Spatial Organization of Neuronal Population Responses in Layer 2/3 of Rat Barrel Cortex

Abstract

Individual pyramidal neurons of neocortex show sparse and variable responses to sensory stimuli in vivo. It has remained unclear how this variability extends to population responses on a trial-to-trial basis. Here, we characterized single-neuron and population responses to whisker stimulation in layer 2/3 (L2/3) of identified columns in rat barrel cortex using in vivo two-photon calcium imaging. Optical detection of single action potentials from evoked calcium transients revealed low spontaneous firing rates (0.25 Hz), variable response probabilities (range, 0 -0.5; mean, 0.2 inside barrel column), and weak angular tuning of L2/3 neurons. On average, both the singleneuron response probability and the percentage of the local population activated were higher in the barrel column than above septa or in neighboring columns. Within the barrel column, mean response probability was highest in the center (0.4) and declined toward the barrel border. Neuronal pairs showed correlations in both spontaneous and sensory-evoked activity that depended on the location of the neurons. Correlation decreased with increasing distance between neurons and, for neuronal pairs the same distance apart, with distance of the pair from the barrel column center. Although neurons are therefore not activated independently from each other, we did not observe precisely repeating spatial activation patterns. Instead, population responses showed large trial-to-trial variability. Nevertheless, the accuracy of decoding stimulus onset times from local population activity increased with population size and depended on anatomical location. We conclude that, despite their sparseness and variability, L2/3 population responses show a clear spatial organization on the columnar scale.

Visually evoked activity in cortical cells imaged in freely moving animals

“…All neurons displayed robust Ca 2ϩ -transients (Fig. 3G), which were similar to action potential (AP)-induced transients measured by conventional MP microscopes in anesthetized (5,6,9) and awake (8,11) animals. Transients in astrocytes were of similar amplitude (peak ⌬F/F 0 of 20-30%) but much slower dynamics (rise times of 2-15 s, durations of 5-100 s; Fig.…”

“…Previously, this approach, which includes both allothetic and idiothetic cues, lead to the discovery of place cells (1), head-direction cells (2), and grid cells (3). Multiphoton (MP) imaging (4) of neurons bulk-loaded with calcium indicators (5-7) allows not only the unambiguous identification and precise anatomical localization of active neurons but also the simultaneous recording of activity in multiple neurons even at very low firing rates (8)(9)(10). Although one major limitation of conventional MP imaging has been the need to firmly hold the skull in position to prevent the brain from moving relative to the microscope objective, some aspects of free movement can be simulated in head-fixed animals by placing them in a virtual reality situation (11).…”

Late Emergence of the Vibrissa Direction Selectivity Map in the Rat Barrel Cortex

“…This process was repeated 200 times to evaluate the standard deviation R of the distribution of the ͉R*͉. The directional selectivity of the neuron was considered as being sufficiently characterized when ͉R͉ exceeded 1.6452 R , meaning that the probability to be isotropic had fallen below 0.1 [threshold used for comparison with previous two-photon study (Kerr et al, 2007)]. The standard deviation of the distribution of angle of the bootstrap R* was also calculated to estimate the uncertainty in the preferred direction of a neuron.…”

“…This is unlikely to arise from a contamination of the neuron fluorescence by the neuropil (Gö-bel and Helmchen, 2007). To separate neuron and neuropil signals, great care was taken to optimize the in-plane and axial resolutions of the microscope and to define conservative ROI at neuronal somata (Göbel and Helmchen, 2007;Kerr et al, 2007) (recordings were included in the analysis of a neuron only when they occurred at the depth of its equatorial plane Ϯ 3 m, Materials and Methods). Absence of contamination was assessed by inspecting the nonsystematic co-occurrence of calcium transients in neurons and neuropil (Fig.…”

“…Only one electrophysiological study has shown evidence for a second map in SIbf of adult rats, representing the vibrissa direction selectivity (Andermann and Moore, 2006). However a subsequent two-photon imaging study has recently established that this second map was absent in 1-month-old rats (Kerr et al, 2007). One possibility for interpreting these opposite results could be the role played by the age of the animals, but assessing it is precluded by the difficulty of comparing electrophysiological results with low neuronal spatial sampling and the exhaustive mapping of biphotonic microscopy.…”