EMBL scientists have found evidence of an unexpected role for retinal cells in pre-processing visual information; their results provide potential opportunity for future prosthetic visual aids.
Scientists have long known that vision is made possible because our brains decode electrical signals that are sent by the retina. However, recent evidence indicates that retinas may do even more. They too may be deciphering what we actually see.
Located at the back of the eye, the retina is a neuronal tissue that receives visual input to the eye and converts it to electrical signals that travel to the brain. Apart from mapping the outside world, the retina extracts visual features, such as colour, contrast, and motion.
The retinal layers contain diverse and specialised sets of cellular components. When light hits the retina, it stimulates photoreceptors, creating an electric signal that is conveyed through other neurons – horizontal, bipolar, and amacrine cells – to the retinal ganglion cells (RGCs).
The RGCs are located on the inner surface of the retina, where they project visual information to the brain via their axons, which make up the optic nerve. These neurons are at the core of the scientists’ recent findings.
RGCs have conventionally been considered as a relay, simply integrating incoming visual signals from the eye and conveying them to the brain. However, recent anatomical studies suggest that RGCs may form a more complex network with other retinal neurons through high-speed communication channels.
Scientists in the Asari Group at EMBL Rome found evidence that RGCs can send feedback signals to other retinal cells, and contribute to local computation of visual stimuli by modulating the output signals from the retina. Their results on such a novel gain control mechanism in the retina have been published in the journal PLOS ONE.
Building on the team’s expertise in computational methods, the scientists developed a biologically inspired mathematical model of the retinal network and derived predictions on how RGCs of the same type modulate each other’s activity. They then conducted experiments to confirm those predictions in mouse and salamander retinas.
“Our brain is a predictive machine,” said Hiroki Asari. “It generates a mental model of the outside world based on past evidence and predicts future sensory inputs. When the prediction differs from actual sensory inputs, the brain uses the discrepancies (or ‘surprises’) to update the mental model and adjust our behaviour accordingly. While the brain cortex is commonly assumed to perform such predictive processing, recent evidence suggests that the retina might compute those visual surprises by some unknown mechanisms. We think that the feedback signalling we found at the level of retinal output may play a key role there.”
Understanding exactly how this feedback pathway helps retinal function creates a way for researchers to potentially develop future prosthetic devices that could faithfully mimic the retina’s visual processing.
The study also shows the benefit of combining experimental and theoretical approaches in neurobiological studies. “Conventionally, scientists run experiments first and then build a model to explain the data,” Asari said. “In contrast, we started with a computational model of the retina and derived predictions on the retinal physiology. Then we tested the prediction by performing electrophysiological experiments. This type of theory-driven approach can be applied to other brain areas to better understand their function and is therefore a promising new direction of research.”
Source article(s)
Feedback from retinal ganglion cells to the inner retina
Vlasiuk A., Asari H.
PLoS One 22 July 2021
10.1371/journal.pone.0254611
Tags: asari, neuroscience, retina, rome, theory at embl, vision
| Online Game Advances Neuroscientific Research The retina is not designed to record the absolute intensity of the light reaching it, but rather to detect the differences in the intensity of the light striking it at different points. | For you to see anything, your eye must first form a precise image of it on your retina. Then the light energy striking your retina must be converted into nerve impulses by the retina's photoreceptor cells. The image can then be processed by your nervous system. This processing does not start in the brain, but instead starts immediately in the retina itself. In fact, anatomists regard the retina as a part of the brain that is located outside it, somewhat the way you may regard your home satellite dish as an integral part of your television receiver.
In addition to this direct pathway from the photoreceptors to the brain, two other kinds of cells contribute to the processing of visual information in the retina. The horizontal cells receive information from the photoreceptors and transmit it to a number of surrounding bipolar neurons. The amacrine cells receive their inputs from the bipolar cells and do the same thing to the ganglion neurons: activate the ones that are in their vicinity. |
The simple cell receptive fields in the primary visual cortex are thought to be the result of the convergence of several adjacent receptive fields of cells in the relay that precedes it, the lateral geniculate nucleus. Note, by the way, that the receptive fields of this nucleus are still circular, like those of its source, the ganglion neurons in the retina. Other cells in the primary visual cortex have "complex" and "hypercomplex" receptive fields with properties that are even more selective. |
The primary visual cortex is the first relay in the visual pathways where information from the two eyes is combined. In other words, a single cell in this cortex may respond just as much to the stimuli presented to one eye as to those presented to the other. |
Layer IV, for example, contains numerous stellate cells, small neurons with dendrites that radiate out around the cell body and receive connections from the lateral geniculate nucleus. Thus this layer specializes largely in receiving information.
Layer I contains very few neurons. It is composed of axons and dendrites from cells in the other layers. With the development of improved staining methods, some of the six layers in the visual cortex have now been classified into sub-layers. |