Week 2 - the retina, LGN + visual pathways

• The retinal image variable resolution is mostly poor and the wrong way up
• a receptive field is a region within the sensory periphery where stimulus can influence the electrical activity of sensory cells
• Each photoreceptor receives a small part of info from the visual landscape
• The cells receptive field is the portion of the retina effecting the signalling of the photoreceptor
• no retinal cell "sees" all of the image. Each one only gets light from a small part of space known as the receptive field
• photoreceptors respond when light falls on their receptive field, whose output takes the form of AP in retinal ganglion cells whose axons form the optic nerve
• Brain cells throughout the visual system have receptive fields. The more we investigate the brain the more structured and specific these receptive fields become
• Retinal ganglion cells - the output of the eye

  
• ganglion cell receptive fields centre - surround

  
• retinal ganglion cells can be divided into on centre and off centre however we only focus on on centre
• on centre cells connect directly to a small group of photoreceptors <10
• The middle portion receives input from photoreceptors that excites the retinal ganglion cell
• The outer portion receives input from photoreceptors inhibiting the retinal ganglion cell
• One part of the receptive field excites the cell and another inhibits the cell. This helps detect changes over spaces or edges
• No light means resting APs
• Centre light means strong output whereas donut of light means no output and full light means resting APs
• the consequence of centre-surround antagonism is "simultaneous contrast illusion" - grey bar light thing
• The off surround of this cell will turn this cell off through inhibition making the bar will look darker than it really is
• the on centre cell on the left side with the dark background will not be inhibited due to there being less light to activate the off surround and the bar will look lighter then what it actually is
• a consequence of the centre surround output from retinal ganglion cells is they process edges. Most are silent, but those receiving edge info are active
• an image with just the edges compared to one with a normal visual will be processed similarly due to the ganglion cells only processing edges.
• Another consequence is the Craik-O’Brien-Cornsweet illusion. Unpacking the colours of the edges and inside differently when in fact theyre the same
• Inhibition over time causes neurons to reduce their activity (save energy)
• Lack of crisp edges mean that spatial locaiton is not signalled well either
• Reduced neural signal due to inhibition and lack of crisp edges allows filling-in of the grey regions by the cortex
• Thus, if a stimulus stays on for a long time it will appear to fade away - "troxler fading"
• When the stimulus is removed the time lag on the inhibition produces the negative after image
• after effects are caused by fatigued cells in the retina responding to light. Visual aftereffects are changes in perception of visual stimulus after adaption to a previous stimulus
• The lateral geniculate nucleus (LGN) shows what happens after visual info leaves the retina

  
• LGN gateway to the cortex

  
• From LGN to the what and where visual pathways. What pathway (ventral) and where (dorsal).

  
• stroke in one of the ventral areas leads to problems recognising faces - prosopagnosia
• strokes in one of the where areas (STS) leads to problems recognising emotions (not faces) - capgras syndrome
• the colour and lightness of objects is not recovered directly from the retinal image byt depend on efficient neural processing
• Many Retinal Ganglion cells also respond only to sudden changes in light over time.
• The LGN is like a gating or re-routing station. The division of signals into specialist pathways starts at the LGN.
• Cortical brain areas specialise in different aspects of vision.