1Institute of Ophthalmology,University College London, London, UK
2Smith-Kettlewell Eye Research Institute, San Francisco, USA
3Jules Stein Eye Institute, University of California Los Angeles, Los Angeles, USA
An extensive network of lateral connections exists within primary visual cortex (V1), yet little is known about its functional contributions because of difficulties in measuring connectivity. One hypothesis is that the spatial extent of the lateral input to each cell is modified by the level of contrast in the visual stimulus. This hypothesis is supported by the well-characterized reduction in spatial summation of individual cells with increasing stimulus contrast.
We measured V1 lateral connectivity during spontaneous activity and with different stimulus contrasts. We recorded local field potentials (LFP) and spikes across a 10x10 electrode array in anesthetized monkeys and cats. We assessed connectivity by computing the average LFP waveforms across the entire array, triggered by spikes at a given reference electrode.
We find that both LFP amplitude and time-to-peak depend on the location of the reference spikes: 1) the amplitude falls with distance in an exponential fashion and 2) the time delay increases linearly with distance, with a speed of 0.2-0.4 m/s. This speed of propagation is consistent with measurements in horizontal connections within V1 and strongly suggests that the strength of correlations between the LFP and spikes measures lateral connectivity.
Stimulus contrast strongly modulates the spatial extent of V1 lateral connectivity. When transitioning from low to high contrast, the rate of spatial decay of the spike-triggered LFP amplitude away from spikes is greatly reduced ( 5 fold in cats, 3 fold in monkeys). These data are consistent with the reduction of spatial summation with increasing contrast, and show that the spatial footprint of the lateral connectivity is greatly reduced for high vs. low contrast stimuli.
We conclude that the influence of lateral connectivity on cortical responses is not constant but rather is modulated by the sensory input, being stronger when sensory stimulation is weak and strongest when the stimuli are absent.