Aberrant cortical activity, functional connectivity, and neural assembly architecture after photothrombotic stroke in mice

Reviewer #1 (Public Review):

Summary: This impressive study by Bandet and Winship uses 2-photon imaging in awake-behaving mice to examine long-term changes in neural activity and functional connectivity after focal ischemic stroke. The authors discover that there are long-lasting perturbations in neural activity and functional connectivity, specifically within peri-infarct cortex but not more distant cortical regions. Overall I thought the study provided important new findings that were supported by compelling data.

Strengths: This is a technically challenging study to perform and the authors show high-quality data. The manuscript was well-written, and the figures were clearly presented. I really like the analytic tools they applied, which were rigorous and provided some novel insights regarding neural activity patterns during movement or rest. The discovery of long-lasting impairments in neural activity/functional connectivity is an important one as it is important for future stroke studies to recognize what problems need to be rectified in the post-stroke brain, even many weeks after injury. They also suggest there is a much more nuanced relationship between macroscopic changes in somatosensory maps and single-cell activity. Overall, I think this is a well-executed study whose primary conclusions were justified by the data presented.

Weaknesses: I found very little in the way of weaknesses. The statistics were notably conservative and are appropriate.

Reviewer #2 (Public Review):

This study investigates the excitability of neurons in the peri-infarct cortex during recovery from ischemic stroke. The excitability of neurons in the peri-infarct cortex during stroke recovery has produced contradictory findings: some studies suggest hyper-excitability to direct-brain stimulation, while others indicate diminished responsiveness to physical stimuli. However, most studies have used anesthetized animals, which can disrupt cortical activity and functional connectivity. The present study used two-photon Ca2+ imaging after focal photothrombotic stroke to examine neural activity patterns in awake mice. The authors found reduced neuronal spiking in the peri-infarct cortex that was strongly correlated with motor performance deficits. Additionally, the authors found disruptions in neural activation, functional connectivity, and assembly architecture in the immediate peri-infarct region but not in the distal cortex regions.

The findings of this study are very important as they show that there is no measurable change in terms of neuronal activation and reorganization in distal regions of remapped cortical response areas after stroke.

However, cortical response areas are calculated using a threshold of 95% peak activity within a trial. The threshold is presumably used to discriminate between the sensory-evoked response and collateral activation / less "relevant" response (noise). Since the peak intensity is lower after stroke, the "response" area is larger - lower main signal results in less noise exclusion. Predictably, areas that show a higher response before stroke than after are excluded from the response area before the stroke and included after.

We suggest a reinterpretation of the findings: much of the non-remapped areas are included when using a within-trial threshold as a criterion, and the absence of increased neuronal activation and reorganization is evidence for this claim. The take-home message of this study should be that we need a much better criterion for what we consider remapping.