Reflectance: the proportion of light of a particular wavelength that a surface reflects
Different materials have different reflectance functions
When a light hits an opaque surface, some light is absored and some light is reflected. The colour (hue) depends on the degree to which the surface reflects different wavelengths.
- When most of the light is absorbed, the surface appears dark
- When most of the light is reflected, the surface appears light
- A surface that reflects long wavelengths more than short or medium, the surface appears reddish/brown/orange
The amount of light of a particular wavelength reflected (proximal stimulus) =
Amount of light of that wavelength x reflectance (distal stimulus)
If we include all wavelengths in the visible range...
Spectral content of light= spectral content of illumination x surface spectral reflectance
A perceptual illusion is a consistent and persistent discrepancy between sensory percept and the distal stimulus, such that the observer is decieved as to the nature of the distal stimulus.
Perceptual illusions occur as a result of normal processing.
A pencil in a glass of water is not a perceptual illusion because it is not a result of processing operations carried out by the nervous system on proximal stimulation, but because water and air refract light differently.
In the case of visual illusions, the brain attempts to intepret the proximal stimulus in terms of the reflectance of the surface, but comes up with the wrong answer. We are decieved as to the true nature of the stimulus as a result of processing carried out by the brain on proximal stimulation.
For example, in this illusion, the brain takes into account the context (e.g. the shadow). We know that shadows make things darker and so the brain tries to compensate, interpreting the square as lighter than it actually is.
Colour blindness (1)
In principle, colour deficiencies can be caused by damage or abnormality anywhere from the retina to the V4, bust most forms are due to abornormalities in the cones.
Roughly 8% of men, and 0.5% of females have some form of colour deficiency. Colour blindness very rarely means seeing in total black and white.
Almost all colour blind people can see in colour but fail to make certain distinctions.
I.e. two lights or surfaces that appear differently coloured to a normal person can look similar to a colour blind person.
Ishihara plates can be used to study colour blindness. Colour blind people fail or take longer to pass the tests.
There are 7 possibilites for colour blindness, but only 5 have ever been found.
Colour blindness (2)
Type 1- Monochromats- the only CB unable to see in colour. Very rare.
1) Cone Monochromats [only S cones + rods]
2) Rod monochromats [no cones, only rods]
Type 2- Dichromats.
1) Protonopia [No L cones],
2) Deuteranopia [no M cones],
3) Tritanopia [no S cones. Quite rare].
Type 3- Anomalous trichromats. Cones have slightly different absorbance spectra than usual. Subtypes:
1) anomalous protonopia [L cones similar to M cones].
2) Anomalous deuteranopia [M cones similar to L cones. The most common type- around 5% cauc males].
Most types of colour blindness occur in around 1% of males and 0.05% of females.
Deciding what to do when we reach an obstacle
It depends on the mode of locomotion, current goals, task requirements, capability, obstacle size, and nature of obstacle.
It is not the absolute size, but the relative size of the object to the person.
Maximum riser height to step over= twice the length of a person's thigh.
Infomaiton is body-scaled- we need infomation about the size of an object in some sort of body size unit.
In studies where participants can choose to step over or go around an object, they stepped over objects less than 0.85 of the lower leg length. They had no awareness of this.
Participants could accurately judge a set of stairs climbable without hands (twice the length of your thigh). This doesn't tell us whether people would actually choose to climb them.
People show a preference for stairs with a riser hight of 25% of their entire leg length. This might be becase it minimises energy output. People could learn this through experience.
Energy minimisation (1)
Active dynamic walking- awlking on the flat involes injecting energy to maintain the natural dynamic cycle.
It has been proposed that people and animals move in a way which not only achives the outcome but keeps metabolic energy costs as low as possible.
The principle of energy minimisation says that during goal directed activities, animals move their bodies in such a way that the energy expanded to achieve a goal is as little as possible.
There are two advantages of PEM- energy minimisation and reducing the chance of tissue damage. Less efficient movement leads to muscle damage on the short term and irreparable wear and tear in the long term.
Direct tests of energy costs- measure energy costs of moving in differnt ways and then determine whether or not people move in a way to expend the least energy
Indirect tests- derive a mathetmatical model from which the minimum energy solution can be derived, and then see whether performance corresponds to the solution.
We can derive a measure of energy consumption during locomotion from the uptake of O2 and the release of CO2. Important early studies on horses found horses used more energy the faster they went.
Energy minimisation (2)
Measures of energy consumption aren't completely useful- we need a measure of energy cost of efficiency that allows us to compare different models of behaving.
Specific cost of transport= energy expended x (weight x distance). Such measures showed that horses switched type of movement (walk, trot, gallop) when the energy cost for each movement became too high.
Minimisation is the preference for a situation in which the least energy costs would be incurred e.g. in climbing stairs. Energy cost was lowest when climbing a step at .26 leg lengths, which corresponds closest to the height of preference.
Gait theory- there is little evidence to suggest that people attempt to reduce energy consumption by flattening the path of their centre of gravity.
Steering a bend
Whatever the technique for turning a vehicle, avoiding under and oversteer is important.
This can depend on physical characteristics of the vehicle, the environment, and steering actions of the driver.
Understeer occurs either when the driver doesn't turn the wheel far enough (amplitude error) or when the driver steers too late (timing error).
Oversteet occurs when the driver turns the wheel too early (amplitude error), or when they turn the wheel too early (timing error.
When asked to change a lane blind, people fail. Visual infomation is needed to prompt the second phase of the manoeuvre. However, people aren't aware of the second stage as it is triggered by visual infomation beneath the level of awareness.
Passing through a gap
People can judge whether they need to turn their shoulders to get through a gap from several metres away. They assess the gap wigth in body scaled units.
Warren and Whang found that the degree of turning increased aproximately linearly with the decreasing gap length (more turning the smaller the gap). They identified the critical gap lenght of 1.3 should widths as the point where someone has to turn their shoulders.
In a second study which asked pps if they could fit through the gap without turning, the crtical gap width was 1.16 shoulder widths proving people can visually discriminate gaps.
To keep the time walking turned to a minimum, the turning movement must be completed just as you arrive at the gap (not before). Thus, if turning takes 200ms then you start turning 200ms before. This is called time to arrival. TTA= distance / walking speed.
Is is not as simple as being able to percieve TTA and sending commands to mucles. We have to take into account the time needed to process stimulus infomation and extract TTA value. It also takes time to transmit command signals from the brain to the muscles. The TTA value at which this whole process should start is called the criterion value.
2 compnents for accurace and effective timing- pre planning of moevment duration (interal) and triggerin command generation at the right moment (stimulus-driven)
Extrinsic timing= determining time of occurance of some characteristic of an action by a stimulus event.
Instrinsic timing= determining the time of occurence of some features of an action by a process internal to the NS.
One of the frontal motor areas.
- PPN- carries signals to spinal cord
- Globus Pallidus
- Caudate nucleus
Neurodegenerative disease. Large neuron death in the substantia nigra specifically in the pars compacta. In the early stages neuron loss is greater on one side meaning symptoms are worse on one side.
Neuron loss in substantia nigra --> decreased output to putamen (less excitatory input) --> reduction in neurones that inhibit internal globus pallidus --> neurons of IGP disinhibted- more active - output increased --> the output is inhibitory. This provides outputs from basal ganglia so inhibiton can explain the lack of voluntary movement that produces the symptoms of bradykinesia, akinesia and hypometria.
Early signs of Parkinsons include increased chance of falling and stooped posture. The most characteristic symptoms is trembling of the upper limbs when at rest at a frequency of around 5Hz.
Bradykinesia- slowness of voluntary movements
Hypometria- increased muscle tone increases resitance to external forces
Akinesia- voluntary movements of inadequate amplitude
Motor actions unaffected by Parkinson's
Akinesia does not affect one type of action: interceptive actions.
Interceptic actions and other actions involving interaction with objects in motion (like hitting a ball) are unaffected. This is paradoxical as they seem more complex and difficult than other movements.
However, these actions are stimulus elicited rather than volitional. It could be that stimulus elicted signals are processes via the dorsal stream of visual processing which does not lead to visual awareness.