Physiology of Photoreceptors
• Vertebrate photoreceptors hyperpolarize and produce graded potentials
• Photoreceptors use glutamate as transmitter
• Bipolar cells can both hyperpolarize and depolarize producing both
ON and OFF responses
• ON bipolar – glutamate is inhibitory
• OFF bipolar – glutamate is excitatory

Receptive Fields
• Record from a single ganglion cell in the retina
• Using small spots of light activate the portion of the visual field that activates the neuron

Schematic Retina Showing a
Receptive Field
Orange are excitatory inputs into the receptive field.
Blue are inhibitory inputs into the receptive field .
Receptors
Horizontal
Cells
Bipolar Cells
Amacrine
Cells
Ganglion Cells
-
+
-

The excitatory connection stimulates and increase in the firing rate in the ganglion cell.
Light strikes a cone with an excitatory connection to the ganglion cell
Number of Action
Potentials:
1

Light now fills the excitatory region of the receptive field.
Number of
Action
Potentials:
12

If light falls in the inhibitory region, the firing rate of cell is reduced.
Number of
Action
Potentials:
8

Lateral Inhibition – Variations in the
On/Off Structure
• Lights on the edge of the field cause a reduction in the background activity of the cell
• On and off neurons
• Center-surround structure – need to examine in light of different channels of information direct to the cerebral cortex

Receptive Fields in the Retina
• Two types of ganglion cells:
– on and off dependent upon the bipolar neurons
• Center Surround structure of the receptive field described by Kuffler
• Best activated by central illumination
• Best inhibited by annular illumination

Different View of Center-Surround
Organization: Parallel Pathways
• Transformation of visual information is evident in the ganglion cells of the retina
• X cells – sustained linear responses
• Y cells – transient, excitatory non-linear responses

P and M Projections to LGN:
Different Physiologic Channels
• P cells in the retina (also known as midget ganglion cells) project to the parvocellular layers (3-6) of LGN
• M cells in the retina (also known as parasol cells) project to the magnocellular (ventral most) layers (1-2) of the LGN
• Intercalated layers are termed koniocellular (dustlike or tiny cells)

Physiology of V1
• Originally studied by
Hubel and Wiesel who demonstrated two types of neurons
– Simple cells – constructed from
LGN on/off cells
– Complex cells – constructed from simple cells

Cortical Complex Cells: Example of Hierarchy of V1
• Strong orientation selectivity in cells
• Moving bars in a specific direction
• NO on/off areas like in simple cells
• Receptive fields were not elongated
• Located in layers
2,3, and 5 which receive input from layer 4 (from ? simple cells)