The antagonistic surround response of bipolar cells is due to photoreceptors that surround those that synapse directly on them. These photoreceptors (which also connect directly with their own bipolar cells) synapse with horizontal cells that participate in complex triadic synapses with many photoreceptors and bipolar cells. The pathway through •Fig. 8.9 Relationshipsamongavisualtarget(long arrow, top),imagesontheretinasofthetwoeyes(middle), andprojectionsoftheganglioncellscarryingvisualinformationabouttheseimages(bottom). Thetargetimageissolargethatitextendsintothemonocularsegmentsoftheeyes,whereonesideofitisseenbyonlytheipsilateraleye.Notehowtheaxonsaresortedinthechiasmsothatallinformationabouttheleftvisualfieldofbotheyesisconveyedtotherightsideofthebrainandallinformationabout therightvisualfieldisconveyedtotheleftside.
continued object movement within the field will not be signaled. P cells respond differently to different wavelengths of light. Because there are blue, green, and red cones, many combinations of color properties are possible, but in fact P cells have been shown to have opposing responses only to red and green or only to blue and yellow (a combination of red and green). These mechanisms can greatly reduce the ambiguity of color detection caused by the overlap in cone color sensitivity and may provide a substrate for the opponency process observations.
M cells, on the other hand, respond with phasic bursts of action potentials to the redistribution of light, such as would be caused by the movement of an object within their large receptive fields. M cells are not sensitive to differences in wavelength but are more sensitive to luminance than P cells are.
Thus the output of the retina consists primarily of ganglion cell axons from (1) sustained, linear P cells with small receptive fields that convey information about color, form, and fine details and (2) phasic, nonlinear M cells with larger receptive fields that convey information about illumination and movement. Both exist in on-center and off-center varieties (
Fig. 8.8
The Visual Pathway
Retinal ganglion cells transmit information to the brain by way of the optic nerve, optic chiasm, and optic tract.
Fig.
8.9 shows the relationships among a visual target, the retinal images of the target in the two eyes, and the projections of retinal ganglion cells to the two hemispheres of the brain. The eyes and the optic nerves, chiasm, and tract are viewed from above.
The visual target, an arrow, is in the visual fields of both eyes (see
Fig. 8.9 ) and, in this case, is so long that it extends into the monocular segments of each retina (i.e., one end of the target can be seen by only one eye and the other end by only the other eye). The shaded circle at the center of the target represents the fixation point. The image of the target on the retinas is reversed by the lens system. The left
CHAPTER 8 The Special Senses half of the visual target is imaged on the nasal retina of the left eye and the temporal retina of the right eye. Thus the left visual field is seen by the left nasal retina and the right temporal retina. Similarly, the right half of the visual target is imaged on and seen by the left temporal retina and the right nasal retina. The lens system also causes an inversion in the vertical axis, with the upper visual field imaged on the lower retina and vice versa.
The axons of retinal ganglion cells may or may not cross in the optic chiasm, depending on the location of the ganglion cell in the retina (see