While some things may be ‘as simple as black and white,’ this has not been the case for the circuits in the brain that make it possible for a person to distinguish black from white. However, researchers are still unsure as to how this information is encoded by neural circuits in the visual cortex, a part of the brain that plays a critical role in building the neural representations that are responsible for sight. Now, researchers at the Max Planck Institute have made it possible for the first time to visualize the activity of hundreds of neurons simultaneously in the living brain to quantify the responses of neurons in the ferret visual cortex to light and dark stimulation. The opensource study is published in the journal Neuron.
Previous studies show that neurons in the retina that provide information to higher centers in the brain respond selectively to light vs dark stimuli. These are ‘ON’ cells that respond selectively to light stimuli and ‘OFF’ cells that respond selectively to dark stimuli, which were shown to form separate parallel channels relaying information to circuits in visual cortex. Based on recording the responses of single cortical neurons, it appeared that as soon as the ON and OFF channels entered the cortex, they converged onto single neurons. Further stages in cortical processing were thought to lead to more and more mixing of the ON and OFF signals, so that individual neurons responded similarly to both dark and light stimuli. However, these results raised the quandary that if the responses of single cortical neurons to dark and light are ambiguous, how does the brain allow a person to perceive these differences. The current study was able to visualize neurons that responded to these stimuli to identify patches of neurons that responded preferentially to dark vs light stimulation.
The current study used wide-field epifluorescence and two-photon imaging to demonstrate a robust modular representation of luminance polarity (ON or OFF) in the superficial layers of the ferret primary visual cortex. Results show polarity-specific domains with both uniform changes in luminance and single light/dark edges, and include neurons selective for orientation and direction of motion. Data findings show the integration of orientation and polarity preference is evident in the selectivity and discrimination capabilities of most layer 2/3 neurons.
Results show that the cortical neurons that respond selectively to the orientation of edges or to the direction of stimulus motion also responded preferentially to dark vs light stimuli. The lab conclude that polarity selectivity is an integral feature of layer 2/3 neurons, ensuring that the distinction between light and dark stimuli is available for further processing in downstream areas.
The team surmise that their findings show information about dark and light is preserved in the responses of most neurons in visual cortex, and it is an integral part of the neural code that cortical circuits use to represent the visual world. For the future, the researchers state that the next challenge is to understand the precise patterns of synaptic connections that enable cortical circuits to construct this modular representation of black and white.
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