Sensation and Perception: The Beginning of the Perceptual Process

Sensation & Perception

The Beginning of the Perceptual Process

• The Perceptual Process Steps:
  • Step 1: Distal stimulus (e.g., a tree).
  • Step 2: Light is reflected and focused to create an image of the tree on the retina.
  • Step 3: Receptor processes: Receptors transform light into electricity.
  • Step 4: Neural processing: Signals travel in a network of neurons.

Light: The Stimulus for Vision

• Electromagnetic Spectrum:
  • Energy is described by wavelength.
  • The spectrum ranges from short wavelength gamma rays to long wavelength radio waves.
  • The visible spectrum for humans ranges from $400$ to $700$ nanometers.
  • Most perceived light is reflected light.

The Eye

• Components:
  • Pupil: Allows light to enter the eye.
  • Cornea: Focuses light.
  • Lens: Focuses light; adjustable.
  • Fovea: Point of central focus on the retina.
  • Optic nerve: Carries visual information to the brain.
  • Retina: Contains receptor cells (rods and cones).
  • Receptor cells: Rods and cones transform light into electrical signals.
  • Optic nerve fibers: Transmit electrical signals to the brain.
  • Pigment epithelium: Layer behind the retina.

The Eye (2)

• The eye contains receptors for vision.
• Light enters the eye through the pupil and is focused by the cornea and lens to a sharp image on the retina.
• Rods and cones are the visual receptors in the retina that contain visual pigment.
• The optic nerve carries information from the retina toward the brain.

The Eye (3)

• Differences Between Rods and Cones
  • Shape:
    • Rods: large and cylindrical
    • Cones: small and tapered
  • Distribution on Retina:
    • Fovea consists solely of cones.
    • Peripheral retina has both rods and cones.
    • More rods than cones in the periphery.

The Eye (3)

• Macular Degeneration:
  • Fovea and small surrounding area are destroyed.
  • Creates a “blind spot” on the 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.

The Eye (5)

• Number of Rods and Cones:
  • 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 the edge of the visual field.
    • The brain “fills in” the spot.

Focusing Light Onto the Receptors

• The cornea (fixed) accounts for about 80% of focusing.
• The lens (adjusts shape for object distance) accounts for the other 20%.
  • Accommodation results when ciliary muscles are tightened, causing the lens to thicken.
    • Light rays pass through the lens more sharply and focus near objects on the retina.

Focusing Light Onto the Receptors (3)

• 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 the lens and weakening of ciliary muscles.
  • Corrective lenses are needed for close activities, such as reading.

Focusing Light Onto the Receptors (4)

• Myopia (Nearsightedness):
  • Inability to see distant objects clearly.
  • Image is focused in front of the retina.
  • Caused by:
    • Refractive myopia: cornea or lens bends too much light.
    • Axial myopia: eyeball is too long.

Focusing Light Onto the Receptors (2)

• Hyperopia (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 Light Energy Into Electrical Energy

• Receptors have outer segments, which contain:
  • Visual pigment molecules, which have two components:
    • Opsin: a large protein.
    • Retinal: a light-sensitive molecule.
• Visual transduction occurs when the retinal absorbs one photon.
  • Retinal changes its shape, which is known as isomerization.

Adapting to the Dark

• Dark adaptation is the process of increasing sensitivity in the dark.
  • Measured by determining a dark adaptation curve.

Measuring the Dark Adaptation Curve

• Three separate experiments are used.
• Method used in all three experiments:
  • Observer is light adapted.
  • Light is turned off.
  • Once the observer is dark adapted, she adjusts the intensity of a test light until she can just see it.

Measuring the Dark Adaptation Curve (cont’d.)

• Experiment for Rods and Cones:
  • Observer looks at a 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.

Measuring Rod Adaptation

• Experiment for Rod Adaptation:
  • Must use a rod monochromat.
  • Results show that sensitivity increases for about 25 minutes and then levels off.

Visual Pigment Regeneration

• Process needed for transduction:
  • Retinal molecule changes shape.
  • Opsin molecule separates.
  • The retina shows visual pigment bleaching.
  • Retinal and opsin must recombine to 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.

Spectral Sensitivity (3)

• Rod Spectral Sensitivity:
  • More sensitive to short-wavelength light.
  • Most sensitivity at $500$ nm.
• Cone Spectral Sensitivity:
  • Most sensitivity at $560$ nm.
• Purkinje Shift:
  • Enhanced sensitivity to short wavelengths during dark adaptation when the shift from cone to rod vision occurs.

Spectral Sensitivity (6)

• Difference in spectral sensitivity is due to absorption spectra of visual pigments.
• Rod pigment absorbs best at $500$ nm.
• Cone pigments absorb best at $419$nm, $531$nm, and $558$nm.
  • Absorption of all cones equals the peak of $560$nm 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 the same size regardless of stimulus intensity.
  • Increase in rate to increase in stimulus intensity.
  • Have a refractory period of $1$ ms - upper firing rate is $500$ to $800$ impulses per second.
  • Show spontaneous activity that occurs without stimulation.

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

• Synapse is the small space between neurons.
• Neurotransmitters:
  • 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.

Transmitting Information Across a Gap (3)

• 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 sent horizontally:
  • Between receptors by horizontal cells.
  • Between bipolar and between ganglion cells by amacrine cells.

Neural Convergence and Perception (2)

• $126$ 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.

Convergence Causes the Rods to Be More Sensitive Than the Cones

• 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.

Early Events Are Powerful

• Hubble space telescope: improvement in image quality demonstrates the importance of early visual processing.

Infant Visual Acuity

• Techniques to Assess:
  • Preferential looking (PL) technique.
  • Visual evoked potential (VEP).