PSY 310: Sensory and Perceptual Processes 1

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1 Prof. Greg Francis and the eye PSY 310 Greg Francis The perceptual process Perception Recognition Processing Action Transduction Lecture 03 Why does my daughter look like a demon? Stimulus on receptors Attended stimulus Environmental stimulus Neural circuits are specialized to process certain types of information Blowing up a part of the image highlights a odd feature about how the brain works Some of the information from the retina crosses on its way to the brain Contralateral processing Neural fibers from eye cross on way to cortex The left hemisphere of the brain takes in input from: the left part of the left eye The left part of the right eye ==> objects to the right of where you are looking The right hemisphere of the brain takes in input from: the right part of the left eye The right part of the right eye ==> objects to the left of where you are looking An fmri brain scan demonstrates that different hemispheres respond to contralateral stimuli Movie: moving wedge 1

2 Prof. Greg Francis The two hemispheres are pretty similar On each trial you saw two oddly constructed faces for most perceptual processing And had to judge which face appeared younger But they differ for cognitive processing The right hemisphere plays a bigger role than the left in judging faces If you look at the middle of an image, the left side goes to the right brain, and the right side goes to the left brain If you look at the middle of an image, the left side goes to the right brain, and the right side goes to the left brain This face might look young, because the judgment is based more on the left side of the image. This face might look old, because the judgment is based more on the left side of the image. The faces are actually mirror images of each other The hemispheres of left-handed people are not as different as for right-handed people Experiment measures the percentage of choices with younger half face on left Handedness Percentage Left 57 Right 67 2

3 Prof. Greg Francis Visual perception involves the detection of light We interpret patterns of light that vary in space and time in a way to learn something about objects in the world This wasn t always obvious People used to think that perception involved some kind of ray going out of the eyes to touch objects The fact that photographs are convincing demonstrates that this idea is not correct is electro-magnetic energy It can be described as a wave The distance between two neighboring peaks of a wave is the wavelength Usually given in nanometers It can be described as a wave The distance between two neighboring peaks of a wave is the wavelength Can also fix a distance and talk about the number of peaks in the wave frequency People are visually sensitive to only a small fraction of the wavelengths of light Different species are sensitive to different wavelengths of light Color The wavelength of light often corresponds to a particular color E.g. blue skies are blue because they scatter low wavelength light more than high wavelength light The non-scattered light reaches our eyes directly (the sun looks yellow, orange, red) The scattered light is what we see when we look to other parts of the sky But be careful! Color is a perceptual experience, not a property of light The magenta and orange have exactly the same wavelength! 3

4 Prof. Greg Francis Illuminant and reflectance We need light to see things, but we are not often interested in sources of light. Illuminant and reflectance We are really interested in objects from an illuminant reflects off of objects A source of light is called an illuminant energy reflected of an object or surface is luminance energy falling onto a surface is illuminance Measuring light intensity Most illuminants produce light at lots of different wavelengths So how do we measure the strength of a source of light? reflects off objects and some of it enters the eye We only sense a small part of the wavelengths Cannot just measure total electro-magnetic energy Who cares about the radio waves or gamma waves? They do not contribute to visual perception. There are special calculations that weights different frequencies to include only those important for visual perception Candelas per square meter Foot-lamberts Cross-section shows more detail is hollow, but filled with clear fluid The cornea is a clear lens So is the lens Cross-section shows more detail enters the pupil The cornea is a clear lens So is the lens They help focus light They help focus light 4

5 Prof. Greg Francis The interior of the eye is hollow, but is filled with a clear liquid passes through the cornea, lens, and fluid and projects on to the back of the eye The retina The image is upside down because the lens inverts the image You change the shape of the lens to make certain the image is sharp on your retina Why is the pupil black? Accommodation (automatic) As you age, the lens loses flexibility and cannot be shaped so well Then you need glasses Much of the visible light that goes into the eye is absorbed by the detection process (next time) What is reflected back comes out of the right back to the source Unless your eye is right where the source of light is located you do not see the light coming out of the eye Why is the pupil red in photographs? With a camera s flash, the camera lens is very close to the source of light (the flash) So it does record some of the light coming out of the eye Thus we get red-eye Thus it looks black 5

6 Prof. Greg Francis Wavelength Luminance Color is a percept Eye Gross anatomy Lens pupil Conclusions Next time More detail on the properties of the retina Detection of light energy Fovea Rods Cones Visual acuity 6