If a cell responds strongly to a given stimulus, how will the cells’s response change following prolonged exposure to that stimulus?
the cell’s response will decrease
What is a cell’s receptive field?
- A cell’s receptive field is the area on the retina which, when stimulated by light, elicits a change in the firing rate of the cell.
- The effect of the stimulus can be either excitatory or inhibitory.
Describe Single Cell Recording
- An electrode, inserted into a neuron (e.g. V1) measures electrical activity
- Activity is that of a SINGLE neuron.
What is the order in which the receptive fields of cells increase in complexity?
from most simple:
- photoreceptors
- ganglion cells
- LGN cells
- Simple cells
- Complex Cells
- Hypercomplex cells
How do cells in V1 respond?
- V1 simple cells respond to oriented bars and edges
○ Respond to lines with particular orientation
Describe the Tilt After-Effect
People perceive that the vertical pattern appears to be tilted in the opposite direction to the tilted pattern.
Explain the 3 components of the Tilt After-Effect
- Orientation tuned neurons respond best to preferred orientation but also respond to other similar orientations
- Perceived orientation determined by distribution of responses across cells to find the degree of orientation. Cells have a preferred orientation
- Adaptation – cell’s response decreases following prolonged activity.
Describe the process of the Tilt After-Effect
1, Start of adaptation tilted line looks tilted
- During adaptation tilted line continues to look tilted, but cells’ responses decrease
- After adaptation vertical line looks tilted due to asymmetrical response distribution
- Size of after-effect depends on difference between adapt
- Wont get an after effect if you test at vertical and adapt at vertical
What evidence does the Tilt After-Effect provide about Spatial Vision?
- Tilt after-effect provides evidence for orientation tuned cells in human visual system
- Shows we can use psychophysics to make inferences into what is happening in the visual pathway.
What evidence does the Size After-Effect provide about Spatial Vision?
Size after-effect provides evidence for size-tuned cells in human visual system.
Explain the Size After-Effect.
- Before adaptation size perceived veritically.
- Adapt to fatter bars
- After adaptation lines look thinner due to asymmetrical response distribution
- During adaptation cells’ response decreases
Define spatial frequency.
= number of bars per unit distance (usually cycles per degree)
○ Fat bars = low spatial frequency
○ Thin bars = high spatial frequency
In a natural images what do spatial frequencies show?
- High spatial freq = fine details
- Low spatial freq = corse info
Define Contrast.
Contrast: difference in luminance between light areas and dark areas of the stimulus
Describe the spatial contrast sensitivity function.
- Not equally sensitive to different spatial frequencies.
- Greater sensitivity to intermediate spatial frequencies = perceived at low contrasts
- Lower sensitivity to high and low spatial frequencies = need higher contrast to be perceived.
Define Spatial frequency channels.
Collections of neurons tuned to the same spatial frequencies (i.e. respond to the same range of spatial frequencies)
How do receptive field sizes change in the periphery?
what does this mean?
Receptive field sizes increase in the periphery.
= contrast sensitivity varies with eccentricity - we can’t see high spatial frequencies in the periphery.
How does receptive field size effect sensitivity to spatial frequency?
- Neurons in our primary visual cortex that have different sizes of receptive fields
○ They are sensitive to different spatial frequencies
Big RF = low spatial frequencies - Therefore acuity is poorer in the periphery
Define Resolution Limit.
Resolution limit = no longer visible.
There are no cells responding to that spatial frequency as no cells have small enough RF.
What does Spatial frequency tell us about size?
- Spatial frequency tells us about size on the retina – it does NOT indicate real size in the world since the projected size depends on distance
- Any line on the retina could have been produced by infinite different lines in the world
- Not consciously aware that the size of the retinal image changes. We think that the thumb is getting closer, not getting bigger.
How does distance and width of bars affect spatial frequency on the retina.
- Bars of different widths at different distances can project -to same SF on retina
- Bars with same width at different distances will project to different SF on retina
Can we perceive both retinal and real size?
○ Stimuli 1 & 2 at different distances from participant
○ Discrimination experiment - which stimulus has thinner bars?
○ Discriminating retinal size – task very hard
○ Discriminating real size – task easy
○ Conclusion – conscious perception in terms of real size
Define Size Constancy
Size constancy: we perceive an object’s real size in the world regardless of distance
Define Orientation Constancy
Orientation constancy: we perceive an object’s orientation in the world regardless of the orientation on the retina
