Abstract
Neurons in the visual system vary widely in the spatiotemporal properties of their receptive fields (RFs), and understanding these variations is key to elucidating how visual information is processed. We present a new approach for mapping RFs based on the filtered back projection (FBP), an algorithm used for tomographic reconstructions. To estimate RFs, a series of bars were flashed across the retina at pseudo-random positions and at a minimum of five orientations. We apply this method to retinal neurons and show that it can accurately recover the spatial RF and impulse response of ganglion cells recorded on a multi-electrode array. We also demonstrate its utility for in vivo imaging by mapping the RFs of an array of bipolar cell synapses expressing a genetically encoded Ca2+ indicator. We find that FBP offers several advantages over the commonly used spike-triggered average (STA): (i) ON and OFF components of a RF can be separated; (ii) the impulse response can be reconstructed at sample rates of 125 Hz, rather than the refresh rate of a monitor; (iii) FBP reveals the response properties of neurons that are not evident using STA, including those that display orientation selectivity, or fire at low mean spike rates; and (iv) the FBP method is fast, allowing the RFs of all the bipolar cell synaptic terminals in a field of view to be reconstructed in under 4 min. Use of the FBP will benefit investigations of the visual system that employ electrophysiology or optical reporters to measure activity across populations of neurons.
Highlights
The visual system processes stimuli through a variety of spatial and temporal filters (Kuffler, 1973) and the results of this processing are described by the receptive field (RF) of neurons at different stages of the visual pathways
The filtered back projection (FBP) method returned a slightly larger value for the major axis (median difference 21 μm, inter-quartile range (IQR) 7 to 58 μm) whereas there was no significant difference for the minor axis. These results demonstrate that no systematic bias was present when the parameters of the spatial RF are estimated with FBP as compared to spike-triggered average (STA)
The position of the RF in visual space varied according to the terminals’ lateral location in the inner plexiform layer (Fig. 8F). These results demonstrate that the FBP method is implemented to map the RFs of visual neurons in imaging experiments and that it should prove useful in other visual areas
Summary
The visual system processes stimuli through a variety of spatial and temporal filters (Kuffler, 1973) and the results of this processing are described by the receptive field (RF) of neurons at different stages of the visual pathways. The first approaches to mapping RFs in the visual system employed spots or bars flashed at various locations while recording spikes from a single neuron (Hubel & Wiesel, 1959). This technique is time-consuming and cannot be applied systematically when recording activity from a population of neurons. STA has been used to map RFs in neurons throughout the visual pathway, from the retina to the cortex (Hubel & Wiesel, 1962; DeAngelis et al 1993; Meister et al 1994; Martinez et al 2005; McAdams & Reid, 2005; Lesica et al 2007; Field et al 2010; Sher & Devries, 2012; Zhao et al 2013)
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