Horizontal or Vertical? How does the mouse retina help to assess an object’s orientation?
Researchers at the Feinberg School of Medicine, part of Northwestern University, have identified two novel types of orientation selective retinal ganglion cells (OS RGCs) in the mouse retina.
These cells are highly selective for vertical or horizontal cardinal orientations, and their morphology and electrophysiology are beginning to give insights into the mechanisms through which mammals calculate orientation.
Orientation selectivity (OS) is a central part of the mammalian visual system and has been well studied over the last 50 years. In 1962, Hubel and Wiesel found neurons in the primary visual cortex (V1) that respond selectively to bars of light in a particular orientation. Since then studies in rabbits and mice have shown that receptive fields in the lateral geniculate nucleus (LGN) project to a single V1 neuron that has OS. However, further research has shown that there is a pathway involved upstream of the V1.
Led by Dr Greg Schwartz, the study recently published in The Journal of Neuroscience confirms the existence of another orientation-selective pathway in rodents upstream of the V1.
By combining two-photon illumination and imaging with electrophysiological recordings and visual stimuli the neuroscientists identified and characterised two subtypes of ON OS RGCs. These cell types were divided by their light response characteristics into horizontal (ON hOS RGCs) and vertical (ON vOS RGCs) based on their orientation preference.
Examination of the dendrites showed the ON hOS RGNs have asymmetric branches aligned with the orientation preference of the response. ON vOS RGNs have symmetric dendrites but still receive OS excitation. Additionally, both ON OS RGCs receive OS inhibition that is not aligned to the dendrites. Therefore, morphology alone doesn’t explain OS computation in the retina.
The authors report that OS inhibition is probably carried by a combination of glycinergic and GABAergic amacrine cells, but it is not clear if the amacrine cells themselves are OS. The origins of OS excitation are harder to identify, and further studies will be required to elucidate how these conductances arise in the retina. It is still uncertain whether OFF OS RGCs are also present in the mouse retina.
To discover whether the use of different OS pathways enables altered perceptions and behaviours more experiments will be needed.
Two-photon illumination, imaging and electrophysiological recordings were performed on a custom built Scientifica Multiphoton Imaging System designed around the SliceScope Pro 6000 electrophysiology rig.
Nath A., Schwartz G.W. Cardinal Orientation Selectivity is Represented by Two Distinct Ganglion Cell Types in Mouse Retina The Journal of Neuroscience (2016) doi: 10.1523/JNEUROSCI.4554-15.2016
Banner Image: Dendritic morphology of (A) horizontal ON OS RGNs and (B) vertical ON OS RGNs (Credit: Dr Greg Schwartz)