Space Raiders

When I was seven my parents bought a ZX81, one of the first (silent, monochromatic, rickety, burnt-smelling) personal computers. My favourite game for it was a Space Invaders clone called Space Raiders. One day while playing it I found that if I squinted, the game speeded up. I couldn’t fathom any reason why this would be so, but nevertheless I was convinced – I squinted my eyes up, the game ran faster. What’s more, turning the TV contrast up or the brightness down had the same effect. What was happening? Obviously I couldn’t affect the speed of the game with my eye muscles, so what was going on? I never found out.