Chapter 6 from LECTURE: Exteroceptive sensory systems: bring info from the external environment to the body
Sensation: the process of detecting a stimulus (receptor)
Perception: the higher order process of integrating, recognizing and interpreting patterns of sensations
Case example: the man who mistook his wife for a hat
Hierarchical Organization: (receptor to thalamus, to thalamic nuclei, to cortex)
Receptors: different for vision, hearing, olfaction, etc. and they project to other structures
Other structures (eg. Thalamic nuclei): then project to the cortex
Cortical Processing: primary sensory cortex (specific to each sense), secondary sensory cortex (by large also specific for sense),
associate cortex (receive input from multiple senses and integrate them together).
Parallel processing. *picture on page 2.
Vision : produces an internal representation of the outside world. Not always an accurate representation of reality.
A perception of reality based on specific receptors (hardware)
o Interspecific differences: eg. Snakes can see infrared, insects see UV light
o Intraspecific differences: eg. Colour blindness
Top-down influence: attention, individual experience, etc. affect perception
o Eg. Same scene viewed differently by different people with different interests
Perception of Light: light is an electromagnetic wave (energy) that can be represented by discrete units/particles of energy called
photons. Very fast, travels at 300 000 km/sec or 3.0 X10 m/s. Visible light for humans: 380-700 nanometers (10 m). Different
wavelengths equal different colours; different intensities are seen as differences in brightness.
Anatomy of the eye:
Iris: contractile tissue: it regulates the size of the pupil
o Wide pupil: more light sensitivity: less acuity
o Narrow pupil: lower light sensitivity: high acuity
o Light sensitivity is the ability to detect an object in dim light, and acuity is the ability to see fine details.
With two eyes:
Lateral: very wide field of view (pigeons’ eyes are so lateral, they can see behind them)
Frontal: smaller field of view
Frontal eyes in Humans: 3D depth and distance due to large overlap, so seen by two eyes from different points of view, so
the input from the two eyes is slightly different, which is called binocular disparity.
Refraction: when light comes from really far, they are practically parallel so they just get bent by the lens onto the retina, I
assume the lens is stretched, flat.
Accommodation: changing shape of lens to adjust to near and far objects using ciliary muscles so light hits the retina properly,
more bending of light.
Presbyopia: reduced ability to focus on near objects with age lens becomes less flexible. Need glasses for reading (convex lens)
Cellular Structure of the Retina: retina is inside out; light must pass through the axons and ganglions to get to receptors which are
embedded in the retinal tissue of the retinal pigment epithelium and who are connected to epithelial cells that take care of them. If
they slip out of the retinal pigment epithelium, they’ll die and you will go blind. Squids have the ‘right way’ retina, that you’d think it
would be like. This poses two problems: distortion because rays of light must cross of the layers of cells, affects precision and acuity of our vision
(fovea is where you have the sharpest vision) because cells are bent to the sides so that the light directly reaches the receptors), and
blind spot (meeting of axons with no receptors to optic nerve; completion fills in the blind spot)
*draw out retina cell layers.
Duplexity Theory of Vision
Photopic vision: during high intensity illumination, CONE mediated
o Low convergence: low sensitivity + high acuity (knows exactly where it comes from, single cell, details)
o More intense signal (light) required to activate retinal ganglion cell.
o Colour vision mediated by cones
o High spatial resolution
Scotopic vision: involved when there is low levels of light, ROD mediated
o High convergence: high sensitivity + low acuity
o In low lighting, the weak signals picked up by several receptors can add up to cause action potential in retina
o Spatial ambiguity: low acuity
Saccades: very quick eye movements (involuntary, scanning vision field) allow fovea (only has cones at the center “sharp
vision”) to get good acuity. Visual system integrates all the information from the scanning to give us the perception of a
single image that is sharp.
Stabilized retinal image: images disappear and then reappears in waves
Transduction: conversion of one form of energy into another
Visual transduction: light into neural signal. Pigments in the eye are ‘opsins’
Rhodopsin: a red pigment in rods
o Dark: red
In the dark rhodopsin is inactive. cGMP keeps Na channels open. Na influx partially depolarizes the cell
membrane. Rods continuously release glutamate.
o Light: bleaching
In+the light rhodopsin: g-protein coupled membrane receptor is activated by light. cGMP is broken down,