Psyc 362 Chapter 6 text Vision Sensory Receptors specialized neurons that detect various physical events and allow us to receive information about the environment NOT the same as receptors for NTs which are specialized proteins that bind with certain molecules Sensory transduction sensory events are transferred into changes in the cell s MP when stimuli influence receptors Receptor Potentials electrical changes that affect the release of NTs and modify the firing rate in neurons with which sensory receptors form synapses Light is simply the part of the Electromagnetic Spectrum between 380nm and 760nm that we can see Color that is perceived depends on hue saturation and brightness hue is determined by wavelength which is affected by the frequency of oscillation of the waves Slower oscillation longer wavelengths looking at a spectrum the visual spectrum shows the range of hues we can detect Brightness is determined by intensity Saturation involves the relative purity of the perceived light if all the radiation is at one wavelength the color is pure fully saturated but if the radiation contains all visible wavelengths it appears white Anatomy Figure 6 3 page 131 The eyes are suspended in bony pockets called orbits and are held there and able to move with the help of sclera six extraocular muscles attached to the tough white outer coat of the eye The attachment is hidden by the conjunctiva Saccadic movements jerky movements abrupt shifts in your gaze from one point to the next Pursuit movement looking at a single spot object while you move the spot object around allowing slower movement of the eyes The cornea outer layer at the front of the eye is transparent to light while the white outer layer of the eye does not permit light entry An opening in the iris pigmented ring of muscles situated behind the cornea known as the pupil regulates the amount of light that enters with changing size Behind the iris is the lens which has a series of transparent layers and is able to alter its shape by contractions of ciliary muscles set of muscle fibers attached to the outer edge of the lens allowing the eye to accommodate focus images of near or distant objects on the retina The Retina the interior lining of the back of the eye where photoreceptor cells both rods and cones are located then has light fall on it here Rods greatly outnumber cones although cones provide us with more visual information such as color detection and fine detail acuity The Fovea in the central region of the retina contains only cones responsible for color vision ability to discriminate light of different wavelengths and mediates the most acute vision Rods are more sensitive to light than cones despite their inability to detect color and be acute the Optic Disk in the retina has many axons gathered that leave the eye through the optic nerve to convey visual information although it does have a blind spot where no receptors are located Three Layers of the retina Photoreceptor layer bipolar cell layer ganglion cell layer The Photoreceptors form synapses with the bipolar cells neurons with two arms connected to the inner and outer layers of the retina which then form synapses with ganglion cells whose axons travel through the optic nerve carrying visual info into the rest of the brain The Retina also contains horizontal and amacrine cells which transmit info in a direction parallel to the surface of the retina combining mesages from adjacent photoreceptors Photoreceptor layer outer segment contains many thin plates of membrane called lamellae The chemicals needed to start the visual perception chain are called Photopigments and are special molecules in the membrane of the lamellae consisting of an opsin protein have several forms and retinal lipid synthesized from vitamin A Rhodopsin a specific opsin breaks into two components rod opsin and retinal when exposed to light changing the color of the opsin so that the light bleaches the photopigment causing a receptor potential change in MP of photoreceptor altering firing rate of bipolar cells Figure 6 5 pg 133 The LGN dorsal lateral geniculate nucleus is the first part of the brain in the thalamus which receives messages from ganglion cells through the optic nerves The LGN contains six layers of neurons each of which receive messages from only one eye The magnocellular layers include the inner two layers which have neurons with larger cell bodies and the outer four layers are called the parvocellular layers There is also a third set of neurons in the koniocellular sublayers found ventral to each of the other layers The different layers are responsible for different types of visual analysis receiving info from different types of ganglion cells The Optic chiasm is a cross shaped connection between the optic nerves under the base of the brain that ascends to the LGN of the opposite side of the brain But each hemisphere receives visual info from both eyes not just the contralateral eye The Calcarine fissure a horizontal fissure in the medial and posterior occipital lobe in the primary visual cortex striate cortex V1 are the next receivers of this visual information as the LGN sends their axons through the optic radiations to V1 Coding info Microelectrodes can be used to record electrical activity of single neurons detecting light and dark The Receptive field RF of a neuron in the visual system is the place in which a visual stimulus must be located to produce a response in an individual neuron depending on the location of the photoreceptors that provide that neuron visual information These RFs of ganglion cells have roughly circular centers surrounded by a ring Simulation of the center or surrounding fields have contrary effects such that on cell are excited by light falling in the center and inhibited if light falls into the surrounding field and off cells respond in the opposite way There are 3 different types of cones which provide us with color vision though Monochromatic vision from rods gives us black and white the theory is that Photoreceptors have trichromatic coding such that color vision that the eye detects different colors because each different type of receptors are sensitive to a single hue The different wavelengths absorbed by photopigments are controlled by the particular opsin a photoreceptor contains since different opsins absorb particular wavelengths The three receptors perceive either blueviolet green or red Color vision issues may be due to a defect in one or