V1 responses are enhanced with visual attention of various forms. An appealing hypothesis about how this happens is: Top-down inputs activate VIP interneurons, which disinhibits pyramidal cells through SOM cell inhibition & causes enhanced/modified responses to the same stimuli.

Ideally to test this, you would need to make an animal do an attention task, find attentionally modulated cells, and transiently switch off or switch on local VIP cells while simultaneously looking at responses of the attentionally modulated pyramidal cells.
And since any circuit manipulation has complicated effects on different cell classes, ideally you should know how multiple different cell classes respond to attention AND to the VIP manipulation in the same tissue (e.g. pyramidal, SOM and PV cells, in addition to the VIPs). Thats exactly what Dylan and team did!

First, we established that attention leads to strong modulation of stimulus response selectivity, and separately, VIP activation leads to strong modulation of pyramidal cell activity. But when we put the two together, there was no interaction. ie, attention and VIP led to independent modulations.

This result was confirmed by silencing VIPs, which left the attentional modulation untouched. So attention signals do not pass through VIP cells. In fact, we found that at the population level, the two modulations were orthogonal.

And because we had measured the activity of VIP, SOM, PV and pyramidals simultaneously, we could show that the changes in different cell classes and their interactions was distinct between attention and VIP modulation.

But how is it possible to have a strong modulation by attention AND by VIP on the SAME cells, but the two not interact? With our amazing collaborators Katharina Wilmes and Claudia Clopath, we found a very specific circuit architecture that can support this property.

Please read and share the preprint, which has much more in there. This was a great team effort led by Dylan Myers-Joseph, and with tremendous contributions from @k47h4, Marian Fernandez-Otero and Claudia Clopath