NEUROBIOLOGY B6

• The dorsal pathway is known as the where pathway
• The dorsal pathway determines object location
• The ventral pathway is known as the what pathway
• The ventral pathway determines object recognition
• Magnocellular LGN neurons project to layer IVCa
• Parvocellular LGN neurons project to later IVCb
• Koniocellular LGN axons bypass layer IV to make synapses in layers II and III
• Information from both eyes is brought together in regions superficial to IVC
• Ocular Dominance

  • Phenomenon where one eye has a greater influence on visual perception and processing compared to the other eye
  • Information from ipsilateral and contralateral eye is kept separate with ocular dominance columns
• Cortical Magnification

  • Uneven representation of different parts of the visual field in the primary visual cortex
  • Central parts of the visual field are over-represented in the cortex compared to the periphery
• Orientation Selectivity

  • The ability of neurons in the visual system to respond selectively to the orientation of visual stimuli oriented in the preferred direction
  • Orientation selective neurons are aligned in columns of the cortex
• Area V1

  • Contains blobs
  • Sensitive to surfaces rather than edges
• Area V2

  • Contains thin and thick stripes
• Blobs

  Bundle of colour-sensitive neurons centred on an ocular dominance column of layer IV
• Thick stripes are involved in

  • Direction movement
  • Binocular disparity
  • Illusory contours
• Thin stripes are specialised for colour
• Simple Cells

  • Respond best to bar of light oriented along length of receptive field
  • Varying receptive fields of magnocellular or parvocellular pathway origins
  • There are more vertically and horizontally oriented simple cells than others
• Complex Cells

  • Large receptive field without fixed excitatory or inhibitory zones
  • Orientation selective and respond most strongly to motion perpendicular to their axis
• Long-Range Horizontal Connections

  • Refer to lateral connections between neurons that extend over relatively large distances within the cortical area
  • Allow communication between non-adjacent neurons within the same cortical region
  • Functions: contour integration, spatial pooling, and contextual modulation of neural responses
  • Crucial in integrating and processing visual information across the visual field
• End-Stopped/Hypercomplex Cells

  • Orientation selectivity and position invariance properties but includes a strong inhibitory area at one end of its bar shaped receptive field
  • These cells will respond to bars of light anyway in its large receptive field
  • Also responds to moving corners or angles
• Direction Selectivity

  Neurons in the visual system responding preferentially to stimuli moving in specific directions

  • Neurons exhibit higher firing rates in response to motion in their preferred direction
  • This selectivity allows the brain to detect and process motion information efficiently
  • Crucial role in tracking moving objects and perceiving motion in the environment
• Features of Direction Selectivity in Neuronal Circuit

  • Spatial and temporal filtering
  • Asymmetric connectivity patterns
  • Feedback inhibition
  • Differential sensitivity to motion features
• Spatial and Temporal Filtering

  Neurons integrate inputs with specific spatial and temporal patterns to detect motion in particular directions
• Asymmetric Connectivity Patterns

  Neuronal circuits exhibit structural biases that enhance responses to motion in specific directions
• Feedback Inhibition

  Inhibitory signals undergo changes over time to refine directional selectivity based on experience
• Differential Sensitivity to Motion Features

  Neurons selectively respond to specific motion characteristics, such as direction or velocity
• Binocular Receptive Fields

  Two receptive fields, one for each eye
• Disparity

  • Difference in location of visual stimuli on the retinas of the left and right eyes
  • Helps brain infer distance of objects and perceive depth in visual scene
  • The brain compares the position of visual stimuli on the retinas of the left and right eyes
• Plasticity and Learning

  • Behavioural experiments showed that kittens reared within only a vertical environment detected mostly vertical objects (and vice versa)
  • More colinear segments more easily detected
  • V1 neuronal responses grow correspondingly
  • After practice, a line with fewer segments stands out more easily and responses in V1 also increase
  • Adaptation is achieved via synaptic changes over time