In mammals, the visual cortex of the brain is the area of the cerebral cortex that processes visual information. The visual cortex is located in the occipital lobe, located at the rear of the head. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and then reaches the visual cortex. The area of the visual cortex that receives the sensory input from the lateral geniculate nucleus is the primary visual cortex, also known as visual area 1 (V1), Brodmann area 17, or the striate cortex. The extrastriate areas consist of visual areas 2, 3, 4, and 5 (also known as V2, V3, V4, and V5, or Brodmann area 18 and all Brodmann area 19).
Both hemispheres of the brain include a visual cortex; the visual cortex in the left hemisphere receives signals from the right visual field, and the visual cortex in the right hemisphere receives signals from the left visual field.
Introduction The primary visual cortex (V1) is located in and around the calcarine fissure in the occipital lobe. Each hemisphere's V1 receives information directly from its ipsilateral lateral geniculate nucleus that receives signals from the contralateral visual hemifield.
Neurons in the visual cortex fire action potentials when visual stimuli appear within their receptive field. By definition, the receptive field is the region within the entire visual field that elicits an action potential. But, for any given neuron, it may respond best to a subset of stimuli within its receptive field. This property is called neuronal tuning. In the earlier visual areas, neurons have simpler tuning. For example, a neuron in V1 may fire to any vertical stimulus in its receptive field. In the higher visual areas, neurons have complex tuning. For example, in the inferior temporal cortex (IT), a neuron may fire only when a certain face appears in its receptive field.
Furthermore, the arrangement of receptive fields in V1 is retinotopic, meaning neighboring cells in V1 have receptive fields that correspond to adjacent portions of the visual field. This spatial organization allows for a systematic representation of the visual world within V1. Additionally, recent studies have delved into the role of contextual modulation in V1, where the perception of a stimulus is influenced not only by the stimulus itself but also by the surrounding context, highlighting the intricate processing capabilities of V1 in shaping our visual experiences.
The visual cortex receives its blood supply primarily from the calcarine branch of the posterior cerebral artery.
The size of V1, V2, and V3 can vary three-fold, a difference that is partially inherited.
Psychological model of the neural processing of visual information
Ventral-dorsal model V1 transmits information to two primary pathways, called the ventral stream and the dorsal stream. The ventral stream begins with V1, goes through visual area V2, then through visual area V4, and to the inferior temporal cortex (IT cortex). The ventral stream, sometimes called the "What Pathway", is associated with form recognition and object representation. It is also associated with storage of long-term memory. The dorsal stream begins with V1, goes through Visual area V2, then to the dorsomedial area (DM/V6) and middle temporal area (MT/V5) and to the posterior parietal cortex. The dorsal stream, sometimes called the "Where Pathway" or "How Pathway", is associated with motion, representation of object locations, and control of the eyes and arms, especially when visual information is used to guide saccades or reaching.
The what vs. where account of the ventral/dorsal pathways was first described by Ungerleider and Mishkin.
More recently, Goodale and Milner extended these ideas and suggested that the ventral stream is critical for visual perception whereas the dorsal stream mediates the visual control of skilled actions. It has been shown that visual illusions such as the Ebbinghaus illusion distort judgements of a perceptual nature, but when the subject responds with an action, such as grasping, no distortion occurs.
Work such as that from Franz et al. suggests that both the action and perception systems are equally fooled by such illusions. Other studies, however, provide strong support for the idea that skilled actions such as grasping are not affected by pictorial illusions and suggest that the action/perception dissociation is a useful way to characterize the functional division of labor between the dorsal and ventral visual pathways in the cerebral cortex.
Primary visual cortex (V1)
The primary visual cortex is the most studied visual area in the brain. In mammals, it is located in the posterior pole of the occipital lobe and is the simplest, earliest cortical visual area. It is highly specialized for processing information about static and moving objects and is excellent in pattern recognition. Moreover, V1 is characterized by a laminar organization, with six distinct layers, each playing a unique role in visual processing. Neurons in the superficial layers (II and III) are often involved in local processing and communication within the cortex, while neurons in the deeper layers (V and VI) often send information to other brain regions involved in higher-order visual processing and decision-making.
Research on V1 has also revealed the presence of orientation-selective cells, which respond preferentially to stimuli with a specific orientation, contributing to the perception of edges and contours. The discovery of these orientation-selective cells has been fundamental in shaping our understanding of how V1 processes visual information.
Furthermore, V1 exhibits plasticity, allowing it to undergo functional and structural changes in response to sensory experience. Studies have demonstrated that sensory deprivation or exposure to enriched environments can lead to alterations in the organization and responsiveness of V1 neurons.
The primary visual cortex, which is defined by its function or stage in the visual system, is approximately equivalent to the striate cortex, also known as Brodmann area 17, which is defined by its anatomical location. The name "striate cortex" is derived from the line of Gennari, a distinctive stripe visible to the naked eye that represents myelinated axons from the lateral geniculate body terminating in layer 4 of the gray matter.
In the case of the striate cortex, the line of Gennari corresponds to a band rich in myelinated nerve fibers, providing a clear marker for the primary visual processing region.
Additionally, the functional significance of the striate cortex extends beyond its role as the primary visual cortex. It serves as a crucial hub for the initial processing of visual information, such as the analysis of basic features like orientation, spatial frequency, and color. The integration of these features in the striate cortex forms the foundation for more complex visual processing carried out in higher-order visual areas. Recent neuroimaging studies have contributed to a deeper understanding of the dynamic interactions within the striate cortex and its connections with other visual and non-visual brain regions, shedding light on the neural circuits that underlie visual perception.
The primary visual cortex is divided into six functionally distinct layers, labeled 1 to 6. Layer 4, which receives most visual input from the lateral geniculate nucleus (LGN), is further divided into 4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives mostly magnocellular input from the LGN, while layer 4Cβ receives input from parvocellular pathways.
The average number of neurons in the adult human primary visual cortex in each hemisphere has been estimated at 140 million. The volume of each V1 area in an adult human is about 5400mm on average. A study of 25 hemispheres from 15 normal individuals with average age 59 years at autopsy found a very high variation, from 4272 to 7027mm for the right hemisphere (mean 5692mm), and from 3185 to 7568mm for the left hemisphere (mean 5119mm), with 0.81 correlation between left and right hemispheres. The same study found average V1 area 2400mm per hemisphere, but with very high variability. (Right hemisphere mean 2477mm, range 1441–3221mm. Left hemisphere mean 2315mm, range 1438–3365mm.)
Function
The initial stage of visual processing within the visual cortex, known as V1, plays a fundamental role in shaping our perception of the visual world. V1 possesses a defined map, referred to as the retinotopic map, which organizes spatial information from the visual field. In humans, the upper bank of the calcarine sulcus in the occipital lobe robustly responds to the lower half of the visual field, while the lower bank responds to the upper half. This retinotopic mapping conceptually represents a projection of the visual image from the retina to V1.
The importance of this retinotopic organization lies in its ability to preserve spatial relationships present in the external environment. Neighboring neurons in V1 exhibit responses to adjacent portions of the visual field, creating a systematic representation of the visual scene. This mapping extends both vertically and horizontally, ensuring the conservation of both horizontal and vertical relationships within the visual input.
Moreover, the retinotopic map demonstrates a remarkable degree of plasticity, adapting to alterations in visual experience. Studies have revealed that changes in sensory input, such as those induced by visual training or deprivation, can lead to shifts in the retinotopic map.
Notably, neurons in V1 have the smallest receptive field size, signifying the highest resolution, among visual cortex microscopic regions. This specialization equips V1 with the ability to capture fine details in the visual input.
In addition to its role in spatial processing, the retinotopic map in V1 is connected with other visual areas, forming a network that contributes to the integration of various visual features and the construction of a coherent visual percept.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). The correspondence between a given location in V1 and in the subjective visual field is very precise: even the 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 (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. 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.