The Beginning of the Perceptual Process

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

Light: The Stimulus for Vision

Electromagnetic spectrum
Energy is described bywavelength.
Spectrum ranges from short wavelength gamma rays to long wavelength radio waves.

Visible spectrum for humans ranges from 400 to 700 nanometers.
Most perceived light is reflected light.
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
Shape
Rods:large and cylindrical
Cones: small and tapered
Distribution on retina
Fovea consistssolely of cones
Peripheral retina has both rodsandcones
More rods than cones in periphery

Diseases that Affect the Retina
Macular degeneration: Fovea and small surrounding area are destroyed, Creates a “blind spot” on retina, Most common in older individuals
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
Blind spot: place where optic nerve leaves the eye
We don’t see it because:
One eye covers the blind spot of the other
It is located at edge of the visual field
The brain “fills in” the spot

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

Accommodation results when ciliary muscles are tightened which causes the lens to thicken
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
objects
Presbyopia:“old eye”
Distance of near point increases
Due to hardening of lens and weakening of ciliary muscles
Corrective lenses are needed for close activities, such as reading

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

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

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

Transforming of Light Energy Into Electrical Energy
Receptors have outer segments, whichcontain:
Visual pigment molecules, which havetwo components:
Opsin, a large protein
Retinal, a light sensitive molecule
Visual transductionoccurs when the retinal absorbs one photon
Retinal changes its shape, which is known as isomerization
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
         until she can just see it

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

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

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

Visual Pigment Regeneration
Process needed fortransduction:
Retinal molecule changes shape (isomerization)
Opsin moleculeseparates
The retina shows visual pigmentbleaching
Retinal and opsin mustrecombineto respond to light
Visual pigment regenerates

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

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

Rod spectral sensitivity
More sensitive to short-wavelength light
Most sensitivity at500 nm
Cone spectral sensitivity
Most sensitivity at560 nm
Purkinje shift: enhanced sensitivity to short wavelengths during dark adaptationwhen the shift from cone to rod vision occurs
Difference in spectral sensitivity is due toabsorption spectra of visual pigments.
Rod pigment absorbs best at500 nm.
Cone pigments absorb best at419nm, 531nm, and 558nm.
Absorption of all cones equals the peak of 560nm in the spectral

sensitivity curve.

Electrical Signals in Neurons

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

Recording Electrical Signals in Neurons

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

Basic Properties of Action Potentials
Action potentials:
Show propagated response.
Remain thesame sizeregardless of stimulus intensity.
Increase in rate to increase in stimulus intensity.
Have a refractory period of1 ms – upper firing rate is 500 to 800 impulses per second.
Show spontaneous activity that occurs without stimulation.

What do we mean by propagated response?

electrophysiological response of axons

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

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

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

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

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

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

Convergence: allows a neuron to receive input from many neurons in a network
126 million rods and cones converge to 1 million ganglion cells.
Higher convergence of rods than cones
Average of 120 rods to one ganglion cell
Average of six cones to one ganglion cell
Cones in fovea have one to one relation to ganglion cells
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.
The trade-off is that rods cannot distinguish detail.

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

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