Neural Coding Flashcards
What is perception?
the detection and interpretation of sensory information that may be used to guide action.
What is sensory information?
patterns of energy, chemical, mechanical or electromagnetic, that correspond to significant entities or events in the environment.
What are sensory processes?
strategies and mechanisms used to detect and encode sensory information.
What are sensory systems?
Categorised based on what form of energy they detect.
Regardless of sensory modality, external energy patterns are transduced to electrical signals in receptor neurons.
What are sensory receptors?
Olfactory: line the throat and nose and have small hairs which allow specific chemicals to bind to them and trigger electrical signals conveyed to the brain.
Auditory: contain cilia which are bent by vibrations in the cochlea of the inner eat which are then converted to patterns of electrical signals.
Muscle stretch: processes embedded in muscle fibres which are distorted when the muscle stretches or extends and then conveyed to the brain as electrical signals.
Cutaneous: embedded in the skin and distorted when pressure is applied which is conveyed to the brain as electrical signals.
Photoreceptors: in the retina and contain stacks of photopigments which interact with light to produce electrical signals which are conveyed to the brain.
What are neurons?
Receptor neurons convert patterns of external energy into internal electrical signals providing the input for the nervous system.
Interneurons process information by mapping patterns of sensory inputs onto the appropriate motor neurons.
Motor neurons control muscles and provide the output of the nervous system.
All have: dendrites, soma, axon and form synapses.
Membrane potential: voltage difference between inside and outside of cell that arises from differences in chemical composition of intracellular and extracellular fluids.
Resting potential: -40mV relative to the outside.
Changes in this potential can be used to transmit information across the cell.
What are graded potentials?
A change in the electrical charge across the entire membrane of a neuron
Continuous variations in the membrane potential could represent continuous variations of values of some information such as intensity of stimulus.
Fast and can be decoded instantaneously.
Small change in voltage at input end should be transmitted instantaneously to output end.
Infinitely graded meaning a wide range of values could be coded in this way.
The effect is omni-directional so that inputs which converge on the cell membrane from several cell sources can be easily summated.
The cell membrane is a poor insulator so change in voltage get weaker with distance from any given point.
This means that graded potentials can only be transmitted over very short distances.
Thus they are only used in dendrites where integrating a large number of inputs from other neurons by means of summation is a priority or in sensory receptors where photoreceptors and interneurons need to integrate response to light rapidly over very short distances.
What are action potentials?
A change in the rate of electrical impulses along axons.
Relies on specialisms in the nerve membrane that mean when the membrane potential is depolarised it generates an active wave of depolarisation along the length of the membrane by successively opening and closing channels in the membrane allowing charged ions to diffuse through.
Only activated if membrane depolarisation is sufficient to reach threshold.
Fixed amplitudes so voltage can’t be used to encode intensity of stimulus.
Can be transmitted over very long distances but travel more slowly than graded potentials.
Travel at fixed speeds dependent on the diameter of an axon and how well insulated it is.
Refractory period where channels are reset meaning there is an upper limit of the rate of action potentials.
Can only encode a limited range of values.
Unidirectional which is an advantage in motor neurons controlling muscles.
Take a relatively long time to decode.
These limitations can be circumvented to some extent by means of place coding and ensemble coding strategies at the cost of having to devote considerably more neurons to the task.
What is Mueller’s Law of specific nerve energies (1835)
Mueller observed that each sensory neuron gives rise to its own characteristic sensation regardless of how it is stimulated.
Pressing hard on his closed eye produced the experience of flashes of light.
Thus increased pressure produced electrical signals that were interpreted by the brain as light.
Therefore it can be deduced that any kind of stimulation of the optic nerve gives rise to the sensation of light.
What did Penfield’s (1950) brain stimulation studies find?
Brain contains no sensory neurons so surgery can be conducted while patients are awake.
Used electrodes to map the functions associated with the cerebral cortex and produced detailed maps of which part of the cortex were associated with different sensory and motor functions.
Each sensory pathway projects to a specific part of the cerebral cortex and direct stimulation of the primary sensory cortex gives rise to its characteristic sensation.
Now use transcranial magnetic stimulation to produce the same results.
What determines the quality of the sensation is what part of the cortex receives the stimulation.
What are phantom limbs and how do they explain how the quality of a stimulus is encoded?
Amputees report the sensation of being touched on their missing arm when their face is stroked.
This is because the part of the somatosensory cortex associated with the face lies immediately next to the part that represents the arm.
In the absence of normal sensory inputs, stroking the face can cause activation in the area of the brain associated with the arm and is subsequently interpreted as coming from the arm.
What is frequency coding?
Intensity is encoded in a number of ways.
The firing rate of action potentials vary according to the stimulus and can be used to encode intensity.
Firing rate varies in proportion to intensity of stimulus.
Economical as one neuron is sufficient to encode a range of values.
Refractory period limits firing rate to 500 spikes per second.
Firing rates take time to decode.
The range of light sensitivities is 10,000 mil and outnumbers the possible firing rates.
What did Hartline and Graham’s (1932) studies with limulus find?
Orthropods so have compound eyes which are convenient for studying optic nerves
Between each facette in the eye is an ommatidium which is the structure that converts light to electrical signals.
It is possible to record frequency of action potentials produced by a single ommatidium in response to light projected onto it.
Firing rate increases as light intensity increases
For every 10 fold increase, the frequency of firing rate increases by 20 spikes per second.
Firing rate is proportional to the logarithm of the intensity of light, hence logarithmic encoding.
This phenomenon is universal in sensory systems.
It is an effective way of mapping a wide range of stimulus intensities onto a small range of neuronal firing rates.
What is weber’s law?
k=∆I÷I
psychophysical correlate of logarithmic coding
Interested in measuring difference thresholds, the smallest differences in intensity in two stimuli that can just be detected.
Two weights
The just noticeable difference depends on the magnitude of the reference weight
The lighter the reference, the smaller the just noticeable difference and vice versa
The ratio of the just noticeable difference to reference stimulus is constant
Each modality has a characteristic weber fraction
Lifted weight 0.02
Light intensity 0.08
Implies log relationship between stimulus intensity and response
Fixed difference in firing rate corresponds to a fixed ratio
What is logarithmic encoding?
Overcomes the limitation of range of firing rates but takes much longer to decode the chain of spikes.
Experimental recordings found that the brain takes 100ms to discriminate between 9 and 10 spikes, which cannot be done reliably.
However it can be estimated by measuring latencies of responses of neurons in the visual system and taking into account the number of synapses involved in the circuit.
For example the anterior inferotemporal cortex responds to images of faces in 80-100ms and involves 10 synapses so cannot possibly be frequency coding even on a logarithmic scale.
