Lecture 7: Sensing the World I - Senses and Vision Flashcards
Define sensation and what does it involve
The capacity to detect a particular physical or chemical stimulus (external or internal). Involves sensory organs (e.g. skin) and afferent nerves.
Define perception and what does it involve
The conscious experience and interpretation of the sensory information. Involves neurons in the central nervous system.
How many senses do humans have and what are they
five senses - audition, olfaction, taste, vision, somatosensory/touch
What are stimuli and how do we detect them
Stimulus: a thing or event that evokes a specific functional reaction in an organ or tissue. It can be physical (sound, visible light, heat, magnetic field and UV light) or chemical (taste, odour, pheromone).
stimulus comes from environment and a sensory organ detects it in the organism.
What do sensory organs do
they perform transduction to convert stimuli into the nervous system language called neuronal activity. Each sensory organ deploys a specific mechanism to transform chemical or physical attributes of stimuli to neuronal activity.
Give the sense, organ and stimulus
1) Somatosensory - skin and tissues - pressure, warmth, cold, pain.
2) Sight - eyes- light
3) Hearing - ears- air vibration
4) Smell -nose- volatile chemicals
5) Taste - mouth - soluble chemicals
Examples of senses animals have that humans do not
- electrolocation (fish)
- ultrasounds (bats - don’t need light)
- infrasound (elephants)
- magnetoreception (pigeon/cows/turtles)
How have elephant fish evolved to adapt to muddy water and sense their environment
Through electrolocation - weakly electric fishes produce and detect small current flows around them and they use these currents to see objects (electrolocation) and communicate with conspecifics (electrocommunication). Tail is electric organ, electroreceptors in the body.
How do cockeyed squid adapt to the dark environment
They have asymmetric vision where the large yellow/red eye looking upward detects predators against the dim sunlight.
The small eye looking downward detects prey bioluminescent signals. Squid floats and swims slightly tilted for this reason.
What are sensory receptors and what are some examples?
Cells that specialise in converting external stimuli (physical or chemical) into neural (electrical) activity.
1)Photoreceptors: detect light - vision
2) Mechanoreceptors: detect movement - sound, texture, blood pressure and muscle stretch.
3) Chemoreceptors: detect chemicals compounds - smell, taste.
4) Nociceptors: detect tissue damage - pain
What are receptive fields?
Every sensory organ or receptor detect a specific part/quality of the world.
e.g. each photoreceptor points to a unique direction, sensing the light present in this point in space.
Do sensory receptors all have the same density and sensitivity?
No, receptor distribution is not homogenous e.g. two-point discrimination test.
Receptors also have thresholds: minimum stimulus intensity that is detected. e.g. auditory receptors to different frequencies.
Does sensory = perception?
No. Sensory information is modified at each relay, allowing for the construction of different aspects of the sensory experience. For the brain, it is more important which nerves are stimulated rather than how as the only input it receives are action potentials through sensory nerves. Our senses tell the brain what is ‘real’.
Define perception in contrast to sensation
Perception involves not only the sensation but also the context, emotional state or memories which can affect how we perceive the same sensory experience. E.g. optical illusions like Rubin’s vase or Muller-Lyer illusion.
Describe human vision abilities
Humans possess highly developed day colour vision but are also capable of detecting light coming from galaxies 2 million light years away from earth.
Our visual cortex extends further than any other sensory cortices - through seeing and perceiving the world around us, we can assess reality but also can produce adaptive behaviours.
Can someone with very damaged visual cortex still navigate themselves?
A patient had intact navigation skills after bilateral loss of striate cortex
How is light detected
Visible light is a stimulus and is a section of the electromagnetic spectrum that human eye can detect.
Light can enter the eye from a source of light or after bouncing on an object.
Describe anatomy of human eye and how does it work
- retina
- fovea
- optic disc (blind spot)
- blood vessel
- optic nerve
- sclera
- cornea
- iris
- pupil
- lens
The iris opens and closes to allow more or less light through the pupil. The cornea and the lens focus the light into the retina, where the image is recreated. - The optic disc allows vessels going in and out and the optic nerve to carry visual information to the brain.
What photoreceptors do humans have in the eye
Cones (~6m) and Rods (~120m) in the retina.
Densely packed cones mainly cover the fovea for acute daylight vision whereas the periphery is covered by rod, which are more sensitive to light than cones (sense very dim light).
What are the three types of cones
Blue, green and red - each type is sensitive to a specific wavelength band.
What is the fovea
Used to focus on something
What happens if you focus on a light in the dark?
You can’t see it if you focus on it but you can if you look away because the rods in the periphery have much higher sensitivity to light + less spatial resolution, whereas the fovea (the focus point) has cones which are less sensitive.
Fovea = lots of light + colour
periphery = dim light
How do photoreceptors work in the retina in a molecular level
In the dark, the photoreceptors aren’t activated so guanylate cyclase (enzyme) releases cyclical GMP (cGMP) which keeps one ion channel open for sodium and calcium to always come in (constant flow), which keeps the photoreceptor depolarised, releasing glutamate.
In light, Rhodopsin (photopigment) detects light and reacts with a series of chemical reactions resulting in destruction of cGMP and therefore the sodium/calcium channel closes, hyperpolarising the cell and therefore inhibiting/reducing glutamate release.
How do photoreceptors and bipolar cells affect transduction in the retina
They do not fire action potentials, they change their membrane potential, which affects the probability of neurotransmitter release.
