Chapter 2: Introduction to the Physiology of Perception

Chapter 2: Physiology of Perception Overview of Questions How are physiological processes involved in perception? How is light transformed into electricity in the eye? How is what we see determined by the properties of the receptors in our retinas?

History of the Mind

Basic Brain Structure The brain has modular organization –The sensory modalities have primary receiving areas Vision - occipital lobe Audition - temporal lobe Tactile senses - parietal lobe

Neurons: Communication & Processing Key components of neurons: –Cell body –Dendrites –Axon or nerve fiber Receptors - specialized neurons that respond to specific kinds of energy –Note: “Receptors” will be used again to describe the structures that capture neurochemical signals

Basics of Neural Signals Neurons are surrounded by a solution of 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

Neuroscience Overview The Brain: The Neuron: Membrane Potential: Action Potential: Synaptic Transmission:

Recording Neural Signals Microelectrodes are used to record from single neurons –Recording electrode is inside the nerve fiber –Null 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

Recording Neural Signals - continued Electrical signals or action potentials occur when: –Permeability of the membrane changes –Na+ flows into the fiber making the neuron more positive –Then K+ flows out of the fiber making the neuron more negative –This process travels down the axon in a propagated response

Neuron and Neural Impulse

Properties of Action Potentials Action potentials remain the same size Increase in stimulus intensity can increase the firing rate of neurons Refractory period is 1 ms - upper firing rate is 500 to 800 impulses per second Spontaneous activity of action potentials occurs without stimulation

Changes in firing rate due to changes in intensity

Synaptic Transmission of Neural Impulses Neurotransmitters are: –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

Synaptic Transmission

Types of Neurotransmitters 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

Light and Vision Reading 3

Light is the Stimulus for Vision Electromagnetic spectrum –Energy is described by wavelength –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

The electromagnetic spectrum

Focusing and Transduction

Windows Light and the Eye

Focusing Images on the Retina The cornea, which is fixed, accounts for about 80% of focusing The lens, which adjusts shape for object distance, accounts for the other 20% –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

Accommodation

Focusing Images on Retina - continued 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

Retinal Processing - Rods and Cones 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 periphery

Figure The distribution of rods and cones in the retina.

Figure 2.16 The mosaic of rods and cones in the peripheral retina of a monkey.

Retinal Processing - Rods and Cones - continued –Number 120 million rods 5 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- Remember, we don’t see with our eyes!

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

Figure 2.18 (a) Simulated macular degeneration and (b) retinitis pigmentosa

Transduction of Light into Nerve Impulses 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 it shape, called isomerization

Figure 2.20 Model of a visual pigment molecule.

Psychophysical Study of Isomerization Experiment by Hecht et al. (1942) –Determine the absolute threshold for detecting a light –Determine how many visual pigment molecules the threshold level light would affect –Results showed that only one photon was needed to excite one visual pigment molecule

Figure 2.21 The observer in Hecht et al.’s (1942) experiment could see a spot of light containing 100 photons. Fifty photons reached the retina, and 7 photons were absorbed by visual pigment molecules. Each of these visual pigment molecules were most likely contained in different rods. Hecht concluded that (1) it takes only 1 photon to activate a rod receptor and (2) we see the light when 7 rod receptors are activated simultaneously. Rods are sensitive!!

Physiological Reaction after Isomerization Isomerization triggers an enzyme cascade –Enzymes facilitate chemical reactions –A cascade means that a single reaction leads to increasing numbers of chemical reactions

Enzyme Cascade: Isomerization of one visual pigment molecule activates about a million other molecules.

Measuring Dark Adaptation 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 Dark Adaptation - continued 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 3 to 4 minutes Then sensitivity levels off for 7 to 10 minutes - the rod-cone break Stage two shows increased sensitivity for another 20 to 30 minutes

Figure 2.23 Viewing conditions for a dark adaptation experiment. The image of the fixation point falls on the fovea, and the image of the test light falls in the peripheral retina.

Measuring Dark Adaptation - continued Experiment for cone adaptation –Test light only stimulates cones –Results show that sensitivity increases for 3 to 4 minutes and then levels off 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 pigment bleaching –Retinal and opsin must recombine to respond to light –Cone pigment regenerates in 6 minutes –Rod pigment takes over 30 minutes to regenerate –What does this mean?

Spectral Sensitivity of Rods and Cones 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

Figure 2.26 (a) The threshold for seeing a light versus wavelength. (b) Relative sensitivity versus wavelength -- the spectral sensitivity curve.

Figure 2.27 Spectral sensitivity curves for rod vision (left) and cone vision (right). The maximum sensitivities of these two curves have been set equal to 1.0. However, the relative sensitivities of the rods and the cones depend on the conditions of adaptation: the cones are more sensitive in the light, and the rods are more sensitive in the dark.

Spectral Sensitivity of Rods and Cones - continued Rod spectral sensitivity shows: –More sensitive to short-wavelength light –Most sensitivity at 500 nm Cone spectral sensitivity shows: –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 of Rods and Cones - continued Difference in spectral sensitivity is due to absorption spectra of visual pigments Rod pigment absorbs best at 500 nm Cone pigments absorb best at 419nm, 532nm, & 558nm –Average of all 3 equals 560nm These match the spectral sensitivity curves Psychophysics (thresholds) can tell us about the physical properties of rods and cones.

Figure 2.29 Absorption spectra of the rod pigment (R), and the short- (S), medium- (M), and long wavelength (L) cone pigments. (From J. K. Bowmaker and H. J. A. Dartnall, “Visual Pigments of Rods and Cones in a Human Retina,” Journal of Physiology, 298, 1980, Copyright © Reprinted with permission of the author.)