Limitations of short range Mexican hat connection for driving target selection in a 2D neural field: activity suppression and deviation from input stimuli.

Dynamic Neural Field models (DNF) often use a kernel of connection with short range excitation and long range inhibition. This organization has been suggested as a model for brain structures or for artificial systems involved in winner-take-all processes such as saliency localization, perceptual decision or target/action selection. A good example of such a DNF is the superior colliculus (SC), a key structure for eye movements. Recent results suggest that the superficial layers of the SC (SCs) exhibit relatively short range inhibition with a longer time constant than excitation. The aim of the present study was to further examine the properties of a DNF with such an inhibition pattern in the context of target selection. First we tested the effects of stimulus size and shape on when and where self-maintained clusters of firing neurons appeared, using three variants of the model. In each model variant, small stimuli led to rapid formation of a spiking cluster, a range of medium sizes led to the suppression of any activity on the network and hence to no target selection, while larger sizes led to delayed selection of multiple loci. Second, we tested the model with two stimuli separated by a varying distance. Again single, none, or multiple spiking clusters could occur, depending on distance and relative stimulus strength. For short distances, activity attracted toward the strongest stimulus, reminiscent of well-known behavioral data for saccadic eye movements, while for larger distances repulsion away from the second stimulus occurred. All these properties predicted by the model suggest that the SCs, or any other neural structure thought to implement a short range MH, is an imperfect winner-take-all system. Although, those properties call for systematic testing, the discussion gathers neurophysiological and behavioral data suggesting that such properties are indeed present in target selection for saccadic eye movements.

Activation of postsynaptic D2 dopamine receptors in the rat dorsolateral striatum prevents the amnestic effect of systemically administered neuroleptics.

Systemically administered antipsychotics bind to dopamine (DA) D2 receptors expressed in both pre- and postsynaptic neurons of different striatal sites and present an amnestic effect on learning and memory of conditioned avoidance responses (CAR). The aim of this study was to test whether blockade of the pre- or post-synaptic D2 receptors of the dorsolateral striatum of rats is the mechanism by which systemically administered antipsychotics present this amnestic effect. CAR learning and memory was evaluated in rats that received i.p. administrations of pre- or postsynaptic doses of the antipsychotic sulpiride combined with intra-DLS infusion of the D2 agonist quinpirole. Intra-DLS quinpirole itself was not amnestic and this effect was prevented by co-administration of presynaptic dose of sulpiride. However, sulpiride was amnestic when administered systemically in a post- but not presynaptic dose. This amnestic effect of sulpiride was prevented by the co-administration of quinpirole into the DLS. These results show that a blockade of postsynaptic D2 receptors in the DLS is necessary and sufficient to produce the amnestic effect of neuroleptics on CARs.