRI experiments which explore the stimulus threshold for conscious perception. See figure 4A of the following article, which shows that a sound can be detected and processed without being consciously noticed. Sounds which are consciously noticed have stronger responses and activity caused by the sound spreads to many more cortical areas.
The Global Neuronal Workspace Model of Conscious Access: From Neuronal Architectures to Clinical Applications, Stanislas Dehaene, Jean-Pierre Changeux, and Lionel Naccache, 2011
Abstract While a considerable body of experimental data has been accumulated on the differences between conscious and non-conscious processing, a theory is needed to bridge the neuro-psychological gap and establish a causal relationship between objective neurophysiological data and subjective reports. In the present review, we first briefly outline the detailed postulates and predictions of our working hypothesis, referred to as the global neuronal workspace (GNW) model. We then compare these predictions to experimental studies that have attempted to delineate the physiological signatures of conscious sensory perception by contrasting it with subliminal processing, using a variety of methods: behavioral, PET and fMRI imaging, time-resolved imaging with ERP and MEG, and finally single-cell electrophysiology. In a final section, we examine the relevance of these findings for pathologies of consciousness in coma and vegetative states.
Although the above article describes a framework for attention, I haven’t finished reading it and can’t say if it agrees with my hypothesis about attention.