Sensation and Perception Sensation and Perception The link

Sensation and Perception

Sensation and Perception • The link between the outer physical world and the inner psychological world: – The means by which we understand the world around us. • Different forms of energy from the physical world are transduced by specialized receptor neurons into neural impulses. – The process is called sensory coding. • It is transmitted to the thalamus which then relays it to the appropriate areas of the cortex. • The cortex then interprets the sensory coding to form our perceptions of the world around us.

The Sensory Systems Sensory System Stimulus Energy Specialized Receptors Vision Electromagnetic energy (light waves) Rods and cones in the back of the eye. Hearing Sound waves Hair cells in a special organ in inner ear. Touch Pressure on the skin Touch receptors embedded in skin. Pain Intense stimuli such as pressure, heat, cold. Pain receptors embedded in skin. Taste Water soluble chemical energy. Taste receptors in taste buds in tongue. Smell Air soluble chemical energy. Olfactory receptors in mucus membranes of nose.

Sensory Systems • Designed to respond to change in environmental stimulus. – Become less sensitive to constant stimulation over time. – Called sensory adaptation. – When a continuous stimulus stops a sensory system will again react more strongly. W. W. Norton

The Visual System How Does it Work?

Electromagnetic Energy www. ucar. edu/learn/images/ spectrum. gif

Light • Visible spectrum ranges from 400 to 700 nanometers (1 millionth of a millimeter). • Below 400 nm is ultraviolet, above 700 nm is infra red. Not visible but affect our systems. • Light can be emitted from a source, or reflected. In either case it varies in intensity (number of photons) and wavelength. • Most light that reaches the eye has been reflected off the objects we are viewing.

The Eye Fovea

The Lens of the Eye Lens Incoming light Focal Point • If the lens of the is working properly the image will be focused directly on the retina. It accommodates to varying sizes and distances of objects to focus the image on the retina at the back of the eye, ideally on the fovea.

Focusing on the Retina

Errors of Refraction Myopia--Nearsightedness Far objects tend to be blurry.

Errors of Refraction Hyperopia--Farsightedness Close objects tend to be blurry.

Errors of Refraction Presbyopia • As we age, the lens of the eye becomes less flexible and therefore less able to accommodate to variations in the light. • We become increasingly farsighted—it becomes difficult to focus clearly on objects that are close up. • Almost everyone over the age of 45 requires corrective lenses for presbyopia.

Eye Movements • Saccades: Constant small jittery movements. Eyes are never still. • Conjugate movements: Eyes move together. • Pursuit movements: Eyes track moving targets.

The Retina • Composed of layers of several types of cells. 1) Light sensitive receptor cells, called rods and cones. 2) Horizontal cells. 3) Bipolar and amacrine cells. 4) Ganglion cells. • Light must pass through all these layers, from 4) to 1) to stimulate the rods and cones.

The Rods • Contain a photopigment called rhodopsin that bleaches (breaks down) when exposed to light. • Degree of bleaching depends on the intensity of the light. • This starts neural activity that is passed from the rods to the horizontal cells, bipolar and amacrine cells, and finally to the ganglion cells. • Rods transmit brightness information—black, grays and white. Are not sensitive to wavelength. • Rhodopsin is constantly restored unless the mammal is malnourished. Requires Vitamin A.

The Cones • Four types, three of which have different photopigments. • Each of the three main ones is maximally sensitive to a different wavelength of light. They are responsible for our colour vision. • The fourth type, reported in 2002, seems to have something to do with our circadian rhythms. Explains why blind people can adjust their sleep/waking rhythms to daylight savings time, and to jet lag.

Rods and Cones

Distribution of Rods and Cones

Distribution Varies--1

Distribution Varies--2

Other Cells of the Retina • The retina includes layers of horizontal, bipolar, and amacrine cells that finally synapse with ganglion cells. • It is the axons of the ganglion cells that form the optic nerve. • The optic nerve leaves the back of the retina at the area of the optic disk, making us blind to light that falls on that small area of the retina. • All these cells continue the sensory coding process. • Approximately 120 million rods and 6 million cones converge on about 1 million ganglion cells in each retina.

Gazzaniga and Heatherton, 2003 W. W. Norton

How Does the Varying Sensitivity Affect Vision

Colour Vision: Properties of Colour Sensation • Hue: varies with wave length • Brightness: sensation without hue, ranges from black through greys to white. • Saturation: depth, or purity of hue,

Colour Vision: Properties of Colour Sensation Hue: varies with wave length • Most hues are composed of several wave lengths. • Three unique hues: blue (465 nm), green (465 nm), yellow (570 nm). Unique red is contrived.

Colour Vision: Properties of Colour Sensation Brightness: sensation without hue, ranges from black through greys to white. • Applies to both achromatic and chromatic colour sensation. • Achromatic—without hue, all greys • Chromatic—with hue, colours can vary in brightness.

Colour Vision: Properties of Colour Sensation Saturation: depth, or purity of hue • Blue wavelengths of different saturations, but same brightness. Darker indicates greater saturation. The lighter are purer (more saturated) blues than the darker.

Colour Vision: Properties of Colour Sensation Colour of certain hue, brightness, and saturation. Different hues, same brightness and saturation. Same hue, same brightness, different saturations.

Spectral Sensitivity of Rods and Cones

Colour Vision: Theories • Young-Helmholtz Trichromatic Theory • Opponent Process Theory • Both are necessary to explain all the facts.

Colour Vision: Mixing Colours • Subtractive colour mixtures: using filters to permit only certain wavelengths to pass through, or mixing pigments. We experience a narrow band of wavelengths. • Additive colour mixtures: Two ranges of wavelength are reflected back to the eye at the same time. Receptors respond to a wider band of wavelengths and the sensation will be that produced by the wider band of wavelengths.

Colour Vision: Trichromatic Theory • Support: It is possible to create any hue experience in our visual system by combining only three wavelengthts of light. • We have three types of cones, each maximally responsive to either long wavelengths (640 nm), green wavelengths (500 nm), or blue wavelengths (460 nm). • The combination of responses in the three types of cones can explain our sensitivity to various wavelengths.

Trichromatic Colour Theory at The Level of The Cones R Red cones fire at 50 G B Green cones fire at 200 Blue cones fire at 175 Transmitting the colour experience of

Colour Vision: Trichromatic Theory • However, trichromatic colour theory cannot explain why we can see colours for wave lengths that are not stimulating our receptors. • These are called after images.

Colour Vision: After Images • After images suggest some kind of opponent process system. • When the ‘red’ receptors tire, we then have the experience of seeing green. • After image research suggests the system is wired in opponent pairs, red-green, blueyellow, and black-white.

Colour Vision: Opponent Process Theory ü Need something to explain: – Reducing numbers of neurons in the system. – We don’t see reddish-green or bluish-yellow. – After images. ü Beyond the level of the cone cells, there are ganglion cells that become excited in response to one set of wave lengths and inhibited for the colour opposite. ü Some are in blue-yellow pairs, some in red-green pairs and some in black-white pairs.

Colour Vision: Opponent Process Theory ü If the receptor cells signal a green tone, some of the ganglion cells will increase their level of firing —excitatory impulse. ü If the signal is a red tone, they will decrease their level of firing-inhibitory impulse. ü Similarly there are ganglion cells that are excited by yellow tones and inhibited by blue tones. ü The degree of excitation or inhibition will depend on the intensity and purity of the wavelengths that originally excited the cone cells.

Trichromatic Theory: At the Level of the Cone Cells This could be one pure wave length but is more likely a mixture of long, intermediate, and short wave lengths. R G B +70 +200 +175 The red, green, and blue cones will signal this wave length with different rates of firing Hypothetical firing levels. The horizontal cells will combine the effect of the cones’ firings and transmit it to the bipolar and amacrine cells, that will, in turn pass on the combined effect to the ganglion cells.

Opponent Process Theory: At the Level of the Ganglion Cells + -- -- ++ + - -- The intermediate bipolar and amacrine cells signal the ganglion cells that this was the light stimulus that started the process. +++ Hypothetical ganglion cells receiving input from many intermediate cells Input from long (red) wave lengths and from short intermediate (green) wave lengths will have opposing effects on the firing of an opponent process redgreen ganglion cell. Similarly, short (blue) and long intermediate (yellow) wave lengths will have opposing effects on the blue-yellow ganglion cells. The firing of an individual cell will depend on the combined effects.

The Sensation of Colour Beyond the Ganglion Cells Optic nerve formed from the axons of the ganglion neurons The axons do not synapse with other neurons until they reach the lateral geniculate nucleus of the thalamus. The opponent process effect is continued here. It is from the thalamus that the combined effect of their many firings is transmitted to the cortex.

Optic Tract (once more)

Colour Blindness ü If one subset of cone cells is inactive, nonexistent, or has the wrong pigment, this will affect colour vision. ü If a subset of the ganglion cells is inactive or nonexistent, this will affect colour vision. ü Similarly, problems with any of the later neurons specialized for transmitting wave length information, or in the receiving areas of the cortex may affect colour vision.

Colour Deficiencies Some Colours Are Missing

Image from: http: //www. psychology. psych. ndsu. nodak. edu/mccourt/ website/htdocs/Home. Page/Psy 460/Color%20 Vision. html

Colour Blindness • There are many different kinds of defective colour vision: – Sex-linked red-green colour blindness results from a defect in either the pigment for the red cones, or that for the green cones. – These defects alter the spectral sensitivity curves for the particular cones involved so that the cones are sensitive to different wave lengths of light that normal. – It results in four different kinds of red-green colour deficiency

Colour Blindness: One Type of Red-Green Deficiency Normal spectral sensitivity Green cones now have spectral sensitivity shifted much closer to that of the red cones.

Red-Green Deficiency Simulation: With only red sensitive cones Picture reflecting primarily red and green wave lengths. Simulation: With only green sensitive cones Probably all of these pictures would be indistinguishable for someone with any red-green deficiency. Images from http: //www. firelily. com/opinions/color. html

Colour Blindness As seen by someone who is not sensitive to blue wave lengths. Picture of poppies and cyclamen as seen with someone with normal colour vision. As seen by someone who is not sensitive to red wave lengths. Images from http: //vischeck. com/examples

Different Colour Vision Across Species Monkey sees Owl sees Human sees