We are capable of drawing a variety of inferences effortlessly, spontaneously, and with remarkable efficiency — as though these inferences are a reflex response of our cognitive apparatus. This remarkable human ability poses a challenge for cognitive science and computational neuroscience: How can a network of slow neuron-like elements represent a large body of systematic knowledge and perform a wide range of inferences with such speed? The connectionist model Shruti attempts to address this challenge by demonstrating how a neurally plausible network can encode a large body of semantic and episodic facts, systematic rules, and knowledge about entities and types, and yet perform a wide range of explanatory and predictive inferences within a few hundred milliseconds. Relational structures (frames, schemas) are represented in Shruti by clusters of cells, and inference in Shruti corresponds to a transient propagation of rhythmic activity over such cell-clusters wherein dynamic bindings are represented by the synchronous firing of appropriate cells. Shruti encodes mappings across relational structures using high-efficacy links that enable the propagation of rhythmic activity, and it encodes items in long-term memory as coincidence and conincidence-error detector circuits that become active in response to the occurrence (or non-occurrence) of appropriate coincidences in the on going flux of rhythmic activity.

Recent theoretical models based on cellular processes in parahippocampal structures show that persistent neuronal spiking in the absence of stimulus input is important for encoding. The goal of this study was to examine in humans how sustained activity in the parahippocampal gyrus may underlie long-term encoding as well as active maintenance of novel information. The relationship between long-term encoding and active maintenance of novel information during brief memory delays was studied using functional magnetic resonance imaging (fMRI) in humans performing a delayed matching-to-sample (DMS) task and a post-scan subsequent recognition memory task of items encountered during DMS task performance. Multiple regression analyses revealed fMRI activity in parahippocampal structures associated with the active maintenance of trial-unique visual information during a brief memory delay. In addition to a role in active maintenance, we found that the subsequent memory for the sample stimuli as measured by the post-scan subsequent recognition memory task correlated with activity in the parahippocampal gyrus during the delay period. The results provide direct evidence that encoding mechanisms are engaged during brief memory delays when novel information is actively maintained. The relationship between active maintenance during the delay period and long-term subsequent memory is consistent with current theoretical models and experimental data that suggest that long-term encoding is enhanced by sustained parahippocampal activity. Key words: memory; parahippocampal; neuroimaging; medial temporal lobe; delayed match to sample; computational modeling

Recent computational modeling and slice physiology studies have suggested that long-term encoding may depend on sustained spiking during brief memory delays in parahippocampal neurons, and that this persistent spiking activity is modulated by effects of acetylcholine at muscarinic receptors. Our recent functional magnetic resonance imaging (fMRI) study has shown that sustained parahippocampal delay period activity during delayed match-to-sample performance in healthy young individuals predicted subsequent memory of visual stimuli on a recognition memory assessment 20 min later (Schon et al., 2004). The current study combined this fMRI paradigm with a pharmacological manipulation to test whether this long-term encoding-related delay activity is reduced in subjects who receive the muscarinic cholinergic antagonist scopolamine before fMRI scanning. Subsequent memory was predicted by sustained activity during brief memory delays bilaterally in the perirhinal/entorhinal cortex, in the right posterior parahippocampal and mid-fusiform gyri, and in the hippocampal body in healthy young individuals without a scopolamine challenge. This activity was reduced in subjects receiving scopolamine. The results are consistent with computational modeling data and behavioral pharmacological studies, suggesting that long-term encoding-related activity may be reduced if cholinergic receptors are blocked by scopolamine. Key words: memory formation; cholinergic; fMRI; medial temporal lobe; delayed match to sample; computational modeling

Acetylcholine may regulate working memory for novel stimuli by activating intrinsic mechanisms for sustained spiking in entorhinal cortical neurons, which have been demonstrated in slice preparations of the entorhinal cortex. Computational modeling demonstrates that loss of the cholinergic activation of intrinsic mechanisms for sustained activity could selectively impair working memory for novel stimuli, whereas working memory for familiar stimuli could be maintained because of previously modified synapses. Blockade of muscarinic cholinergic receptors and selective cholinergic lesions has been shown to impair encoding in delayed matching tasks. However, previous studies have not compared explicitly the role of cholinergic modulation in working memory for novel versus familiar stimuli. Here, we show that lesions of the cholinergic innervation of the entorhinal cortex selectively impair delayed nonmatch to sample performance for novel odors, whereas delayed nonmatch to sample for familiar odors is spared. This indicates an important role for cholinergic innervation of the entorhinal cortex in working memory for novel stimuli. Key words: acetylcholine; episodic buffer; sustained activity; computational modeling; working memory; persistent spiking

The temporal correlation hypothesis proposes using distributed synchrony for the binding of different stimulus features. However, synchronized spikes must travel over cortical circuits that have varying-length pathways, leading to mismatched arrival times. This raises the question of how initial stimulus-dependent synchrony might be preserved at a destination binding site. Earlier, we proposed constraints on tolerance and segregation parameters for a phase-coding approach, within cortical circuits, to address this question [22]. The purpose of the present paper is twofold. First, we conduct simulation experiments to test the proposed constraints. Second, we explore the practicality of temporal binding to drive a process of long-term memory formation based on a recruitment learning method [15].

The orbital frontal cortex appears to be involved in learning the rules of goal-directed behavior necessary to perform the correct actions based on perception to accomplish different tasks. The activity of orbitofrontal neurons changes dependent upon the specific task or goal involved, but the functional role of this activity in performance of specific tasks has not been fully determined. Here we present a model of prefrontal cortex function using networks of integrate-and-fire neurons arranged in minicolumns. This network model forms associations between representations of sensory input and motor actions, and uses these associations to guide goal-directed behavior. The selection of goal-directed actions involves convergence of the spread of activity from the goal representation with the spread of activity from the current state. This spiking network model provides a biological implementation of the action selection process used in reinforcement learning theory. The spiking activity shows properties similar to recordings of orbitofrontal neurons during task performance.

The temporal correlation hypothesis proposes using distributed synchrony for the binding of different stimulus features. However, synchronized spikes must travel over cortical circuits that have varying-length pathways, leading to mismatched arrival times. This raises the question of how initial stimulusdependent synchrony might be preserved at a destination binding site. Earlier, we proposed constraints on tolerance and segregation parameters for a phase-coding approach, within cortical circuits, to address this question (Gnay and Maida, 2001). The purpose of the present paper is twofold. First, we conduct simulation experiments to test the proposed constraints. Second, we explore the practicality of temporal binding to drive a process of long-term memory formation based on a recruitment learning method (Feldman, 1982).

The temporal correlation hypothesis proposes using distributed synchrony for the binding of different stimulus features. However, synchronized spikes must travel over cortical circuits that have varying-length pathways, leading to mismatched arrival times. This raises the question of how initial stimulus-dependent synchrony might be preserved at a destination binding site. Earlier, we proposed constraints on tolerance and segregation parameters for a phase-coding approach, within cortical circuits, to address this question [1]. The purpose of the present paper is twofold. First, we conduct simulation experiments to test the proposed constraints. Second, we explore the practicality of temporal binding to drive a process of long-term memory formation based on a recruitment learning method [2].

this memory depend upon the flow of activity across sets of synaptic connections, allowing specific populations of neurons in region CA3 to evoke activity in other specific populations of neurons. See Figure 4 for description of flexible retrieval within an associative network

phenomena observed in unit recordings from awake animals, including match suppression, non-match enhancement, and non-match suppression. Key words: delayed match to sample; delayed non-match; stellate cells; pyramidal cells; medial entorhinal cortex; afterhyperpolarization; working memory; biophysical modeling; computer simulation; nonspecific cationic current I NCM Lesions of the entorhinal and perirhinal cortices impair performance in delayed non-match to sample (DNMS) tasks in both non-human primates (Zola-Morgan et al., 1993; Leonard et al., 1995) and rats (Otto and Eichenbaum, 1992). In delayed nonmatch to sample tasks, stimuli are presented sequentially, and animals must respond to a particular stimulus if that stimulus does not match the previously presented stimulus. The role of the entorhinal cortex in these tasks may involve activation of muscarinic cholinergic receptors, because performance in delayed matching tasks is impaired by systemic injections of muscarinic choline