Lecture 6 - Primary Visual Pathway

Question 1

Primary visual pathway

Answer 1

From eyes (retina) to primary visual cortex (striate cortex, V1) in occipital lobe

Question 2

Information-processing stages in primary visual pathway

Answer 2

• Hierarchical process
• Neurons code more and more complex elements of visual pathway
• Photoreceptors = detect light in visual field

Question 3

Experimental strategy to reveal mechanisms of visual preception

Answer 3

• By studying the different neuronal responses at different stages of the visual pathway, one may gain understanding of the different stages of visual information processing that mediate visual perception
• Put electrodes at different stages and see how neurons respond to visual information
• Provide visual stimuli to visual field and plant electrodes at different points in visual pathway
• Many of these studies done under anaesthesia to animals as don’t need to be conscious

Question 4

Seminal contributions to our understanding of visual information processing

Answer 4

• First time a group called themselves a neurobiology group
• David Hubel
• Thorsten Wiesel
• Nobel Prize in Physiology or Medicine 1981
• For discoveries concerning information processing in the visual system

Question 5

Photoreceptors

Answer 5

• In the retina
• Both contain photopigments = light changes configuration in photopigment
• Translate light into electrical signal in photoreceptor
• Mammalian photoreceptor inverted

Question 6

Rods

Answer 6

• More abundant (ca. 120 million in human retina)
• No colour (i.e. wave length) discrimination
• Sensitive in low light levels
• Higher density in periphery (don’t look directly at dim stars)
• Track high-rate changes (see flicker of 60Hz monitor from corner of your eyes)

Question 7

Cones

Answer 7

• Less abundant (ca. 6 million in human retina)
• 3 types discriminate different wavelengths (S, M, L) – mutations in these photopigments leads to colour blindness
• Less sensitive to low light
• Higher concentration in fovea
• Cannot follow rapid changes (can’t see 60-Hz flicker when directly looking at monitor)

Question 8

Photoreceptors and bipolar cells

Answer 8

• Photoreceptors and bipolar cells vary their voltage as they are stimulated (analogue signal), whereas all subsequent cells vary spike rate (all-or-nothing, digital signal)
• Photoreceptor detection of light is translated into excitation or inhibition of retinal ganglion cells via bipolar cells

Question 9

Receptive fields of visual neurons

Answer 9

• The portion of the retina/visual field in which visual stimulation will evoke a change in the firing rate of a given visual neuron
• Substructure of a receptive field: a description of how visual stimuli need to be presented in the receptive field of a visual neuron in order to evoke firing-rate changes

Question 10

Retinal ganglion cells

Answer 10

• Still in retina
• Receive input from multiple photoreceptors (via bipolar cells)
• ON-OFF centre-surround receptive fields
• Light presented in ‘ON’ regions excites cell, and light in ‘OFF’ regions inhibits cell
• ON and OFF regions are organised in ‘centre-surround’ fashion
• Response rate of cell is based on the sum of stimulation in ON region minus stimulation in OFF region
• Enhancement of contrast and boundaries
• Neurons in the lateral geniculate body respond to visual stimuli in similar ways to retinal ganglion cells (understand what happens at next stage)
• Cells have a baseline firing rate, which presentation of visual stimulus changes
• When stimulation ends, don’t go back to baseline firing rate but go over in opposite direction
• ‘ON’ = rebound effect with slightly lower firing rate (taking light away)

Question 11

Functional significance of centre-surround fields

Answer 11

• The world has lots of things that stay constant, and we don’t need to keep responding to them – what counts most are changes and boundaries
• Responding only to changes and boundaries (edges) is efficient
• The luminance of features is represented relative to their surround
• This helps preserve appearance of objects regardless of light levels in the environment (newspaper looks basically the same in a dark room and in sunlight, despite hugely different levels of overall reflected light)
• However, it can also result in illusions

Question 12

Colour sensitivity of retinal ganglion and LGN neurons

Answer 12

• Retinal ganglion and LGN (lateral geniculate) cells receive inputs from cones (that are differentially sensitive to different wavelengths) and are sensitive to colour
• Colour-sensitive retinal ganglion and LGN neurons have receptive fields that show centre-surround colour opponency

Question 13

Function significance of colour opponency

Answer 13

Not clear
- However, colour opponency, together with firing-rate adaptation (rebound effects) in retinal ganglion cells can explain negative colour afterimages
- Green (red on, green off) – take green away = rebound effect makes us see red

Question 14

Primary visual cortex

Answer 14

• Information passed from lateral geniculate body to primary visual cortex
• Striate cortex, V1 (‘stripy cortex’)
• Sits around calcarine fissure between the two hemispheres

Question 15

Orientation-selective cells in V1

Answer 15

• Most V1 neurons respond to elongated stimuli with specific orientation in their receptive field
• Two main types of orientation-sensitive V1 neurons

Question 16

Simple cells

Answer 16

• Fields have inhibitory and excitatory regions
• Can be thought of as combining inputs from ON and OFF cells in lateral geniculate nucleus

Question 17

Complex cells

Answer 17

• Fields have no discrete ON and OFF regions (can excite the neuron from anywhere)
• Best response to moving stimuli (reflecting response adaptation)
• Can be thought of as combining inputs from simple cells

Question 18

Retino-topic map

Answer 18

• Orderly mapping of retina/visual field onto visual cortex
• Each part of retina responds to specific part of visual field

Question 19

Modules

Answer 19

• V1 is divided into small columnar modules that combine neurons sensitive to different aspects of stimuli presented in a small part of the visual field

Question 20

Further processing of visual information

Answer 20

• To result in perception and memory of the ‘holistic’ visual properties of whole objects and visuals scenes, the visual information from the modules in V1 needs to be combined and further processed
• This processing takes place in the visual association cortices (V2-V5, inferior temporal cortex, posterior parietal cortex) and other regions

Question 21

Blindsight

Answer 21

• Subjects with lesions to primary visual cortex and apparent ‘blindness’ can show appropriate responses to visual stimuli of which they are not ‘conscious’
• Examples of such ‘blindsight’ include: ‘looking’ (i.e., moving the eyes) or pointing toward visual stimuli; detection of movement; etc.
• ‘Blindsight’ highlights that, apart from the primary visual pathway that is critical for conscious vision, there are additional visual pathways
• Recent study suggests that direct LGN projections to extrastriate cortex are critical for blindsight (Schmid et al., 2010)
• ‘Blindsight’ also highlights that the brain can perform visual information processing which can guide subjects’ behaviour without their conscious awareness
• Cowey and Stoerig (1991)