Saturday, September 4, 2010

The New Executive Brain 3: Tests of Frontal Function

In real world, we choose a path and learn information we need; in memory tests we are given a list of words and told to memorize.  Difference is role of frontal lobes.  Hence, animal experiments show a more pronounced role for frontal memory than human experiments.  CVLT allows semantic clustering, which (postulated) may be due to frontal activity.  Most working memory tests lack a selection component.   Frontal role is to select information and hold it online and manipulate it as long as is needed.

Courtney et al. (Cereb Cortex 1996) showed faces on a grid then another set of faces with "what" question and "where" question, which respectively, activated ventral and dorsal visual stream representations within the frontal lobes (inferior and superior). 

"How does an organism decide what information is important enough to be kept online" is a question that has been ignored in almost all neuropsychological paradigms.  Goldberg divides decision making into veridical and ambiguous.  Veridical is "I spend 300 of 1000 dollars, how much is left?"  Ambiguous or adaptive is "Shall I wear gray, blue, or white?"  What is the answer?  Gertrude Stein asked "What is the question?"  Disambiguation may requires prioritizing (remember Buridano's donkey from Luria's books).  Individual must be able to disambiguate in multiple ways and switch set ( "White is for spring, but white is not allowed at school so I will wear gray").  Veridical decisions find the truth, adaptive ones find what is best for the organism. 

In Cognitive Bias Test, individual is given a card and asked to pick the one "they like the most."  Then they were asked to find the choice "most similar to the target" then the one "most different from the target."  Frontal lobe is required only for the first condition, the only one requiring disambiguation.  Frontal lesions did not affect the second and third conditions.  In Alz disease, the disambiguating test declined much faster.  Verdejo found same in individuals with substance abuse (altered adaptive but not veridical decision making).

Males made choices on CBT in a context dependent way, that is when the targets changed, so did their choices.  Females made context independent choices, that is their choices were stable regardless of target.  This is first account of gender differences in adaptive decision making.  Males with damaged right frontal lobe performed in an extremely context dependent manner, and males with damaged left frontal lobes performed in an extremely context independent manner. In females, damage to either frontal lobe caused context dependent choices.  The Wisconsin Card Sorting Test failed to differentiate laterality of the lesion. 

The effects of posterior lesions were less robust than those of frontal lesions, but were nonetheless dimorphic.  In males, posterior temporal lesions followed frontal lesions:  left sided lesions made males more context independent, and right sided lesions made males more context dependent.  In females, posterior lesions were opposite frontal lesions: on either side, they made decision making more context independent. 
Goldberg argues that in male laterality is more articulated, but in the female, front-back differences are more articulated. 

In functional imaging experiments, when asked to process verbal information, males coactivated front and back in same hemisphere, whereas females coactivated homologous regions across hemispheres.  Males have greater functional differentiation of the hemispheres, whereas females have greater functional integration of hemispheres-- note they also have bigger callosa.  However, males have larger longitudinal fasciculi within hemispheres (see book for citations). 

An overwhelming number favored similarity over novel among humans on the CBT (unlike other primates who picked the novel object).  However, that is only true among righthanders.  Among lefthanders, many picked the choice that differed from the target, not the one similar to it.  J Cogn Neurosci 1994. EG suspects that the 9:1 balance in favor of right handers represents evolutionary needs for stability versus innovation.  Functional roles of left handers may be reversed on CBT (Ibid.). 

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