Visual Perception and Memory

• How is visual processing hierarchical?

  processing is hierarchical as the complexity of the visual representation increases throughout the visual pathway
• How does visual processing have functional differentiation?

  at different stages of visual info processing, different neurons/brain regions are used for processing different properties of visual stimuli
• What are simple features of visual processing?

  • light intensity
  • light wavelength
  • 2D position in field
• What are complex visual representation in visual processing?

  • an intergration of visual information made of form, surface, spatial relationships, and movement
  • also intergrated with other sensory modalities
• How do simple features become complex representations?

  combination and elabortion of simple features via parallel channels leads to more complex representations
• Beyond the V1, visual info is passed to...

  the occipital lobe (extrastriate cortex), then the inferior temporal lobe
• What is the occipital lobe made up of?

  primary visual cortex (V1), and the extrastriate cortex (V2, V3, V4, V5)
• What is the inferior temporal cortex made up of?

  posterior IT, and anterior IT
• What is the key feature of the extrastriate cortex?

  • neurons here signal ‘global’ properties of visual scenes and objects rather than ‘component’ properties
  • e.g. there is holistic signalling of dynamic form in V3, colour in V4, and motion in V5
• How is colour actually perceived?

  Perceived colour of an object depends not only on the wavelength reflected by object, but also on wavelength reflected by the surroundings
• Why do we perceive colour this way?

  evolutionary adaptation to retain colour constancy- perceived colour of object does not change when viewed in different lights
• How do neurons in V4 perceive colour?

  • Some neurons in V4 are ‘colour’-sensitive
  • this means they respond to wavelengths in the centre of their receptive field, depending on the wavelengths reflected from the background
• How do neurons in V1 and V2 perceive colour?

  they are only wavelength-sensitive
• What is global/pattern motion vs component motion?

  • selective cells in V1 register component directions of objects
  • cells in V5 register the true, overall direction
• What are visual streams?

  After the V1, visual processing is mediated by 2 streams that are anatomically and functionally different (dorsal and ventral)
• What is the dorsal stream?

  Visual pathway specialized for spatial perception (visuo-spatial, 'where is an object') and guiding actions (visuo-motor, 'how do we interact with object')
• What is the ventral stream?

  Visual processing pathway for object recognition ('what is this object')
• What is the posterior parietal lobe?

  Part of the brain responsible for spatial awareness
• Where is the dorsal stream?

  starts in V1 and extends to the posterior parietal lobe
• Where is the ventral stream?

  starts in V1 and extends to inferior temporal cortex
• Lesions in the inferior temporal cortex leads to...

  impaired object recognition (due to damage to ventral stream), but normal object location recognition (due to no damage in dorsal stream)
• Lesions in the posterioir parietal lobe leads to...

  imparied object location recognition (due to damage to dorsal stream), but normal object discrimination (due to no damage to ventral stream)
• What is visual agnosia?

  Impaired recognition or identification of visually presented objects or stimuli
• What is optic ataxia?

  Impaired reaching and grasping due to damage in the parietal lobe
• What is the key evidence that the dorsal stream is involved in spatially guided action?

  • patients with brain damage to occipital/temporal lobes have visual agnosia but still have intact visually guided action
  • but patients with brain damage to posterior parietal lobe have optic ataxia but still have intact visual function
• What is the DF case study?

  patient DF with ventral-stream lesions has visual agnosia, but no optic ataxia (showing dorsal stream must be involved in visuo-spatial guided action)
• What are the key features of the inferior temporal cortex?

  • recieves input from extrastriate cortex
  • forms final stage of visual processing in the ventral stream
• What is special about neurons in the inferior temporal cortex?

  • they respond very selectively to specific shapes and objects
  • these responses to info stay the same depsite changes in size, orientation, etc of the object, and in absence of visual object
• What are face cells?

  type of neurons in the inferior temporal lobe which show highly selective responses to individual faces
• What are grandmother neurons?

  Hypothetical neurons that respond to specific individuals, such as grandmothers, face cells are compared to these neurons
• What does the medial temporal lobe include?

  hippocampus, amgydala, multiple cortices etc
• What is the medial temporal lobe?

  The end of visual-processing hierarchy, combines inputs from dorsal and ventral stream, and receives input from other sensory modalities
• What complex/ multi-modal representations does the medial temporal lobe generate?

  • complex spatial representations generated by encoding relations between many visual stimuli
  • multimodal representation of experiences and facts (episodic and sematic memory)