Cosyne 2007 Workshops
February 26-27, 2007
The Canyons, Utah
Speaker Name
Matthew A. Smith, Postdoctoral Fellow, Center for the Neural Basis of Cognition, Carnegie Mellon University
Talk Title
Stimulus and distance dependence of neuronal correlation in macaque primary visual cortex
Talk Abstract
Common synaptic input is thought to give rise to both correlated trial-to-trial response variability and synchronous spiking in nearby cortical neurons. As a way to determine the mechanisms the source of this common input, we measured how correlation between neurons depends on the similarity of their tuning properties, the cortical distance between them, and the orientation of the stimulus.
We recorded from the primary visual cortex (V1) of anesthetized, paralyzed macaque monkeys using two techniques: (1) pairs of electrodes spaced 300 to 500 microns apart, and (2) A 100-electrode array. The array consisted of a 10 x 10 grid with 400 micron spacing between adjacent electrodes. Electrode length was 1.0 mm and the array was implanted 0.6 mm into cortex, resulting in superficial layer recordings. We presented sinusoidal grating stimuli drifting in different directions to elicit neuronal responses.
We measured correlation on two different time scales. Spike count correlation (rsc, or "noise" correlation) was measured over the course of the entire stimulus presentation (1.28 seconds). Synchrony (precise temporal correlation) was measured by examining the spike-train cross-correlogram (CCG) peak height near zero time lag. We found that spike count correlation did not change with stimulus orientation. Although rsc was smaller for widely separated neurons, it remained significantly above zero even at separations greater than 4 mm. The peak height of the CCG was strongest for the orientation that produced the best response for the pair of neurons. Similar to spike count correlation, CCG peak height decreased with distance, but at a faster rate.
Together, our results suggest that sharp synchrony and spike count correlation on long timescales arise from different mechanisms. The dependence of synchrony on distance and stimulus orientation suggests it is primarily (but not exclusively) driven by common input arising from short-range cortical circuitry. The spatial extent of spike count correlation and its lack of dependence on stimulus orientation suggest it is mediated by connections which cover large regions of cortex.
