• Lateral inhibition:fig 3.2/1-2. Lateral inhibition enhances contrast between stimuli.
• Retinal ganglion cells:fig 3.2/3-4.
• Ganglion cell receptive fields have a center and antagonistic surround. The receptive field of most ganglion cells is not homogenous but is divided into two parts: a circular zone at the center, called the receptive field center, and the remaining area of field, called the surround. Two classes of ganglion cells can be distinguished by their response to a small spot of light applied at the center of their receptive field. ON center cells signal rapid increases in light intensity, and OFF center cells signal rapid decreases.
• What is the purpose of the center-surround structure? The absolute amount of light reflected by objects is relatively uninformative because it is determined by the intensity of the light source. The information required to detect objects is contained mainly in variations in the light intensity across the visual scene. The firing rate of ganglion cells provides a measure of the difference in the intensities of light illuminating the center and surround. In this way information about differences in intensities is enhanced and directly transmitted to higher centers.

fig 3.2/1  lateral inhibition (ref 12)	fig 3.2/2  lateral inibition (ref 2, 185, 7.5) Two cones, A and B, and lateral neural connections (highly schematic). Inhibitory connections are indicated by minus signs and green. What happens if the overall stimulation is increased? The signals from each of the receptors tend to increase, but so does their ability to inhibit each other. The net result is that the overall illumination is largely ignored. What happens if the illumination at only A is increased?The enhanced signal from A further inhibits that from B. However, because B does not receive any extra illumination, its inhibitory effect on A remains unchanged. The effect of an increased illumination at A is thus an increased signal from the more highly stimulated A and a decreased signal from the more inhibited B. The brain is made more aware of this difference in illumination at A and B than it would be without lateral inhibition. The result is that an edge, where the light intensity changes rapidly from brighter to darker, is made more noticeable (edge enhancement), while an overall illumination change is not so apparent.
fig 3.2/3  ON center ganglion cell (ref 3, 67, 2.35) Response of a retinal ganglion cell in the cat's retina to stimulation (a) outside the receptive field (area A on the screen); (b) inside the excitatory area of the cell's receptive field (area B); and (c) inside the inhibitory area of the cell's receptive field (area C). The excitatory-center-inhibtory-surround receptive field is shown on the far right without the screen.	fig 3.2/4  stimulus size increases (ref 3, 67, 2.36) Response of a cell with an excitatory-center-inhibtory-surround receptive field (ON center cell). The area stimulate dwith light is indicated by the shading, and the response to the stimulus is indicated by the records below each receptive field. This cell responds best to stimulation that is the size of the receptive field center. (In a simple way, when light falls within the excitatory region, it will add to the total stimulation, and increase the firing rate of the cell. When light falls in the inhibitory region, it will subtract from the total stimulation, and decrease the cell's firing rate.)
fig 3.2/5.  simultaneous  contrast (ref 3, 71, 2.41) Simultaneous contrast. The two center squares reflect the same amount of light into your eyes but, because of the simultaneous contrast effect, look different. Psychologically you can say that the effect is caused by the difference in the backgrounds. But what is the physiological mechanism behind this effect?	fig 3.2/6  Hermann grid Explanation of the Herman grid based on center-surround antagonism, and receptive field size.

KEYTERMS
    lecture: center/surround receptive field,  lateral inhibition, contrast, ON-center receptive field, OFF-center receptive field.
    book, Chapter three: Bloch's law, crowding effect, dark adaptation, duplex theory, eccentricity, Hermann grid, intensity, lateral inhibition, lightness, lightness contrast, Mach bands, mesopic, photopic, scotopic, photon, Purkinje shift, resolution, retinal ganglion cells, Ricco's law, spatial summation, temporal summation, visual acuity
 
 

STUDY QUESTIONS:

• How can you relate Mach bands and receptive field structure?
• What receptive field property is most related to the Hermann grid?
• How is the receptive field structure of retinal ganglion cells related to retinal image processing?
• If you want to emphasize spatial resolution in a visual system what would you have to pay for that? (think about the relationship between spatial resolution and sensitivity)
• How are magno and parvo retinal ganglion cells contributing to spatial resolution and sensitivity?

• The receptive field of a cell has an excitatory center and  inhibitory surround. We will observe the highest firing rate from this cell when we a. stimulate only the center with a light increment; b. stimulate only the surround with a light increment; c. stimulate the entire receptive field with a light increment; d. stimulate outside the receptive field.
• Which of the following phenomena are thought to be due to lateral inhibition? a. Mach bands; b. the Hermann grid; c. neither a nor b;d. both a and b.
• In simultaneous lightness contrast you may see a lightness difference between two grey disks because a. they are on different backgrounds; b. they have different physical intensities; c. of the responses of retinal ganglion cells; d. both a and c.
• The rod system is ____ sensitive than the cone system in the dark because a. less; the rod receptors are more spread out than cone receptors; b. less; of the way the rods diverge as they synapse on the ganglion cells; c. more; of spatial summation; d. more; each rod synapses on its own bipolar cell.