Sensory Systems

1. General Properties of Sensory Systems

Important Conceptual Principals for Understanding Sensory Systems

1.What is Reception ?

2.What is Transduction ?

3.What is Coding?

How a stimulus event is represented in the brain.  This representation is not a passive passive event.  It involves

a.  Filtering

b. Abstracting

c. Integrating

Coding helps tell us about the stimulus in the environment (e.g., color, shape, spatial location)

A Sensory Systems Requires Different Receptors to Discriminate among Forms of Energy

1.Johannes Muller and the Doctrine of Specific Nerve Energies:  Proposed the idea that each sensory system had its own receptor and neural pathway and that these systems are independent.

2. We know that these are not "Nerve Energies" but rather electrical impulses that travel along specific axons in the brain

A Sensory System Should Discriminate Among Different Intensities of Stimulation

1.What would the optimal sensory system design look like ?

2.Organisms are biologically programmed to optimally respond to stimulus change.  Why is this the case ?

A Sensory System Should Respond Reliably

What does it mean for a sensory system to respond reliably ?  We know what it means for a person to be reliable or a tool to perform reliably, but what does this mean with respect to sensory systems.

The idea is that when a stimulus from the environment is presented a complex representation of the stimulus is produced in the brain, it is important that each time the same stimulus is presented that the same sensations are produced.

How could you design a system that is more reliable ?

Have parallel but redundant circuits

A Sensory System Should Respond Rapidly

If we think about natural selection, those sensory systems that responding more rapidly, would be more likely to confer an advantage upon the organism, and therefore be selected.  At the same time you want the sensory system to be reliable to increase the likelihood of survival.

A Sensory System Should Suppress Extraneous Information

For the purpose of Survival, it is also important that a sensory system is not only reliable and fast, but must also be able to get rid of information that is not useful at the time to the organism.

What are some ways that this is accomplished ?

Vary the response threshold.

Adaptation

There are other ways that sensory systems accomplish this.

For a sensory system to be optimal, it must be able to detect the difference between stimulus characteristics, be reliable, be fast and be able to filer out extraneous stimuli.

Different Species Detect Different Aspects of the World with Similar Sensory Systems

Discuss what we mean by this.  What are some examples ?

Sensory systems of different animals have different ranges for which they respond to the physical energy.

Sensory Processing Begins in Receptor Cells

1.Describe the process of Reception and Sensory Transduction.

2.Describe the initial stage of sensory processing, the generator potential.

Lowenstein (1971) conducted an experiment to test the idea that receptors have electrical potentials that change in a graded fashion –which were called generator potentials.

He mechanically stimulated the Pacinian corpuscle (a receptor that is found throughout the body in the skin and muscles and is very densely distributed around the tissue surrounding the abdominal cavity.

Lowenstein (1971) continued.  What he did was to mechanically stimulate the Pacinian corpuscle, and this resulted in a change in the electrical potential.  The magnitude of this change was directly proportional to the strength of the stimulus-----this is a graded potential called a generator potential.

How does this work in the Pacinian corpuscle ?

1.Mechanical Stimulation deforms the copuscle

2.This leads to mechanical stretch of the tip (Hillock) of the axon

3.Mechanical effects open the ion channels in the membrane of the axon and there is an influx of sodium producing graded potentials.

4.When the threshold of activation is reached—generator potential is great enough, an action potentials occurs.

Sensory Events are Represented by Neural Codes

What to we mean by this idea ?  The stimulus in the environment contacts your sensory system in the form of  a specific type of physical energy –this has to be transduced and then at some point coded so that the stimulus can be represented in the brain as, for example a red light, rather than a blue light.

What are the ways that sensory information is coded in the brain

1.We must be able to code for differences in stimulus intensity.  Remember that I said that one way that we can code for stimulus intensity is by the rate of firing of the neuron, however, this must be incomplete.  Most sensory neruons can only fire at a maximum rate of a few hundred AP's per sec.

If we have this limitation on the rate at which sensory neurons can fire, then how can we code for stimulus intensity that exceeds the maximum rate of firing?

1.  Multiple neurons acting in parallel that have different thresholds of activation.

2.  The principle of coding called RANGE FRACTIONATION is a variant of the idea of multiple neurons working in parallel.  Here we see different neurons that are maximally specialists in responding to a specific range in the intensity scale.

Stimulus Type and Coding

Within a given stimulus humans can discriminate a variety of stimulus qualities, we can ask the question, what kind of coding underlies these different qualities ?

The initial step in this process has to do with the concept of labeled line –that is receptors that can only detect certain types of physical energy and specific pathways along which the information travels.

Examples

1.Somatosensory Events ---in this case the location of the stimulus is coded by which receptors on the body are stimulated.  The properties of the spatial location of the stimulus is represented by labeled lines.  Note that there must be an orderly relationship between the receptors that are stimulated and the neurons that are stimulated along the way to the information represented in the cortex. ----There must be some kind of "Map"

2.How is the spatial location of auditory information represented or olfactory information.  These are bilateral sensory systems.  Relative time the information arrives at one side versus the other.

Receptor responses can decline with maintained stimulation

1.Concept of Sensory Adaptation

Tonic Receptors: Slow or non-existent decline in frequency of AP

Phasic Receptors:  Show a rapid decrement in the frequency of AP.

Sensory Inputs can be Suppressed

1.Accessory Structures:  Example the eyelid

2.Another way to suppress sensory information is via neural control.  An example of this would be the descending inhibition of pain that occurs via your own opiates.

Successive Levels of the CNS Process Sensory Information

Concept or Receptive Fields and Sensory Processing

Example of Plasticity in Sensory Systems

The Visual System

1. Stimulus for Vision

Electromagnetic radiation--- wide range continually bombarded; organisms only detected a small portion of these waves; for visible light the band is between 360 and 700 nm

What is the Light Source?

A. Direct Light Sources which Emit Waves
1. Sun
2. Light Bulb

B. Indirect light which is reflected accounts for most light that goes to our eyes from objects. The light that is reflected provides information concerning the objects properties.

 

Characteristics of the Visual Stimulus

How can we specify the perceptual stimulus

1. Distal Stimulus--- Stimulus out in the environment at a distance from observer.

We can measure Physical Size and Physical Distance

Proximal Stimulus--- stimulus on the observers retina specify retinal size
 

Figure Showing the difference between the Distal and the Proximal Stimulus

Typically retinal size is determined by the visual angle rather that the mm of the object on the retina.

The Receptive Organ: Basic anatomy of the eye
 

Orbits: eyes are suspended in orbits which are bony pockets in front of skull.

Eyes moves by six extraocular muscles attached to Sclera: Tough fibrous outer coat of eye.

Muscles are hidden by Conjunctiva muscous membranes line eyelid fold back to attach to the eye. (For instance parent  contact lens that has slipped of the cornea from falling behind the eye.)

Sclera: outer layer of most of the eye, opaque, light is not entered here.

Cornea: outer layer at front of eye transparent admits light.  Pupil: amount of light that enters is regulated by the size of the  pupil.

 The of the pupil is formed by the opening in the iris. The iris consists of a ring of muscles situated behind the cornea. Two  bands of muscles regulate the size of the pupil. Contraction of the DIALATOR muscle enlarges the pupil and contraction of the SPHINCTER muscle reduces the size of the pupil. The belladonna alkaloid atropine produces dialation of the pupil by  relaxing the sphincter of the iris.

The lens is situated immediately behind the iris consists of a series of transparent onion-like layers. The shape of the lens can be altered by contraction of the ciliary muscles. Normal tension of the elastic fibers that suspend the lens reslt in the lens being relatively flat. When the lens focuses an image from a distance object the lens is flat. Movement of the ciliary muscles  determines whether the lens focuses images that are near or far. This process is called accomodation. The light waves must  be focused on the retinal surface for the image to be in focus. The figure below represents the processd of accomodation.

The Vitreous Humor: When the light passes  through the lens the light must now pass through  bulk of the eye which contains the vitreous  humor, a clear and gelatinous substance the gives the eye its bulk. Light then falls on the  photoreceptors in the back of the eye. The rods and cones are contained here in the retina.

Basic Anatomy of the Eye: Cross Section of the Retina

Distribution of the Rods and Cones in the Retina 

Distribution of Rods and Cones

Fovea Located directly in line of sight contains only cones

Looking directly at an object results in the object being projected on the Fovea. Fovea small only objects with a visual angle of .5 or < have image falling directly on Fovea.

6 million cones in retina- most cones in peripheral area of Retina.

Rods in the peripheral Retina out number cones by 20:1

Blind spot on Retina; no photoreceptors found.

 

Functional Significance of Neural Connections of Rods and Cones to Bipolar and Ganglion Cells

1. A number of rods and cones share a common ganglion cell: in the periphery and convergence operates. One consequence of this is spatial summation whereby the likelihood of a ganglion cell. firing is increased.
Therefore you need less light to activate the ganglion and generate an action potential that travles along the optic nerve to the primary visual cortext in the occipital lobe.

It is also the case that a single rod requires less energy for activation than a single cone.

Because of the convergence seen with rods sensitivity is higher; and the fact that an individual rod requires less energy for the activation than a cone aids in the masgnitude of sensitivity.  There is a trade off however, and that is that the rods have poor activity "information" is lost; Detail is lost. In the cones there is no convergence. Therefore less sensitivity, but activity is high!!! much more detail is seen.

Connections of the Cones to the Bipolar Cells

Rod outer segment contains rhodopsin embedded within Lamellae

Single rod contains 10 million photopigment molecules

Lamellae; thin plates of membrane; Photopigments found in the lamella

Describe Details of the Retinal Circuitry

Amacrine or horizontal cells send generator potentials parallel to retinal surface. Receptors, bipolar cells, horizontal cells and Amacrine cells produce generator potential (graded potentials) not action potentials. Only Ganglion Cells produce, action potential.

Basis for the Generation of a Receptor Potential

Rhodopsin Molecule breaks into two parts when a photon of light strikes opsin=protein and retinal=lipid

Photopigment of human rods is Rhodosin---rod opsin and retinal retinal synthesized from vitamin A

When the eye is kept in the dark the regenerative part of the cycle begins. Vitamin A joins with opsin to reconstitute Rhodopsin;

Dowling and Wald (1960) deprived rats of vitamin A for 8 weeks.  Rats were significantly less sensitive to low levels of light. If one presents a critical deficiency in Vitamin A  Pathological insensitivity to dim light. Sensitivity decreased radically 1000 times
 

What happens when a molecule of Rhodopsin is exposed to light?? A single photon is enough to trigger the release of energy.

Rhodopsin breaks into rod, opsin, and retinal: Rhodopsin changes from a pink hue to pale yellow; light bleaches the photopigment!

What happens when rhodopsin is split apart? Receptor potential is generated:

Rate at which the photoreceptor releases transmitter changes

The resting and receptor potential in the Rods

Ion channels are usually open during resting. Ion channels permit influx of cations, thus resting membrane potential is less polarized compared to other neurons, -40mv vs -70mv. Sodium, calcium, and magnesium ion channels remain open by cGMP. When light photons strike a photoreceptor hyperpolarization of
 the membrane occurs; Rhodopsin molecule splits and cations no longer enter; membrane hyperpolarizes and releases less  transmitter substance phosphodiesterase activated--- cGMP destroyed, channels close. Photoreceptors continuously release  neurotransmitter; membrane potential changes amount of  neurotransmitter released depolarization- higher release  hyperpolarization- less release during a receptor potential,  hyprepolarization reduces the amount of neurotransmitter released. During the resting state the increased amount of neurotransmitter normally released from the photoreceptor  hyperpolarizes the dendrites of the bipolar cells.

Accordingly, a reduction in the release of the neurotransmitter results in the  membrane of the bipolar cell to depolarize. Light hyperpolarizes the photoreceptors and depolarizes the bipolar cell resulting in depolarization of the ganglion cells increasing the rate of firing The figure below shows one way in which transduction of the rod receptor potential occurs. Note that it is now thought that  photoreceptors are continuously releasing glutamate and that the G protein called transducin activates the enzyme phosphodiesterase.

Figure illustrating the Generation of a Receptor Potential

Measurement of a Receptor Potential and the consequences at  the Bipolar Cell and the Ganglion Cell

 More complete illustration of the Process that Underlies the Rod Resting Potential and the Rod Receptor Potential

Neural Pathways from The Retina to Other Areas of the Brain

How is Perception of Brightness Mediated or Determined by Neural Circuits ?

What is the difference between light Intensity and Brightness ?

How is Contrast Physical Contrast Different from Perceptual Contrast ? 

Mach in the 1870's using observational techniques showed that our perception of brightness of two areas next to each other does not always match the distribution of light intensity across the same area. 

Stated another way what this means is that we may see two areas next to each other as brighter but that a light meter will tell us that the same amount of light is reflected from them. 

When looking at the previous figure we see all of the strips on the left as brighter.  What can we conclude from this observation ?  If stripe [a] and [b] are not next to each other we see the edges as having the same level of brightness.  Thus, our perception is consistent with a light meter.

The fact that the stripes are next to each other must be having a physiological effect that produces this perceptual phenomena known as Mach bands. 

It was not until the 1950's that the work of Hartline, Wagner and Ratliff (1956)  facilitated our understanding of what Mach was surmising as the physiological mechanism for the Mach band. 

Perception of Brightness and Contrast

Hartline,Wagner and Ratliff (1956) reported that the Mach band phenomena was due to lateral inhibition.  Lateral inhibition is a neural process whereby cells in the retina laterally inhibit adjacent cells.

How did Hartline et al., (1956)  study lateral inhibition ?  They used the limulus (Horseshoe Crab) because of its special eye.  The limulus eye is special because it is made up of hundreds of tiny ommatidia.  Each ommatidium has a small lens on the eyes surface located over a single receptor.  The lens and the receptor are about the diameter of a pencil point.  This allows the researcher to be able to illuminate with light a single receptor an measure the output without affecting adjacent receptors. The Figure in the next slide illustrates the conditions that are required to produce lateral inhibition in the limulus eye.  Illuminating receptor [A] and recording from nerve fiber [A] shows an increase in the output from this fiber.  When nearby receptors [B] are now illuminated with light the output from [A] is decreased.  Finally when the light intensity is increased at [B] the out from [A] is further decreased.

Diagram Showing the Recording of the Rate of the Action Potential from a Ganglion Cell in the Retina of the Limulus Eye.

Graph showing the differences in the distribution of light as measured by a light meter (Physical Intensity) and the way we perceive the intensity (brightness)

Conclusion

Perception is the result of processing by the nervous system.  Thus, the physical information that is coming in from the environment is modified.  This is one way that we code for brightness and contrast. This helps us to exaggerate edges. It helps us to sharpen contours.In some sense then, the truth about the world is exaggerated.  Remember, contours are areas where change occurs and we need to know about these types of features in our world.

Our perception is not an exact replica of the energy in the environment.  In Mach Bands the physical contrast across the stripe at the edges is the same but the perceptual contrast is different. 

Therefore perceptual contrast can exist without physical contrast.

The Nature of Receptive Fields in the Visual System

Knowing about receptive field of a cell helps us to better understand how we perceive information in the world.

The Seminal Work on Receptive Fields (Kuffler, 1953) and Barlow (1953)

General Procedure: Moved a small spot of light across the receptive field and recorded the rate of the action potential from a single ganglion cell.

Results: The results demonstrated that the receptive field had a circular center with a ring around it.  In later studies, similar results were found when recording from bipolar cells

Two Types of Receptive Fields Found in Bipolar and Ganglion Cells

On-Center/Off-Surround Off-Center/On-Surround

How do these receptive fields function when a stimulus activates the center and the surround of the field ?   The next slide will show this graphically.

Organization of Primate Lateral Geniculate Nucleus in the Thalamus

The primate LGN has six layers:

Four dorsal (outer layers) called the parvocellular (small cells)

Two Ventral (inner layers) called magnocellular (larger cells)

The Next Slide Graphically Shows the Six Layers of the LGN

Characteristics of the Connections Between the LGN and Retinal Cells and the Nature and Role of the Receptive Fields

Magnocellular Layers:  Large receptive fields.  The axons that innervate these layers of the LGN come from diffuse retinal bipolar cells.  The axons that innervate the bipolar cells come from several photoreceptors.  We see the idea of convergence here.  A large proportion of the magnocellular cells in the LGN do not play any role in color discrimination.  This can be inferred because they fail to show differential rates of firing to different wavelengths of light.

Parvocellular layers: Small receptive fields.  The axons that innervate these layers come from the midget bipolar cells.  These cells do show differential sensitivity to wavelength.

LGN cells of all six layers have concentric receptive fields. 

A Closer Examination of the Primate Ganglion Cells

Leventhal (1979) and Perry and Colleagues (1984) localized two distinct types of ganglion cells called the M and P-----The M ganglion cells project there axons to the magnocellular layers of the LGN whereas the P ganglion cells project there axons to the parvocellular layers of the LGN. 

M ganglion cells: These cells have dendrites with large fields (they take up a relatively large amount of area) and therefore they can have many bipolar cells synapse with them ---they have large receptive fields

Functionally, these ganglion cells can detect stimuli with low contrast.  They are also sensitive to motion and they response transiently, have large diameter axons and conduct AP's very rapidly.

P ganglion cells: Relatively small cells, have a small dendritic branching and have small receptive fields.

Functional: Require high contrast stimuli to generate action potentials. Do not respond well to stimuli with motion.  They do show differential responses to different wavelengths and thus are important in color perception

These cells have sustained responses to the stimuli that activate them.

1.If you shine light on a specific area of the retina, and then measure the firing rate of a cell in the receptive field found in the lateral geniculate nucleus of the thalamus, and the firing rate increases; will that same spot of light be effective in activating cells in the primary visual cortex?  No ----Why is this the case?  The shape of the receptive field is different and responds to more specific elongated stimuli such as lines or bars, or to stimuli that are moving.

2.Examples of elongated stimuli.

Seminal work on characterizing the receptive fields of the visual cortex was conducted by Hubel and Wiesel (1959).

Types of cells found in the visual cortex

1.Simple Cortical Cells:  These cells would show an increase in the rate of firing to lines or bars of a particular angle.  That is, depending on there orientation in the visual field, the specific cells the the visual cortex would increase their firing rate.  Therefore, there must be groups of cells that would increase their firing rate to all possible orientations of lines in the visual field. Often called edge detectors.

2.Complex Cortical Cells:  Respond best to a particular orientation of a line that is moving across the visual field.

3.Hypercomplex Cortical Cells:  The receptive fields have an inhibitory area at each end.  The highest rate of firing occurs if the line is limited in length, as the length increases, and one of the inhibitory areas is activated, the rate of firing will decrease.

Procedure for Measuring Action Potentials from a Neuron in the Primary Visual Cortex that are Classified as Simple Cells.

Examples of Receptive Fields in the Brain

Perception of Contrast in a Scene

1.  We can think of the changes from light to dark in a scene as varying according to a sine wave function. 

Notice that the high spatial frequenccy is filtered the detail or sharpness is lost but there is still high contrast between areas.  When the low spatial filter is applied then there are sharp edges, but a lot of the the contrast is lost.

Conclusions

From a neural level of analysis, how can we perceive the complex scenes in our world with respect to contrast?

The brain must be able to analyze all possible spatial frequencies from high frequency to low frequency.

There is evidence that the firing rate of simple cortical cells are more accurately tuned to spatial-frequency than to the widths of the bars.

Form Perception and its Neural Basis

Cells in area V4 are also differentially sensitive to wavelength.  This finding suggests a role in color perception

2. Cells in area V5 are specialized for the perception of motion.

3. Cells that are found in the inferior temporal visual cortex respond best to complex forms.

4. What is the procedure that is used to try and determine what dimensions of the stimuli the cell is firing to?

5. There is also an area of cells in the prefrontal cortex that shows an increase in the rate of firing  to faces but not other visual stimuli. 

6. Pathway underlying the visual identification of stimuli:  From primary visual cortexàthrough temporal cortical regions to àprefrontal region.  

Perception of Subjective Contours

Here the black circles are covered and the subjective contours are lost.

Theories and Physiological Evidence for Color Coding

1.Trichromatic Theory of Color (Young-Helmholtz)

2.Opponent Process Theory (Hering)

3.Bringing both views together

I. Deduction of Physiological Mechanisms of Color vision through the use of
Psychophysical techniques

1. Color matching experiment forms basis for a trichromatic theory of color vision.

500---Test 440, 560, 640---Comparison---mix together in appropriate proportions so that the comparison resembles looks like the
test.

This cannot be done with only two other wavelengths.

These data suggest a trichromatic theory of color vision!!!

Young (1802) Proposed Trichromatic Theory Helmoltz (1852) Further explicated theory

Called Young-Helmholtz Theory

Young (1802) Three different kinds of receptors in retina, each receptor different sensitivity to light of a specific spectral comparison; When stimulated by wavelength, neural activity accounts for experience of color.

Herman von Helmholtz (1806) revised Young's proposal and postulated a distinct spectral sensitivity curve for each of the three receptors. Questioned the notion of a single wavelength activating one receptor can give rise to experience of color and instead argued that receptors were maximally sensitive to wavelengths that correspond to blue, green and red, but that each receptor responds to a spectrum of wavelengths. All hues result from the distribution of the spectral lights. Mixtures can be produced by the appropriate contributions of a three receptor system.

Color vision depends on three receptor mechanisms each having different spectral sensitivities; that is they respond differentially to different wavelengths,--- absorb different amounts of pigment by cones.

Explanation of Theory

Light of a given wavelength stimulates three receptors to different degrees. Ratio of activity in three receptor mechanisms determines the perception of color.

Three mechanisms short- wave mechanism (S), medium wavelength (M) Long-wavelength (L); region of spectrum which most sensitive.

The figure below illustrates the response to the three types of retinal mechanisms proposed by the trichromatic theory
 

If we expose to the retina a light of 500 nm we see a response of 1.3 from (S) mech; 9.0 from (M) mechanism and 6.0 from  the (L) mechanism. According to the trichomatic theory the ratio of 1.3 to 9.0 to 6.0 of activity determines the perception of  a particular color. Given that we know the ratio of activity of  the three receptors mechanism we should be able to determine  the perception of color from the response of each of these receptors!! This theory allows us to predict what color we will  see if we combine lights of different colors. The following  figure visually represents the ratio of activity between the S, M and L Cones.
 

Example: Project spot of red light onto spot of green light what is the resultant color? (1) Green produces large R in M receptor(2) Red produces large R in L receptor. If we take these together we see large R in M receptor; Large R in L receptor and small R in S receptor: therefore we will see Yellow: According to the trichromatic theory this is also what we see if we mix lights.  What happens if we mix blue light and yellow light?? Blue = Large activity in S receptors and Yellow = Large activity in Both M and L receptors. Given this; according to the trichromatic theory we should perceive white, however, this is not correct when we mix lights we see green! What happens when we take a 500 nm test stimulus and match a 420 nm + 500 nm + 640 nm so that we see both patches of light as the same. Test- 500= 1.3, 9.0, 6.0 Comparison- 420+560+640= 1.3,9.0,6.0. Same ratio of activity output, response of these receptor types. Two physically different stimuli have similar physiological effects.

Physiological Basis for a trichromatic theory

It is indeed the case the the three cones have different absortion spectra. That is, they maximally absorb a single wavelength but have a range of wavelengths that each cone absorbs. As you vary the wavelength from that which is maximally absorbed, the absorbtion decreases. See the figure below to graphically illustrate the principal.

I. Opponent- Process Theory

a. Hering's Version of the Theory

Postulates three independent mechanisms that underline color vision; three pairs of visual processes; give rise to three pairs of unique sensory qualities. Each pair is opponent in nature; opposite physiological processes and sensory experience. Pairs---- Yellow- Blue, Red- Green, White- Black. If one subsystem of the pair is stimulated more than the other member than will be seen; if both are equally stimulated cancel each other out leaving gray. We can experience red- blues; green- blues never yellow- blues.Because each pair is mutually opponent; never have both yellow and blue mix. Red- Green System Lower threshold; than Yellow-
Blue

Hurvich and Jameson (1955, 1974)

Proposed three photoreceptors (cones) eac