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Functional Significance of Long-Term Potentiation for Sequence Learning and Prediction

Abbott — 2008-03-28T10:43:33-00:00

Cereb. Cortex, Vol. 6, No. 3. (1 May 1996), pp. 406-416.

Population coding, where neurons with broad and overlapping firing rate tuning curves collectively encode information about a stimulus, is a common feature of sensory systems. We use decoding methods and measured properties of NMDA-mediated LTP induction to study the impact of long-term potentiation of synapses between the neurons of such a coding array. We find that, due to a temporal asymmetry in the induction of NMDA-mediated LTP, firing patterns in a neuronal array that initially represent the current value of a sensory input will, after training, provide an experienced-based prediction of that input instead. We compute how this prediction arises from and depends on the training experience. We also show how the encoded prediction can be used to generate learned motor sequences, such as the movement of a limb. This involves a novel form of memory recall that is driven by the motor response so that it automatically generates new information at a rate appropriate for the task being performed. 10.1093/cercor/6.3.406

Forward and reverse hippocampal place-cell sequences during ripples

Kamran Diba — 2007-10-08T17:01:09-00:00

Nature Neuroscience, Vol. 10, No. 10. (02 September 2007), pp. 1241-1242.

Dynamics of Population Code for Working Memory in the Prefrontal Cortex

EH Baeg — 2007-10-10T14:34:46-00:00

Neuron, Vol. 40, No. 1. (25 September 2003), pp. 177-188.

Some neurons (delay cells) in the prefrontal cortex elevate their activities throughout the time period during which the animal is required to remember past events and prepare future behavior, suggesting that working memory is mediated by continuous neural activity. It is unknown, however, how working memory is represented within a population of prefrontal cortical neurons. We recorded from neuronal ensembles in the prefrontal cortex as rats learned a new delayed alternation task. Ensemble activities changed in parallel with behavioral learning so that they increasingly allowed correct decoding of previous and future goal choices. In well-trained rats, considerable decoding was possible based on only a few neurons and after removing continuously active delay cells. These results show that neural activity in the prefrontal cortex changes dynamically during new task learning so that working memory is robustly represented and that working memory can be mediated by sequential activation of different neural populations.

An hypothesis regarding the cortical mechanisms of operative memory

AS Batuev — 2007-10-10T13:33:54-00:00

Neuroscience and Behavioral Physiology, Vol. 23, No. 2. (1 March 1993), pp. 130-134.

Investigations of the neuronal activity of the cerebral cortex of monkeys during the performance of a delayed spatial choice made it possible to formulate an hypothesis regarding the neuronal systems providing for operative memory. One system functions on the principle of relay race-reverberation transmission of information. During the action of a sensory signal a population of spatially selective “sensory” neurons is excited. By the delay period (operative memory) this information is transmitted to a population of “memory” neurons. The delay period is quantized in time segments in the course of which individual populations of cells are involved in relays in the reverberation activity. Each of these populations comprises a “neuronal trap” in which the excitation circulates for 1.5–2 sec. At the end of the delay period switching of the excitation to a different population of cells takes place, which are associated with the preparation of a goal-directed movement (the “neurons of the motor programs”). Another system of neurons assures the reliability of the transitional phases of the above-named processes, specifically: 1) of the switchings of information from the “sensory” neurons to the “memory” neurons and subsequently to the neurons of the “motor programs”; 2) the reflection of the entire period of operative memory without relay race-reverberation; and 3) the preservation of the signal information in the activity of a unified neuronal population right up to the moment of the performance of the goal-directed movement.

Dopamine Enhances Spatiotemporal Spread of Activity in Rat Prefrontal Cortex

Susanta Bandyopadhyay — 2007-06-21T14:46:23-00:00

J Neurophysiol, Vol. 93, No. 2. (1 February 2005), pp. 864-872.

Dopaminergic modulation of prefrontal cortex (PFC) is important for neuronal integration in this brain region known to be involved in cognition and working memory. Because of the complexity and heterogeneity of the effect of dopamine on synaptic transmission across layers of the neocortex, dopamine's net effect on local circuits in PFC is difficult to predict. We have combined whole cell patch-clamp recording and voltage-sensitive dye imaging to examine the effect of dopamine on the excitability of local excitatory circuits in rat PFC in vitro. Whole cell voltage-clamp recording from visually identified layer II/III pyramidal neurons in rat brain slices revealed that, in the presence of bicuculline (10 microM), bath-applied dopamine (30-60 microM) increased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by weak intracortical stimulus. The effect was mimicked by the selective D1 receptor agonist SKF 81297 (1 microM). Increasing stimulation resulted in epileptiform discharges. SKF 81297 (1 microM) significantly lowered the threshold stimulus required for generating epileptiform discharges to 83% of control. In the imaging experiments, bath application of dopamine or SKF 81297 enhanced the spatiotemporal spread of activity in response to weak stimulation and previously subthreshold stimulation resulted in epileptiform activity that spread across the whole cortex. These effects could be blocked by the selective D1 receptor antagonist SCH 23390 (10 microM) but not by the D2 receptor antagonist eticlopride (5 microM). These results indicate that dopamine, by a D1 receptor-mediated mechanism, enhances spatiotemporal spread of synaptic activity and lowers the threshold for epileptiform activity in local excitatory circuits within PFC. 10.1152/jn.00922.2004

Dopaminergic Modulation of Local Network Activity in Rat Prefrontal Cortex

Susanta Bandyopadhyay — 2007-06-21T14:46:13-00:00

J Neurophysiol, Vol. 97, No. 6. (1 June 2007), pp. 4120-4128.

Dopamine modulates prefrontal cortex excitability in complex ways. Dopamine's net effect on local neuronal networks is therefore difficult to predict based on studies on pharmacologically isolated excitatory or inhibitory connections. In the present work, we have studied the effects of dopamine on evoked activity in acute rat brain slices when both excitation and inhibition are intact. Whole cell recordings from layer II/III pyramidal cells under conditions of normal synaptic transmission showed that bath-applied dopamine (30 microM) increased the outward inhibitory component of composite postsynaptic currents, whereas inward excitatory currents were not significantly affected. Optical imaging with the voltage-sensitive dye N-(3-(triethylammonium)propyl)-4-(4-(p-diethylaminophenyl)buta-dienyl)pyridinium dibromide revealed that bath application of dopamine significantly decreased the amplitude, duration, and lateral spread of activity in local cortical networks. This effect of dopamine was observed both with single and train (5 at 20 Hz) stimuli. The effect was mimicked by the D1-like receptor agonistR(+)-6-chloro-7,8-dihydroxy-1-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (1 microM) and was blocked by R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (10 microM), a selective antagonist for D1-like receptors. The D2-like receptor agonist quinpirole (10 microM) had no significant effect on evoked dye signals. Our results suggest that dopamine's effect on inhibition dominates over that on excitation under conditions of normal synaptic transmission. Such neuromodulation by dopamine may be important for maintenance of stability in local neuronal networks in the prefrontal cortex. 10.1152/jn.00898.2006

Encoding of movement fragments in the motor cortex.

NG Hatsopoulos — 2007-05-16T00:33:20-00:00

J Neurosci, Vol. 27, No. 19. (9 May 2007), pp. 5105-5114.

Previous studies have suggested that complex movements can be elicited by electrical stimulation of the motor cortex. Most recording studies in the motor cortex, however, have investigated the encoding of time-independent features of movement such as direction, velocity, position, or force. Here, we show that single motor cortical neurons encode temporally evolving movement trajectories and not simply instantaneous movement parameters. We explicitly characterize the preferred trajectories of individual neurons using a simple exponential encoding model and demonstrate that temporally extended trajectories not only capture the tuning of motor cortical neurons more accurately, but can be used to decode the instantaneous movement direction with less error. These findings suggest that single motor cortical neurons encode whole movement fragments, which are temporally extensive and can be quite complex.