this paper, we examine in detail the potential role of time as a variable in the combinatorial representation of sensory stimuli by this part of the brain. We focus on processing of odors to which the animal has been exposed, i.e., we do not consider here the responses evoked by the first one to three presentations of an unfamiliar odor. We find that the temporal firing patterns of individual neurons, as well as the synchronization of firing across groups of neurons, are stimulus specific. In other words, each odor appears to be represented not simply by an ensemble of synchronized neurons but by a progressive and odor-specific transformation of that ensemble, so that each neuron synchronizes with several others only during one or more precise epochs of the ensemble response. We, thus, propose that oscillations in the olfactory nervous system occur, at least here, in parallel with a slower code that is distributed both in time and across many neurons

. We study some mechanisms responsible for synchronous oscillations and loss of synchrony at physiologically relevant frequencies (10-200 Hz) in a network of heterogeneous inhibitory neurons. We focus on the factors that determine the level of synchrony and frequency of the network response, as well as the effects of mild heterogeneity on network dynamics. With mild heterogeneity, synchrony is never perfect and is relatively fragile. In addition, the effects of inhibition are more complex in mildly heterogeneous networks than in homogeneous ones. In the former, synchrony is broken in two distinct ways, depending on the ratio of the synaptic decay time to the period of repetitive action potentials (øs=T ), where T can be determined either from the network or from a single, self-inhibiting neuron. With øs=T ? 2, corresponding to large applied current, small synaptic strength or large synaptic decay time, the effects of inhibition are largely tonic and heterogeneous neurons spike relativ...

for interneurons. The observed differences in intrinsic the frequency preference between pyramidal cells and interneurons have implications for rhythmogenesis and information transmission between populations of cortical neurons. Introduction Recent analysis of the neuronal spike trains in the lateral geniculate nucleus suggests that the information from the retina to the visual cortex may be transmitted by precise spike times in addition to that carried by the firing rate (1). Cortical neurons are capable of precisely initiating spikes in response to broadband fluctuating stimuli both in vitro (2; 3) and in vivo (4), although the significance of single spike timing in the cortex is debated. This raises the issue of how the cortex could take advantage of this information (5; 6). The rhythmic activity observed in the cortex may reflect internal cortical mechanisms for synchronizing populations of cortical neurons (7; 8). In this study the spike-time reliability of

We analyze the control of frequency for a synchronized inhibitory neuronal network. The analysis is done for a reduced membrane model with a biophysically based synaptic influence. We argue that such a reduced model can quantitatively capture the frequency behavior of a larger class of neuronal models. We show that in different parameter regimes, the network frequency depends in different ways on the intrinsic and synaptic time constants. Only in one portion of the parameter space, called phasic, is the network period proportional to the synaptic decay time. These results are discussed in connection with previous work of the authors, which showed that for mildly heterogeneous networks, the synchrony breaks down, but coherence is preserved much more for systems in the phasic regime than in the other regimes. These results imply that for mildly heterogeneous networks, the existence of a coherent rhythm implies a linear dependence of the network period on synaptic decay time and a much weaker dependence on the drive to the cells. We give experimental evidence for this conclusion.

Gamma frequency field oscillations (40–100 Hz) are nested within theta oscillations in the dentate–hilar and CA1–CA3 regions of the hippocampus during exploratory behaviors. These oscillations reflect synchronized synaptic potentials that entrain the discharge of neuronal populations within the �10–25 msec range. Using multisite recordings in freely behaving rats, we examined gamma oscillations within the superficial layers (I–III) of the entorhinal cortex. These oscillations increased in amplitude and regularity in association with entorhinal theta waves. Gamma waves showed an amplitude minimum and reversed in phase near the perisomatic region of layer II, indicating that they represent synchronized synaptic potentials impinging on layer II–III neurons. Theta and gamma oscillations in the entorhinal cortex were coupled with theta and gamma oscillations in the dentate hilar region. The majority of layer II–III neurons “The ultimate physical substrate of memory formation and consolidation resides in alteration in synaptic efficacy, which then alters the patterns of activity in large aggregate collections of cortical neurons ” (Hebb, 1949). It is the modifications in the patterns of activity in large aggregate collections of neurons that have relevance to the cognitive operations of the mammalian brain. The means by which large collections of neurons, within and across vast regions of the brain, can effectively interact is not well understood. The occurrence and transient sychronization of neurons to local field oscillations has been demonstrated in many neural

To explain how stimuli cause consciousness, we have to explain causality. We can't trace linear causal chains from receptors after the first cortical synapse, so we use circular causality to explain neural pattern formation by self-organizing dynamics. But an aspect of intentional action is causality, which we extrapolate to material objects in the world. Thus causality is a property of mind, not matter.

ABSTRACT: Carbachol, a muscarinic receptor agonist, produced three distinct spontaneous oscillations in the CA3 region of rat hippocampal slices. Carbachol concentrations in the 4–13 �M range produced regular synchronized CA3 discharges at 0.5–2 Hz (carbachol-delta). Higher concentrations (13–60 �M) produced short episodes of 5–10 Hz (carbacholtheta) oscillations separated by nonsynchronous activity. Concentrations of carbachol ranging from 8–25 �M also produced irregular episodes of high-frequency discharges (carbachol-gamma, 35–70 Hz), in isolation or mixed with carbachol-theta and carbachol-delta. At carbachol concentrations sufficient to induce carbachol-theta, low concentrations of APV reversibly transformed carbachol-theta into carbachol-delta. Higher concentrations of D,L-2-amino-5-phosphonopentanoic acid (APV) reversibly and completely blocked carbachol-theta. A systematic study of the effects of carbachol shows that the frequency of spontaneous oscillations depended nonlinearly on the level of muscarinic activation. Field and intracellular recordings from CA1 and CA3 pyramidal cells and interneurons during carbachol-induced rhythms revealed that the hippocampal circuitry preserved in the slice was capable of spontaneous activity over the range of frequencies observed in vivo and suggests that the presence of these rhythms could be under neuromodulatory control. Hippocampus

Abstract. Do cortical neurons operate as integrators or as coincidence detectors? Despite the importance of this question, no definite answer has been given yet, because each of these two views can find its own experimental support. Here we investigated this question using models of morphologically-reconstructed neocortical pyramidal neurons under in vivo like conditions. In agreement with experiments we find that the cell is capable of operating in a continuum between coincidence detection and temporal integration, depending on the characteristics of the synaptic inputs. Moreover, the presence of synaptic background activity at a level comparable to intracellular measurements in vivo can modulate the operating mode of the cell, and act as a switch between temporal integration and coincidence detection. These results suggest that background activity can be viewed as an important determinant of the integrative mode of pyramidal neurons. Thus, background activity not only sharpens cortical responses but it can also be used to tune an entire network between integration and coincidence detection modes. Keywords: cerebral cortex, synaptic background, computational model, operating mode

Abstract. Temporal precision of spiking response in cortical neurons has been a subject of intense debate. Using a canonical model of spike generation, we explore the conditions for precise and reliable spike timing in the presence of Gaussian white noise. In agreement with previous results we find that constant stimuli lead to imprecise timing, while aperiodic stimuli yield precise spike timing. Under constant stimulus the neuron is a noise perturbed oscillator, the spike times follow renewal statistics and are imprecise. Under an aperiodic stimulus sequence, the neuron acts as a threshold element; the firing times are precisely determined by the dynamics of the stimulus. We further study the dependence of spike-time precision on the input stimulus frequency and find a non-linear tuning whose width can be related to the locking modes of the neuron. We conclude that viewing the neuron as a non-linear oscillator is the key for understanding spike-time precision. Keywords: computational model, cortical neurons, Type I membrane, frequency-locking

humans by sensory stimuli and may be involved in working memory. Phenomenologically similar �3 � oscillations can be evoked in hippocampal slices by strong two-site tetanic stimulation. Weaker stimulation leads only to two-site synchronized �. In vitro oscillations have memory-like features: (1) EPSPs increase during �3�; (2) after a strong one-site stimulus, twosite stimulation produces desynchronized �; and (3) a single synchronized �3 � epoch allows a subsequent weak stimulus to induce synchronized �3�. Features 2 and 3 last �50 min and so are unlikely to be caused by presynaptic effects. A previous model replicated the �3 � transition when it was assumed that K � conductance(s) increases and there is an ad hoc increase in pyramidal EPSCs. Here, we have refined the model, so The generation of � (10–30 Hz) oscillations in cortical structures appears to be inextricably linked with the generation of � (30–70