Deviations from an ideal image are called aberrations. An ideal optical image satisfies the following three conditions.  1) A point object creates a point image.  2) A plane object perpendicular to the optical axis creates a plane image.  3) A figure on a plane perpendicular to the optical axis creates an image similar to itself.  There are two kinds of aberration: monochromatic aberrations and chromatic aberrations. Spherical aberration, coma, and astigmatism cause images to be blurred. Field curvature and distortion deform the image of a figure. These are referred to as Seidel aberrations and monochromatic aberrations.  Chromatic aberrations are the result of the refractive index of a medium being a function of the wavelength. There are two kinds of chromatic aberration: longitudinal chromatic aberrations and lateral chromatic aberrations.  The sine condition is not an aberration but rather is a very important requirement for correction of coma by a lens. It is expressed as follows:  β=n₁sin u₁/(n₂sin u₂),  where β is lateral magnification and n₁,u₁ and n₂,u₂ are the index and slope angle of a ray in object and image space, respectively.

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In this review, I present a brief discussion about optical coherence tomography (OCT) technology currently under development (OCT angiography, polarization-sensitive OCT, OCT-assisted ocular surgery, ultra-wide-field OCT). These OCT technologies have the potential to completely change the world of ophthalmology in the near future.

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Functional magnetic resonance imaging (fMRI) is one of the main methods for the study of visual function in the human brain. fMRI can measure human brain activation noninvasively by detecting the blood oxygenation level dependent (BOLD) signal, which reflects changes in the deoxyhemoglobin concentration accompanying neural activation. This signal, however, is an indirect measure of neural activation, so we have to be careful in designing experiments and analyzing data to interpret the signal as brain activation. In this review, principles of MRI and fMRI measurements and characters of their signals are explained with several notes regarding spatial and temporal resolution and experimental design. As an example of functional brain imaging with fMRI, retinotopic mapping and separation of the early visual fields are shown.

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Recent advances in non-invasive brain activity measurement technologies and quantitative analysis techniques have enabled quantitative modeling and decoding of human visual representation. Here I review the trends and technical aspects related to decoding human visual experiences using functional MRI.

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Recent advances in diffusion magnetic resonance imaging (dMRI) and tractography have enabled us to identify a major visual white matter pathway in the living human brain and measure the tissue properties. Correlations between function and diffusion measures have been reported. This article treats the basic theory of dMRI and tractography. We review visualization of the major visual pathway using dMRI and tractography and also discuss limitations and future perspectives.

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Understanding the tissue properties of the living human brain is essential to elucidation of the relationship between anatomical properties and information processing in the brain. Recent advancements of quantitative MRI (QMRI) have opened a new avenue for quantitative measurement of tissue properties of the living human brain. Here, I describe an overview of recent progress in QMRI methods as well as the latest studies examining the tissue properties of gray and white matter in the human cerebral cortex using QMRI methods.

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