An examination of the participation of neurons in readout from memory

Emergence of dynamic memory traces in cortical microcircuit models through STDP

Numerous experimental data suggest that simultaneously or sequentially activated assemblies of neurons play a key role in the storage and computational use of long-term memory in the brain. However, a model that elucidates how these memory traces could emerge through spike-timing-dependent plasticity (STDP) has been missing. We show here that stimulus-specific assemblies of neurons emerge automatically through STDP in a simple cortical microcircuit model. The model that we consider is a randomly connected network of well known microcircuit motifs: pyramidal cells with lateral inhibition. We show that the emergent assembly codes for repeatedly occurring spatiotemporal input patterns tend to fire in some loose, sequential manner that is reminiscent of experimentally observed stereotypical trajectories of network states. We also show that the emergent assembly codes add an important computational capability to standard models for online computations in cortical microcircuits: the capability to integrate information from long-term memory with information from novel spike inputs.

From synchronous neuronal discharges to subjective awareness?

For practical clinical purposes, as well as because of their deep philosophical implications, it becomes increasingly important to be aware of contemporary studies of the brain mechanisms that generate subjective experiences. Current research has progressed to the point where plausible theoretical proposals can be made about the neurophysiological and neurochemical processes which mediate perception and sustain subjective awareness. An adequate theory of consciousness must describe how information about the environment is encoded by the exogenous system, how memories are stored in the endogenous system and released appropriately for the present circumstances, how the exogenous and endogenous systems interact to produce perception, and explain how consciousness arises from that interaction. Evidence assembled from a variety of neuroscience areas, together with the invariant reversible electrophysiological changes observed with loss and return of consciousness in anesthesia as well as distinctive quantitative electroencephalographic profiles of various psychiatric disorders, provides an empirical foundation for this theory of consciousness. This evidence suggests the need for a paradigm shift to explain how the brain accomplishes the transformation from synchronous and distributed neuronal discharges to seamless global subjective awareness. This chapter undertakes to provide a detailed description and explanation of these complex processes by experimental evidence marshaled from a wide variety of sources.

Space-time correlations of neuronal firing related to memory storage capacity

Most viable theories of memory require some form of synaptic modification dependent on the correlation of pre- and postsynaptic neuronal firing (which we will denote as the Hebb hypothesis). We show here that a possible consequence of this hypothesis is that the storage capacity of a network of highly interconnected neurons is related to the number of synapses and that this implies that the network can be excited into many different time sequence of firing patterns of assemblies of neurons. The important role played by the assembly (as defined by E. R. John) is discussed in detail. A modified Hebb hypothesis is proposed. The crucial experiments to test the model involve the use of two (or more) extracellular microelectrodes to record, simultaneously, the firing activity of several neurons and thus determine the spatial and temporal cross correlations after presenting a mature animal with a variety of stimuli.

Evoked potential-unit relationships in behaving cats

Adult cats were implanted with a movable microelectrode, and trained to respond differentially to two different frequencies of light flicker. Unit responses and evoked potentials were recorded along trajectories in the visual cortex and lateral geniculate nucleus. Although strong correlations are shown to exist between certain components of the evoked potential and peaks in the poststimulus histogram, it is demonstrated that it is impossible to specify a causal, predictive relationship.