1.2 Primary visual cortex

The primary visual cortex (V1) is the first stop for visual information in the brain. Located in the occipital lobe, V1 processes basic features like edges, contours, and shapes. It's crucial for conscious visual perception and has been extensively studied in humans and animals.

V1's unique structure and organization allow it to analyze various aspects of visual input. From its retinotopic map to its specialized neurons for orientation, spatial frequency, and color, V1 lays the foundation for complex visual processing. Understanding V1 is key to grasping how we perceive and interpret the visual world.

Primary visual cortex (V1)

• V1 is the first cortical area in the visual processing hierarchy receives visual information from the lateral geniculate nucleus (LGN) of the thalamus
• V1 is crucial for conscious visual perception processes basic features of the visual scene such as edges, contours, and simple shapes
• V1 activity has been studied extensively in humans and animal models provides insights into the neural basis of visual perception and its relationship to artistic representations

Location of V1 in the brain

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• V1 is located in the occipital lobe at the posterior pole of the cerebral cortex
• V1 surrounds the calcarine sulcus in the medial surface of the occipital lobe
• V1 is also known as Brodmann area 17 or striate cortex due to the distinctive stripe of myelinated fibers in layer 4 (line of Gennari)
• V1 receives direct input from the lateral geniculate nucleus (LGN) of the thalamus via the optic radiation
• V1 projects to higher-order visual areas in the ventral and dorsal streams of visual processing (V2, V3, V4, MT)

Retinotopic organization of V1

• V1 is retinotopically organized meaning that neighboring cells in V1 respond to neighboring locations in the visual field
• The retinotopic map in V1 is inverted and magnified central vision (fovea) occupies a disproportionately large area compared to peripheral vision
• The upper and lower visual fields are represented in the lower and upper banks of the calcarine sulcus, respectively
• The left and right visual hemifields are represented in the right and left hemispheres, respectively
• The retinotopic organization of V1 can be mapped using functional MRI (fMRI) and visualized on inflated or flattened cortical surfaces

Layers of V1

• V1 has a six-layered structure typical of neocortex with distinct input, output, and processing layers
• Layer 4 is the main input layer receives feedforward input from the LGN
  • Layer 4C receives magnocellular (M) input from the LGN
  • Layer 4Cβ receives parvocellular (P) input from the LGN
• Layers 2/3 and 5/6 are the main output layers send projections to higher-order visual areas and subcortical structures
• Layers 2/3 and 5/6 also contain intrinsic connections within V1 mediate lateral interactions and feedback from higher-order areas

Simple vs complex cells in V1

• Simple cells in V1 respond optimally to oriented edges or bars at a specific location in the visual field
  • Simple cells have elongated receptive fields with distinct excitatory and inhibitory subregions
  • Simple cells exhibit linear spatial summation of inputs from the LGN
• Complex cells in V1 also respond to oriented edges or bars but are less sensitive to the exact position or phase of the stimulus
  • Complex cells have larger receptive fields than simple cells and show nonlinear spatial summation
  • Complex cells receive input from multiple simple cells with similar orientation preferences
• The distinction between simple and complex cells was first described by Hubel and Wiesel in their Nobel Prize-winning work on the cat visual cortex

Orientation selectivity of V1 neurons

• Many neurons in V1 are selective for the orientation of edges or gratings in the visual field
• Orientation selectivity emerges from the alignment of LGN inputs to V1 neurons
  • LGN neurons have circular receptive fields and respond to spots of light
  • V1 neurons receive input from multiple LGN neurons arranged in a line resulting in an elongated receptive field
• Orientation selectivity is organized in columns perpendicular to the cortical surface
  • Neurons in a given column prefer similar orientations which change gradually across the cortical surface
• Orientation selectivity is thought to be important for detecting contours and boundaries in the visual scene
• The columnar organization of orientation selectivity in V1 has inspired artistic representations such as the work of Piet Mondrian

Spatial frequency tuning of V1 neurons

• V1 neurons are also selective for the spatial frequency (SF) of gratings or textures in the visual field
• SF selectivity refers to the preference for patterns with a specific spacing or periodicity (e.g., wide vs. narrow stripes)
• SF selectivity arises from the size and spacing of LGN inputs to V1 neurons
  • Neurons with small receptive fields prefer high SFs (fine details)
  • Neurons with large receptive fields prefer low SFs (coarse patterns)
• SF selectivity is organized in columns similar to orientation selectivity
• The range of SFs represented in V1 spans several octaves from low to high
• SF tuning is thought to be important for analyzing the scale and texture of visual objects and scenes

Color processing in V1

• V1 receives input from color-opponent neurons in the parvocellular layers of the LGN
• Color-selective neurons in V1 respond preferentially to specific colors or color contrasts
  • Some neurons respond to red-green or blue-yellow color opponency
  • Other neurons respond to color-luminance edges or color-orientation combinations
• Color selectivity in V1 is organized in patches or blobs in layers 2/3
  • Blobs contain high concentrations of cytochrome oxidase a metabolic enzyme
  • Blobs are surrounded by interblobs which are more selective for orientation than color
• The segregation of color and orientation processing in V1 has been linked to the parallel processing of visual information in the ventral and dorsal streams
• The representation of color in V1 has been studied in the context of color perception and its relationship to art and aesthetics

Contrast sensitivity of V1 neurons

• V1 neurons are sensitive to the contrast or relative luminance of visual stimuli
• Contrast sensitivity refers to the ability to detect small differences in luminance between adjacent regions of the visual field
• V1 neurons exhibit a sigmoidal contrast response function
  • Response increases gradually at low contrasts and saturates at high contrasts
  • The steepest part of the function determines the neuron's contrast threshold
• Contrast sensitivity varies across V1 neurons and is influenced by factors such as:
  • Receptive field size and eccentricity (higher sensitivity for central vision)
  • Spatial frequency tuning (higher sensitivity for optimal SFs)
  • Adaptation to prevailing contrast levels (contrast gain control)
• Contrast sensitivity is thought to be important for detecting edges, textures, and other visual features in low-contrast or noisy environments

Temporal dynamics of V1 responses

• V1 neurons exhibit temporal dynamics in response to visual stimuli
• The temporal response of V1 neurons can be characterized by:
  • Response latency: the delay between stimulus onset and the start of the neural response (typically 30-50 ms)
  • Response duration: the time period over which the neuron remains active (typically 100-200 ms)
  • Response adaptation: the gradual decrease in response amplitude with prolonged stimulation
• The temporal response of V1 neurons is influenced by factors such as:
  • Stimulus contrast (shorter latency and higher peak response for high-contrast stimuli)
  • Stimulus size and position (longer latency and duration for larger or peripheral stimuli)
  • Attention and task demands (modulation of response amplitude and timing by top-down factors)
• The temporal dynamics of V1 responses have been studied using techniques such as single-unit recording, local field potentials, and EEG/MEG in humans

Feedback connections to V1

• V1 receives feedback connections from higher-order visual areas in addition to feedforward input from the LGN
• Feedback connections to V1 originate from areas such as V2, V4, MT, and the frontal eye fields
• Feedback connections are thought to modulate V1 activity based on top-down factors such as:
  • Attention and task relevance (enhancement of responses to attended stimuli)
  • Contextual modulation (integration of local features with global context)
  • Predictive coding (generation of predictions about incoming visual input)
• Feedback connections to V1 have been implicated in phenomena such as:
  • Figure-ground segregation (enhancement of responses to figure vs. background)
  • Perceptual filling-in (interpolation of missing or occluded information)
  • Visual imagery and hallucinations (activation of V1 in the absence of external input)
• The role of feedback connections in V1 has been studied using techniques such as cooling or inactivation of higher-order areas, and neuroimaging of top-down modulation

Role of V1 in visual perception

• V1 is necessary but not sufficient for conscious visual perception
• Lesions or inactivation of V1 can cause blindness or visual field defects (e.g., scotomas)
• Activation of V1 alone does not guarantee conscious perception (e.g., in blindsight or visual suppression)
• V1 is thought to provide the "building blocks" of visual perception by extracting basic features such as edges, contours, and colors
• Higher-order areas build upon the information from V1 to construct more complex representations of objects, scenes, and categories
• The role of V1 in visual perception has been debated in theories such as:
  • Hierarchical models (V1 as a low-level feature detector)
  • Predictive coding models (V1 as a comparator of bottom-up and top-down signals)
  • Global workspace models (V1 as a contributor to conscious access)
• The relationship between V1 activity and visual perception has been studied using techniques such as binocular rivalry, visual masking, and transcranial magnetic stimulation (TMS)

V1 plasticity and learning

• V1 exhibits plasticity and learning in response to changes in visual experience
• Plasticity in V1 can occur at different timescales:
  • Short-term plasticity (seconds to minutes) such as adaptation and perceptual learning
  • Long-term plasticity (hours to days) such as ocular dominance plasticity and perceptual learning
• Plasticity in V1 is influenced by factors such as:
  • Developmental stage (higher plasticity in early life, critical periods)
  • Sensory experience (deprivation, enrichment, training)
  • Neuromodulatory systems (acetylcholine, norepinephrine, dopamine)
• Examples of V1 plasticity and learning include:
  • Ocular dominance plasticity (changes in eye preference after monocular deprivation)
  • Perceptual learning (improved discrimination of visual features with training)
  • Crossmodal plasticity (recruitment of V1 by other sensory modalities in blind individuals)
• V1 plasticity and learning have been studied using techniques such as single-unit recording, optical imaging, and neuroimaging before and after visual experience manipulation

Disorders affecting V1 function

• V1 dysfunction can lead to a variety of visual disorders and impairments
• Examples of disorders affecting V1 function include:
  • Amblyopia (lazy eye) - reduced visual acuity and contrast sensitivity due to abnormal visual experience during development
  • Strabismus (misaligned eyes) - disruption of binocular integration and stereopsis due to misalignment of the eyes
  • Achromatopsia (color blindness) - inability to perceive colors due to loss of cone photoreceptors or color-selective neurons in V1
  • Visual agnosia (inability to recognize objects) - disruption of object recognition despite intact V1 function
  • Visual hallucinations - perception of visual stimuli in the absence of external input, often associated with V1 hyperactivity or disinhibition
• V1 disorders can have significant impacts on quality of life and daily functioning
• Treatment of V1 disorders may involve optical correction, visual training, or neuromodulation techniques such as TMS or neurofeedback

Artistic representations of V1 activity

• V1 activity has inspired various artistic representations and styles
• Examples of artistic representations of V1 activity include:
  • Op art (optical art) - use of geometric patterns and illusions to create perceptual effects related to V1 processing (e.g., Bridget Riley, Victor Vasarely)
  • Pointillism - use of small, distinct dots of color to create the impression of form and texture, related to the color processing in V1 blobs (e.g., Georges Seurat, Paul Signac)
  • Abstract expressionism - use of gestural brushstrokes and color fields to evoke emotional responses, related to the orientation and color selectivity of V1 neurons (e.g., Jackson Pollock, Mark Rothko)
• Artistic representations of V1 activity can provide insights into the perceptual and aesthetic aspects of visual processing
• Collaborations between artists and neuroscientists have explored the relationship between V1 activity and artistic expression, such as the work of Margaret Livingstone and Bevil Conway on the neural basis of color perception in art

V1 in non-human primates vs humans

• V1 has been studied extensively in non-human primates such as macaques and marmosets
• V1 in non-human primates shares many similarities with human V1 in terms of:
  • Retinotopic organization and magnification of central vision
  • Orientation and spatial frequency selectivity
  • Color processing in cytochrome oxidase blobs
  • Laminar organization and feedforward/feedback connectivity
• However, there are also some differences between non-human primate and human V1, such as:
  • Size and position of V1 relative to other visual areas
  • Density and distribution of different cell types and receptors
  • Degree of plasticity and learning in response to visual experience
• Non-human primate studies have provided valuable insights into the structure and function of V1, but caution is needed when extrapolating to humans
• Comparative studies of V1 across species can inform our understanding of the evolution and development of visual processing in the brain