SENSORY SYSTEMS
What is a Sensory System? Window to physical energies. Give rise to sensory perceptions.
General plan of sensory pathways: receptor, thalamus (diencephalon), cortex (telencephalon).
Basic function of sensory pathways: transduction and coding.
Transduction: transformation of physical energy into neuronal activity. Occurs in Receptor neurons.
Coding: Information about stimuli are represented (coded) in the patterns of activity (action potentials) of the neurons: intensity, duration.
Different sensory systems: sensory modalities:
Vision
Audition
Somatosensory
Taste
Smell
Why do we have different modalities when the result of transduction of sensory stimuli for all modalities is the same: action potentials?
Theory of specific sensory energies by Johannes Muller, 1826. Labeled pathways.
Some receptors transduce and encode: somatosensory & olfaction
Other receptors only transduce (do not generate action potentials): vision, audition, taste.
RECEPTIVE FIELD: The activity of all sensory neurons can be modulated by stimulating a certain region of the sensory surface. This region is called the Receptive Field of the neuron.
THE VISUAL SYSTEM (Chap. 6)
Web links for Visual System:
http://epunix.biols.susx.ac.uk/Home/George_Mather/Linked%20Pages/Physiol/index.html
The visual ssystem is one of the most studied sensory systems.
Functions of the visual system:
- Responds to questions: What (recognition) and Where (location)?
-Generates 3-D perception of the world from a 2-D retinal image.
Illusions, hallucinations.
Visible light: Narrow portion of the spectrum of electromagnetic radiation. Waves (wavelength) or photons (particles).
-Speed: 300,000 Km/sec; 186,000 miles/sec. Does not need a medium (air, water).
-Visible spectrum: wavelength extends from 760 (red end) to 380 (blue end) nanometers (nm, billionth of a meter: 10 to -9). (FIG 6.2). It can vary for different species.
-Wavelengh of light associated with color perception. Frequency.
-Intensity of light associated with brightness.
Eye. It is an optical instrument similar to a photographic camera. Development of optic cup from diencephalon.
It forms a 2-D image of the world onto the retina (FIG 6.4)
Retina contains photoreceptors that tranduce light
Having two eyes gives us the ability to create 3-D perceptions (depth perception through binoculat vision).
The eyes can be moved with the help of three pairs of muscles.
The two eyes move conjointly.
The eyes can accomodate to objects viewed at different distances.
Accomodation is possible by changes in the shape of the lens.
THE RETINA. Consist of 5 layers of neurons, from external to internal (Figs. 6.5, 6.6).
Receptors cells (rods and cones) (Figs. 6.7, 6.8)
Horizontal cells
Bipolar cells
Amacrine cells
Ganglion cells
Inside out organization of retina. Because of this, light must pass through other layers before reaching the receptors. This problem is minimized by the fovea (Fig. 6.6). Also, there is need for the optic disk (blind spot). The problem posed by the blind spot is solved by completion (do the demo in page 136).
Why, then, do we have an inside out retinal organization? It is believed that it facilitates the nutrition and turn over of receptors: closest to choroid, a vascular black lining in the back of eyeball.
Blind spot or Optic Disk. Has no receptors. Area where axons from ganglion cells leave the eyeball (SEE DEMO IN PAGE 136).
Fovea, 0.33 mm in diameter, has high density of receptors (cones), allows us to see in great detail, that is, with high acuity (Fig. 6.6).
PHOTORECEPTORS:
Rods (about 120 million) and Cones (about 8 million) (Figs. 6.7, 6.8)
Duplexity theory of vision: Two Receptors= Two visual systems:
one for daylight and one for dim light. Some animals only have one, e.g., rats and other nocturnal rodent.
Photopic system: Cone driven system:
High acuity (detail, day vision)
Poor sensitivity
private line: little convergence from cone to ganglion cell (Fig. 6.8)
Cones are concentrated in the fovea (Fig. 6.9).
Color (3 types of cones most sensitive to red, green and blue)
Scotopic system: Rod driven system
High sensitivity, night vision (up to one photon!!) (star gazing)
Poor acuity
party line: large convergence from rod to ganglion cells (Fig. 6.8). Spatial summation.
Rods are found outside the fovea (Fig. 6.9)
No color (only one type of rod)
Spectral sensitivity of rods and cones (FIG. 6.10)
Purkinje effect: blue flowers appear brighter at dusk
yellow & red flowers appear brighter during day
Importance of eye movements.
If cones are restricted to the fovea, how do we perceive the entire visual field? We scan the visual field with quick eye movements called saccades (temporal integration).
Do the demonstration at the bottom of page 138: perception of color and detail decrease beyond 20 degrees from the fixation point. However, if you move your eyes, perception of color and detail occurs throughout the field.
Stabilized retinal images dissapear! Neurons need change to respond.
Phototransduction (Fig. 6.12)
Photoreceptors have outer segments, inner segments and synaptic terminals.
Rods: Outer segment has discs.
Cones: outer segments has invaginations
Phototransduccion has been studied most extensively in rods.
Discs have rhodopsin, a photopigment made up of retinal (from Vit A) and opsin, a protein.
Dark current: at night, cGMP (cyclic guanosine monophosphate) keeps Na+ channels open, keeping the membrane potential depolarized!
Glutamate transmitter is released at the synaptic terminal
Light activates (splits) rhodopsin and starts a cascade of chemical reactions.
Cascade decreases, by hydrolisis, the concentration of cGMP.
Na+ channels close spontaneously: membrane potential hyperpolarizes (more negative inside).
Release of glutamate is stopped (see Fig. 6.12).
Visual pathway (FIG. 6.13) (see also Web Site above):
Optic nerve, optic chiasm, optic tract, dLGN, optic radiations, visual cortex.
Retinal ganglion cells synapse with neurons in the dorsal lateral geniculate nucleus (dLGN) of the thalamus (diencephalon).
dLGN neurons synapse with neurons in layer 4 of the primary visual cortex (V1, area 17, striate cortex) in the occipital lobe.
Visual field: Monocular and binocular portions.
Visual Thalamus:
dorsal Lateral Geniculate Nucleus: dLGN
First relay station
Layered structure (number of layer bottom-up).
The layers are monocular (input from one eye only):
ipsilateral eye (same side) or contralateral (opposite side) eye.
The layers are retinotopically organized, that is, they contain a map of the retina (as we shall see, most parts of the visual system are retinotopically organized)
The layers are in register.
Visual Cortex (in occipital, parietal and temporal lobes). (Fig. 6.26)
6 layers of neurons (I-VI, top-bottom)
Primary visual cortex (V1, striate cortex)
Neurons in the dLGN project to layer IV in V1.
Layer IV in V1 is monocular
Ocular dominance columns in V1
Binocular cells appear in layers II-III of V1
Layer V projects to the superior colliculus (mesencephalon):
Layer VI in V1 projects back to the dLGN.
Visual information gets processed in V1, in the Occipital lobe.
Neurons in layers II-III in primary visual cortex (V1) send information to other, non-primary areas (association, extrastriate areas), located in the Temporal and Parietal lobes. More than 30 in the monkey, and possibly more in humans. (Fig. 6.29).
Seeing Edges. The perception of an edge is the perception of a contrast between two adjacent areas of the visual field.
Lateral Inhibition and Contrast Enhancement (Fig. 6.14).
The phenomenon of contrast enhancement is explained by lateral inhibition among photoreceptors (Fig. 6.15).
Properties of visual neurons
Receptive field of a neuron: Area of visual field within which it is possible to influence the firing of a neuron (Fig. 6.16).
In the retina:
ON bipolar: depolarizes with light.
OFF bipolar: hyperpolarizes with light.
Combined, ON and OFF responses produce the receptive fields of retinal ganglion neurons and dLGN neurons:
These receptive fields show concentric, center-surround organization, either:
Center ON, surround OFF
Center OFF, surround ON
These neurons code for contrast (Fig. 6.17).
Input from dLGN to cortex forms SIMPLE cells in cortex (convergence) (FIG. 6.18).
Several simple cells converge and form COMPLEX cells in cortex (receptive field larger, no clear ON-OFF regions, respond to motion).
Receptive fields of cortical neurons are selective (Fig. 6.19):
They respond to specific orientations, direction of motion, velocity, depth (binocular disparity), etc.
Columnar Organization of Primary Visual Cortex
Orientation Columns (Fig. 6.21).
Hypercolumn (Fig. 6.22, name not used in book) : portion of cortex that has the machinery to analyze all properties of stimuli (orientation, motion, etc), for each point in the visual field
Seeing Color
Component (trichromatic) theory (Thomas Young, 1802; Hermann von Helmholtz, 1852).
There are three different kinds of photoreceptors (cones), each with a different spectral sensitivity (Fig. 6.23), and the color of a particular stimulus results from the ratio of activity in these three kinds of receptors.
This theory is based on the observation that any color of the visible spectrum can be matched by a mixing together of three different wavelengths of light in different proportions. This can be accomplished with any three wavelengths, provided that the color of any of these wavelengths cannot be matched by a mixing of the other two.
MECHANISMS OF PERCEPTION, CONSCIOUS AWARENESS, AND ATTENTION. CHAP. 6.6, 7.1
Visual neurons respond to features of the stimuli that are important for answering the questions What and Where. (Fig. 6.30)
Example: emergence of orientation selective cells.
Principles of Sensory System Organization
Hierarchical Organization: From receptors to association cortex (Fig. 7.1)
Functional segregation: Cortical areas specialize in different kinds of analysis of the visual image.
Paralell Processing: WHAT AND WHERE are processed in different pathways which originate in the retina (Figs. 6.30, 6.31).
Magnocellular pathway:
-large cells
-respond to motion, spatial information (localization)
-processing in parietal cortex (dorsal stream)
Cortical lesions: case of LM: deficit in the perception of movement.
Movement is also useful for the perception of the form of objects.
-breaking camouflage
-biological motion: Johansson figures (lights in joints)
Check this Site for Biological Motion demo: http://www.biomotionlab.ca/Demos/BMLwalker.html
Balint Syndrome:
Localization deficit due to lesion of parieto-occipital cortex:
-Ataxia: Deficit in the visual guidance of movements: error in visually reaching for objects.
-Ocular Apraxia: Deficit in the ocular scanning of images. Patients cannot fixate.
-Simultagnosia. Perception of only one object at the time, even if they are together.
Parvocellular pathway: -small cells
-respond to shape, color, detail
-processing in temporal cortex (ventral stream).
Lesions produce Visual Agnosias or failure to recognize visual stimuli (acuity may be normal):
Prosopagnosia: failure to recognize faces:
“The man who mistook his wife for a hat” by Oliver Sacks