Effects of Verbal Working Memory Load on Corticocorical Connectivity Modeled by Path Analysis of Functional Magnetic Resonance Imaging Data

(Honey et al., Neuroimage, 2002)

12.04.19.(Thu)Summarized by Joon Shik Kim

Computational Models of Intelligence

Blood Oxygen Level Dependent (BOLD) Signal

• Amount of oxygen molecules bound to hemoglobin.

• If a specific task is performed, re-lated brain regions are activated and the need for more oxygen leads to BOLD signal increase.

Task

• In a control task, subjects viewed a series of 14 letters with an interstim-ulus interval of 2 s.

• Subjects pressed the mouse button if the currently presented letter was the same as that presented n trials previously. If n=1, we call the task as 1 back task. If n=2, 2 back task and if n=3, 3 back task.

Generation of fMRI time se-ries

Brain Activation Maps

Structural Equation Modeling (SEM) (Bullmore et al., 2000)

1( )I K

1( ) ( ) ( )T TI K I K

where q denotes the number of path coefficients to be estimated and C denotes the sample covariance.

Model by Automated Search

Path Coefficient

Significant Path Coefficient

Right Left

Discussion (1/3)

• Manipulation of cognitive demand in a verbal working memory task is as-sociated with concomitant changes in corticocortical connections.

• Under both 1-back and 2-back condi-tions, there was a significant connec-tions from the left inferior frontal gyrus to the left posterior parietal cortex.

Discussion (2/3)

• The projection from the left inferior frontal gyrus to the left posterior parietal cortex may be important in mediating ar-ticulatory rehearsal.

• Increasing n from 1 to 2 increases the load on articulatory rehearsal. This ma-nipulation was associated with the emer-gence of a second significant connection between the inferior frontal gyrus and posterior parietal cortex via SMA.

Discussion (3/3)

• Our model did not provide a good fit to the 3-back data.

• The change in functional integration in-duced by an increase in load from 2- to 3-back may require qualitative recon-struction of the path diagram.

• Enhanced prefrontoprefrontal connectiv-ity under the 2-back condition may re-flect greater demand for executive process.