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who_controls_the_attentional_enhancement_of_targets_and_the_blocking_of_distracters [2015/09/18 13:45]
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who_controls_the_attentional_enhancement_of_targets_and_the_blocking_of_distracters [2015/09/18 13:47] (current)
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 By means of a cue, one of these two streams will be indicated as the task-relevant one, and the other one will be the distracter. The streams are continuously present but their content varies over time. There are two stimulus categories; for concreteness,​ we assume them to be letters and digits. 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 task-relevant stimulus streams indicates the hand (left or right) with which the participant must press the corresponding button after the go-signal. 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 the so-called //​cue//​.  ​ By means of a cue, one of these two streams will be indicated as the task-relevant one, and the other one will be the distracter. The streams are continuously present but their content varies over time. There are two stimulus categories; for concreteness,​ we assume them to be letters and digits. 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 task-relevant stimulus streams indicates the hand (left or right) with which the participant must press the corresponding button after the go-signal. 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 the so-called //​cue//​.  ​
  
-{{ |Figure 2. Snapshot of an example varying-contrast stimulus}}+{{:​varyingcontraststimulus.png?​400 ​|Figure 2. Snapshot of an example varying-contrast stimulus}}
  
 For answering our research questions, it is required that target enhancement and distracter suppression are manipulated independently,​ and this not the case for the trial type just described. In fact, because both target and distracter are continuously present they are perfectly anti-correlated. Therefore, we now change the stimulus streams such that target and distractor become uncorrelated. The resulting new stimulus type is called a //​varying-contrast stimulus//, and a snapshot is shown in Figure 2, which shows a moment at which only the right stream has a low contrast. The contrast of both streams continuously varies over time, and this can be achieved by degrading the letters and digits (arrays of square pixels whose grey-values can be adjusted to achieve the appropriate contrast; see Fig. 2). Over time, for each of the streams, the contrast is modulated rhythmically between maximum and zero contrast. The rhythmic contrast modulations of the two streams have different frequencies (e.g., 1 Hz for the one and 1.5 Hz for the other stream), which results in a zero temporal correlation between target enhancement and distracter suppression (if the time window is sufficiently long). This is a form of frequency tagging, which has the important advantage that a frequency domain analysis of the electrophysiological data allows for a separation of the neurophysiological correlates of target enhancement and distracter suppression (involving both cognitive control processes and their consequences over sensory areas). This separation can be performed effectively by means of a regression analysis. Importantly,​ within each of the two streams, the stimuli will vary across the two stimulus categories, each of which is associated with one response side. This has several advantages: (1) it requires a continuous engagement over the course of a trial, and (2) it allows to investigate stimulus selection during both stable and switching motor preparation. The latter is likely to be a crucial variable for cognitive control, whose neuronal substrate is closely related to the motor system. For answering our research questions, it is required that target enhancement and distracter suppression are manipulated independently,​ and this not the case for the trial type just described. In fact, because both target and distracter are continuously present they are perfectly anti-correlated. Therefore, we now change the stimulus streams such that target and distractor become uncorrelated. The resulting new stimulus type is called a //​varying-contrast stimulus//, and a snapshot is shown in Figure 2, which shows a moment at which only the right stream has a low contrast. The contrast of both streams continuously varies over time, and this can be achieved by degrading the letters and digits (arrays of square pixels whose grey-values can be adjusted to achieve the appropriate contrast; see Fig. 2). Over time, for each of the streams, the contrast is modulated rhythmically between maximum and zero contrast. The rhythmic contrast modulations of the two streams have different frequencies (e.g., 1 Hz for the one and 1.5 Hz for the other stream), which results in a zero temporal correlation between target enhancement and distracter suppression (if the time window is sufficiently long). This is a form of frequency tagging, which has the important advantage that a frequency domain analysis of the electrophysiological data allows for a separation of the neurophysiological correlates of target enhancement and distracter suppression (involving both cognitive control processes and their consequences over sensory areas). This separation can be performed effectively by means of a regression analysis. Importantly,​ within each of the two streams, the stimuli will vary across the two stimulus categories, each of which is associated with one response side. This has several advantages: (1) it requires a continuous engagement over the course of a trial, and (2) it allows to investigate stimulus selection during both stable and switching motor preparation. The latter is likely to be a crucial variable for cognitive control, whose neuronal substrate is closely related to the motor system.