Perception

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Early understanding

  • The first knowledge of the biological underpinnings of vision came from studies of people who had impaired vision due to large-scale brain damage from strokes, tumours, and gunshot wounds. From this, it was established that the brain areas dedicated to visual abilities are in or near the occipital cortex, at the back of the head.
  • Very little was known about the specific neural events that resulted in vision until the development of single-cell recording techniques in the 1950s. This gave vision scientists their first glimpses of neural information processing, and allowed them to exame these mechanisms neuron by neuron
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Visual perception

  • All of our 3D vision is generated from a 2D microscopic array of nerve cells at the back of our eyes
  • A 2D image of what we look at it focused onto these nerve cells upside-down and left-right reversed. The nerve cells convert the 2D image into a 2D pattern of neural firings - cells that have a bright part of the world imaged onto them fire less, while cells that have a dark part of the world imaged onto them fire more
  • Visual perception is the process of acquiring knowledge about environmental objects and events by extracting information from the light they reflect and using prior knowledge
  • Visual perception is based on the light that objects reflect. Light is the carrier of information. In the dark, we can't see anything. Light bounces around in 3D space, structured by the 3D objects around us. We only see the 2D projection of this light on the backs of our eyes
  • Vision is determined by a mixture of both bottom-up and top-down processes
  • Blindsight is a phenomenon that shows that the visual cortex is necessary but not sufficient for conscious awareness
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Perception as a constructive act

  • Vision isn't always accurate because it is based on assumptions that can be wrong. Thus, vision isn't a "clear window into reality", rather it is an actively constructed, meaningful model of the environment.
  • These vision assumptions are either learnt by the species or by the individual. Assumptions are embedded in the interconnections between cells in the nervous system. They can be physical assumptions or concerned with the meaning or functional significance of an object. They have great power to distort our perceptions when the information available is poor.
  • The 3D assumption: perceiving an array of 3D objects provides the basis for expecting what we'd see if we were to move to another vantage point into the scene, from which new object surfaces come into view. A 3D model frees us from having to re-perceive everything from scratch as we move around the world. A 3D model transcends momentary stimulus information and enables us to plan our actions
  • The assumption that objects endure and remain constant: Objects don't cease to exist when we blink or when they are otherwise removed from view.
  • The assumption that the background is stationary: Our eyes are constantly fixating on different parts of a scene, causing the image of the scene, including its background, to jump around on the back of the eye. Regardless, we don't perceive the background to have moved/be moving.
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Information from eyes to brain

  • Information from the right side of space is focused on the left of the left eye and the left of the right eye and passed to the left hemisphere.
  • Left and right eye information is fed into different layers of cells in the lateral geniculate nucleus. It is combined within single cells in the visual cortex.
  • The mapping from eye to visual cortex is topographical. Nearby regions in the retina project to nearby regions in the visual cortex. However, the central region of the world is even more strongly represented than in the eye.
  • There is no image in the brain. There is only an array of nerve cells firing more or less strongly, depending on the pattern of light emanating from the part of the visual field each codes or represents.
  • Retinal ganglion cells respond to spots of light surrounded by dark. Cells in the primary visual cortex respond to edges where one side of the edge is light and the other dark.
  • Cells in regions of the brain further removed from the eyes respond to even more complex features. They can respond to these features largely independently of where the features occur in the visual field/on the retina.
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Pathways in the brain

  • Some information doesn't travel via the visual cortex, but is relayed to the superior colliculus in the brain stem.
  • It is suggested that information arriving at the primary visual cortex is relayed onto higher cortical regions via a dorsal 'where' or 'how' pathway and a ventral 'what' pathway that perhaps combines at the front of the brain
  • Patients who've suffered strokes in the temporal lobe exhibit visual agnosia: a deficit in identifying objects by sight. Those who've suffered strokes in the parietal lobe exhibit unilateral neglect: an inability to attend to objects in the half of the visual field opposite to their brain damage.
  • Having different modules in the brain may support our ability to process and attend to different visual features selectively
  • An influential proposal is that attention to a spatial location binds the representations of different features belonging to an object at that location into unified representation.
  • Since spatial attention is thought to be supported by the parietal cortex, integrated perception of an object may well involve concerted activity of the whole brain.
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Ganglion cells

  • The first retinal cells to be analysed for their spatial properties were the ganglion cells.
  • Kuffler (1953) and Barlow (1953) began recording the firing rates from indivdual ganglion cells. While the cell's activity was being recorded, different images were presented to the animal's retina to determine which ones made the cell fire and which ones did not. Kuffler recorded from ganglion cells in cats, and Barlow from those in rabbits, but they essentially found the same thing: firing rate was highest for a spot of light of a particular size at a particular position on the retina. If the size of the spot was either increased or decreased, the firing rate diminished. These results suggested some sort of antagonism between an inner circle and a surrounding ring.
  • They respond to spots of light surrounded by dark. They respond weakly if the whole field of vision is full of light.
  • In further studies, researchers mapped the complete receptive fields of these ganglion cells, and found 2 distinct types. On-centre cells caused spike discharges when the light at the centre of the receptive field was turned on (excitatory response). Off-centre cells caused discharges when the light at the centre was turned off (inhibitory response).
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Lateral Geniculate Nucleus

  • Axons from the ganglion cells exit the eye through the optic nerve, pass through the optic chiasm, where some of them cross to the opposite side, and finally synapse onto cell bodies in the laternal geniculate nuclei (LGN) of the thalamus. LGN cells have centre-surround receptive fields that are much like those of the retinal ganglion cells, but somewhat larger and with a stronger inhibitory surround.
  • Ganglion cells form a 2D sheet parallel to the receptor surface of the retina and receive input just from nearby cells in their own eye. In contrast, the LGN is a 3D structure and received input from both eyes.
  • The LGN is laminar (layered), consisting of many 2D sheets of neurons. Each LGN is constructed of 6 distinct layers of cells that are then folded over. The lower 2 layers are called the magnocellular layers (Latin 'magnus' meaning large), as they contain large cell bodies. The upper 4 layers are called parvocellular (Latin 'parvus' meaning small) because they contain small cell bodies. The magnocellular cells are sensitive to differences in contrast, aren't very selective to colour, have relatively large receptive fields, and exhibit a transient response to appropriate changes in retinal stimulation, that begins and ends quickly. The parvocellular cells are insensitive to contrast, highly selective to colour, have small receptive fields, and exhibit a more sustained response to changes in retinal stimulation
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Lateral Geniculate Nucleus pt. 2

  • This specialisation of function has led researchers to speculate that the magno cells consitute a specialised neural pathway for processing motion and depth information, whereas the parvo cells constitue a separate pathway for processing colour and shape.
  • Although the LGN as a whole receives input from both eyes, each layer gets signals from only one eye of the opposite side of the head. Each layer is laid out spatially like the retina of the eye from which it receives input.
  • This is called retinotopic mapping because it preserves the relative location of cells from retina to LGN: nearby regions on the retina project to adjacent regions of the LGN. This kind of spatial mapping is a common feature of higher levels of the visual nervous system.
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Perception and the brain

  • Spatial distributions of intensities in an image is coded in a pattern of neural firings in a 2D array of 128m light receptor cells. It is densest at the centre (Fovea) where objects fall when the eye is focused. It is interrupted at the optic nerve (where light receptor array are gathered and leave eye).
  • Blindspot: With a blindspot, the brain constructs a model of what is there rather than what is actually there. Objects of the same size at different distances project different sized images. Objects of different sizes at different distances project identical images.
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