Stimuli reduce the dimensionality of cortical activity

TL;DR

This study investigates how stimuli influence the effective dimensionality of neural activity in sensory cortex, revealing that stimuli reduce dimensionality growth and are explained by a clustered network model with bounds related to correlations.

Contribution

It introduces a model explaining how stimuli constrain neural activity dimensionality and predicts bounds based on correlations and clustering, validated by empirical data.

Findings

Dimensionality grows linearly with ensemble size, faster during ongoing activity.

Stimuli reduce the growth rate of neural activity dimensionality.

A clustered network model predicts an upper bound on dimensionality related to correlations.

Abstract

The activity of ensembles of simultaneously recorded neurons can be represented as a set of points in the space of firing rates. Even though the dimension of this space is equal to the ensemble size, neural activity can be effectively localized on smaller subspaces. The dimensionality of the neural space is an important determinant of the computational tasks supported by the neural activity. Here, we investigate the dimensionality of neural ensembles from the sensory cortex of alert rats during period of ongoing (inter-trial) and stimulus-evoked activity. We find that dimensionality grows linearly with ensemble size, and grows significantly faster during ongoing activity compared to evoked activity. We explain these results using a spiking network model based on a clustered architecture. The model captures the difference in growth rate between ongoing and evoked activity and predicts a characteristic scaling with ensemble size that could be tested in high-density multi-electrode recordings. Moreover, the model predicts the existence of an upper bound on dimensionality. This upper bound is inversely proportional to the amount of pair-wise correlations and, compared to a homogeneous network without clusters, it is larger by a factor equal to the number of clusters. The empirical estimation of such bounds depends on the number and duration of trials. Together, these results provide a framework to analyze neural dimensionality in alert animals, its behavior under stimulus presentation, and its theoretical dependence on ensemble size, number of clusters, and pair-wise correlations in spiking network models.