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Even salient sensory stimuli are sometimes not detected. What goes wrong in the brain in that case? Here we show that a late (> 100-ms) component of the neural activity in the primary visual cortex of the monkey is selectively suppressed when stimuli are not seen. As there is evidence that this activity depends on feedback from extrastriate areas, these findings suggest a specific role for recurrent processing when stimuli are reaching a perceptual level. Further results show that this perceptual level is situated between purely sensory and decision or motor stages of processing.

The present results reveal unique contributions of the different cortical layers to the formation of a visual percept. This new blueprint of laminar processing may generalize to other tasks and to other areas of the cerebral cortex, where the layers are likely to have roles similar to those in area V1.

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Neurons in the primary visual cortex (V1) receive feedforward input from the thalamus, which shapes receptive-field properties. They additionally receive recurrent inputs via horizontal connections within V1 and feedback from higher visual areas that are thought to be important for conscious visual perception. Here, we investigated what roles different glutamate receptors play in conveying feedforward and recurrent inputs in macaque V1. As a measure of recurrent processing, we used figure-ground modulation (FGM), the increased activity of neurons representing figures compared with background, which depends on feedback from higher areas. We found that feedforward-driven activity was strongly reduced by the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), whereas this drug had no effect on FGM. In contrast, blockers of the NMDA receptor reduced FGM, whereas their effect on visually driven activity varied with the subunit specificity of the drug. The NMDA receptor blocker 2-amino-5-phosphonovalerate (APV) caused a slight reduction of the visual response, whereas ifenprodil, which targets NMDA receptors containing the NMDA receptor NR2B subunit, increased the visual response. These findings demonstrate that glutamate receptors contribute differently to feedforward and recurrent processing in V1 and suggest ways to selectively disrupt recurrent processing so that its role in visual perception can be elucidated.T he areas of the primate visual cortex are arranged in a hierarchy, with feedforward connections propagating information from lower to higher areas and feedback connections carrying information in the opposite direction, back to the lower areas (1). Feedforward and recurrent processing differ greatly in function. Feedforward connections drive neurons in the visual cortex. They shape the receptive field of neurons, causing the rapid formation of tuning properties such as orientation and direction selectivity (2) and even sensitivity to complex objects such as faces. In contrast, recurrent input, carried by feedback connections from higher areas and by horizontal connections within a visual area, is thought not to drive neurons but to mediate modulatory, contextual effects (3, 4). Recurrent connections have been suggested to be involved in attention (5-7), figure-ground segregation (8-11), and conscious visual perception (12, 13), although their precise function is not well understood. One problem in studying the role of recurrent connections has been the lack of a tool to selectively inhibit them without disrupting feedforward processing. Some studies blocked the source of recurrent processing by suppressing activity in higher-level visual areas using cooling or injections of GABA while recording in lower-level areas (9,14,15). The results of these studies vary because some showed strong effects on V1 activity (15) and a decrease in contextual modulation (9), whereas others showed no effect (14). The exact effect of recurrent input into V1, therefore, remains to be elucidated.Why might feed...

A neural correlate of working memory in the monkey primary visual cortex Lamme, V.A.F.; Suphr, H.; Spekreijse, H.

We have analyzed the early development of the main hippocampal afferents in the mouse. Following injections of the lipophilic tracer 1-1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in the entorhinal cortex, entorhinal axons were observed for the first time in the hippocampus at E15, in the white matter. At E17, entorhinal fibers arborized within the stratum lacunosum-moleculare. At subsequent stages entorhinal axons formed dense networks that were restricted to their appropriate termination zone in the lacunosum-moleculare. The first axons invading the fascia dentata were noticed at E19, their density increasing at later stages. These axons were mainly present in the outer molecular layer. This onset of entorhinohippocampal projections was corroborated by retrograde labeling data after injections in the hippocampus. Commissural fibers first entered the contralateral hippocampus at E18, their number increasing at the following stages. Commissural axons arborized within the stratum oriens and radiatum in the hippocampus proper. In the fascia dentata, the earliest commissural fibers were seen at P2, terminating in the inner zone of the molecular layer and in the hilus. We conclude that developing entorhinal and commissural axons show a high degree of laminar specificity from the earliest stages of formation, which is compatible with the notion that distinct subsets of early maturing neurons populating the hippocampal plexiform layers may attract particular fiber systems. Hippocamposeptal fibers develop at E15, before the first septal fibers can be detected in the hippocampus. These early hippocamposeptal fibers originated from nonpyramidal neurons and terminated in the medial septal area, which is the main source of septal afferents to the hippocampus. In contrast, septohippocampal fibers were not seen in the hippocampus until E17. At perinatal stages, the hippocamposeptal connection reshapes, sending axons to the dorsolateral septal area as the innervation of the medial septum becomes less conspicuous. This sequence suggests that hippocampal neurons pioneer the formation of septohippocampal connections.

During neural development, specific recognition molecules provide the cues necessary for the formation of initial projection maps, which are reshaped later in development. In some systems, guiding cues for axonal pathfinding and target selection are provided by specific cells that are present only at critical times. For instance, the floor plate guides commissural axons in the spinal cord, and the subplate is involved in the formation of thalamocortical connections. Here we study the development of entorhinal and commissural connections to the murine hippocampus, which in the adult terminate in nonoverlapping layers. We show that two groups of pioneer neurons, Cajal-Retzius cells and GABAergic neurons, form layer-specific scaffolds that overlap with distinct hippocampal afferents at embryonic and early postnatal stages. Furthermore, at postnatal day 0 (P0)-P5, before the dendrites of pyramidal neurons develop, these pioneer neurons are preferential synaptic targets for hippocampal afferents. Birthdating analysis using 5'-bromodeoxyuridine (BrdU) pulses showed that most such early-generated neurons disappear at late postnatal stages, most likely by cell death. Together with previous studies, these findings indicate that distinct pioneer neurons are involved in the guidance and targeting of different hippocampal afferents.