Koji Ikezoe, Yoshiya Mori, Kazuo Kitamura, Hiroshi Tamura and Ichiro
Relationship between Orientation Map and Strength of Orientation Tuning in Macaque V1 revealed by in vivo 2-photon calcium imaging
In the primary visual cortex (V1), neurons are tuned to orientation of bar
or grating presented in the visual field. The strength of orientation tuning
varies among neurons. Neurons with similar preferred orientations are clustered
and arranged across cortex systematically according to their preferred orientation.
Neurons with similar tuning strength are also clustered. However, the relationship
between these two functional architectures is unclear and controversial. Some
researchers reported that tuning strength of neurons was weaker in a heterogeneous
region about preferred orientation such as orientation pinwheels than that
in a homogenous region such as orientation domains. Others reported that there
was no difference in tuning strength between neurons in a heterogeneous region
and in a homogenous region. One reason for the disagreeing results is low spatial
resolution of their recording techniques including extracellular single-unit
recording and intrinsic signal optical imaging. Low spatial resolution may
cause incorrect estimation of heterogeneity of preferred orientation in a local
region and of relative location of neuron in the region.
Here, we applied in vivo 2-photon calcium imaging techniques to V1 of anesthetized macaque monkeys to investigate the relationship between strength of orientation tuning of neurons and a local structure of orientation preference map. In vivo 2-photon calcium imaging techniques has the necessary spatial resolution to monitor individual neurons' positions and their visual responses. We found that although neurons with similar preferred orientations were clustered in most part of the cortex, neurons were surrounded by neurons with heterogeneous orientation preferences in some regions. In regions where preferred orientation of neurons rapidly changes across the cortex, neurons had low orientation selectivity. Neurons in regions where neurons shared similar preferred orientation had both low and high orientation selectivity. Thus, orientation selectivity is partially predictable from the local structure of the orientation map.
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