Jan 16 Neuroimaging

• Sensations and perceptions are the result of a pathway that begins with stimulus energy such as light, sound, smell, etc., and ends with perceptions in the brain's visual, auditory, and olfactory areas.
• Exteroceptive sensations are any form of sensation that results from stimuli located outside the body detected by sensory organs.
• Interoceptive sensations are sensations from inside our body, including:

  • proprioception (sense of where our limbs are in space)
  • nociception (sense of pain due to body damage)
  • equilibrioception (sense of balance)
• Dancers have increased interoceptive accuracy, as evidenced by their ability to estimate heart rate more accurately than non-dancers.
• Senses can switch, as evidenced by:

  • Synaesthesia: a neurological condition in which one sense automatically triggers the experience of another sense
  • such as hearing colors or seeing time
• You 'expect' to see real words in a sentence
• Color perception depends on the wavelengths of light that fall on our retina and our expectation (from experience) of how objects look under contexts of illumination.
• Synesthesia represents the importance of individual differences, as how Joe with chromesthesia experiences the visual world likely “sounds” different than how Barb experiences the visual world.
• Synesthesia encourages a view that brains are organized as “talking” circuits, as it is explained as cross-talk between processing regions for different senses.
• Synesthesia illustrates the dominance of visual input.
• The visual system consists of:

  1. Early visual processing, which involves sensation and the eyes and the optic nerve
  2. Late visual processing, which involves perception and the visual cortex or occipital lobe
• Early visual processing involves the pathway: light waves enter the eye, projected onto the retina, the retina forms an inverted image, and later processes turn this image around.
• Retina photoreceptors convert light to electrical activity, with:

  • Rods for low light levels for night vision
  • Cones for high light levels for detailed color vision
• The electrical signal is sent to bipolar cells, sent on to the ganglion cells.
• Primary Visual Cortex is a specialized region that processes specific visual attributes or features such as edges, angles, color, and light.
• The dorsal pathway, which is located in the occipital to parietal lobes, processes location, space, movement information.
• Bottom-up processing: the influence of information from the external environment on perception, from the eyes to the visual cortex.
• Visual stimuli is altered at many stages of the processing pipeline, including inversion, compression, and within the primary visual cortex.
• Damage to the ventral stream results in intact dorsal stream but impaired performance on visual guided action (picking up an object appropriately).
• In the cortex, visual input is broken down, processed separately and then combined to form a perception of an entity.
• Neuroimaging studies show separation of what and where pathways.
• We use assumptions about what we expect to see to guide perception, knowledge about assumptions about how the world works affects perception.
• Information from the eyes to the brain is compressed.
• Perceptual filling-in is a process in later visual processes in the brain that provides the missing information by 'interpolating' visual information from surrounding areas.
• The thalamus (lateral geniculate nucleus, LGN) is the way-station for visual information.
• Top-down processing: the influence of knowledge (expectations, context and goals) on perception, from higher processing brain regions (prefrontal cortex or higher visual processing areas) back to the sensory organs.
• The left and right visual fields can compensate for each other's blindspot.
• Constructivist Theory of Perception relies on the influence of top down processes to vision.
• Rods are mostly located in the periphery, making the periphery of the visual field less detailed and less accurate.
• Damage to the dorsal 'where' pathway results in intact ventral stream but impaired performance on visual object recognition or matching tasks.
• The signal from the eyes exits through the optic nerve to the brain for later visual processing.
• Our context activates expectations (schemas, knowledge) affects perception.
• The optic nerve of each eye transmits information to both hemispheres, resulting in contralateral representation

  • where the left visual field is perceived via the right hemisphere
  • the right visual field is perceived via the left hemisphere
• The optic nerve, which exits to the brain, passes through the photoreceptor layer, meaning there are no photoreceptors at the exit location, resulting in no vision.
• Perception is an "illusion" according to the Constructivist Theory of Perception.
• Visual Association Areas interpret visual signal, assign meaning.
• The reality we perceive is a construction of the brain.
• Photoreceptors are concentrated in the fovea, a small area on the central part of the visual field, making the center of the visual field most detailed.
• The ventral pathway, which is located in the occipital to temporal lobes, processes shape, size, visual details.
• Goodale & Milner (1991) argue that the dorsal and ventral pathway represent "perception" and "action".