Perception opportunity 1 notes2

The Beginning of the Perceptual Process

Starting at the Beginning \n Light, the Eye, and the Visual Receptors \n Focusing Light on the Receptors \n Receptors and Perception \n Electrical Signals in Neurons \n Neural Convergence and Perception

Light: The Stimulus for Vision

Electromagnetic spectrum \n Energy is described by==wavelength.== \n Spectrum ranges from short wavelength gamma rays to long wavelength radio waves.

Visible spectrum for humans ranges from 400 to 700 nanometers. \n Most perceived light is reflected light. \n Light enters the eye through the pupil

1. The cornea and lens focus the light to a sharp image on retina
2. The eye contains receptors for vision
3. These receptors are called rods and cones
4. The receptors contain visual pigment
5. The optic nerve carries information from the retina toward the brain

The Eye (cont'd.)

Differences between rods and cones \n Shape \n Rods:large and cylindrical \n Cones: small and tapered \n Distribution on retina \n Fovea consistssolely of cones \n Peripheral retina has both rodsandcones \n More rods than cones in periphery

Diseases that Affect the Retina \n Macular degeneration: Fovea and small surrounding area are destroyed, Creates a “blind spot” on retina, Most common in older individuals \n Retinitis pigmentosa: Genetic disease, Rods are destroyed first, Foveal cones can also be attacked, Severe cases result in complete blindness

Number: about 120 million rods and 6 million cones \n Blind spot: place where optic nerve leaves the eye \n We don’t see it because: \n One eye covers the blind spot of the other \n It is located at edge of the visual field \n The brain “fills in” the spot

How Does the Light Get Focused Onto the Receptors? \n Thecornea, which is fixed, accounts for about 80%of focusing \n Thelens, which adjusts shape for object distance, accounts for the \n other20%

Accommodation results ==when ciliary muscles are tightened which causes the lens to thicken== \n Light rays pass through the lens more sharply and focus near objects on retina

The near point occurs when the lens can no longer adjust for close \n objects \n Presbyopia:“old eye” \n Distance of near point increases \n Due to hardening of lens and weakening of ciliary muscles \n Corrective lenses are needed for close activities, such as reading

Myopia or nearsightedness:inability to see distant objects clearly \n Image is focused in front of retina \n Caused by: \n Refractive myopia: cornea or lens bends too much light. \n Axial myopia: eyeball is ==too long==.

Hyperopia or farsightedness: inability to see nearby objects clearly \n Focus point is behind the retina

Usually caused by an eyeball that is ==too short== \n Constant accommodation for nearby objects can lead to eyestrain and headaches

Transforming of Light Energy Into Electrical Energy \n Receptors have outer segments, whichcontain: \n Visual pigment molecules, which havetwo components: \n Opsin, a large protein \n Retinal, a light sensitive molecule \n Visual transductionoccurs when the retinal absorbs one photon \n ==Retinal changes its shape, which is known as== isomerization \n Transduction is the conversion of physical energy into electrical nrg

1. Adapting to the Dark

   1. Dark adaption is the process of increasing sensitivity in the dark.

      1. Measured by determining a dark adaptation curve

   Measuring the Dark Adaptation Curve

   1. Methodology

      1. Observer is light adapted
      2. Light is turned off
      3. Once the observer is dark adapted, she adjusts the intensity of a test light \n until she can just see it

   Experiment for rods and cones: \n Observer looks at fixation point but pays attention to a test light to the side \n Results show a dark adaptation curve: \n Sensitivity increases in two stages \n Stage one takes place for three to four minutes \n Then sensitivity levels off for seven to ten minutes – the rod-cone break \n Stage two shows increased sensitivity for another 20 to 30 minutes

Measuring Cone Adaptation \n Experiment for cone adaptation \n Test light only stimulates cones \n Results show that sensitivity increases for three to four minutes and then levels off \n Where do you think the subject is looking in this set up?

Measuring Rod Adaptation \n Experiment for rod adaptation \n How are we going to measure just rods? \n Results show that sensitivity increases for about 25 minutes and then levels off

Visual Pigment Regeneration \n Process needed fortransduction: \n Retinal molecule changes shape (isomerization) \n Opsin moleculeseparates \n The retina shows visual pigment%%bleaching%% \n Retinal and opsin must==recombine==to respond to light \n Visual pigment regenerates

Spectral Sensitivity \n Sensitivity of rods and cones to different parts of the visual spectrum \n Use monochromatic light to determine threshold at

different wavelengths. \n Threshold for light is lowest in the middle of the spectrum. ==1/threshold== = sensitivity, which produces the spectral sensitivity curve.

Rod spectral sensitivity \n More sensitive to short-wavelength light \n Most sensitivity at==500 nm== \n Cone spectral sensitivity \n Most sensitivity at==560 nm== \n Purkinje shift: enhanced sensitivity to short wavelengths during dark adaptationwhen the shift from cone to rod vision occurs \n Difference in spectral sensitivity is due toabsorption spectra of visual pigments. \n Rod pigment absorbs best at500 nm. \n Cone pigments absorb best at^^419nm, 531nm, and 558nm.^^ \n Absorption of all cones equals the peak of 560nm in the spectral

sensitivity curve.

Electrical Signals in Neurons

Key components of neurons: \n Cell body \n Dendrites \n Axon or nerve fiber \n Sensory receptors: \n Specialized neurons that respond to specific kinds of energy

Recording Electrical Signals in Neurons

Small electrodes are used to record from single neurons. \n Recording electrode is inside the nerve fiber. \n Reference electrode is outside the fiber. \n Difference in charge between them is ==-70 mV \n ==This negative charge of the neuron relative to its \n surroundings is the resting potential.

Basic Properties of Action Potentials \n Action potentials: \n Show propagated response. \n Remain the@@same size@@regardless of stimulus intensity. \n Increase in rate to increase in stimulus intensity. \n Have a refractory period of^^1 ms – upper firing rate is 500 to 800 impulses per second.^^ \n Show spontaneous activity that occurs without stimulation.

What do we mean by propagated response?

electrophysiological response of axons

Chemical Basis of Action Potentials \n Neurons are surrounded by a solution containing ions. \n Ions carry an electrical charge.

Sodium ions (Na+): positive charge \n Chlorine ions (Cl-): negative charge \n Potassium ions (K+): positive charge

Electrical signals are generated when such ions cross the membranes of neurons. \n ^^Membranes have selective permeability.^^

Transmitting Information Across a Gap \n Synapseis the small space between neurons. \n Neurotransmittersare: \n Released by the presynaptic neuron from vesicles \n Received by the postsynaptic neuron on receptor sites \n Matched like a key to a lock into specific receptor sites \n Used as triggers for voltage change in the postsynaptic neuron \n Excitatory transmitters cause depolarization. \n Neuron becomes more positive.

Increases the likelihood of an action potential \n Inhibitory transmitters cause hyperpolarization. \n Neuron becomes more negative. \n Decreases the likelihood of an action potential

Neural Convergence and Perception \n Rods and cones send signals vertically through: \n Bipolar cells \n Ganglion cells \n Ganglion axons \n Signals are senthorizontally: \n Between receptors by horizontal cells \n Between bipolar and between ganglion cells by amacrine cells

Convergence: allows a neuron to receive input from many neurons in a network \n 126 million rods and cones converge to 1 million ganglion cells. \n Higher convergence of rods than cones \n ==Average of 120 rods to one ganglion cell== \n %%Average of six cones to one ganglion cell%% \n Cones in fovea have one to one relation to ganglion cells \n Rods are more sensitive to light than cones.

Rods take less light to respond.

Rods have greater convergence, which results in summation of the inputs of many rods into ganglion cells increasing the likelihood of response. \n The trade-off is that rods cannot distinguish detail.

Lack of Convergence Causes the Cones to Have Better Acuity \n All-cone foveal vision results in high visual acuity. \n One-to-one wiring leads to ability to discriminate details. \n The trade-off is that cones need more light to respond than rods.

Infant Visual Acuity \n Preferential looking (PL) technique \n Visual evoked potential (VEP)

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