A method includes presenting an image in a head-mounted display (HMD) controlled by a user. The image is rendered with a transfer function that defines one or more presentation characteristics of voxels in the image. Eye sensors in the HMD are used to determine a region of interest being viewed by the user for longer than a predetermined amount of time. The region of interest comprises a subset of voxels included in the image. The transfer function is automatically adjusted to modify one or more of the presentation characteristics of at least the subset of voxels in response to the region of interest. The presentation of the image in the HMD is modified to present the subset of voxels with modified presentation characteristics.