Do we slow down when we get conflicting information ?

One of the things that distinguishes humans from lower animals is their ability to select task-relevant information in cluttered sensory environments. This ability goes beyond merely selecting the information to which one has to respond; it also involves the ability to deal with the conflict that emerges when multiple stimuli elicit incompatible responses. This response conflict results from insufficient selection between the different stimuli. Of course, at some point, this conflict has be resolved; otherwise, no response would be given. The dominant view is that the conflict is resolved by a stronger stimulus selection (i.e., enhancing task-relevant information and blocking distractors). One of the central questions in cognitive neuroscience is how the brain achieves this selective gating of sensory information in interaction with the detection of response conflict.

In this project, we will investigate the processes that allow us to deal with the conflict between such stimuli that are associated with different behavioural responses. These processes are jointly denoted as conflict monitoring. A well accepted behavioural index of conflict monitoring is the so-called conflict effect: the reaction time (RT) difference between stimulus configurations in which the components elicit different responses (incongruent stimuli) and stimulus configurations in which the components elicit the same response (congruent stimuli). The typical observation is that the RT to incongruent stimuli is larger than the one to the congruent stimuli. This has been observed in several tasks, such as the flanker, the Stroop and the Simon task, each of which involves a somewhat different stimulus configuration.

From the conflict effect, it has been concluded that the monitoring of response conflict (i.c., its detection and subsequent resolution) is an automatic process, triggered by stimulus incongruency. However, there is no conclusive evidence for the automaticity of this process. As an alternative hypothesis, I put forward that people can typically make use of spatial attention to prevent response conflict. Therefore, I predict that, when spatial attention is deployed to one of the stimulus components, the conflict effect is no longer observed.

Figure 1. Snapshot of an example stimulus stream

To investigate this hypothesis, we will use a novel experimental paradigm. In this paradigm, the participant fixates a dot at the centre of the screen while stimulus streams are presented in the left and the right visual field. These streams are composed of stimuli that change continuously over the course of time. There are two stimulus categories; for concreteness, we assume them to be letters and digits. The participant responds using two buttons, one for his left and one for his right hand. Each of the two buttons is associated with one stimulus category (letters or digits), and the participant has to press this button when the fixation dot increases size (the so-called go-signal). A snapshot of an example stimulus stream is shown in Figure 1. Over the course of time, the stimulus streams change, with letters being replaced by digits or other letters, and vice versa for digits that are being replaced.

At any point in time, one of the two stimulus streams indicates the hand (left or right) with which the participant must press the corresponding button after the go-signal (an increase in the size of the fixation dot). This is called the relevant stimulus stream. The relevant stimulus stream is indicated by the colour of the fixation dot, for example, with yellow denoting left and blue denoting right. The colour of the fixation dot is called the cue. The two stimulus streams can present conflicting information (a letter in one stream and a digit in the other), and the question now is whether this increases the RT as compared to when no conflicting information is presented (two letters or two digits). Our hypothesis is that this will only be the case if there is uncertainty in the information provided by the cue. To manipulate this uncertainty, we will change the cue in the course of the trial. Thus the fixation dot colour continuously changes in the course of a trial, instructing the participant to switch his attention from left-to-right and vice versa. These changes force the participant to be continuously engaged in the course of a trial. These continuous changes in the stimulus streams and the attentional cue mimic the continuous changes in sensory input in daily life, as well the behavioural relevance of this input.

To manipulate the uncertainty in the information provided by the cue, it is not sufficient to switch the cue in a discrete fashion (from yellow to blue to yellow …). Instead, the attentional cue must be varied in a continuous fashion (from yellow over green to blue over green …). By continuously varying the attentional cue, we continuously change the side to which spatial attention is directed. The largest uncertainty about where attention should be directed is when the fixation dot’s colour is exactly halfway between yellow and blue (i.e., green) and the smallest uncertainty is when the colour is one of the two extremes of the continuum (yellow and blue).

We will use this stimulus type to investigate our hypothesis that the conflict effect is no longer observed when spatial attention is deployed to one of the stimulus streams. More specifically, we predict that the size of the conflict effect increases with the degree of uncertainty about the relevant stimulus stream. This prediction can be evaluated by plotting the conflict effect as a function of the cue color, which ranges from yellow over green to blue. The conflict effect is predicted to be maximal for green and minimal for yellow and blue. In other words, we predict an inverted U-shape relation between cue color and conflict effect.

This project will be supervised by Eric Maris.