Dear Bastiaan,
I am familiar with the principles of optical flow in driving, which means that the point headed for BY THE CAR stays relatively steady in the visual experience whereas the visual experience off to the sides swooshes by, with the part of the world off to the sides of vision moving the fastest. The gradient of optical flow velocity is centered on the point of the world the car is headed towards, where the velocity is least, and from there back along each side (sort of like a boat wake off the prow)the velocity increases smoothly to a maximum at the sides. The driver on a straight road has nothing to do to keep the car headed straight. On a curving road, he keeps the car headed into the middle of the farthest part of the lane in front that the speed of the car and responsiveness of the steering requires. The hand on the steering wheel keeps the top of the steering wheel aimed into this part of the lane.
This visual control uses peripheral vision to sense the differential optical flow in relation to where the eyes are looking with central vision. There is a very different "visual feel" of looking straight ahead driving on a straight road versus driving straight but looking off to the side while passing some cows. The oddness comes from the fact that the gaze off to the side does not result in a stable visual experience, as the world swooshes by thru the steadily aimed gaze of central vision AND the experience of peripheral vision to the rear and in front of central vision (looking left out the window, the rear is to the left and the front to the right) does not have a symmetrical optic flow velocity pattern.
The sort of parallel line experience you refer to works fine if you are moving the gaze from the feet up and outward away by raising the neck, but is not straightforward when you scan the gaze sideways from the feet off to a target to the side. In this case, the peripheral experience that counts is not the sides but above and below the "spotlight" of central vision. The "higher" above central vision, the less clear the visual experienece, and the "lower" below central vision, the less clear the visual experience. This fuzziness above and below central vision creates a "channel" for the eye scan sideways that has something quite similar to parallel lines. So long as the scan of the gaze proceeds along inside this channel, and does not skew out of the channel, your head turn is headed "straight down the lane" so to speak.
Noticing the skewing is something to get used to a learn so that you get a warning when your head turn on the golf course is not correct. The skew warns you that you are about to get misperceptions of the line, and hence of the target.
The ProAim horizontal "channel" is defined by two parallel lines. The real channel is not this explicit, but is indicated by the symmetry of fuzziness of focus above and below central vision. It's an awareness of a specific region of peripheral vision. The lines in the ProAim glasses are not fuzzy, mostly because they are in fact close to central vision on the lens but appear farther above and below when projected as if onto the grass 4.5 feet or so away. You CAN practice not skewing the parallel lines by using the glasses over a string line, but without the glasses you need to train an awareness of the symmetry of peripheral visual experience above and below central vision WHILE SCANNING along plain old ground.
Cheers!
Geoff Mangum
Putting Theorist and Instructor
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