EXTERNAL NOISE DISTINGUISHES ATTENTION MECHANISMS

We developed and tested a powerful method for identifying and characte rizing the effect of attention on performance in visual tasks as due t o signal enhancement, distracter exclusion, or internal noise suppress ion. Based on a noisy Perceptual Template Model (PTM) of a human obser ver, the method adds increasing amounts of external noise (white gauss ian random noise) to the visual stimulus and observes the effect on pe rformance of a perceptual task for attended and unattended stimuli. Th e three mechanisms of attention yield three ''signature'' patterns of performance, The general framework for characterizing the mechanisms o f attention is used here to investigate the attentional mechanisms in a concurrent location-cued orientation discrimination task. Test stimu li - Gabor patches tilted slightly to the right or left - always appea red on both the left and the right of fixation, and varied independent ly, Observers were cued on each trial to attend to the left, the right , or evenly to both stimuli, and decide the direction of tilt of both test stimuli, For eight levels of added external noise and three atten tion conditions (attended, unattended, and equal), subjects' contrast threshold levels were determined. At low levels of external noise, att ention affected threshold contrast: threshold contrasts for non-attend ed stimuli were systematically higher than for equal attention stimuli , which were, in turn, higher than for attended stimuli. Specifically, when the rms contrast of the external noise is below 10%, there is a consistent 17% elevation of contrast threshold from attended to unatte nded condition across all three subjects, For higher levels of externa l noise, attention conditions did not affect threshold contrast values at all. These strong results are characteristic of a signal enhanceme nt, or equivalently, an internal additive noise reduction mechanism of attention. (C) 1998 Elsevier Science Ltd. All rights reserved.