CNP SCAP
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Contents
Basic Task Description
The goal of this task is to probe behavior across different levels (loads) of a spatial working memory capacity task. There is evidence that spatial working memory is impaired in schizophrenia, and the deficit may be a core feature of the disease. From our previous schizophrenia twin studies we have evidence that the spatial working memory deficit (as assessed by a version of this task) may be heritable (Glahn et al, 2003). There is further evidence that performance on this task may differentiate between bipolar patients with and without a history of psychosis (Glahn, 2006).
The Cannon lab has a great deal of experience with this task, and it has been administered in a very wide variety of populations at a number of sites. We have administered it to adolescents with schizophrenia, prodromal subjects, aging Finnish twins with and without schizophrenia, chronic schizophrenia patients, bipolar patients, patients with neurofibromatosis-1, and healthy controls. The version that we have administered most frequently is the one used in the LA2K, a block design task with 4 memory loads (1,3,5 and 7 locations).
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_VCAP.
This task differs in structure from the LA5C SCAP task design used in the scanner.
First, subjects undergo a training period
Sample Text
During the SDRT, subjects were shown a target array of 1, 3, or 5 yellow circles positioned pseudorandomly around a central fixation. After a fixed delay, subjects were shown a single green circle and were required to indicate whether that circle was in the same position as one of the target circles. A relatively long stimulus presentation was used to allow subjects to fully encode the target array, minimizing a potential encoding on the basis of set size interaction. Likewise, decision or selection requirements were kept constant across set sizes to reduce possible effects of set size on response processes. Trial events included a 2-sec target-array presentation, a 3-sec delay period, and a 3-sec fixed response interval. A central fixation was visible throughout each of the 36 trials (12 per memory set size).
Glahn, 2003
Task Structure Detail
Loads: 1, 3, 5 or 7 dots
There are 12 trials per load, the trials are blocked in sets of 2, and analyzed in those blocks.
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
Each trial lasts 9 seconds. TR=3s
Task Schematic
Task Parameters Table
Stimuli
Dependent Variables
The primary variables of interest are mean correct at each load, mean reaction time at each load, and overall working memory capacity.
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 (SCAP_CAPACITY1, SCAP_CAPACITY3, SCAP_CAPACITY5, SCAP_CAPACITY7), 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 (SCAP_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).
1*(scap_corr_hit1 + (1 - scap_corr_fa1) - 1)
3*(scap_corr_hit3 + (1 - scap_corr_fa3) - 1)
5*(scap_corr_hit5 + (1 - scap_corr_fa5) - 1)
7*(scap_corr_hit7 + (1 - scap_corr_fa7) - 1)
The code for generating the summary statistics viewed on this page was written in Stata, as follows:
//ALL SUBJECTS
//get breakdown of numbers in each group:
bysort status: summarize ptid
bysort la5ccontrol: summarize ptid
bysort gender: summarize ptid
//see how many complete cases there are.
count if scap_max_capac !=.
//exclusions
generate float scap_percent_corr= (scap1_correct_sum+ scap3_correct_sum+ scap5_correct_sum+ scap7_correct_sum)/48
count if scap_percent_corr <=.5
count if scap_trial_count < 48
count if scap_max_capac < 1
count if scap_average_corr > 6000 & scap_average_corr !=.
//create variable summing exclusion criteria
generate float useable_scap= 1 if scap_trial_count ==48 & scap_percent_corr>.5 & scap_max_capac>0 & scap_average_corr < 6000
count if useable_scap==1
//SUBJECTS WITH USEABLE DATA
bysort status: summarize ptid if useable_scap==1
bysort la5ccontrol: summarize ptid if useable_scap==1
bysort gender: summarize ptid if useable_scap==1
summarize age scap_percent_corr scap_average_corr scap_max_capac if useable_scap==1
bysort status: summarize scap_max_capac if useable_scap==1
bysort la5ccontrol: summarize scap_max_capac if useable_scap==1
bysort gender: summarize scap_max_capac if useable_scap==1
//COMPLETERS WITH USEABLE DATA
generate float comp_useable_scap= 1 if status==2 & scap_trial_count ==48 & scap_percent_corr>.5 & scap_max_capac>0 & scap_average_corr < 6000
count if comp_useable_scap==1
bysort status: summarize ptid if comp_useable_scap==1
bysort la5ccontrol: summarize ptid if comp_useable_scap==1
bysort gender: summarize ptid if comp_useable_scap==1
summarize age scap_percent_corr scap_average_corr scap_max_capac if comp_useable_scap==1
bysort status: summarize scap_max_capac if comp_useable_scap==1
bysort la5ccontrol: summarize scap_max_capac if comp_useable_scap==1
bysort gender: summarize scap_max_capac if comp_useable_scap==1
//OUTLIERS
sktest scap_max_capac scap_average_corr scap_percent_corr
histogram scap_average_corr if comp_useable_scap==1, title(Average Reaction Time) xtitle(Average Reaction Time) scheme(s2mono)
histogram scap_percent_corr if comp_useable_scap==1, title(Average Percent Correct) xtitle(Percent Correct (0-1 x 100)) scheme(s2mono)
histogram scap_max_capac if comp_useable_scap==1, title(Maximum Capacity) xtitle(Percent Correct (0-1 x 100)) scheme(s2mono)
Data Distributions
References
Cannon et al, 2005, Archives of General Psychiatry
Glahn et al, 2003, Biological Psychiatry
Glahn et al, 2002, Neuroimage
Glahn et al, 2006, Bipolar Disorders