Abstract not found

... This article models how these interactions help visual cortex to realize: (1) the binding process whereby cortex groups distributed data into coherent object representations; (2) the attentional process whereby cortex selectively processes important events; and (3) the developmental and learning processes whereby cortex shapes its circuits to match environmental constraints. New computational ideas about feedback systems suggest how neocortex develops and learns in a stable way, and why top-down attention requires converging bottom-up inputs to fully activate cortical cells, whereas perceptual groupings do not.

Abstract not found

Some forms of synaptic plasticity depend on the temporal coincidence of presynaptic activity and postsynaptic response. This requirement is consistent with the Hebbian, or correlational, type of learning rule used in many neural network models. Recent evidence suggests that synaptic plasticity may depend in part on the production of a membrane permeant-diffusible signal so that spatial volume may also be involved in correlational learning rules. This latter form of synaptic change has been called volume learning. In both Hebbian and volume learning rules, interaction among synaptic inputs depends on the degree of coincidence of the inputs and is otherwise insensitive to their exact temporal order. Conditioning experiments and psychophysical studies have shown, however, that most animals are highly sensitive to the temporal order of the sensory inputs. Although these experiments assay the behavior of the entire animal or perceptual system, they raise the possibility that nervous systems may be sensitive to temporally ordered events at many spatial and temporal scales. We suggest here the existence of a new class of learning rule, called apredictiue Hebbian learning rule, that is sensitive to the temporal ordering of synaptic inputs. We show how this predictive learning rule could act at single synaptic connections and through diffuse neuromodulatory systems.

The role of N-methyl-D-aspartate (NMDA) receptors in cat visual cortex was studied as a function of both layer and age by iontophoresis of the NMDA antagonist (D)-2-amino-5phosphonovaleric acid (APV). Effects on both visual responses and spontaneous activity were observed. In superficial layers (II and Ill), D-APV reduced visual responses substantially at all ages. lontophoresis of D-APV with 10 nA of ejecting current for 2-3 min was sufficient to reduce the response to approximately one third of control levels. The magnitude of the reduction did not vary with age. In granular and deep layers (IV, V, and VI), D-APV affected the visual response in young animals but only spontaneous activity in older animals. On average, visual responses were reduced to about half at 20-23 days of age and to about 75 % at 4 weeks of age but in most cases were not significantly affected

I. Temporal response characteristics of neurons were sampled in fine spatial grain throughout the hand representations in cortical areas 3a and 3b in adult owl monkeys. These monkeys had been trained to detect small differences in tactile stimulus frequencies in the range of 20-30 Hz. Stimuli were presented to an invariant, restricted spot on a single digit. 2. The absolute numbers of cortical locations and the cortical area over which neurons showed entrained frequency-following responses to behaviorally important stimuli were significantly greater when stimulation was applied to the trained skin, as compared with stimulation on an adjacent control digit, or at corresponding skin sites in passively stimulated control animals. 3. Representational maps defined with sinusoidal stimuli were not identical to maps defined with just-visible tapping stimuli. Receptive-field / frequency-following response site mismatches

Neuronal activity is important for both the initial formation and the subsequent refinement of anatomical and physiological features of the mammalian visual system. Here we examine recent evidence concerning the role that spontaneous activity plays in axonal segregation, both of retinogeniculate afferents into eye-specific layers and of geniculocortical afferents into ocular dominance bands. We also assess the role of activity in the generation and plasticity of orientation selectivity in the primary visual cortex. Finally, we review recent challenges to textbook views on how inputs representing the two eyes interact during the critical period of visual cortical plasticity.

Supervised learning procedures for neural networks have recently met with considerable success in learning difficult mappings. So far, however, they have been limited by their poor scaling behaviour, particularly for networks with many hidden layers. A promising alternative is to develop unsupervised learning algorithms by defining objective functions that characterize the quality of an internal representation without requiring knowledge of the desired outputs of the system. Our major goal is to build self-organizing network modules which capture important regularities in the environment in a simple form. A layered hierarchy of such modules should be able to learn in a time roughly linear in the number of layers. We propose that a good objective for perceptual learning is to extract higher-order features that exhibit simple coherence across time or space. This can be done by transforming the input representation into an underlying representation in which the mutual information between ...

During a critical period of postnatal development of the mammalian visual cortex, synaptic connections are susceptible to use-dependent modifications. Synaptic connections strengthen if pre- and postsynaptic elements are active simultaneously and postsynaptic depolarization is sufficient to allow for the activation of A'-methyl-D-aspartate (NMDA)-receptor-gated conductances. By contrast, synaptic gain decreases if postsynaptic activation exceeds a critical threshold and presynaptic afferents are not capable of activating NMDAreceptor-dependent conductances. These processes lead to selective stabilization of connections between neuronal elements which often exhibit correlated activity and thus modify connectivity according to functional criteria. It is suggested that such experience-dependent selection of circuits serves different purposes at different levels of visual processing. At the input stage to the striate cortex it contributes to optimize the match between the representations of the two eyes. At a later stage of processing it participates in the development of selective connections between cortical columns and thereby serves to establish neuronal

The development of sensory maps is thought to require an activity-dependent structural rearrangement of afferent terminal arbors within the CNS which recreates the topographic relations of sensory somata present in the periphery. In the frog retinotectal projection, activation of the NMDA receptor plays a role in this structural plasticity. Exposure of the optic tectum of tadpoles to NMDA receptor antagonists results in a rearrangement of retinal ganglion cell arbors so that their organization into a topographic projection and eye-specific stripes is disrupted (Cline et al., 1987; Cline and Constantine-Paton, 1989). Exposure of the optic tectum to the receptor agonist, NMDA, increases the eye-specific segregation of these arbors (Cline et al., 1987). We examined the projection of the supernumerary retina and the morphology of individual retinal afferent arbors of