ATTENTIONAL EFFORT MODULATED BY TASK-DIFFICULTY

It has been assumed that stimulus discrimination in a visual task is p erformed with fixed attentive effort. Here we show that attention to t he same pair of stimuli can be modulated by varying the task difficult y when a task requires the discrimination of only a small number of di fferent stimuli. We used a matching-to-sample paradigm, where a test s timulus is presented after a sample stimulus. When both stimuli Gabor gratings have identical orientations (''matching'' trial) the required response is different from when they have different orientations (''n on-matching'' trial). The task difficulty was manipulated by changing the orientation difference between sample and test stimuli for non-mat ching trials. Difficult non-matching probe trials were embedded within an easy block of trials (easy environment), and vice versa for easy p robe trials. Detectability (d') differences for the same pairs of stim uli (probe trials) in the two environments were calculated as a measur e for change in attentional effort, regardless of changes in likelihoo d ratios (beta). Our results show an increase in d' during the difficu lt task, for both types of probe trials, in paradigms that contained a small number of stimulus combinations. Thus a modulation in attention al effort along a single discrimination dimension is revealed. However it is restricted by the number of stimulus combinations, due to the l imited capacity of the attention available for each stimulus combinati on.