Vision Flashcards
Absolute threshold
the weakest level of stimulus that can be detected
Difference threshold
the smallest detectable change in a stimulus.
Observer’s threshold:
Measured using either the method of adjustment or the method of constant stimuli.
transduction process
light energy hits the photoreceptors (rods and cones) and is converted into action potentials
Order of cells in the visual pathway
Photoreceptors (rods and cones), horizontal, bipolar, amacrine and ganglion cells
Characteristics of cone cells (5)
- specialised for colour and detail, daylight
- High density
- High visual acuity
- There are 3 types: S-cones, M-cones and L-cones,
- named to reflect the wavelength of light they are excited by.
- 1:1 with each ganglion cell.
Characteristics of Rods (6)
- specialised for light and general shape, darkness
It is found in the peripheral vision (which lacks detail and colour).
- Low visual acuity (vague information collected).
- Highly sensitive
- Active many ganglion cells, resulting in a general, dull signal.
- Many Rods to one ganglion cell.
Graded potentials
primarily generated by sensory input, causing a change in the conductance of the membrane ofthe sensory receptor cell.
Graded responses
changes in membrane potential depending on how much neurotransmitter is received and can be depolarizing or hyperpolarizing.
Firing rate
Action potentials per second
Rate Coding (4 points)
- Same amplitude, regardless of stimulus intensity (graded potentials).
- Firing rate increases in line with increases in stimulus intensity.
- The size of the stimulus is not indicated by the size of the signal but by the frequency of the signals.
- Therefore, changes in firing rate indicate stimulus intensity.
Limitation of Rate Coding
if a neuron’s response to a given level of stimulation was fixed, it could only distinguish a narrow range of levels of stimulation.
In reality, the range of perceivable intensity is often greater than the range of firing rates of ganglion cells.
Dichromic Vision (3)
- present in most mammals.
- Only L and S cones.
- Single wavelengths are confused with white light.
Trichromatic vision (3)
- Humans evolved to split L into L and M (explaining the considerable overlap).
- See mixtures of red and blue or red and yellow.
- Do not see mixtures of red and green or blue and yellow.
Combinations in Opponent Process Theory (3)
Four primaries:
- Green is the opponent to red
- Blue is the opponent to yellow
- (also black vs white)
Pattern Coding: Colour Vision (2)
Perception reflects the relative responses of 3 classes of cone photoreceptors rather than individual cone types.
At post-receptoral levels of coding, known as pattern coding, this information is coded by cells that reflect the balance (or imbalance) in responses in L versus M cones. or in S-cones versus L and M cones.
Pattern coding:
- L vs. M cones
- S vs. L+M cones
Explain Centre-Surround (Spatial) Antagonism and affect of stimulating centre, surround and both (5)
In ommatidia (optical units), individual cell activation triggers inhibition in neighbouring cells,
so receptive fields are activated by light, whilst surrounding cells are deactivated by this activation in the receptive field.
- Stimulation to centre only = increased firing rate
- Stimulation to surround only = decreased firing rate
- Stimulation to centre and surround = little or no activation.
Explain the principle of Univariance (4)
- Cell responses are univariant, meaning they vary along a single dimension.
This can result in the firing rate either increasing or decreasing.
- Various factors can influence this rate, such as orientation or salience (the degree to which an object stands out from its surroundings).
- Consequently, the firing rate of one neuron in isolation doesn’t provide comprehensive information. Its response is ambiguous.
How to disambiguate Firing Rates (3)
Pattern Coding is utilised by comparing the outputs of several similar cells.
The relative responses of mechanisms tuned to different features provide an unambiguous signal.
Pattern coding is used for orientation, depth, motion, and many other aspects of vision, just as the relative response ratios of L, M, and S cones are used in colour vision.
Explain the Principle of Adaptive Independence (3)
Neurons selective for one stimulus feature can be selectively adapted without adapting other neurons.
That is, individual mechanisms can be adapted independently.
This fact allows us to make inferences about the underlying neurophysiology.
Explain the mechanism of aftereffects (5)
- Ganglion cell receptive fields respond equally to an edge or line of any orientation.
- If we combine the outputs of several ganglion cells with neighbouring receptive fields.
- Allow adaption to take place and then test perception.
- After Effects are indicative of pattern coding.
- Sensitivity to luminance decreases with decreasing contrast.
Define Heuristics
Rules of thumb about the world and uses these to disambiguate perception.
Explain Depth Perception (2)
- Retina images are 2-dimensional (2D).
- 3-dimensional (3D) vision arises from using depth cues such as binocular disparity.
Examples of Monocular (single-eye needed) Cues
Motion Parallax Cues
Relative Size (Texture Cues)
Familiar Size Cue (Heuristic)
Interposition (Heuristic)
Convergence (Heuristic)
Height in Plane/Scene (Heuristic)
Facial Convexity (Heuristic)
Light/Shadow/Shading (Heuristic)
