Functional imaging methods monitor neural activity by measuring hemodynamic signals. These are more closely related to local field potentials (LFPs) than to action potentials. We simultaneously recorded electrical and hemodynamic responses in the cat visual cortex. Increasing stimulus strength enhanced spiking activity, high-frequency LFP oscillations, and hemodynamic responses. With constant stimulus intensity, the hemodynamic response fluctuated; these fluctuations were only loosely related to action potential frequency but tightly correlated to the power of LFP oscillations in the gamma range. These oscillations increase with the synchrony of synaptic events, which suggests a close correlation between hemodynamic responses and neuronal synchronization.
The corpus callosum (CC) provides the main route of communication between the 2 hemispheres of the brain. In monkeys, chimpanzees, and humans, callosal axons of distinct size interconnect functionally different cortical areas. Thinner axons in the genu and in the posterior body of the CC interconnect the prefrontal and parietal areas, respectively, and thicker axons in the midbody and in the splenium interconnect primary motor, somatosensory, and visual areas. At all locations, axon diameter, and hence its conduction velocity, increases slightly in the chimpanzee compared with the macaque because of an increased number of large axons but not between the chimpanzee and man. This, together with the longer connections in larger brains, doubles the expected conduction delays between the hemispheres, from macaque to man, and amplifies their range about 3-fold. These changes can have several consequences for cortical dynamics, particularly on the cycle of interhemispheric oscillators.axons ͉ cerebral cortex ͉ corpus callosum ͉ information transfer ͉ interhemispheric T he increased size of the human brain and its anatomical asymmetry and functional lateralization suggest that connections between the hemispheres must have undergone a substantial degree of reorganization in primate evolution. The timing of interhemispheric interactions is probably a crucial constraint in this reorganization (1). However, although some data suggest a progressive slowing down of interhemispheric communication in larger brains (1, 2), other data maintain that the speed of interhemispheric communication scales with brain size (3, 4). In this study, we examined interhemispheric connections in the macaque, chimpanzee, and human. The results reconcile the 2 views presented above and open unique perspectives on the role of long corticocortical connections in cortical dynamics and computation. ResultsIn cross-sections of the chimpanzee corpus callosum (CC), the intensity of myelin staining was found to vary in the anterior-toposterior direction, suggesting that larger and more myelinated axons would be found in the middle of the body and in the anterior part of the splenium. Indeed, the diameter of axons was found to increase progressively from anterior to the midbody and to decrease again further posterior [supporting information (SI) Fig. S1]. Thicker axons were also found in the anterior and lower part of the splenium. This pattern resembled that described in the macaque (5) and human (6) CC.To understand if the differences in axonal size relates to the origin of the CC axons, as suggested by LaMantia and Rakic (5), in 3 long-tailed macaques (Macaca fascicularis), 9 cortical sites (prefrontal, premotor, somatosensory, parietal, and visual areas) were injected with biotinylated dextran amine (BDA) (Fig. 1 A and B).Each injection labeled a discrete cluster of axons in the CC. As expected from previous anatomical (7,8) and imaging (9) work, the position of the axonal clusters in the CC corresponded to the anteroposterior location of the injection...
Every act of information processing can in principle be decomposed into the component operations of information storage, transfer, and modification. Yet, while this is easily done for today's digital computers, the application of these concepts to neural information processing was hampered by the lack of proper mathematical definitions of these operations on information. Recently, definitions were given for the dynamics of these information processing operations on a local scale in space and time in a distributed system, and the specific concept of local active information storage was successfully applied to the analysis and optimization of artificial neural systems. However, no attempt to measure the space-time dynamics of local active information storage in neural data has been made to date. Here we measure local active information storage on a local scale in time and space in voltage sensitive dye imaging data from area 18 of the cat. We show that storage reflects neural properties such as stimulus preferences and surprise upon unexpected stimulus change, and in area 18 reflects the abstract concept of an ongoing stimulus despite the locally random nature of this stimulus. We suggest that LAIS will be a useful quantity to test theories of cortical function, such as predictive coding.
Gap junctions are common between cortical GABAergic interneurons but little is known about their quantitative distribution along dendritic profiles. Here, we provide direct morphological evidence that parvalbumin-containing GABAergic neurons in layer 2/3 of the cat visual cortex form dense and far-ranging networks through dendritic gap junctions. Gap junction-coupled networks of parvalbumin neurons were visualized using connexin36 immunohistochemistry and confocal laser-scanning microscopy (CLSM). The direct correspondence of connexin36-immunopositve puncta and gap junctions was confirmed by examining the same structures in both CLSM and electron microscopy. Single parvalbumin neurons with large somata (Ն200 m 2 ) formed 60.3 Ϯ 12.2 (mean Ϯ SD) gap junctions with other cells whereby these contacts were not restricted to proximal dendrites but occurred at distances of up to 380 m from the soma. In a Sholl analysis of large-type parvalbumin neurons, 21.9 Ϯ 7.9 gap junctions were within 50 m of the soma, 21.7 Ϯ 7.6 gap junctions in a segment between 50 and 100 m, 11.2 Ϯ 4.7 junctions between 100 and 150 m, and 5.6 Ϯ 3.6 junctions were in more distal segments. Serially interconnected neurons could be traced laterally in a boundless manner through multiple gap junctions. Comparison to the orientation-preference columns revealed that parvalbumin-immunoreactive cells distribute randomly whereby their large dendritic fields overlap considerably and cover different orientation columns. It is proposed that this dense and homogeneous electrical coupling of interneurons supports the precise synchronization of neuronal populations with differing feature preferences thereby providing a temporal frame for the generation of distributed representations.
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