CNP VCAP

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Revision as of 16:45, 27 June 2011 by Katie (Talk | contribs) (Basic Task Description)

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Basic Task Description

This task was designed by Ty Cannon and David Glahn as a verbal analogue to the SCAP (spatial capacity task). It has been run in a number of local samples, including Aftercare (first episode), ABBRC (adolescent psychosis), CAPPS (prodromal), and NF1 (neurofibromatosis).

This task manipulates verbal working memory load using arrays of letters. The loads (3, 5, 7, 9) are selected to be behaviorally analogous to the CNP_SCAP loads.

Task Procedure

This task manipulates working memory load. The loads were selected to be of parallel difficulty, in terms of performance, to the loads in the CNP_SCAP.

Basic Task Info Loads: 3, 5, 7 or 9 lowercase letters

Trial layout:
1. Fixation for 1sec
2. Target array for 2 sec
3. Delay period for 3 sec
4. Probe array (single dot) for 3 seconds

First, subjects undergo an instruction and training period (4 trials), then the task begins and the subject performs 48 trials (12 trials per load, randomly selected).

Sample Text
We employed a modified Sternberg item recognition task (Sternberg, 1966 S. Sternberg, High-speed scanning in human memory, Science 153 (1966), pp. 652–654. View Record in Scopus | Cited By in Scopus (758)Sternberg, 1966). A target set of 3, 5, 7, or 9 yellow uppercase consonants was displayed for 2 s, followed by a 3-s fixation. A green uppercase probe then appeared for 3 s, followed by 1 s of fixation before the next trial. Subjects indicated whether the probe matched the target set. The task included 12 trials per load, which were clustered by load into two-trial long blocks; presentation order of the blocks was counterbalanced, but remained the same for all subjects.
Karlsgodt, 2007

Task Structure Detail

There are 12 trials per load, the trials are blocked in sets of 2 and analyzed in those blocks.

Trial layout:
1. fixation for 1 second
2. target array (multiple uppercase letters) for 2 seconds
3. delay period (fixation) for 3 seconds
4. probe array (single lowercase letter) for 3 seconds.

Each trial lasts 9 seconds, the TR=3seconds, there are 180 TRs making the task 9 minutes long.

Task Schematic

Vcap bigschematic.jpg

Task Parameters Table

Stimuli

Dependent Variables

Vcap variables.jpg
Vcap calc variables.jpg

Cleaning Rules

1. If any of the calculated variables are missing, that subject should be listed for exclusion.
2. If Trial Count is not =48, then that subject should be listed for exclusion.
3. If overall performance (average percent correct) is below 50% (chance, in this task), that subject should be listed for exclusion.
4. It is possible for a subject to have one or more capacities be a negative number (VCAP_CAPACITY3, VCAP_CAPACITY5, VCAP_CAPACITY7, VCAP_CAPACITY9), which usually results from too few true positives in a condition. However, no capacity should be greater than the associated load (i.e., no one can have >capacity=5 on load 5). And, if the average capacity is negative (VCAP_MAX_CAPAC), that subject should be listed for exclusion. However, it is highly unlikely, if not impossible, to achieve this without being already excluded in rule #2.
5. If average RT is greater than 6000ms that subject should be listed for exclusion.

It is worth considering dropping people who have a capacity lower than the lowest load (for instance on SCAP, a capacity of less than 1) but that is up to user discretion.

If you want to know if your group looks approximately like it should, in every sample run so far, accuracy decreases by load, and reaction time increases. This should be the first group level analysis, to quickly verify everything is correct. It would be highly unexpected to have a deviation from this pattern, even though the position along the y axis (overall accuracy or RT) may vary, especially in patient groups.

Code/Algorithms

Capacity is calculated according to the formula by Cowan: k=n*(H+CR−1), where k=capacity, n=load, H = hits (out of those responded to), CR= correct rejections (out of those responded to).

3*(vcap_corr_hit3 + (1 - vcap_corr_fa3) - 1)
5*(vcap_corr_hit5 + (1 - vcap_corr_fa5) - 1)
7*(vcap_corr_hit7 + (1 - vcap_corr_fa7) - 1)
9*(vcap_corr_hit9 + (1 - vcap_corr_fa9) - 1)

Data Distributions

Vcap all data tables.jpg

Vcap all useable tables.jpg

Vcap comp useable tables.jpg

Vcap comp useable outlier tables.jpg

Vcap percorr.jpg

Vcap rt.jpg

Vcap capac.jpg

References

Karlsgodt, 2009, Schizophrenia Research
Sanz, 2009, Psychiatry Research
Karlsgodt, 2008, Biological Psychiatry