Nature of Sensation and Perception PDF

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structure of the eye, sensation and perception...

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Nature of Sensation and Perception • The only input our brain receives from the “real” world is a series of action potentials passed along the neurons of our various sensory pathways. • How nerves can turn energy, such as light waves, into nerve impulses is understood. • The pathways those nerve impulses take to reach the brain are also known. • Less well understood is how we perceive one set of nerve impulses as a representation of the world. Sensory Receptors • Specialized cells that transduce (convert) sensory energy (e.g., light) into neural activity • Each sensory system’s receptors are designed to respond only to a narrow band of energy. • Vision: Light energy produces chemical energy. • Auditory: Air pressure produces mechanical energy. • Somatosensory: Mechanical energy • Taste and olfaction: Chemical molecules • Receptive field • Region of sensory space (e.g., skin surface) in which a stimulus modifies a receptor’s activity. Receptor Density and Sensitivity • Sensory receptors are not evenly distributed across the body or its organs. • Density is important for determining the sensitivity of a sensory system. • Example: more tactile receptors on the fingers than on the arm • Differences in receptor density determine the special abilities of many animals. • Example: olfactory ability of dogs Neural Relays • All receptors connect to the cortex through a sequence of intervening neurons. • Information is modified at various stages in the relay, allowing the sensory system to mediate different responses. • Neural relays allow sensory systems to interact. Sensory Coding and Representation (part 1) • Sensory information is encoded by action potentials that travel along peripheral nerves to the CNS. • Presence of a stimulus can be encoded by an increase or a decrease in discharge rate. • Increase or decrease can encode stimulus intensity. Sensory Coding and Representation (part 2) • The neocortex represents the sensory field of each modality—vision, hearing, touch, smell, and taste—as a spatially organized neural representation of the external world. • A topographic map is a neural–spatial representation of the body or of the areas of the sensory world perceived by a sensory organ.

• In mammals, each sensory system has at least one primary cortical area. • These may project to secondary areas. Perception • Sensation : • Registration of physical stimuli from the environment by the sensory organs • Perception: • Subjective interpretation of sensations by the brain • Our visual experience is not an objective reproduction of what is out there; rather, it is a subjective construction of reality that is manufactured by the brain. Perceptual Illusions • (A) Edgar Rubin’s ambiguous reversible image can be perceived as a vase or as two faces.

Visible Light and the Structure of the Eye (part 1) • Light is electromagnetic energy that we see. • Range of electromagnetic energy visible to humans • About 400 nanometers (violet) to 700 nanometers (red) • Nanometer (nm): onebillionth of a meter

Visible Light and the Structure of the Eye (part 2) • Cornea • Clear outer covering • Iris • Opens and closes to allow in more or less light • The hole in the iris is the pupil. • Lens • Focuses light • Bends to accommodate near and far objects • Retina • Where light energy initiates neural activity

Structure of the Retina (part 1) • Retina • Light-sensitive surface at the back of the eye; consists of neurons and photoreceptor cells • Translates light into action potentials • Discriminates wavelengths (colors) • Works in a wide range of light intensities • Fovea • Region at the center of the retina that is specialized for high acuity • Receptive field at the center of the eye’s visual field Central Focus • This cross section through the retina (A) shows the depression at the fovea—also shown in the scanning electron micrograph (B) —where photoreceptors are packed most densely and where our vision is clearest.

Acuity Across the Visual Field Fovea • Vision is better in the center of the visual field than at the margins, or periphery. • When we look at the center, letters at the periphery must be much larger than those in the center for us to see them as well. The difference is partly due to the fact that photoreceptors are more densely packed at the center of the retina, in a region known as the fovea.

Structure of the Retina (part 2) • Blind spot • Region of the retina (known as the optic disc) where axons forming the optic nerve leave the eye and where blood vessels enter and leave • Has no photoreceptors Example: eraser and pencil on the table; eraser vanishes when pencil is moved along the table. • Your visual system solves the blind spot problem. • Your optic disc is in a different location in each eye. • The optic disc is lateral to the fovea in each eye; it lies to left of the fovea in the left eye and to the right of the fovea in the right eye. Because the two eyes’ visual fields overlap, the right eye can see the left eye’s blind spot and vice versa.

• Papilledema • Swollen optic disc • May be due to high intracranial pressure (tumor or brain infection) or inflammation of the optic nerve (optic neuritis) • Can cause loss of vision Photoreceptors The retina’s photoreceptor cells convert light energy first into chemical energy and then into neural activity. Rods • More numerous than cones • Sensitive to low levels of light (dim light)

• Used mainly for night vision • One type of pigment only

Cones • Highly responsive to bright light • Specialized for color and high visual acuity • In the fovea only • Three types of pigment Three types of cone pigments (all rods have the same pigment) • Absorb light over a range of frequencies, but their maximal absorptions are • 419 nm (blue, or short wavelength) • 531 nm (green, or middle wavelength) • 559 nm (red, or long wavelength) • There are approximately equal numbers of red and green cones but fewer blue cones....