A specific form of cognitive rigidity following excitotoxic lesions of the basal forebrain in marmosets

Evidence for executive dysfunction in autism

A group of young people with autism (ranging in ability from high functioning to moderately learning disabled), and ability-matched control groups of (i) non-autistic individuals with moderate learning disabilities, and (ii) normally developing children, were presented with two tests of executive function: the Intra-dimensional/Extra-dimensional set-shifting task and the Tower of London planning task. These tests were graded in difficulty and included internal control measures. On each task, the autistic group was differentially impaired with respect to both control groups. Moreover, this impairment was specific to the stages of each task which placed greatest demands upon executive control. This evidence for executive dysfunction in autism is discussed in the context of Norman and Shallice's (Centre for Human Information Processing Technical Report 99, 1980) "Supervisory Attentional System" model of frontal function.

Nicotinic and muscarinic modulations of excitatory synaptic transmission in the rat prefrontal cortex in vitro

The importance of the cholinergic innervation of the neocortex in cognitive functions has been shown in a number of clinical and animal studies. Until recently, attempts to study the mode of action of acetylcholine in the neocortex have concentrated on muscarinic effects, whereas cholinergic actions mediated by nicotinic receptors have been difficult to demonstrate. The present work was undertaken to study the mechanism of action of nicotinic agents on cortical neurons and compare it to muscarinic effects by means of intracellular recordings in a slice preparation. The study was performed in the prelimbic area of the rat prefrontal cortex, a cortical region particularly involved in cognitive processes. Recordings were made from pyramidal cells located in layers II/III and synaptic potentials were evoked by stimulation of superficial cortical layers. Iontophoretic applications of nicotinic agonists (nicotine, dimethylphenylpiperazinium, cytisine) increased the amplitude of the monosynaptic excitatory postsynaptic potential mediated by non-N-methyl-D-aspartate glutamate receptors in 14% (22/159) of cells. This effect was abolished by the selective nicotinic blocker, neuronal bungarotoxin (IC50 = 0.6-0.7 microM) and by dihydro-beta-erythroidine (IC50 = 20-30 microM), whereas hexamethonium, mecamylamine, curare and alpha-bungarotoxin were ineffective. The nicotinic agonists did not change resting membrane potential, input resistance or current-voltage relationship. They also did not affect the depolarizations produced by glutamate applied by iontophoresis in the somatic or dendritic area. In contrast, the muscarinic agonists (muscarine, acetyl-beta-methylcholine) decreased the amplitude of the excitatory postsynaptic potential in 100% of the neurons tested. Atropine was more effective (IC50 = 0.08 microM) than pirenzepine (IC50 = 2 microM) to antagonize the muscarinic action. These effects were observed in the absence of any direct postsynaptic change in membrane potential or input resistance, provided that the site of the iontophoretic application was more than 100 microM distant from the soma. The muscarinic agonists did not influence the actions of iontophoretically applied glutamate. These results suggest that nicotinic and muscarinic agonists modulate excitatory synaptic transmission mediated at dendritic sites by non-N-methyl-D-aspartate glutamate receptors, possibly through a presynaptic action. Thus ascending cholinergic systems may take part in information processing in the prefrontal cortex through the control of ongoing excitation to pyramidal cells.

Stimulus-bound perseveration after frontal ablations in marmosets

Marmosets with bilateral ablations of either the lateral or ventral surface of the frontal lobes were found to perseverate on object but not spatial serial reversal. They also perseverated on reversal of a visuospatial task where different stimuli required different spatial responses. No differences were found between the two lesion groups. Since the control animals showed mild perseveration on spatial but not object serial reversal it is argued that frontal ablations do not lead to perseveration of a natural tendency but rather that object and visuospatial perseveration are forms of stimulus-bound behaviour which do not occur when the animal is performing a spatial task in which stimulus position is irrelevant. Perseveration was reduced by pretreatment with a dopamine-blocking drug (haloperidol). It is suggested that information from temporal lobe mechanisms, involved in long-term memory, and from frontal lobe mechanisms, which exert shorter acting influences on behaviour, compete within the basal ganglia to determine stimulus choice. The ascending dopaminergic pathway may modulate the balance between these competing factors.