Abstract
How the brain preserves information about more than one stimulus at a time remains poorly understood. We recently showed that when more than one stimulus is present, single neurons may fluctuate between coding one vs. the other(s) across some time period. A critical unanswered question is whether and how any such fluctuations are coordinated across the neural population. Here, we analyzed the spike count (“noise”) correlations observed between pairs of visual cortex (V1, V4) neurons under a variety of conditions. We report that when two separate grating stimuli are presented simultaneously, distinct distributions of positive and negative correlations emerge in V1, depending on whether the two neurons in the pair both respond more strongly to the same vs. different individual stimuli. Neural pairs that shared the same stimulus preference were more likely to show positively correlated spike count variability whereas those with different preferences were more likely to show negative correlations, suggesting that the V1 population response to one particular stimulus may be enhanced over the other on any given trial. Distinct distributions of spike count correlations based on tuning preferences were also seen in V4 for adjacent stimuli, but were not observed in either structure for single stimuli or when the two gratings were superimposed and formed a single plaid. The effects were most pronounced among pairs of neurons that showed fluctuating activity. These findings support the interpretation that the pattern of correlated fluctuations is related to the segregation of individual objects in the visual scene.
Significance Statement How the brain separates information about multiple objects despite overlap in the neurons responsive to each item is not well understood. Here we show that pairs of neurons in primate V1 and V4 show unique patterns of correlated firing when there are multiple objects in the visual scene. These patterns were most pronounced in neurons that showed fluctuating activity consistent with switching between encoding each object across time (i.e. time division multiplexing). Both strongly positive and strongly negative correlations were observed, depending on whether the neurons in the pair exhibited similar or different stimulus preferences. These patterns were absent when only one object was presented, suggesting that they may play a key role in preserving information about multiple items.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Additional data from V4 has been included; the author list has been expanded.
