Visual receptive field mapping using filtered back projection

A recently published paper in The Journal of Physiology shows how filtered back projection (FBP), an algorithm often used for tomographic reconstruction (e.g. in CAT scans), can be used to map the spatial and temporal receptive fields (RFs) of retinal neurons. It also explains the advantages of using FBP over the traditional spike-triggered average (STA) method for mapping RFs.

The STA for each neuron is calculated as the average stimulus preceding a spike and provides an estimate of the linear component of a neurons RF. This can be done with a whole population of neurons in parallel using multi-electrode arrays, but there are a number of drawbacks to this method:

• It can be very time-consuming particularly for cells that have a low mean spike rate
• The temporal resolution is restricted to the refresh rate of the monitor delivering the stimulus
• The Gaussian white noise stimulation does not appear to stimulate all necessary cell types

Understanding variations in the RFs of different neurons is useful for learning how visual information is processed and having an efficient and accurate method of measuring RFs will help improve research in this field.

In order to create an improved procedure to measure RFs, researchers – led by Professor Leon Lagnado at the University of Sussex – studied whether FBP could be used to overcome the problems of the STA technique.

In CAT scanners X-rays are passed through objects to measure the density of the part of the object that the beam goes through. By measuring the density with parallel beams over different angles it is possible to reconstruct the shape of the whole object using the Radon transform and FBP algorithm.

Professor Lagnado and his colleagues have developed a way to map RFs of Retinal Ganglion Cells (RGCs) using a similar process. Instead of using X-rays they used light or dark bars flashed on to the retina and instead of assessing density they measured the neuronal spike response strength with multi-electrode arrays (MEAs).

Professor Lagnado said: "We hope that the technique described in this study will be useful to those studying the visual system by electrophysiology or by imaging, especially when they are making recordings across large populations of neurons. A fundamental property of any neurons within the retina or visual cortex is it's receptive field and it is important to have methods that allow these to be mapped accurately but within a short time, and for many neurons simultaneously. This approach is very well suited to mapping receptive fields when using multiphoton imaging to measure neural responses using genetically-encoded calcium indicators."

This method was both faster and had a higher temporal resolution than traditional STA calculations. It could also be used on several RGC subtypes that the STA method cannot measure. It is also possible to measure the spatial components of the RF for ON and OFF components of retinal neurons independently.

Using a Scientifica two-photon microscope, the team tested whether the FBP method could be used to map RFs of retinal ganglion cells in vivo by imaging calcium reporter proteins located at the synapses of bipolar cells. They used GCaMP6 to measure presynaptic terminals of bipolar cells. By flashing a bar across the field of view and detecting the change in Ca2+ at terminals over this area they were able to calculate the RFs of neurons using the same FBP procedure.