Download Mind, Brain & Behavior

Mind, Brain & Behavior
Monday
February 17, 2003
Retina

Two kinds of photoreceptors (Table 22-1):





Cones – wavelength specific (perceive color) and
detail, see in daylight, detect fast flicker
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.
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.
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.
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.
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.
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.
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.
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.
Edge Detection

Ganglion cells respond to contrast and change in
visual input.




Center-surround (on-off) receptive field.
Bipolar cells also have center-surround receptive
fields.
Neurons in the visual cortex have rectilinear
receptive fields with excitatory and inhibitory zones.
Complex cells provide positional invariance.
Complex Forms, Motion

Processing of form occurs outside the visual
cortex – inferior temporal cortex.



Not organized retinotopically.
10% selective for specific images (hands, faces).
Processing of motion occurs in middle
temporal area (MT or V5), then parietal lobe.

Used for seeing moving objects, pursuit eye
movements, guidance of bodily movement