OFF: 'Visions' while meditating

[Anthony Nelson-Smith]

Several subscribers made helpful comments when I last mentioned meditation problems, awhile ago, so I'm not too apologetic to come back for more...   Often, after relaxing into meditation mode, I can see behind my closed eyelids a duller version of the effect one experiences when pressure is applied to the eyeballs:  a series of indefinite shapes in either a greenish pale purple or a dull orange against the dark, purplish black background.  These either swirl away in a spiral or expand from the lower edge of my vision in a semicircle, like a ripple in a pond.  Their movement doesn't closely match any movements which my eyeballs might make, nor do they pulse to match my heartbeat.  I believe that I might be in alpha mode by the time they appear.  Can anyone suggest how these 'visions' arise? I often have problems suppressing extraneous thoughts (what my t'ai chi master calls 'your mind out shopping') but these 'visions' either appear when my mind is fairly blank or, perhaps, even override such thoughts.   I'm still pretty much a novice so far as productive meditation is concerned, so I'd be grateful for comments from such masters of the art who, I'm sure, are out there somewhere!                                       Tony N-S.