Saturday, April 21, 2007

Advances in understanding ventromedial prefrontal function

Fallows LK. The accountant joins the executive. Neurology (views and reviews) 2007; 68:991-995.
VM frontal lobes encode value that impacts decision making. It assesses factors such as risk, delay and ambiguity. The Iowa Card test shows impairment of decision making in patients with lesions in this area. VMF is impaired to behavior adjustment when payoffs to choices change. Whereas nucleus accumbens is critical for the magnitude of reward, the VMF is for whether the event will occur. VMF is required for experience of regret. VMF lesioned patients show disappointment over a sad outcome, but not autonomic changes, SCR's, and "maximizing" strategies investing more time and effort in establishing an outcome. Thus impulsive choices may not be risk-seeking, but lack of risk aversion.

Tuesday, February 13, 2007

Basal ganglia and memory

Packard MG and Knowlton BJ. Learning and memory functions of the basal ganglia. Annu Rev. Neurosci. 2002; 25:563-593. This article reviews the role of the basal ganglia, especially the dorsal striatum, in learning and memory. The hypothesis is that the basal ganglia mediates a form of learning in which stimulus-response (S-R) associations or habits are acquired. These can be dissociated from the cognitive or declarative medial temporal lobe memory systems invluding the hippocampus. During learning, the two systems maybe activated simultaneously and competitively inhibit each other.
Tibits picked up from the article:
1. Wise et al. (1996 Crit Rev Neurobiol) hypothesized that the frontostriatocortical loops train the cortex to produce learned responses in the presence of patterned sensory information.

2. Neurochemically dopaminergic input from the substantia nigra and ventral tegmentum innervate the dorsal and ventral striatum, whereas glutamatergic input from cortex, thalamus, amygdala and hippocampus are excitatory. Gaba-ergic output from medium spiny neurons in neostriatum exist, and interneurons, many are cholinergic.

3. The "patch" compartment of the striatum (striosomes) have low Ach, and high opiates and substance P. The matrix compartment has cholinergic and somatostatin containing neurons. Dopaminergic input from the ventral tegmentum innervates primarily patch, whereas dopaminergic input from SN innervates primarily matrix. Amygdala and hippocampus innervate primarily patch, whereas cortex and thalamus innervate primarily matrix. White et al. hypothesize that the matrix primarily mediates mnemonic functions (1989 Life Sci).

4. In rats, a win-shift and win-stay model double dissociates. Dorsal striatal lesions impair win stay and hippocampal-fimbriae-fornix lesions impair win shift model (win stay measures working memory; rats have to visit each bay once to get reward; win shift requires them to learn to visit a bay with a stimulus eg a light). With reinforcer devaluation (pair food reward with nauseating lithium chloride injections) rats continue to approach illuminated bays. This suggests the caudate win stay task involves acqusition of a S-R (light approach) not a stimulus-stimulus (light food) reward behavior.

Monday, February 12, 2007

Cerebellar cognitive affective syndrome

Schmahmann JD, Sherman JC. The cerebellar cognitive affective syndrome. Brain 1998; 121:561-579. The features include deficits in executive, visual-spatial, and linguistic performance with disordered regulation of affect. Sertraline helps.

Sunday, February 11, 2007

Right hemisphere contributions to language

Bookheimer same article.

Some functions the right hemisphere contributes to language include metaphor, connotative meaning , prosody, and non literal interpretation of meaning. Kang et al. (1999) showed right anterior inferior pars triangularis is important in interpreting semantic context of speech. Wernicke's homolog area in right hemisphere also is important. Caplan and Dapretto (2001) found the right hemisphere important in topic maintenance. Meyer et al. found right frontal lobe (area 44/45) and STG were important in repairing anomalies in a sentence. Other functions: prosody, tone discrimination, emotional face recognition,

Content specific organization of semantics

see same Bookheimer review article quoted above.

The large literature suggests that frontal lobe semantic regions are modality and content independent, but that in the temporal lobe, semantic content is highly organized and spatially segregated. While there are some inconsistencies regarding the location of the categories of the stimuli, regions of focal activity reflect different dimensions of object naming and knowledge including visual features, ans associations with object uses and semantically related objects.

Objects that are manipulable activate brain regions associated with reaching and grasping; objects that move activate visual motion detector areas; and objects that must be discriinated from exemplars of similar objects (like faces) activate visual form recognition areas. Martin and Chao (2001) argue that anterior temporal lobe integrates areas serving increasing uniqueness. Bookheimer argues that the integration of these areas might be served by the anterior inferior frontal gyrus on the left (IFG again).

In general, animals activate lateral fusiform/occipital gyri; tools the left middle temporal gyrus (MTG) or area 21 and medial fusiform gyrus. The areas tend to be continuous rather than discrete on f MRI tests. Caramazza (2000) reviews why.

Broca's aphasia an update using PET/fMRI

Bookheimer S. Functional MRI of language: new approaches to understanding the cortical organization of semantic processing. Annu Rev Neurosci 2002; 25:151-188. [review article]

The deficits traditionally included in Broca's aphasia involve deficits of articulation, sequential production of speech, sentence production, syntax, naming, and comprehension of complex syntactic structures. Alexander et al. (1990) found that damage to the white matter underlying Broca's area was necessary to produce the whole syndrome. Functional MRI has shown that "large module" theories are incorrect, but that the language model is organized into a large number of relatively small but tightly clustered and interconnected modules with unique contributions to language processing.

fMRI has identified functional heterogeneity in the inferior frontal lobule (IFG) corresponding to specific aspects of language. Three separate regions correspond to syntax (both productive and comprehension), semantics and phonology. It is involved in executive aspects of semantic processing, involving semantic working memory, semantic search, or drawing comparisons among semantic concepts in working memory. Petersen et al (1988) identified the anterior inferior portion of the IFG in semantic processing (area 47 of Brodmann), meaning processing semantic relationships between words and phrases, or retrieving semantic information. Posterior superior parts of IFG are important for attention and selection. Area 44 (as well as the temporal lobe) was activated during tasks involving Jabberwocky sentences, thought to be important for syntactic attentional tasks. Cortical stimulation of area 44 disrupts phoneme monitoring, accessing and sequencing (Ojemann &7 Mateer, 1979). f MRI suggested phoneme (v. tone) comprehension occurs in area 44/45 while word meaning tasks activate temporal lobes (demonet, 1992). Increased blood flow in posterior IFG occurs when subjects make judgments about embedded (ambiguous) phonemes. Zatorre (1992) using PET showed posterior IFG engaged when subjects made a phonological v. pitch discrimination judgment.

Non language tasks done by BA include tone discrimination (Muller, 2001) especially anterior inferior IFG (areas 45/47). Area 44 is important for imagery of motion