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Temporal coding modulates visual processing in retinal bipolar cells
Researchers in the US have discovered that ON cone bipolar cells assist image processing by using temporal coding to tune the way each subtype responds to light stimuli.
Image processing begins in the bipolar cells of the retina. These neurons synapse directly from photoreceptors, creating connections with either rods or cones. There is only one type of rod bipolar cell but more than 10 cone bipolar cell subtypes. Bipolar cells can also be categorised by how they react to the glutamate released by photoreceptors. They are designated ON or OFF bipolar cells depending on whether they have metabotropic or ionotropic glutamate receptors, respectively. When light hits a photoreceptor cell it hyperpolarises, releasing less glutamate. This causes ON bipolar cells to depolarise and OFF bipolar cells to hyperpolarise.
In this recent study, Dr Tomomi Ichinose at the Washington University of Medicine, St. Louis, and colleagues from Wayne State University School of Medicine, Detroit, looked at ON cone bipolar cell subtypes and how they temporally encode information from photoreceptors.
Temporal coding in sensory systems refers to the phenomenon that it is not just the rate of action potential firing in the neuron that is responsible for its output but the precise timing of the action potentials and fluctuations in the firing rate that lead to the complete neural code.
For this paper, they identified 6 subtypes of bipolar cells by looking at their morphology, in particular their terminal ramification patterns in the inner plexiform layer (IPL) of the retina. These were subtype 5s, 5f, 6, 7, 8 and XBC ON cone bipolar cells.
These cells were isolated from the rest of the network by various methods to ensure that any observed differences were caused by signal modulation within the bipolar cells. The photoreceptors were then stimulated with sinusoidal light (from 0.3 to 20 Hz) or step-pulse light stimuli using a CoolLED pE-2 light source. The resulting light-induced EPSPs (L-EPSPs) were recorded using whole-cell recordings carried out on a Scientifica SliceScope Pro 2000 system using a Molecular Devices amplifier and digitiser.
In response to sinusoidal light XBC and subtype 5f, 6 and 7 were bandpass filtering with reduced amplitude at lower and higher frequencies. Subtype 7 cells were the most highly tuned with the narrowest bandwidth and highest low cut-off point. Subtype 5s and 8 are low-pass filtering with only a mildly reduced L-EPSP at higher frequencies.
In response to step-pulse stimuli subtypes 5s and 8 had a sustained output and subtypes 5f, 6, 7 and XBCs had a transient effect. The different temporal coding of these outputs is expected to have a significant effect on the next level of processing.
This is the first study to show how sub-type dependent temporal coding occurs in the mouse retina. Using improved techniques and better technology the results of this experiment help to further our understanding of parallel processing in the visual system. Further explaining how bipolar cells begin to integrate visual information and pass this on to ganglion cells and then to higher levels of processing. With further experimentation signal modulation in other subtypes, as well as horizontal and amacrine cells, will help us to understand how light is encoded by the retina before it is sent to the brain.
Ichinose T, Fyk-Kolodziej B, Cohn J (2014) Roles of ON cone bipolar cell subtypes in temporal coding in the mouse retina The Journal of Neuroscience doi: 10.1523/JNEUROSCI.3965-13.2014