Difference between revisions of "CNP Stop Signal"

Revision as of 05:52, 16 May 2011

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Basic Task Description

The Stop-signal task is a widely used measure of response inhibition and is based on a horse-race model of stopping, which assumes that independent go and stop processes race against one another to determine whether a response is executed or inhibited (Logan and Cowan, 1984; Logan, 1994) (though the independence assumption can be relaxed (Boucher et al., 2007). The primary dependent variable of the task, Stop-signal reaction time (SSRT), provides an individualized measure of inhibitory control. In this task, participants are presented with a series of Go stimuli to which they are instructed to respond quickly. This speeded reaction time task establishes a prepotency to respond. On a subset of trials, the Go stimulus is followed, after a variable delay, by a stop-signal, to which participants are instructed to inhibit their response. The onset of a the stop signal is varied and depends on the participant’s performance, such that it is increased after a previous failure to inhibit and decreased after a previous inhibition. This one-up/one-down tracking procedure ensures that participants inhibit on approximately half of all stop trials, and the horse-race model allows for the estimation of stop-signal reaction time (SSRT), an individualized measure of a participant’s inhibitory ability that controls for difficulty level.

Task Procedure

In this version of the Stop-signal task, participants were shown a series of go stimuli (“X” or “O”) in the center of the screen and were told to press the left arrow button on the keyboard when they saw an “X” and to press the right arrow keyboard button when they saw an “O” (Go trials). On a subset of trials, a stop-signal (a 500 Hz tone presented through headphones) was presented a short delay after the go stimulus appeared and lasted for 250 ms (Stop trials). Participants were instructed to respond as quickly and accurately as possible on all trials, but to withhold their response on Stop trials (on trials with the tone). Participants were instructed that stopping and going were equally important.

On Stop trials, the delay of the onset of the stop-signal, or stop-signal delay (SSD), was varied, such that it was increased after the participant successfully inhibited in response to a stop-signal (making the next stop trial more difficult), and decreased after the participant failed to inhibit in response to a stop-signal (making the next stop trial less difficult). Each SSD increase or decrease was in 50 ms intervals. The SSD values were drawn from two interleaved staircases (or ladders) per block, resulting in 10 trials in each staircase for a total of 20 Stop trials per block, with values starting at 200 and 300 ms for ladders 1 and 2, respectively. At the end of the first block, the last SSD time from each staircase was then carried over to be the initial SSD for the second block. This one-up/one-down tracking procedure ensured that subjects successfully inhibited on approximately 50% of inhibition trials. Also as a result, difficulty level is individualized across subjects and both behavioral performance and numbers of successful stop trials are equated across subjects.

All participants received training on the Stop-signal task in the form of two practice blocks. The first practice block contained 32 random Go trials only to establish prepotency to respond. A second practice block contained 22 Go trials and 10 stop trials presented randomly. Subjects were required to score over 50% accuracy on Go trials per practice block in order to continue. Each experiment block contained 128 trials per block, 96 of which were Go trials and 32 of which were Stop trials (16 from Ladder 1, and 16 from Ladder 2), each presented randomly. Participants completed 2 blocks for a total of 256 trials.

All trials started with a 500 ms fixation cross in the center of the screen and had a 1000 ms fixed response interval. Subjects were allowed to respond at the start of stimulus presentation until the end of the 1000 ms fixed response interval before the next trial began with fixation. Each trial was separated by a fixed 100 ms delay.

On each trial, the screen was black with white characters. The letters were written in 26 size with Courier New font type.

Task Structure Detail

This is what we had worked on before, but could use updating. We'd like to capture a schema that can handle each of the tasks in the CNP, so please think general when editing -fws

• Task Structure (please given an overview of the task procedures here [i.e., overall design, block, trial, and within-trial event structure and timing])
  • procedure
    • block:
    • trial:
• Stimulus Characteristics
  • sensory modality (e.g., visual, auditory, somatosensory, gustatory, olfactory):
  • functional modality (e.g., linguistic, spatial, numerical, categorical):
  • presentation modality (e.g., human examiner, paper, computer display, headphones, speaker):
• Response Characteristics
  • response required -
    • effector modality (e.g., vocal, manual, pedal):
    • functional modality (e.g., words, drawing, writing, keypress, movement):
  • response options (e.g., yes/no, go/no-go, forced choice, multiple choice [specify n of options], free response)-
  • response collection (e.g., examiner notes, keyboard, keypad, mouse, voice key, button press)-
• Assessment/Control Characteristics
  • timing-
  • control/baseline-
  • other?

Task Schematic

Task Parameters Table?

Stimuli

Dependent Variables

Cleaning Rules

Code/Algorithms

Data Distributions

References