1. Mind, Brain & Behavior
Wednesday
February 12, 2003

2. How is Information Encoded?
Patterns of discharge of action potentials:
Frequency code -- strength is conveyed by frequency (faster rates of firing)
Population code – strength is also conveyed by how many neurons are responding within a group
Slow vs fast adapting receptors differentiate duration of a stimulus.
Labeled line code -- modality is encoded by the particular receptor type.

3. Sharpening of Stimuli
Lateral inhibition – a mechanism that combines excitatory and inhibitory components to enhance a signal.
The difference between stimulation of one part of a receptive field and another is strengthened.
Reduces the likelihood that a stimulus at the edges of a field will activate a relay neuron.
Enhances the acuity of the sensory system.

4. Modality-Specific Mechanisms
In addition to these shared properties, different sensory modalities perform different functions and have mechanisms for doing so.
Specific receptors in somatic sensory system are specialized for different kinds of input.
Population codes are most important to taste and smell.
Timing and intensity of signals is important to localizing sounds in space.

5. Construction of the Visual Image
Chapter 21

6. Gestalt Principles of Organization
Visual perception is a creative process.
Gestalt laws describe how the brain builds percepts from details of an image:
Similarity
Proximity
Figure-ground (winner-take-all strategy) – reversible figures
Good continuation (creative filling in) – triangle illusion

7. Other Illusions
Illusions are “misreadings” of visual information that reveal how the brain applies assumptions to the information it receives.
Muller-Lyer illusion
Size constancy illusion
Ames Room

8. The Visual Pathway
Information goes from the retina to the LGN (thalamus) to the visual cortex (striate cortex).
From the striate cortex information goes forward to adjacent areas of the cortex (extrastriate cortex) where motion, depth, color, form and pattern are processed.
Pathways to the striate cortex are sequential.
Pathways past the striate cortex are parallel.

9. Retina Two kinds of photoreceptor cells:
Cones – wavelength specific (perceive color) and detail
Rods – perceive motion, require less illumination and see in black and white
Cones are concentrated in the fovea.
Rods are concentrated on the periphery.
Cones and rods send axons to ganglion cells.

10. Types of Ganglion Cells
Magnocellular (M cells) – large cells that receive input from rods.
Parvocellular (P cells) – small cells that receive input from cones.
Blob pathway – concerned with color perception.
Interblob pathway – concerned with shape/form.
Both types synapse on layers within the LGN (lateral geniculate nucleus) of the thalamus.

11. Mapping Within the LGN
Optic nerve carries information from ganglia to LGN. Crosses at optic chiasm.
Separate layers are maintained for each eye and for each type of cell (M and P).
Interneurons project from areas of the LGN to striate cortex.

12. Mapping in the Striate Cortex
Separate layers from LGN to striate cortex are maintained in ocular dominance columns.
M and P cells enter the cortex at different levels of layer 4 of the visual cortex.
Information is combined by pyramidal cells that synapse at higher levels in the striate cortex.
Input from both eyes is combined at layer 3.

13. Extrastriate Pathways
Parallel processing of visual information from the striate cortex.
Three pathways:
Color processing – P blob cells, goes from V1 to V2, then V4, then inferior temporal cortex.
Shape processing, depth perception – P interblob cells, goes from V1 to interior temporal cortex.
Motion & spatial relations – M cells, V1 to V2, then MT (V5), to parietal cortex.

14. Equiluminance
Brightness is held constant – permits study of the contribution of color to perception.
Results:
Brightness, not color, is important to motion detection, perspective, relative sizes, depth perception, figure-ground relations, visual illusions.
Motion is a cue for distinguishing among objects.
Things that move together belong together.

15. Visual Agnosias
Existence of distinct agnosias for aspects of perception suggests that these abilities are localized to areas selectively damaged.
Achromatopsia – good perception of form despite inability to distinguish hues.
Prosopagnosia – inability to recognize faces as particular people (identity). Can recognize that it is a face, and tell the parts.

16. Binding Mechanisms
How is information from the separate, parallel pathways brought together and associated?
Treisman & Julesz – combination requires attention.
A pre-attentive process detects the major outline of an object.
An attentive process notices, selects & highlights combinations of features.
Maintained in separate global and detailed maps.