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)
Explain Binocular (two-eye required) Cues (3)
- Two images (diplopic) are fused together.
- The eyes are offset (in different positions), producing two different images (as when you cover one eye).
- Using the information from two eyes is like taking a larger sample on which to base your judgements.
Analogy: asking two people who witnessed a crime from different angles what happened to make a judgement about what happened.
Explain the Binocular Disparity Cue (2)
- A large disparity between the two images = close by
- A small disparity between the two images = far away
- Example: close your eyes in turn, looking at your finger close up and at a distance.
Binocular Convergence Cue (3)
Eyes turn (converge) to see something up close, and this causes strain or pressure on the eye muscles.
- More strain/pressure = closer object
- Less strain/pressure = far object
Explain The Motion Aftereffect (3)
arises from the antagonism between opposite directions of motion, meaning that motion is also pattern-coded.
motion perception arises from the average of motion signals for different directions.
- Motion is also ambiguous.
Describe Depth Rigidity (Heuristic)
The distance/relationship between two points on an object is assumed to remain constant (the shape isn’t deformed), even when that requires a change in the depth of one or more of these points.
How is Motion and Depth Disambiguated?
Vision incorporates heuristics to disambiguate depth and motion information.
Where Adaption Takes Place in the Brain, Monocular and Binocular (2)
- Retina and lateral geniculate nucleus (LGN) → Monocular Adaption
- Primary Visual Cortex and Extrastriate cortex → Binocular Adaption
Explain how to test for Interocular transfer (IOT) (4)
Test whether adaption is monocular or binocular:
- Adapt one eye
- Test aftereffects in the other eye (colour and luminance).
- Finding: aftereffect in the adapting eye, but not the other.
- Conclusion: adaptation occurs in monocular cells at the early stages of vision.
Define Perceptual Constancy
The perception of constant properties of an object despite changes in properties such as luminance, size and orientation. Vision automatically makes compensatory adjustments to perception according to current conditions.
Mechanism of Size Constancy (4)
Emmert’s Law
An object’s size can’t be established from the retinal image.
Retinal images of an object decrease in proportion to the distance from the viewer.
- Emmert (1881) states that we should expect the perceived size of the retinal image to be scaled up in proportion to its perceived distance.
Our perception of size relies on the 3D interpretation of the scene, not just retinal size (e.g., the corridor illusion, the Titchener illusion, and the Ponzo illusion).
Extrastriate Processing in the V2 (2)
- Deals with illusionary, ‘going beyond’ the retinal input.
- Perceptual ‘filling-in’ of illusory contours.
Role of V4, V5 and V8 brain areas (4)
- V4 specialises in colour vision. V8 may also be involved.
- Damage around anterior V4/V8 causes acquired cerebral achromatopsia or ‘Cortical Colour Blindness’.
- Area MT / V5 responds to motion.
- V5 codes the global motion of objects, not just local motion signals. Evidence of motion blindness following damage to V5.
Describe Change Blindness (2)
- A change in a stimulus yields a perceptual ‘transient’ (temporary effect), a signal that normally heightens our awareness of objects at the location of a change.
- We fail to notice the change when transients are everywhere in a scene or when there are no transients.
Describe Parallel Vision and test for it (2)
We can process the whole scene using simple features. However, conscious perception reflects limited information visual attention selects rather than early visual processing.
Visual Search ‘pop out’ effect of parallel search.
What do after-effects and illusions (afterimages) reveal about the neural mechanisms of vision? (4)
result from shifts in our perception of the characteristics of visual stimuli that do not reflect reality.
- They reveal features of the visual pathway responsible for translating visual stimuli into our perceptions of the world.
- Adaption
- Pattern coding
Explain the waterfall illusion (many)
- The Waterfall Illusion: Motion Aftereffect:
- Period of adaption to the initial stimulus (a waterfall moving downwards).
- A stationary waterfall is presented.
- It is perceived to be moving upwards, in the opposite direction.
- This is the result of spatial pattern coding, in which;
- Receptor cells sensitive to downward motion are activated.
- They become less responsive as they ‘adapt’ to the stimulus.
- The static waterfall image activates an equal response from receptor cells sensitive to all motion directions.
- Activation in the receptor cells associated with downward movement remains constant.
- Visual fatigue of these cells means they are less active than receptive cells sensitive to opposing directions of motion.
- Net bias in cell responses leads to the perception of upward motion in the static object.
Features of human colour vision (3)
- Human colour vision is a trichromatic system:
- Pattern coding of light waves.
- Three categories of wavelength.
- Specialised cone cells (S, M and L).
Explain the Negative Afterimage (many)
- The stimulus is viewed (adapted to).
- Look at a plain white surface (neutral stimulus).
- A scheme of opposing colours is visible.
- Supports the opponent process theory of colour
- Red and green.
- Yellow and blue.
- …are opposingly excitatory to receptor cells (cones).
-
Therefore:
- Yellow stimulus is presented.
- Receptor cells will be active in response.
- Adaption occurs.
- Focus on a neutral stimulus.
- All receptor cells are activated equally.
- Activation of yellow-sensitive cones remains constant but decreases relative to cones sensitive to other colours.
- Resulting in a net bias towards blue.
- Blue is perceived (afterimage).
