= Primary visual cortex
{title2=V1}
The <primary visual cortex>, often abbreviated V1, is an early cortical stage of visual processing. Its <neurons> have <receptive fields>: changing a stimulus in a particular part of the visible scene can change their <spike trains>. <Orientation selectivity> adds a further distinction: at the same location, a properly aligned bar can evoke a strong response while a rotated bar evokes little. <Simple cells> make the dependence on local position and <visual contrast> especially clear. See the original experiments under <orientation selectivity> and <simple cell>.
In their 1962 study, David Hubel and Torsten Wiesel recorded from forty cats under thiopental anesthesia, maintained light anesthesia while monitoring cortical electrical activity, and used succinylcholine to stabilize the eyes. White-light patterns were projected onto a screen $1.5\,\mathrm m$ away. <Extracellular single-unit recording> with tungsten <electrodes> sampled individual units; <receptive fields> were mapped separately for each eye. This connected controlled stimulus geometry to individual <spike trains>. https://www.gatsby.ucl.ac.uk/teaching/courses/tn1-2020/additional/systems/JPhysiol-1962-Hubel-106-54.pdf[Hubel and Wiesel, Receptive fields, binocular interaction and functional architecture in the cat's visual cortex (1962), pp. 107–108: methods]{ref}
These acute, anesthetized-cat recordings do not directly measure human perception or responses during awake behavior. The useful idea is a local description of structure: a population can distinguish where a feature lies, which way it is aligned, and which side is brighter. The <signed spatial filter model of a simple cell> turns that idea into a computable example. One <neuron>'s preferred stimulus does not establish recognition of an entire object. Likewise, <orientation selectivity> in one <neuron> and an orderly map of preferences across <primary visual cortex> are different findings: rodents can have strong <orientation selectivity> without the orderly arrangement typical of some other <species>. https://pubmed.ncbi.nlm.nih.gov/17720489/[Dario L. Ringach, Specificity and randomness in the visual cortex (2007): authored review of species differences]{ref} The existing <correlation-based development of visual cortical maps> and <orientation-map self-organization> sections describe hypotheses about how organization develops.
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