Pathway from Retina to Cortex
- Signals from the retina travel through the optic nerve to the
- – Lateral geniculate nucleus (LGN)
- – Primary visual receiving area in the occipital lobe (the striate cortex)
- – And then through two pathways to the temporal lobe and the parietal lobe
- – Finally arriving at the frontal lobe
- Optic chiasm: where one optic nerve crosses the other and where info is swapped from one visual field to the opposite hemisphere
Processing in the LGN
- LGN cells have center-surround receptive fields. (central excitatory area and peripheral inhibitory area. Helps to be accurate at location and detect edges)
- • Major function of LGN is to regulate neural information from the retina to the visual cortex. (visual librarion- sorting info from retina in a linear predictive order before sending to retina)
- – Signals are received from the retina, the cortex, the brain stem, and the thalamus (feedback from the brain. Visual cortex moderates)
- – Signals are organized by eye (whether info came from right/left eye) , receptor type (whether it came from rod/cone), and type of environmental information.
- (a) Inputs and outputs of an LGN neuron. The neuron receives signals from the retina and also receives signals from the cortex,
- from elsewhere in the thalamus (T), from other LGN neurons (L), and from the brain stem. Excitatory synapses are indicated by Y’s and inhibitory ones by T’s.
- (b) Information flow into and out of the LGN. The sizes of the arrows indicate the sizes of the signals.
- Bottom up and top down processing
Organization in LGN
- Each LGN has six layers
- – Each layer receives signals from only one eye
- • Layers 2, 3, and 5 receive signals from the ipsilateral eye (eye same side of LGN)
- • Layers 1, 4, and 6 receive signals from the contralateral eye (eye thats in the other hemisphere
- • Thus, each eye sends signals to both LGNs and the information for each eye is kept separated.
- Retinotopic map: each place on the retina corresponds to a place on the LGN
- Determining retinotopic maps: record from neurons with an electrode that penetrates the LGN obliquely
- Stimulating receptive fields on the retina shows the location of the corresponding neuron in the LGN
- For every physical location in the real world, there is a location in the LGN and they are located sequentially.
The Map on the Striate Cortex
- Cortex shows retinotopic map, too.
- – Electrodes that recorded activation from a cat’s visual cortex show:
- • Receptive fields on the retina that overlap also overlap in the cortex.
- • This pattern is seen using an oblique penetration of the cortex.
- Corresponds to real world image
Neurons in Striate Cortex- simple
- Simple cortical cells
- – Side-by-side receptive fields (sequential)
- – Respond to spots of light
- – Respond best to bar of light oriented along the length of the receptive fieldIdentifies edges of things and trying to tell us patterns
- Orientation tuning curves
- – Shows response of simple cortical cell for orientations of stimuli
- (a) The receptive field of a simple cortical cell.
- (b) This cell responds best to a vertical bar of light that covers the excitatory area of the receptive field. The response decreases as the bar is tilted so that it also covers the inhibitory area.
- (c) Orientation tuning curve of a simple cortical cell for a neuron that responds best to a vertical bar (orientation = 0)
Neurons in Striate Cortex - continued
- Complex cells
- – Like simple cells
- • Respond to bars of light of a particular orientation
- – Unlike simple cells
- • Respond to movement of bars of light in specific direction
In the table only responds to 45 degree going towards the right
Neurons in Striate Cortex - end-stopped cells
- – Respond to:
- • Moving lines of specific length
- • Moving corners or angles
- – No response to:
- • Stimuli that are too long
- This example cell prefers medium corners going from bottom right to top left
- Neurons that fire to specific features of a stimulus
- Pathway away from retina shows neurons that fire to more complex stimuli
- Cells that are feature detectors:
- – Simple cortical cell
- – Complex cortical cell
- – End-stopped cortical cell
Properties of Neurons in the optic nerve, LGN and Cortex
- Neurons tuned to specific stimuli fatigue when exposure is long.
- Fatigue or adaptation to stimulus causes
- – Neural firing rate to decrease
- – Neuron to fire less when stimulus immediately presented again
- Selective means that only those neurons that respond to the specific stimulus adapt
Method for Selective Adaptation
- Measure sensitivity to range of one stimulus characteristic
- Adapt to that characteristic by extended exposure
- Re-measure the sensitivity to range of the stimulus characteristic (see how adaptation has changed perception of the stimuli)
Stimulus Characteristics for Selective Adaptation
- Gratings are used as stimuli
- – Made of alternating light and dark bars
- – Angle relative to vertical can be changed to test for sensitivity to orientation
- – Difference in intensity can be changed to test for sensitivity to contrast
- (a) Gratings that vary in orientation. (b) A vertical grating. The contrast is high for the gratings on the left, low for the ones on the right.
- Stare at a certain orientation for 10 min then test using (b)
Method for Contrast Sensitivity
- Measure contrast threshold by decreasing intensity of grating until person can just see it.
- • Calculate the contrast sensitivity by taking 1/threshold.
- • If threshold is low, person has high contrast sensitivity.
Method for Orientation Sensitivity
- Measure contrast sensitivity to different orientations (should all be roughly equal)
- • Adapt person to one orientation, using a high contrast grating
- • Re-measure sensitivity to all orientations
- • Psychophysical curve should show selective adaptation for specific orientation if neurons are tuned to this characteristic
- Results: increase in contrast threshold for that orientation (adaptation)
- We can link perception and physiology.
- We are testing their perception of the lines. The physiology of cells is tuned like b) and the perception very closes matches therefore we can confidently say that the physiology that we see drives our perception. These cells drive the way we see the world
Selective Rearing Experiments
- Animals are reared in environments that contain only certain types of stimuli
- – Neurons that respond to these stimuli will become more predominate due to neural plasticity.
- – Blakemore and Cooper (1970) showed this by rearing kittens in tubes with either horizontal for vertical lines.
- – Both behavioral and neural responses showed the development of neurons for the environmental stimuli and the loss of others
- Links perception to physiology
- Neurons develop through experience, they're plastic.
Organization in Columns - continued
- – Ocular dominance columns
- • Neurons in the cortex respond preferentially to one eye.
- • Neurons with the same preference are organized into columns.
- • The columns alternate in a left-right pattern every .25 to .50 mm across the cortex.
- – Hypercolumns contain:
- • A single location column
- Big set of cells
- • Left and right dominance columns
- • A complete set of orientation columns (0 to 180 degrees)
- • This is called the “ice-cube” model.
- Visual neurons are highly organised
- Functional magnetic resonance imaging (fMRI)
- – Haemoglobin carries oxygen and contains a ferrous molecule that is magnetic
- – Brain activity takes up oxygen, which makes the haemoglobin more magnetic
- – fMRI determines activity of areas of the brain by detecting changes in magnetic response of haemoglobin
- Rotating image of wedge covers all the hypercolumns (all diff orientation, location) then code location with colours on areas of brain
- Eg. when wedge is 0 to30 degrees, the area highlighted is blue etc
- Linking perceptual location in real world to physical location in your brain
- V1: primary visual location area- can see the whole world. From green to blue, everything inside there is map of V1
- Everyone's brain is slightly different
- V4 V5: high level, look at things as a whole
Lesioning or Ablation Experiments
- Lesioning/ablation: a part of the brain was destroyed, inactivated or removed. Mostly with cats or monkeys
- First, an animal is trained to indicate perceptual capacities.
- Second, a specific part of the brain is removed or destroyed.
- Third, the animal is retrained to determine which perceptual abilities remain.
- The results reveal which portions of the brain are responsible for specific behaviours.
What and Where Pathways
- Ungerleider and Mishkin experiment
- – Object discrimination problem
- • Monkey is shown an object
- • Then presented with two choice task (which object did they just see?)
- • Reward given for detecting the target object
- In order to complete this experiment, the monkey has to visually identify and discriminate between two objects. Only way to do this is through vision (cant touch, hear, taste)
- – Landmark discrimination problem
- • Monkey is trained to pick the food well next to a cylinder
- So must identify and move the cylinder to find food
- To complete this, monkey needs to locate object in visual space
- Using ablation, part of the parietal lobe was removed from half the monkeys and part of the temporal lobe was removed from the other half (where they thought these processes happened)
- – Retesting the monkeys showed that:
- • Removal of temporal lobe tissue resulted in problems with the object discrimination task - What pathway
- • Removal of parietal lobe tissue resulted in problems with the landmark discrimination task - Where pathway
Specific where/what pathways
- What pathway also called ventral pathway
- • Where pathway also called dorsal pathway
- • Both pathways:
- – originate in retina and continue through two types of ganglion cells in the LGN.
- – have some interconnections.
- – receive feedback from higher brain areas.
- Magno: means big- has very big receptive field. Need to see big space to be able to locate
- Parvo: small- small receptive field. To identify objects, only need to look specifically at object (smaller receptive field)
Where pathway may actually be "How" pathway
- – Dorsal stream shows function for both location and for action.
- – Evidence from neuropsychology
What and How Pathways - Neuropsycholgical Evidence
- Behavior of patient D.F.
- – Damage to ventral pathway due to gas leak
- – Not able to match orientation of card with slot
- – But was able to match orientation if she was placing card in a slot
- – Other patients show opposite effects
- – Evidence shows double dissociation between ventral and dorsal pathways
- Performance of D.F. and a person without brain damage for two tasks: (a) judging the orientation of a slot, only pretend where to go (placed at random, no what pathway); and (b) placing a card through the slot (physically) got much better results as she knew where to put it
Modularity: Structures for Faces, Places, and Bodies
- Module - a brain structure that processes information about specific stimuli
- – Inferotemporal (IT) cortex in monkeys
- • Responds best to faces with little response to non-face stimuli
- – Temporal lobe damage in humans results in prosopagnosia. (face blindness, can't tell people apart)
- • Evidence from humans using fMRI show:
- – Fusiform face area (FFA) responds best to faces.
- – Parahippocampal place area (PPA) responds best to spatial layout.
- – Extrastriate body area (EBA) responds best to pictures of full bodies and body parts.
- – Lateral occipital complex (LOC): objects
- Know this from brain scanning images
Evolution and Plasticity: Neural Specialization
- Evolution is partially responsible for shaping sensory responses:
- – Newborn monkeys respond to direction of movement and depth of objects (no depth/movement in womb therefore prewired)
- – Babies prefer looking at pictures of assembled parts of faces
- – Thus “hardwiring” of neurons plays a part in sensory systems
- Experience-dependent plasticity in humans
- • Brain imaging experiments show areas that
- respond best to letters and words.
- • fMRI experiments show that training results
- in areas of the FFA responding best to:
- –Cars and birds for experts in these areas
- –Greeble stimuli
- (a) Greeble stimuli used by Gauthier. Participants were trained to name each different Greeble. (b) Brain responses to Greebles and faces before and after Greeble training.
- FFA isn't necessarily tuned to faces, its tuned to things to find important to tell visual differences between. It just so happens that every one of us, have faces as the top of our list to tell visual differences between.
Babies hardwire to look at faces (no special skill over any other animal), because constantly looking at faces, this is then training and fines tunes FFA