HOW does the human visual system 'bind' different fragments in the vis
ual scene to create enduring representations of objects(1-5)? A visual
illusion known as 'metacontrast'(6-9) or backward masking provides co
mpelling evidence that perception is not instantaneous and that it occ
urs sequentially in distinct stages, If a solid white target square is
displayed for 50 ms in a tachistoscope, switched off, and followed by
a 50 ms display of two flanking mask squares, remarkably, subjects re
port seeing only the two flanking squares: the first square is simply
not 'seen'. By plotting the magnitude of masking as a function of the
delay between the target and mask (the stimulus onset asynchrony), one
can obtain a characteristic 'U'-shaped function(7) with optimum maski
ng occurring at about 50 ms, and no masking with synchronous target an
d mask presentations or at delays higher than 300 ms. The illusion is
also highly sensitive to elementary stimulus dimensions such as colour
, orientation and spatial frequency(8), and it has been suggested(10)
that it is based on 'low level' autonomous visual mechanisms rather th
an cognitive processes, Here we describe a novel visual stimulus that
demonstrates that metacontrast can be strongly modulated by 'top down'
influences such as voluntary visual attention.