Retina, thalamus (LGN) and cortex; the visual field map in primary visual cortex.
Enlarged respresentation of the fovea.
Hemidecussation at the optic chiasm: two retinas, one cortical map, one world (Newton).
An implication of topographic projection: receptive fields are small.
BUT the projection is not strictly one to one (dendritic trees have a spread), so neurons pick up from multiple receptors. Also: “gap junctions” connect adjacent receptors and horizontal cells.
What is the role of horizontal cells and amacrine cells?
Experimental setup
Results: a “Mexican hat” profile. Center-surround antagonism (= lateral inhibition)
Retinal microelectrode recording: RF is centered on cell body
Separate on- and off-systems respectively detect onset and offset of center stimulation at the bipolar cell and later levels
Sign-conserving and sign-inverting synapses in the retina
Horizontal cells and (many) amacrine cells are agents of lateral inhibition, mediated by sign-inverting synapses
Simultaneous contrast, Mach Bands, Hermann grid
Selective response to edges means economy of representation: a cartoon-like line drawing needs less information
The “trigger feature” concept, illustrated by “bug detector” retinal ganglion cells in frogs…rather than simply registering light or contrast, the firing of such a neuron may convey something important to the animal.
Diversity among bipolar cells: on- and off-center types for lighter, darker objects. Advantages of this over alternatives: each nerve impulse can be significant, a potential call to action.
More diversity among ganglion cells: anatomy(dendritic trees)
X (=(i = P) and Y (= a = M) cells for focal,
ambient vision respectively
X cells signal the total light falling in their receptive field, wherever it falls within the field, so an X cell may not respond when a light is shifted within its receptive field. Y cells detect any change (how?)
X and Y cells are not truly specialized detectors (not like bug detectors): “ensemble coding”/”distributed representation” means the joint activity of many cells is needed to represent a useful message.