4.3 Visual system: from retina to cortex

The visual system is a complex network that processes light into meaningful information. From the retina to the cortex, it involves specialized cells and structures that work together to create our visual experience. This intricate system allows us to perceive color, motion, and depth.

Understanding the visual system is crucial for grasping how we interpret the world around us. It showcases the brain's ability to transform raw sensory input into complex perceptions, highlighting the sophisticated nature of our sensory systems.

Retina Structure and Function

Photoreceptor Cells and Visual Processing

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• The retina is a light-sensitive layer at the back of the eye that contains photoreceptor cells (rods and cones) and performs initial processing of visual information
• Rods are responsible for low-light and peripheral vision, enabling vision in dim conditions (nighttime)
• Cones are responsible for color vision and visual acuity, allowing for detailed perception of objects and colors (daytime vision)
• The fovea is a small region in the center of the retina that contains a high density of cones and is responsible for high-acuity vision, enabling tasks such as reading and facial recognition

Retinal Cells and Signal Transmission

• Bipolar cells, horizontal cells, and amacrine cells in the retina perform initial processing and integration of visual information before sending signals to the brain via the optic nerve
  • Bipolar cells receive input from photoreceptors and transmit signals to ganglion cells
  • Horizontal cells provide lateral inhibition, enhancing contrast and edge detection
  • Amacrine cells modulate the activity of bipolar and ganglion cells, contributing to complex visual processing
• Ganglion cells in the retina receive input from bipolar cells and transmit action potentials to the brain, with different types of ganglion cells (M and P cells) carrying different aspects of visual information
  • M (magnocellular) cells are sensitive to motion and low-contrast stimuli
  • P (parvocellular) cells are sensitive to color and high-contrast stimuli

Retinotopic Organization in Vision

Topographic Mapping of Visual Field

• Retinotopic organization refers to the spatial arrangement of visual information in the brain that corresponds to the layout of the retina
• The visual field is mapped onto the primary visual cortex (V1) in a topographic manner, with adjacent points in the visual field represented by adjacent neurons in V1
• The fovea, which is responsible for central vision, is overrepresented in the visual cortex compared to the peripheral regions of the retina, reflecting its importance in high-acuity vision

Preservation of Spatial Relationships

• Retinotopic organization is maintained in the lateral geniculate nucleus (LGN) and other early visual areas, allowing for the preservation of spatial relationships in visual processing
• This organization enables the brain to efficiently process and integrate visual information while maintaining the relative positions of objects in the visual field
• Retinotopic maps in higher-order visual areas become increasingly complex and abstract, reflecting the processing of more intricate visual features and object representations

Lateral Geniculate Nucleus and Primary Visual Cortex

Lateral Geniculate Nucleus (LGN) Organization and Function

• The lateral geniculate nucleus (LGN) is a thalamic relay station that receives input from the retina and sends projections to the primary visual cortex (V1)
• The LGN is organized into six layers, with the left and right eyes' inputs segregated into different layers, allowing for the integration of binocular information
• The magnocellular (M) and parvocellular (P) pathways, which carry different aspects of visual information (motion, color), remain segregated in the LGN, maintaining parallel processing of visual features

Primary Visual Cortex (V1) Structure and Processing

• The primary visual cortex (V1) is the first cortical area to receive visual input and is organized into six layers, with different layers processing different types of visual information
• V1 neurons exhibit orientation selectivity, responding preferentially to stimuli with specific orientations (vertical, horizontal, oblique), enabling the detection of edges and contours
• Ocular dominance columns in V1 alternate between input from the left and right eyes, allowing for the integration of binocular information and the perception of depth
• V1 performs initial processing of visual features such as edges, contours, and basic shapes before sending information to higher-order visual areas for more complex analysis

Higher-Order Visual Areas for Complex Processing

Ventral and Dorsal Visual Streams

• Higher-order visual areas, such as V2, V4, and the inferotemporal cortex (IT), process increasingly complex aspects of visual information
• The ventral visual stream, which includes areas like V4 and IT, is involved in object recognition and visual memory, processing features such as color, shape, and faces
  • V4 is sensitive to color and complex shapes, contributing to object recognition
  • IT contains neurons that respond selectively to specific objects or categories (faces, hands)
• The dorsal visual stream, which includes areas like the middle temporal (MT) and posterior parietal cortex, is involved in processing motion, depth, and spatial relationships for guiding actions
  • MT is sensitive to motion and direction, playing a crucial role in perceiving moving stimuli
  • Posterior parietal cortex integrates visual, proprioceptive, and motor information for spatial awareness and guiding actions

Receptive Field Properties and Top-Down Modulation

• Higher-order visual areas exhibit larger receptive fields and increased selectivity for complex visual stimuli compared to early visual areas, enabling the processing of more abstract and global visual information
• Feedback connections from higher-order areas to earlier visual areas allow for top-down modulation of visual processing based on attention, expectation, and memory
  • Attention can enhance the responses of neurons in early visual areas to relevant stimuli
  • Prior knowledge and expectations can influence the interpretation of ambiguous visual input
• The interaction between bottom-up sensory processing and top-down cognitive influences enables the flexible and context-dependent perception of the visual world