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Lecture 7

Lecture 7 - Jan 30.doc

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PSYC 211
Yogita Chudasama

PSYC211 Lecture 7 - Jan. 30 The Stimulus: • Our eyes detect the presence of light The eyes respond to waves of electromagnetic energy between 380 and 760nm in • length • Wavelength is determined by three dimensions: • Hue is an attribute of perceived colour (e.g. Red) • Brightness provides intensity to the colour Saturation is the relative purity of the perceived light • Example of Colours with the Same Dominant Wavelength but Different Levels of Saturation or Brightness: Movement of the Eyes: • Vergence movements keep both eyes fixed on the same target • Rapid, jerky saccadic movements shift your gaze from one point to another • Pursuit movements allow us to maintain a moving object Anatomy of the Eye (I): • The conjunctiva is a mucous membranes that line the eyelid • The cornea is the outer, front layer of the eye. It is transparent and admits light • The iris is a pigmented ring of muscles • The lens consists of several transparent layers. The ciliary muscles can change the shape of the pupil to allow the eye to focus, a process known as accommoda- tion • The pupil regulates the amount of light entering the eye. It is an opening in the iris • The sclera is opaque and does not permit entry of light • The interior lining of the eye is the retina. Photoreceptors called rods and cones are located here. Rods are sensitive to low light intensity (i.e. Vision in the dark). Cones are essential for colour vision therefore useful in bright light and daytime vi- sion • Light passed through the lens and crosses the vitreous humour, a clear, gelati- nous fluid • The central region of the retina is the fovea. It contains the highest number of cones • Site of blind spot is the point at which the optic nerve exits through the back of the eye. It has no receptors The Route Within the Retina: • Light passes through transparent cells and stimulate the photoreceptors located at the back of the eye • Photoreceptors then send messages to bipolar and ganglion cells located closer to the centre of the eye • The ganglion cells’ axons loop around each other and travel back to the brain Cellular Structure of the Retina: • Horizontal and amacrine cells are interneurons without axons. They combine messages from adjacent neurons • Amacrine cells connect adjacent ganglion cells with bipolar cells • The horizontal cells interconnect adjacent photoreceptors with bipolar cells Fovea and Periphery of Retina: • The fovea is a tiny area specialized for visual acuity (sharpness of an image) • Cones are prevalent in the fovea • Each cone in the fovea connects to a single bipolar cell which in turn connect to a single ganglion cell • Thus, receptors in the fovea can register the exact location of the input • In the periphery, several receptors converge onto the bipolar and ganglion cells • Precise location and shape of input is heavily impeded • However, the periphery enables the perceptions of faint lights • Foveal vision is sensitive to detail • Peripheral vision is sensitive to dim light Eyes like a Hawk: • Most birds have two fovea’s per eye, one pointing ahead and one pointing to the side • The extra fovea enables perception of detail in the periphery • Hawks and other predatory birds have a greater density of visual receptors on the top half of their retina (looking down) than on the bottom half (looking up) • This enables the bird to see below them in great detail during flight • A behavioural consequence of how receptors are arranged on the retina • When looking up, the bird has to turn its head almost upside down to see above it- self Photoreceptors: • Photoreceptors are light sensitive neurons located in the retina. Their function is to transduce light (or photon energy) into electrical potentials • Rods •Prevalent in peripheral retina •Sensitive to low light intensity •Are monochromatic (sensitive to light OR dark) •Provide poor acuity • Cones •Prevalent in fovea •Sensitive to moderate-high light intensity •Are trichromatic (sensitive to colour) •Provide excelle
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