Despite the lack of direct evidence, it is generally believed that top-down signals are mediated by the abundant feedback connections from higher-to lower-order sensory areas. Here we provide direct evidence for a top-down mechanism. We stained the visual cortex of the ferret with a voltage-sensitive dye and presented a short-duration contrast square. This elicited an initial feedforward and lateral spreading depolarization at the square representation in areas 17 and 18. After a delay, a broad feedback wave (FBW) of neuron peak depolarization traveled from areas 21 and 19 toward areas 18 and 17. In areas 18 and 17, the FBW contributed the peak depolarization of dendrites of the neurons representing the square, after which the neurons decreased their depolarization and firing. Thereafter, the peak depolarization surrounded the figure representation over most of areas 17 and 18 representing the background. Thus, the FBW is an example of a well behaved long-range communication from higher-order visual areas to areas 18 and 17, collectively addressing very large populations of neurons representing the visual scene. Through local interaction with feedforward and lateral spreading depolarization, the FBW differentially activates neurons representing the object and neurons representing the background.T he current view of perception and cognition is that they rely on three main brain mechanisms, each supported by the existence of particular anatomical connections: bottom up, i.e., processing by early sensory areas, which is conveyed to higherorder areas; lateral processing through horizontal connections within an area; and top-down modulatory influences exerted by the rather extensive anatomical connections from higher-order sensory areas to the cortex in early sensory areas. Despite the fact that these top-down connections have been known for Ͼ25 years (1), and despite an overwhelming number of reports in which one could interpret the observations as presumed effects of top-down modulations, there is still no direct physiological evidence revealing the mechanism(s) by which higher-order sensory areas alter the computations of neurons in early sensory areas (2). That is, there is no evidence how, when, and where the top-down inputs alter the computation of neurons in early sensory areas. Further, the relative importance and timing of local lateral computations and top-down effects in previous studies of object perception are not obvious (3-5).Here, we define top-down modulation as a mechanism by which higher-order sensory areas through their connections influence computations of neurons in early sensory areas. These connections typically target neurons in upper (supragranular) layers or in lower (infragranular) layers within these early areas. Theoretically, it has been proposed that lateral interactions and the eventual feedback from higher-order visual areas would be finely timed to engage a large population of supragranular neurons in areas 17 and 18 in a cooperative computation of the visual stimulus and its surrou...
SummaryUnderstanding neural computation requires methods such as 3D acousto-optical (AO) scanning that can simultaneously read out neural activity on both the somatic and dendritic scales. AO point scanning can increase measurement speed and signal-to-noise ratio (SNR) by several orders of magnitude, but high optical resolution requires long point-to-point switching time, which limits imaging capability. Here we present a novel technology, 3D DRIFT AO scanning, which can extend each scanning point to small 3D lines, surfaces, or volume elements for flexible and fast imaging of complex structures simultaneously in multiple locations. Our method was demonstrated by fast 3D recording of over 150 dendritic spines with 3D lines, over 100 somata with squares and cubes, or multiple spiny dendritic segments with surface and volume elements, including in behaving animals. Finally, a 4-fold improvement in total excitation efficiency resulted in about 500 × 500 × 650 μm scanning volume with genetically encoded calcium indicators (GECIs).
Slow wave activity (SWA) is a characteristic brain oscillation in sleep and quiet wakefulness. Although the cell types contributing to SWA genesis are not yet identified, the principal role of neurons in the emergence of this essential cognitive mechanism has not been questioned. To address the possibility of astrocytic involvement in SWA, we used a transgenic rat line expressing a calcium sensitive fluorescent protein in both astrocytes and interneurons and simultaneously imaged astrocytic and neuronal activity in vivo. Here we demonstrate, for the first time, that the astrocyte network display synchronized recurrent activity in vivo coupled to UP states measured by field recording and neuronal calcium imaging. Furthermore, we present evidence that extensive synchronization of the astrocytic network precedes the spatial build-up of neuronal synchronization. The earlier extensive recruitment of astrocytes in the synchronized activity is reinforced by the observation that neurons surrounded by active astrocytes are more likely to join SWA, suggesting causality. Further supporting this notion, we demonstrate that blockade of astrocytic gap junctional communication or inhibition of astrocytic Ca2+ transients reduces the ratio of both astrocytes and neurons involved in SWA. These in vivo findings conclusively suggest a causal role of the astrocytic syncytium in SWA generation.
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