The Role of Attention in the Detection of Luminance Changes: Endogenous versus Exogenous Cueing

Many visual tasks have been shown to be influenced by the attentional state of the observer. However the conditions under which attention has effects is still unclear. Here we report upon a series of experiments where the observer has to react to changes in the luminance of a target amongst many distractors. We have systematically manipulated the nature of the cue (endogenous vs exogenous) and the task to be performed (detection of changes vs discrimination of direction of change).

Stimuli consisted of a number of circles upon a screen. At some point in time one of the circles changed in luminance and subjects reacted as quickly as possible. Typically half the circles were red and half green. Subjects could be cued to attend to a particular colour by instruction (endogenous) or by a brief flash of the lines that joined the same coloured elements (exogenous). In most cases the cue was appropriate on 80% of the trials and hence we could compare response times between valid and invalid trials.

Our results show that (1) for simple detection endogenous cues were ineffective whereas exogenous cues provided a small advantage for the valid trials, and (2) for discrimination of direction of change endogenous cues provided a small advantage, whereas exogenous cues provided a large advantage for the valid trials. It appears that both cueing type and task type modulate the attentional effects on this ‘preattentive’ task.