Ikezoe, K., Mori, Y., Kitamura, K., Fujita, I.
Irregularity in arrangement of orientation-selective neurons in monkey V1 revealed by in vivo 2-photon calcium imaging.
Single-unit recordings and conventional optical imaging techniques have revealed that neurons tuned to similar orientations of visually presented bars or gratings are clustered and form columnar structures in the monkey primary visual cortex (V1). The preferred orientation of the neurons shifts systematically across the cortical surface. However, these conclusions come from conventional recording techniques with spatial resolutions of 0.1 mm, which is insufficient to describe the arrangement of the orientation-selective neurons at the cellular level. Furthermore, single-unit recordings and signals recorded by conventional optical imaging are less sensitive to neurons with low firing rates. Analysis of the orientation map at single cellular resolution and unbiased sampling of neurons, irrespective of the firing rate, is necessary to consider the function and development of the map. Here, we examined the arrangement of neurons according to their preferred orientation at single-cell spatial resolution by using in vivo calcium imaging with a 2-photon microscope in V1 of analgesized and immobilized macaque monkeys. We presented a set of drifting square-wave gratings as visual stimuli. Of the 714 neurons from 2 monkeys, 354 (49.6%) showed selectivity for the orientation of the gratings. To quantify the smoothness of the changes in the preferred orientation across the cortical surface, we calculated the correlation between the absolute preferred orientation difference in two given orientation-selective neurons and their spatial separation along the single direction where the correlation took its maximal value. In all 7-recorded regions (< 250 Êm x 250 Êm), absolute difference in the preferred orientation correlated with the physical separation between the neurons (p < 0.01, Spearman's rank correlation; 0.36 } 0.16, mean } standard deviation), meaning that the preferred orientation was mapped systematically as a whole. However, the absolute difference of nearby (less than 50 Êm) neurons was 21.7 (median), and neurons in 26% of the nearby neuron pair had preferred orientations that differed by more than 45. Based on the arrangement of the orientation-selective neurons, we inferred that the recorded regions did not include pinwheel centers. In summary, the preferred orientation of neurons is systematically mapped at a macroscopic level in monkey V1. Nevertheless, the orientation map contains markedly irregular transitions at the cellular level.