Impact of cortical plasticity on information signaled by populations of neurons in the cerebral cortex

Optical recording of suprathreshold neural activity with single-cell and single-spike resolution

Signaling of information in the vertebrate central nervous system is often carried by populations of neurons rather than individual neurons. Also propagation of suprathreshold spiking activity involves populations of neurons. Empirical studies addressing cortical function directly thus require recordings from populations of neurons with high resolution. Here we describe an optical method and a deconvolution algorithm to record neural activity from up to 100 neurons with single-cell and single-spike resolution. This method relies on detection of the transient increases in intracellular somatic calcium concentration associated with suprathreshold electrical spikes (action potentials) in cortical neurons. High temporal resolution of the optical recordings is achieved by a fast random-access scanning technique using acousto-optical deflectors (AODs). Two-photon excitation of the calcium-sensitive dye results in high spatial resolution in opaque brain tissue. Reconstruction of spikes from the fluorescence calcium recordings is achieved by a maximum-likelihood method. Simultaneous electrophysiological and optical recordings indicate that our method reliably detects spikes (>97% spike detection efficiency), has a low rate of false positive spike detection (< 0.003 spikes/sec), and a high temporal precision (about 3 msec). This optical method of spike detection can be used to record neural activity in vitro and in anesthetized animals in vivo.

Impact of cortical plasticity on patterns of suprathreshold activity in the cerebral cortex

There are many cellular and synaptic mechanisms of plasticity in the vertebrate cortex. How the patterns of suprathreshold spiking activity in a population of neurons change because of this plasticity, however, has hardly been subjected to experimental studies. Here, we measured how evoked patterns of suprathreshold spiking activity in a cortical network were modified by cortical plasticity with single-cell and single-spike resolution. To record patterns of activity in the rodent barrel cortex, we used optical methods to detect suprathreshold activity from up to 40 neurons simultaneously. Pairing of two inputs resulted in a long-lasting modification of the cortical responses evoked by one of the inputs. The results indicate that plasticity rules on the network level inherit properties from synaptic plasticity rules but are also determined by the functional synaptic architecture, as well as the computations carried out in cortical networks. The largest determinants of the modified cortical responses were those observed when inducing changes by pairing the two inputs. On the single-neuron level, the modified responses only weakly reflected those observed when pairing the two inputs for induction of plasticity. Despite the weak reflection on the cellular level, however, the modified patterns reflected the pairing patterns to the degree that a simple decoding mechanism-a linear separator-correctly discriminated the modified responses from other patterns of activity.