Early modulation of visual input: A study of attentional strategies

Despite agreement among many attentional theories that processing resources are limited and allocated according to task demands, controversy continues about the locus of selectivity. Studies of spatial orientation of attention suggest an early effect. These results, however, can be explained instead by effects of decision processes. The present study avoids this difficulty by directly manipulating attention in a dual-task paradigm and by using SDT to dissociate sensory tuning from criterion shifts. Ten subjects judged whether two lines to the left of fixation were the same or different in length; they also judged two lines presented simultaneously to the right. In a given block of 64 trials, the subject was to allocate 80%, 50%, or 20% of attention to one pair of lines and the rest to the other. On every trial, the subject judged both pairs. Results showed that d' increased from 0.77 with 20% allocation to 1.69 with 80%, indicating that sensitivity is modulated by attentional instructions. These results are predicted quantitatively by Luce's sample-size model.