All manner of sensory "tricks" can occur during Zazen. Some are quite interesting, as seems this one. We may note them, but do not particlarly encourage them in our little corner of Buddhist meditation. ...
In Zen Practice, we have to be careful of certain games the mind will play during Zazen once in awhile ... including unusual visual and auditory sensations, brief periods of paranoia or panic, memories arising from deep down in our subconscious. We are not used to the stillness and quiet of Zazen, and it lets certain memories, emotions, fears and like psychological states rise to the surface ... or allows some things (spots in our eyes that are always there even though not usually noticed, background sounds) to be noticed that are usually blocked out by all the noise and busyness in our heads, senses and around us.
Once, during a Sesshin, I became irate inside because I felt the monk at Sojiji sitting next to me was "encroaching on my space". I once had a little Buddha pop out of the wall and chat with me for several minutes (I pinched myself ... he stayed!), and felt like I was floating in the air. Once I think I heard the ants walking across the floor really loud! It is common during Sesshin, because of the strains involved, the "sensory deprivation", to experience such things as emotional swings, hearing becoming so sharp you can be disturbed by an ant walking across the room, strange bodily sensations such as feelings of floating or being giant sized, and paranoia. I think what you describe fits into this category.
If it is just once in awhile
... and if you are aware of this, and it was not too overwhelming ... then I do not think it cause for worry. If it becomes too overwhelming, break off that sitting and take a little time off until you cool down. If it becomes a regular event, or too profound, that may be a sign of something else that needs to be approached. But, once in awhile ... I would not be concerned.
We tend to call such things "Makyo
", defined as follows (by Daido Loori Roshi). He speaks of hallucinatory like experiences ...
In Zen, hallucinations are called makyo. It is not unusual for practitioners sitting in meditation for long periods of time to experience makyo. Some people feel like they are levitating, others see visions of the Buddha bathed in light, some hear sounds or voices. This in itself is not a problem. The problem arises when we confuse these experiences with enlightenment. When students come to me in dokusan to give me elaborate description of their makyo, a common response from me could be something like, “Oh, don’t worry about it—it will go away. Maybe you’re not sitting straight.” In other words, don’t attach to it. But if a dream is real, why isn’t makyo real? Are dreams, makyo, enlightenment and delusion the same, or are they different?
We learn from all these experience ... we learn how the mind is like a theatre, and creates our experience of the life-world.
I also posted this once ...
Sensory deprivation, and really paying attention to objects of sight that we usually do not pay attention to (the patterns on the carpet, for example) can have such an effect. These things usually are connected to the mechanics of the visual sense, and often beyond our control. It is just an optical illusion.
A dry as toast, but good book on the topic is Dr. Austin's Zen and the Brain ... he has a discussion of all manner of hallucinations here (from about page 373).
d%3D8ywrjDa0vZ8C%26dq%3Dzen%2Band%2Bthe%2Bbrain%26 printsec%3Dfrontcover%26source%3Dbn%26hl%3Den%26ei %3DvAK1SfK1MZiy6wPt34S6BQ%26sa%3DX%26oi%3Dbook_res ult%26resnum%3D5%26ct%3Dresult&ei=vAK1SfK1MZiy6wPt 34S6BQ&usg=AFQjCNEkHKnyAAZJFZ60JYBO-txzdjUW2Q&sig2=gpC5vxamEJTUDyJTKBdEnA">http://www.google.com/url?sa=t&source=w ... DyJTKBdEnA
Seeing patterns on the carpet or wall you are looking at, and floor undulation, is kind of like this effect produced by a bad carpet:
http://www.moillusions.com/2007/11/wavy ... usion.html
Another common effect is to see "spots in the eyes". Most are there all along (floating impurities, early cataracts and such of the eyeball itself), but we just do not notice them until we sit still. Many are just the "cones and rods" of the eye that were there all along. The cones and rods of color, for example, are always present in our eyes, but we do not give them notice so often in day to day life. In Zazen, what is always there just stands out sometimes, and the brain plays some tricks by seeing "connect the dot" patterns.
The eyes contain cones and rods for color that we usually do not notice (but, if you look at any object closely, you will see little dots of color, much like the picture tube of a color tv):
The sensory deprivation effect at staring at the white surface just brings the little dots to our attention, and they play pattern tricks in the brain.
Like a new pair of glasses, the brain will adjust and soon not notice the dots as much. Maybe we are subconsciously looking for the patterns, and thus noticing the patterns. If we just forget about them, they usually go away.
However, visual hallucinations are common in Zazen. Not a worry, nor of any particular importance other than as an amusement, possibly with a small lesson about how we create the world through the senses:
Hallucinations and Illusions
Kornfield (1979, 1983) noted that there was a strong correlation between student reports of higher levels of concentration during insight meditation, when the mind was focused and steady, and reports of altered states and perceptions. He reported that unusual experiences, such as visual or auditory aberrations and hallucinations, and unusual somatic experiences, are the norm among practiced meditation students. Walsh (1978) reported that he experienced hypnagogic hallucinations, and Goleman (1978-79) reported visionary experiences during deep meditation. Shimano and Douglas (1975) reported hallucinations similar to toxic delirium during zazen.
... Earlier, Deikman (1966a) reported that during meditation on a blue vase, his subjects' perception of color became more intense or luminous, and that for some of them the vase changed shape, appeared to dissolve, or lost its boundaries. Maupin (1965) reported that meditators sometimes experience "hallucinoid feelings, muscle tension, sexual excitement, and intense sadness."
The contemplative literature contains numerous descriptions of the perceptual distortion produced by meditation. It is called makyo in Zen Buddhist sources, and is characterized in some schools as "going to the movies," a sign of spiritual intensity but a phenomenon that is regarded to be distinctly inferior to the clear insight of settled practice. In some Hindu schools it is regarded as a product of the sukshma sharira, or "experience body," in its unstable state, and in that respect is seen to be another form of maya, which is the illusory nature of the world as apprehended by ordinary consciousness.
In a similar manner, St. John of the Cross described the false enchantments that may lure the aspirant in prayer, warning that "devils may come in the guise of angels."  In his allegory of the spiritual journey, The Pilgrim's Progress, John Bunyan described Christian's losing his way by following a man who says he is going to the Celestial City but instead leads him into a net. In all the great contemplative manuals, one is taught that detachment, equanimity, and discrimination are required for spiritual balance once the mind has been opened and made more flexible by prayer and meditation. Illusions and hallucinations, whether they are troubling or beatific, are distractions—or signposts at best—on the way to enlightenment or union with God.
Move along folks ... nothing to look at here! :-)
Actually, it is all a fine lesson in how the body-mind-self-world
are all interconnected.
What you are seeing is a fine lesson on how objects are all interconnected. Truly, in this world, one things does blend into another, and only the mind cuts it apart into pieces. Learn that lesson, and return to just sitting.