Facilitation of letter recognition by an annulus in a metacontrast paradigm

The recognition of a single letter is found to improve when it is surrounded by an annulus in close proximity. This facilitatory effect is obtained only when annulus and target durations are equal and onsets are simultaneous. When the annulus is increased in size, the facilitatory effect is found first to increase in magnitude and then to decline; no interaction between target letter and annulus is observed for large diameter annuli (72'+). The results are interpreted with reference to a lateral inhibitory model of spatio-temporal interactions.