Because our brains do such a remarkable job of representing the world around us, it is easy to forget that vision, like all of our sensations, is the product of complex patterns of electrical activity in large populations of neurons. Neuroscientists have a good understanding of how single neurons respond to visual stimuli, and have used this information to predict the behavior of large populations of neurons. But the best way to determine the pattern of population activity evoked by sensory stimulation would be to visualize it directly, an approach made possible with recent developments in brain imaging technology. Using these techniques, a team of scientists lead by Dr. David Fitzpatrick, CEO and scientific director of the Max Planck Florida Institute, examined what occurs in the visual cortex – the part of the brain that processes visual information – when an object being viewed abruptly changes from moving in one direction to moving in another direction. The results published in the September 7 issue of The Journal of Neuroscience were surprisingly different from what was expected from single neuron recordings, and demonstrate the power of brain imaging for understanding mechanisms of neural circuit function.
The researchers used voltage-sensitive dye imaging to literally capture neural circuits in the act of responding to a visual stimulus that abruptly changed its direction of motion. When objects moved in a constant direction, the relative responses from the entire population of cells accurately reflected the object’s direction; but when the objects abruptly changed direction, the population response initially failed to accurately encode the object’s direction of motion. In fact, the population response signaled a sequence of different motion directions that were not present in the stimulus before arriving at a response consistent with the new direction. The complex population dynamics associated with a change in motion direction could be accounted for by considering only the delay and persistence of neuronal response to stimulus change, a result that was not anticipated from single neuron recordings.
The population response to changes in stimulus direction suggests that there would be perceptual distortions associated with rapid changes in direction, and this is indeed the case. For example, an object that undergoes an instantaneous change in direction is perceived as having a curved trajectory, consistent with the sequence of directions that are evident in the population response. “While population coding mechanisms may place constraints on the accuracy with which abrupt changes in direction of motion can be represented by cortical circuits,” Fitzpatrick said “the short lived distortions in visual perception are likely an acceptable tradeoff that balances the need for accuracy with the innumerable advantages that are afforded by distributed coding mechanisms.”