Lecture 6 - Signals to sensation Flashcards
The senses
Convey specific information about some source of physical energy
Each sensory modality has specialised receptors for transduction
Electrical impulses and chemical synapses to communicate to brain - must be changed to this from stimulus
Labelled line- informs brain about only a certain stimuli
The Pacinian corpuscle- ductile receptors, large and flat, detect pressure and stretching on skin -> highly specialised just to touch
Each sensory modality has evolved to fit animal’s need
Intensity coding
Location coding
Coding for specific object properties (colors, shapes, pitch)
Sensory receptors
All animals have specialised body parts that are sensitive to some forms of energy
Sensory Receptor Organs
Act as filters of the environment: they detect and respond to some events and not others
Stimulus
Event that affects the sensory organ
Allow signal in
Specific to each modalitiy, even within the modality
Specialised sensory receptor organ for different stimuli
Brain - coherant representation of stimuli by summing all the stimuli
Principles of sensory coding
All do same thing for different stimuli -> environmental signals -> neural activity (chemincal/ electrical impulses)
Methods for studying sensory processing:
- Psychophysics: behavioral testing to establish the sensitivity of a sensory system and the “rules” of its operation.
- Electrophysiological recording: from single neuron or small groups of neurons along the sensory pathway to find out how the neural circuitry gives rise to the perceptual abilities.
- Neuroimaging in humans: perceptual tasks to identify the brain areas responsible.
What must be sensed
Qualitative features (colour or odorant) -> modality
Quantitative features (magnitude) -> intensity
Temporal features (duration or frequency) -> duration
Spatial location -> where is it?
All estimated at once -> separate neural pathways specialized for estimating different types of stimulus features
Areas of the Occipital Cortex
V1: segregates pattern vision from motion signals
V2: 3D vision
V3: shape perception
V4: colour area and shape perception
V5: motion area seeing camouflage, more complex patterns
Function of a sensory system
Detection
Weak signals can be detected without the animal being able to finely discriminate any of its features.
This is often referred to as estimation.
Discrimination
Features of the signal
Adequate stimuli
Type of stimulus for which a given sensory organ is particularly adapted
Transducer function of receptors
The process in which a stimulus energy is transduced into the electrical response
Stimulus -> Electrical Response -> Brain
Sensory processing
Sensory processing starts with receptor cells
A give receptor cell is specialized to detect particular energies or chemicals
Upon exposure to the stimulus, a receptor cell converts that energy into a change in electrical potentials across its membrane
Changing the signal -> SENSORY TRANSDUCTION
Signal -> Collection -> Transduction -> Provoking -> Action
Transduction of mechanical signals
Excitatory events:
- mechanical stimulation deforms the corpuscle
- deformation of the corpuscle stretches the tip of the axon
- stretching the axon opens mechanically gated ion channels in the membrane, allowing sodium ions to enter
- when the receptor potential reaches the threshold amplitude, the axon produces an action potential
What do sensory receptors do
- Sensory transduction
They transduce [transform/translate] the energy of a stimulus into a change in membrane potential - Amplification
They strengthen the energy of the stimulus
The action potential conducted from the eye to the brain contains 100,000 times more energy than the few photons of light that stimulated the receptor - Transmission
Action potentials from receptors, or from neurons connected to receptors, reach the CNS - Integration
Receptors contribute to the processing of a signal.
For example many receptors show sensory adaptation.
This term means that they respond less during continued stimulation
4 attributes of stimulus
Modality
Nature of a stimulus -> labelled lines
Different receptor for different feeling (pain vs. touch completely different)
Stimuli to receptor specificity
Same organ (e.g. skin) -> detect and process multiple stimuli types simultaneously (e.g. touch and pain)
Independent pathways for touch and pain
Shape depends on function
Intensity
Estimated intensity not linear function of the actual intensity
Power law = Relation between stimulus strength and perceived stimulus intensity
Respond to range of intensity -> neuron can code intensity by changing the frequency of action potential transmitted (best to mid-low range)
Stimulus strength increase = new neurons are recruited -> Intensity represented by the number of activated cells
Range Fractionation = different receptors are specialist in particular segment (fractions) of an intensity scale
Different neurones respond to the different intensity -> if low intensity then low threshold neurone responds
Summation of neurone responses
Allows faster responses representing all intensities dependent on number and instensity of neurones recruited
Location
The position of an object or event (either outside or inside the body)
Localising external events in order to know the location of stimuli, even before it has a chance to harm
Each sensory receptor activates pathways that convey unique positional information Receptive fields vary in size
Smaller receptive fields = GREATER ACUITY
Duration Adaptation Tonic receptors: Produce constant rate of firing as long as the stimulus is applied -> no adaptations Pain Phasic receptors: Burst of activity but quickly reduce firing rate (adapt) if stimulus is maintained -> fire when on and again when off Sensory adaptation
Some stimuli need to respond imediatley and some don’t need respond at all -> receptor adapt to inform brain about duration
Receptors in somatosensory system vs. auditory and visual systems
Somatosensory system: receptor is a specialized peripheral element that is associated with the peripheral process of a sensory neuron
Auditory and visual systems: a distinct type of receptor cell is present.
Auditory -> Receptor synapses directly on the ganglion cell
Visual -> Interneuron receives synapses from the photoreceptor and in turn synapses on the retinal ganglion cell.
Labelled line concept
Information already labelled as certain type due to sensory receptor which recieved it -> reason for the name as labelled line down which the impulses travel such that when they arrive at the brain, the brain knows the type
Touch vs. pain receptors
Skin full of different receptor types
Touch receptors
Pain receptor -> if touch this then instead feel pain
Experiment involving providing adequate stimulus
If stimulate after receptor, but before brain (in the neurone) -> cause the brain to feel as though they have been touched where the receptor is
Insert small needle into the nerve (e.g. easy in feet) -> deliver electrical impuleses -> sensory neurone that connected to is ductile -> feel as though being touched on skin, if was pain -> feel as though painful pins and needles
Receptive fields
A region of space in which a stimulus will alter the neuron firing rate
Subthreshold stimuli can be summed by the secondary neuron
The receptor of each primary neuron can pick up information from a specific area (receptive field)
Receptive fields can be irregular in shape and overlap with receptive fields of other neurons
All very close together
Random organisation- overlapping -> allows greater localisation of exactly where stimuli is
