I’m about to submit a paper on LFP receptive fields in V4 and in the interest of flow had to scrap a couple of figures. We measured temporal impulse response functions of LFPs using a standard sparse flashed bar paradigm. We used two Utah arrays to measure the signals (96 electrodes arranged in a 10×10 grid, minus the corners; electrode spaced 400 microns apart). Here are the temporal impulse response function from 50 to 240 ms in the first array:

And in the second array:

It’s intriguing that the response modulates slowly across the array. My feeling is that the changes in polarity, in particular, are due to the fact that the straight array cannot follow the curvature of cortex. Thus, some electrodes are in deeper layers and some are in more superficial layers, leading to the appearance of current sinks and sources. It would be interesting to try to infer what these layers are based on this information as well as knowledge of other properties of the RFs (especially RF size, which is linked to integration radius).