Here we ask whether visual response pattern varies with position in the cortical microcircuit by comparing the structure of receptive fields recorded from the different layers of the cat's primary visual cortex. We used whole-cell recording in vivo to show the spatial distribution of visually evoked excitatory and inhibitory inputs and to stain individual neurons. We quantified the distribution of 'On' and 'Off' responses and the presence of spatially opponent excitation and inhibition within the receptive field. The thalamorecipient layers (4 and upper 6) were dominated by simple cells, as defined by two criteria: they had separated On and Off subregions, and they had push-pull responses (in a given subregion, stimuli of the opposite contrast evoked responses of the opposite sign). Other types of response profile correlated with laminar location as well. Thus, connections unique to each visual cortical layer are likely to serve distinct functions. How does connectivity in striate cortex correlate with receptive field structure and, ultimately, with neural selectivity for elements of the visual scene? Anatomical studies show that each of the six cortical layers has a unique pattern of inputs and outputs 1-4. Thus, it is possible to investigate the function of specific components of the cortical microcircuit by comparing neural response patterns at different laminar positions 5-20. We took this approach to ask whether there are response properties exclusive to the first stage of cortical integration, where new response properties such as orientation sensitivity emerge 12. Early studies suggested that orientation selectivity depends on the structure of the simple receptive field, an arrangement of elongated On and Off subregions with an antagonistic effect on one another 12 , 21-23. This idea came from observations of responses evoked by stimuli placed at different positions in visual space. For instance, a bright contour aligned lengthwise with an On subregion produced strong excitation that diminished when the stimulus was rotated towards the orthogonal angle or was moved sideways to cover larger portions of an adjacent Off subregion 12. The geometry of the simple cell's response was thought to result from an orderly pattern of convergence from On and Off thalamic relay cells 12 , 23-26 .

Thalamic relay cells transmit information from retina to cortex by firing either rapid bursts or tonic trains of spikes. Bursts occur when the membrane voltage is low, as during sleep, because they depend on channels that cannot respond to excitatory input unless they are primed by strong hyperpolarization. Cells fire tonically when depolarized, as during waking. Thus, mode of firing is usually associated with behavioral state. Growing evidence, however, suggests that sensory processing involves both burst and tonic spikes. To ask if visually evoked synaptic responses induce each type of firing, we recorded intracellular responses to natural movies from relay cells and developed methods to map the receptive fields of the excitation and inhibition that the images evoked. In addition to tonic spikes, the movies routinely elicited lasting inhibition from the center of the receptive field that permitted bursts to fire. Therefore, naturally evoked patterns of synaptic input engage dual modes of firing.

Here we explore inhibitory circuits at the thalamocortical stage of processing in layer 4 of the cat's visual cortex, focusing on the anatomy and physiology of the interneurons themselves. Our immediate aim was to explore the inhibitory mechanisms that contribute to orientation selectivity, perhaps the most dramatic response property to emerge across the thalamocortical synapse. The broader goal was to understand how inhibitory circuits operate. Using whole-cell recording in cats in vivo, we found that layer 4 contains two populations of inhibitory cells defined by receptive field class--simple and complex. The simple cells were selective for stimulus orientation, whereas the complex cells were not. Our observations help to explain how neurons become sensitive to stimulus orientation and maintain that selectivity as stimulus contrast changes. Overall, the work suggests that different sources of inhibition, either selective for specific features or broadly tuned, interact to provide appropriate representations of elements within the environment.

Thalamic relay cells fire action potentials that transmit information from retina to cortex. The amount of information that spike trains encode is usually estimated from the precision of spike timing with respect to the stimulus. Sensory input, however, is only one factor that influences neural activity. For example, intrinsic dynamics, such as oscillations of networks of neurons, also modulate firing pattern. Here, we asked if retinal oscillations might help to convey information to neurons downstream. Specifically, we made whole-cell recordings from relay cells to reveal retinal inputs (EPSPs) and thalamic outputs (spikes) and then analyzed these events with information theory. Our results show that thalamic spike trains operate as two multiplexed channels. One channel, which occupies a low frequency band (<30 Hz), is encoded by average firing rate with respect to the stimulus and carries information about local changes in the visual field over time. The other operates in the gamma frequency band (40–80 Hz) and is encoded by spike timing relative to retinal oscillations. At times, the second channel conveyed even more information than the first. Because retinal oscillations involve extensive networks of ganglion cells, it is likely that the second channel transmits information about global features of the visual scene.

Abstract-Continuous growth in network link rates poses a strong demand on high speed IP lookup engines. While Ternary Content Addressable Memory (TCAM) based solutions serve most of today's high-end routers, they do not scale well for the next-generation [1]. On the other hand, pipelined SRAMbased algorithmic solutions become attractive. Intuitively multiple pipelines can be utilized in parallel to have a multiplicative effect on the throughput. However, several challenges must be addressed for such solutions to realize high throughput. First, the memory distribution across different stages of each pipeline as well as across different pipelines must be balanced. Second, the traffic on various pipelines should be balanced.In this paper, we propose a parallel SRAM-based multipipeline architecture for terabit IP lookup. To balance the memory requirement over the stages, a two-level mapping scheme is presented. By trie partitioning and subtrie-to-pipeline mapping, we ensure that each pipeline contains approximately equal number of trie nodes. Then, within each pipeline, a fine-grained node-to-stage mapping is used to achieve evenly distributed memory across the stages. To balance the traffic on different pipelines, both pipelined prefix caching and dynamic subtrie-topipeline remapping are employed. Simulation using real-life data shows that the proposed architecture with 8 pipelines can store a core routing table with over 200K unique routing prefixes using 3.5 MB of memory. It achieves a throughput of up to 3.2 billion packets per second, i.e. 1 Tbps for minimum size (40 bytes) packets.