Abstract
Diverse sensory systems, from audition to thermosensation, feature a separation of inputs into ON (increments) and OFF (decrements) signals. In the Drosophila visual system, separate ON and OFF pathways compute the direction of motion, yet anatomical and functional studies have identified some crosstalk between these channels. We used this well-studied circuit to ask whether the motion computation depends on ON-OFF pathway crosstalk. Using whole-cell electrophysiology, we recorded visual responses of T4 (ON) and T5 (OFF) cells, mapped their composite ON-OFF receptive fields, and found that they share a similar spatiotemporal structure. We fit a biophysical model to these receptive fields that accurately predicts directionally selective T4 and T5 responses to both ON and OFF moving stimuli. This model also provides a detailed mechanistic explanation for the directional preference inversion in response to the prominent reverse-phi illusion. Finally, we used the steering responses of tethered flying flies to validate the model's predicted effects of varying stimulus parameters on the behavioral turning inversion.
Highlights
In both invertebrate and vertebrate visual systems, neuronal signals bifurcate into parallel pathways, within which many neurons preferentially encode luminance increments (ON) or luminance decrements (OFF) within 1 to 2 synaptic layers downstream of photoreceptors.[1]
Using static stimuli of the preferred contrast (PC; bright for T4, dark for T5), we showed that both cell types share a similar spatiotemporal receptive field (RF)
We found that responses of T4 and T5 cells to moving bars of their non-preferred contrast (NC; dark for T4, bright for T5) were still directionally selective
Summary
In both invertebrate and vertebrate visual systems, neuronal signals bifurcate into parallel pathways, within which many neurons preferentially encode luminance increments (ON) or luminance decrements (OFF) within 1 to 2 synaptic layers downstream of photoreceptors.[1]. Similarities between the computations carried out in the fly and vertebrate visual systems suggest that understanding ON-OFF crosstalk in the Drosophila motion circuit could reveal fundamental aspects of visual processing and may uncover more general, conserved aspects of sensory processing
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