random-dot tests, also due to visual crowding which is based on interaction effects from adjacent visual contours. Fine stereopsis is important for fine-motor tasks such as threading a needle. The stereopsis which an individual can achieve is limited by the level of visual acuity of the poorer eye. In particular, patients who have comparatively lower visual acuity tend to need relatively larger spatial frequencies to be present in the input images, else they cannot achieve stereopsis. Fine stereopsis requires both eyes to have a good visual acuity in order to detect small spatial differences, and is easily disrupted by early visual deprivation. There are indications that in the course of the development of the visual system in infants, coarse stereopsis may develop before fine stereopsis and that coarse stereopsis guides the vergence movements which are needed in order for fine stereopsis to develop in a subsequent stage. Furthermore, there are indications that coarse stereopsis is the mechanism that keeps the two eyes aligned after strabismus surgery. Static and dynamic stimuli It has also been suggested to distinguish between two different types of stereoscopic depth perception: static depth perception (or static stereo perception) and motion-in-depth perception (or stereo motion perception). Some individuals who have strabismus and show no depth perception using static stereotests (in particular, using Titmus tests, see this article's section on contour stereotests) do perceive motion in depth when tested using dynamic random dot stereograms. One study found the combination of motion stereopsis and no static stereopsis to be present only in exotropes, not in esotropes. Research on perception mechanisms There are strong indications that the stereoscopic mechanism consists of at least two perceptual mechanisms, possibly three. Coarse and fine stereopsis are processed by two different physiological subsystems, with a coarse stereopsis being derived from diplopic stimuli (that is, stimuli with disparities well beyond the range of binocular fusion) and yielding only a vague impression of depth magnitude. Coarse stereopsis appears to be associated with the magno pathway which processes low spatial frequency disparities and motion, and fine stereopsis with the parvo pathway which processes high spatial frequency disparities. The coarse stereoscopic system seems to be able to provide residual binocular depth information in some individuals who lack fine stereopsis. Individuals have been found to integrate the various stimuli, for example stereoscopic cues and motion occlusion, in different ways. How the brain combines the different cues – including stereo,