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Abstract
How are visual scenes encoded in local neural networks of visual cortex? In rodents, visual cortex lacks a columnar organization so that processing of diverse features from a spot in visual space could be performed locally by populations of neighboring neurons. To examine how complex visual scenes are represented by local microcircuits in mouse visual cortex we measured visually evoked responses of layer 2/3 neuronal populations using 3D two-photon calcium imaging. Both natural and artificial movie scenes (10 seconds duration) evoked distributed and sparsely organized responses in local populations of 70–150 neurons within the sampled volumes. About 50% of neurons showed calcium transients during visual scene presentation, of which about half displayed reliable temporal activation patterns. The majority of the reliably responding neurons were activated primarily by one of the four visual scenes applied. Consequently, single-neurons performed poorly in decoding, which visual scene had been presented. In contrast, high levels of decoding performance (>80%) were reached when considering population responses, requiring about 80 randomly picked cells or 20 reliable responders. Furthermore, reliable responding neurons tended to have neighbors sharing the same stimulus preference. Because of this local redundancy, it was beneficial for efficient scene decoding to read out activity from spatially distributed rather than locally clustered neurons. Our results suggest a population code in layer 2/3 of visual cortex, where the visual environment is dynamically represented in the activation of distinct functional sub-networks.
Highlights
Mouse visual cortex shares fundamental features such as retinotopy, receptive field types, orientation tuning, and ocular dominance plasticity with visual cortices of higher mammalian species (Hubener, 2003)
Somatic calcium transients in neuronal populations were measured with the calcium indicator OGB1 using 3D laser scanning (Göbel et al, 2007) (Figure 1B; see Methods; n = 12 populations from eight mice; 70–150 neurons per population)
These findings indicate that the measured calcium transients represent the underlying neuronal firing patterns and that the deconvolved calcium transients are reliable estimates of neuronal spike rates
Summary
Mouse visual cortex shares fundamental features such as retinotopy, receptive field types, orientation tuning, and ocular dominance plasticity with visual cortices of higher mammalian species (Hubener, 2003). A salt-andpepper organization of orientation preference exists in layer 2/3 (Ohki et al, 2005; Mrsic-Flogel et al, 2007; Sohya et al, 2007) These neurons can produce highly selective action potential output in response to drifting gratings, even though synaptic inputs onto their dendrites are more broadly tuned (Jia et al, 2010; Medini, 2011). Such selective responses of cortical neurons suggest that in spite of large receptive fields and high overlap of dendritic and axonal arbors of neighboring neurons (Hellwig, 2000) there may exist a specific micro-organization. To better understand local processing of the visual scenery in intermingled networks of neighboring neurons with diverse tuning properties, further characterization of such functional sub-networks is essential
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