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{{technical|date=September 2016}}
The first stage of visual processing in the cortex is called V1.
V1 has a very well-defined map (the retinotopic map) of the spatial information in vision. For example, in humans, the upper bank of the calcarine sulcus (in the occipital lobe) responds strongly to the lower half of the visual field (below the center), and the lower bank of the calcarine to the upper half of the visual field. In concept, this retinotopic mapping is a projection of the visual image from the retina to V1.
This retinotopic organization in V1 is crucial for preserving the spatial relationships present in the external world. Neighboring neurons in V1 respond to adjacent portions of the visual field, maintaining a systematic representation of the visual scene. This mapping is not only limited to the vertical axis but extends horizontally, allowing for the preservation of both horizontal and vertical relationships in the visual input.
V1 has a very well-defined map (''the [[retinotopy|retinotopic]] map'') of the spatial information in vision. For example, in humans, the upper bank of the [[calcarine sulcus]] (in the occipital lobe) responds strongly to the lower half of [[visual field]] (below the center), and the lower bank of the calcarine to the upper half of visual field. In concept, this [[retinotopy|retinotopic]] mapping is a projection of the visual image from [[retina]] to V1.<ref name= kepler1604 >Johannes Kepler (1604) Paralipomena to Witelo whereby The Optical Part of Astronomy is Treated (Ad Vitellionem Paralipomena, quibus astronomiae pars optica traditvr, 1604), as cited by A.Mark Smith (2015) From Sight to Light. Kepler modeled the eye as a water-filled glass sphere, and discovered that each point of the scene taken in by the eye projects onto a point on the back of the eye (the retina).</ref> The correspondence between a given location in V1 and in the subjective visual field is very precise: even the [[Blind spot (vision)|blind spots]] of the retina are mapped into V1. In terms of evolution, this correspondence is very basic and found in most animals that possess a V1. In humans and other animals with a [[Fovea centralis|fovea]] ([[Cone cell|cones]] in the retina), a large portion of V1 is mapped to the small, central portion of visual field, a phenomenon known as [[cortical magnification]].<ref>{{cite thesis |last1=Barghout |first1=Lauren |title=On the Differences Between Peripheral and Foveal Pattern Masking |date=1999 |type=Masters |publisher=University of California, Berkeley|location=Berkeley, California}}</ref> Perhaps for the purpose of accurate spatial encoding, neurons in V1 have the smallest [[receptive field]] size (that is, the highest resolution) of any visual cortex microscopic regions.▼
Furthermore, the retinotopic map exhibits a high degree of plasticity, adapting to changes in visual experience. Studies have shown that alterations in sensory input, such as through visual training or deprivation, can lead to shifts in the retinotopic map, reflecting the brain's ability to reorganize in response to environmental demands.<ref>Wu, Fangfang et al. “A Comprehensive Overview of the Role of Visual Cortex Malfunction in Depressive Disorders: Opportunities and Challenges.” Neuroscience bulletin vol. 39,9 (2023): 1426-1438. doi:10.1007/s12264-023-01052-7</ref>
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The tuning properties of V1 neurons (what the neurons respond to) differ greatly over time. Early in time (40 ms and further) individual V1 neurons have strong tuning to a small set of stimuli. That is, the neuronal responses can discriminate small changes in visual [[Orientation (mental)|orientations]], [[spatial frequencies]] and [[color]]s (as in the optical system of a [[camera obscura]], but projected onto [[retina]]l cells of the eye, which are clustered in density and fineness).<ref name= kepler1604 /> Each V1 neuron propagates a signal from a retinal cell, in continuation. Furthermore, individual V1 neurons in humans and other animals with [[binocular vision]] have ocular dominance, namely tuning to one of the two eyes. In V1, and primary sensory cortex in general, neurons with similar tuning properties tend to cluster together as [[cortical column]]s. [[David Hubel]] and [[Torsten Wiesel]] proposed the classic ice-cube organization model of cortical columns for two tuning properties: [[ocular dominance columns|ocular dominance]] and orientation. However, this model cannot accommodate the color, spatial frequency and many other features to which neurons are tuned {{Citation needed|date=November 2011}}. The exact organization of all these cortical columns within V1 remains a hot topic of current research. The mathematical modeling of this function has been compared to [[Gabor transform]]s.{{Citation needed|date=May 2023}}
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