Lecture 4
Neural pathways:
RGC's project axons to brain. Where axons leave = optic disk = blind
spot!
RGC axons collect at optic disk => blind spot (demo)
Pathway to brain: One principle of brain organization is that the left side of
the brain tends to represent events on the right side and so forth. Vision is no
exception. In the case of vision, though, the left visual field (not the left eye) is
represented on the right side and vice versa.
Visual field defects:
transect left optic nerve => no vision in left eye, visual field of right eye =
fine, loss of left peripheral vision (vision normally subserved only by left eye)
transect optic chiasm => loss of crossed pathways = no medial retina input
= no peripheral vision, monocular vision but different eyes for left & right halves
transect left optic tract=> blindness in right visual field
Response properties of neurons in these pathways
Single unit recording:
Insert an electrode into the brain, and measure the activity of individual
neurons. Listen in on what the neuron is saying.
Correlation: this is a correlative technique -- try to find a correlation
between the neuron's firing patterns and external sensory stimuli, behavioral
responses, or, the internal mental/cognitive state of the subject. The last is very
hard to do because we have no way of measuring internal state in the absence of
some overt expression of that state (i.e. verbal report). So, we've made more
progress in sensory and motor areas than in cognition.
This technique can be done in awake animals & humans (though need
direct access to brain), because the brain has no internal sensation, no pain
receptors.
Start with retinal ganglion cells.
RGC's have receptive fields
- present spots of light all over a screen
- monitor neural responses to that stimulus
-> cell discharges only when light is located @ a certain position
= receptive field
rods, cones, etc. have them too
= direct consequence of the optics
Map RF in detail, discover
Center-surround organization
RGC's = either excited or inhibited by light
usually both
on center, off surround
off center, on surround
Consequence:
Large spot => weak response
Small spot => larger response
Remember the structuralists, who were seeking to reduce perception to its
elemental components? it seems that at the very earliest stage of visual neural
processing, the elemental components of neural processing are small spots or
pixels if you will.
How would you wire one of these up?
Not all RGC’s are alike!
Responses vary:
On vs. off center/surround
Latency
Sustained vs. Transient
Receptive field size
Color (wavelength sensitivity)
Contrast sensitivity
In short, there is SPECIALIZATION for processing the visual scene in
different ways!
(demo tape)
Next stop:
LGN
= lateral geniculate nucleus
part of thalamus = gateway to the cortex
Multiple layers (6 in primates?)
Each layer:
input from 1 eye = monocular
retinotopically organized!
from one class of ganglion cells (X or Y)
X = parvocellular (small cells) – small receptive fields –
“What” pathway
Y = magnocellular – larger receptive fields – “Where”
pathway
- properties of X & Y are maintained in LGN, i.e. X =
acuity, Y = sensitivity/motion
- parvocellular = primarily primates, probably evolved
later
layer 1 (bottom)
contra-M
ipsi-M
ipsi-P
contra-P
ipsi-P
contra P
layer 6 (top)
retinal topography