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Chapter 4

Chapter 4 - Sensation and Perception - without hearing.docx

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PSYC 1010
Jennifer Steeves

Sensation and Perception • Sensation – The processes of sensory responding and of the sensory receiving areas of the brain • Perception – Mental operations that organize sensations into meaningful patterns • E.g. – Sensation: image on the back of the eye ; Perception: a pencil • Sensory Receptors – Transduce physical stimulus into neural signal • Sensory Neurons - Carry impulses to the CNS (usually via the thalamus) • Sensory Areas The Visual System • The stimulus is visible light The Dimensions of Colour • Although we tend to think of colour in terms of colour names, colour is a multi- dimensional experience • Each of these dimensions is associated with a different physical property of light • There is a need for a system that allows for colours to be described accurately and reproduced reliably Light Waves Vary in: • Amplitude (luminance) which affects brightness – perceived intensity of the target • Wavelength which affects colour (hue)  Shorter wavelength = higher frequency (violet) • Purity which affects saturation  Degree of whiteness in target (pale/vivid – or in between) Registered by receptors in the eye How the Eye is Like a Camera • Uses light • Goes through pupil  Like the camera aperture • Focused by the cornea and the lens (70% focusing is done by the cornea)  Like the camera lens • Focused on the retina  Like the film  Retinal image is flipped up-down and left-right Key Eye Structures • Lens – focuses the light rays falling on the retina • Pupil – regulates the amount of light passing to the rear of the eye • Retina – the neural tissue lining the inside back surface of the eye  Periphery – Outside the fovea • Optic Disk – a hole in the retina that corresponds to the blind spot – no photoreceptors  Retinal ganglion cells and blood vessels leave the retina • Ganglion cells – fire action potentials; axons form optic nerve • Fovea – a tiny spot in the centre of the retina where visual acuity is greatest – focus  not blocked by blood vessels or other cell types Visual Photoreceptors (in the retina) • Organized into receptive fields • Transduce photons of light into neural signals – sends to the ganglion cells • Each type contains a different type of photopigments • Don’t fire action potentials • Hyperpolarize to light • Rods play a key role in night and peripheral vision and greatly out-number cones  Dense outside fovea; can see in dim light; low visual acuity; 1 type(no colour perception) • Cones play a key role in day colour vision and provide greater acuity than rods  Need bright light; dense near the fovea; 3 types (trichromatic) • 3 layers horizontally and 2 layers vertically • Receptive Fields are collections of rods and cones that funnel signals to specific visual cells in the retina or the brain Visual signals are sent onward to the brain Visual pathways and processing • Main visual pathway – engage in parallel processing of stimulus input  Parvocellular channel  Magnocellular channel • Second visual pathway – handles coordination of visual input with other sensory input • Primary visual cortex – handles the initial cortical processing of visual input (in occipital lobe) • Feature detectors – neurons in the visual cortex that respond selectively o specific features of complex stimuli • After processing in the primary visual cortex, visual input is routed to other cortical areas along the where pathway (dorsal stream) and the what pathway (ventral stream) Eye Movements • If the whole retina was densely packed with cones, we’d need an enormous brain to process all the information • Solution: move the fovea around to extract info across the scene • People move their eyes about 3-4 times/second Retinotopy • There is a “map” in the visual cortex – occipital lobe • Adjacent parts of the retina map to adjacent parts of the visual cortex Optical Illusions • Discrepancy between the appearance of a visual stimulus and its physical reality • The Müller-Lyer illusion, the Ponzo illusion, and the moon illusion, show that perceptual hypotheses can be wrong and that perception is not a simple reflection of objective reality • How the eye isn’t like a camera eye: Adaptation; Illusion; Stereo depth perception Adaptation • The visual system adapts – The sensitivity changes with prolonged exposure (less sensitive) or lack of exposure (more sensitive) to a stimulus • Dark Adaptation – When you stay in the dark, you become more sensitive to light  Greater availability of receptor photochemicals. 7-8 min for cones and ~25min for rods • Light Adaptation – Lose sensitivity in bright scene; happens very rapidly Afterimages • You can have different levels of adaptation in different parts of the retina • This can lead to afterimages Motion Aftereffect • Consider 2 neurons coding motion in a similar location in space, one that likes upward motion and on that likes downward motion • When seeing downward motion, the downward neuron fires a lot and equilibrium is lost • When that’s done, the downward neuron relaxes and the upward one takes over. So you see upward motion Illusions False Conclusions:  Vision is unreliable – illusions rarely fool us in everyday  Illusions are rare – everything is an illusion… but it’s a damn good one! • The brain is a Statistician – tries to find the best fit; sometimes it’s wrong Colour Perception • Subtractive colour mixing works by removing some wavelengths of light, leaving less light • Additive colour mixing woks by putting more light in the mixture than any one light • Trichromatic theory – the eye has 3 groups of receptors sensitive to wavelengths associated with red, green, and blue • Opponent process theory - receptors make antagonistic responses to three pairs of colours • Conclusion – the evidence suggests that both these theories are necessary to explain colour perception Form Perception • The same visual input can result in very different perceptions • Form perception is selective, as the phenomenon of inattentional blindness demonstrates • Some aspects of form perception depend on feature analysis, which involves detecting specific elements and assembling them into complex forms • Gestalt principles, such as figure and ground, proximity, closure, similarity, simplicity, and continuity, help explain how scenes are organized into discrete forms • Form perception often involves perceptual hypotheses, which are inferences about the distal stimuli that could be responsible for the proximal stimuli sensed Depth Perception • Binocular cues – clues about distance based on differing views of the two eyes • Retinal disparity – refers to the fact that the right and left eyes see slightly different views of objects • Monocular cues – clues about distance based on the image in either eye alone • Pictorial cues – monocular cues that can be given in a flat picture, such as linear perspective, texture gradi
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