THE ORGANIZATION OF THE TURTLE INNER RETINA .2. ANALYSIS OF COLOR-CODED AND DIRECTIONALLY SELECTIVE CELLS

Color coding and directional selectivity (DS) of retinal neurons were studied in the Pseudemys turtle by using similar intracellular recordi ng and staining techniques as in the preceding paper (J. Ammermuller a nd H. Kolb, 1995, J. Comp. Neurol. 358:1-34). Color-coded responses we re elicited by red (621 or 694 nm), green (525 or 514 nm), and blue (4 55 nm) light flashes. In addition to red/green and yellow/blue types o f chromaticity horizontal cells, in our sample of 305 identified cells we found that 17% of bipolar cells, 6.5% of amacrine cells, and 18% o f ganglion cells exhibit color-coded responses. DS responses were foun d in 37% of the tested ganglion cells and 41% of the tested amacrine c ells. Two morphologically identified bipolar cell types, B10 and B11, were red-ON/blue-OFF and red-OFF/green,blue-ON, respectively. Of five identified amacrine cell types, three were red-OFF/blue-ON center (A1, A3, A23b), one was red-OFF/green-ON center (A32), and one (A33) was d ouble color-opponent of red-ON/blue-OFF center:red-OFF/blue-ON surroun d. Five ganglion cell types had variously color-coded centers (G14 and G24) or surrounds (G3 and G18), including one type, G6, that was doub le color-opponent (red-OFF/green-ON center:red-ON/green-OFF surround). Responses to colors were found primarily in the sustained responses o f bipolar and ganglion cells. However, in amacrine cells, transient co mponents of the response also showed color dependence. Red-OFF-center responses were found in ganglion cells that were in a position to make connections at the strata 2/3 border with the red-OFF bipolar cell (B 11); red-ON-center responses occurred in ganglion cells with branches in stratum 4 of the IPL where the red-ON-center bipolar (B10) ended. B lue-ON-center signals appeared to be processed mainly in strata 1-2/3, and blue-OFF-center signals in strata 3-5 of the IPL, with contributi ons of amacrine cells and bipolar cells. Labeled DS amacrine cells cou ld be identified as A9, A20, and A22, and ganglion cells as G19, G20, and G24. The latter type (G24) showed DS and color coding. All respons e types (ON-center, OFF-center, ON-OFF) were encountered. DS amacrine cells were monostratified near the middle of the IPL, whereas DS gangl ion cells were mono-, bi-, and multistratified, although all DS gangli on cells had one feature in common: they had dendrites in stratum 1 of the IPL. We discuss our findings in terms of the importance of color and motion discrimination tasks associated with complex ganglion cell physiology in the retina of this species. (C) 1995 Wiley-Liss, Inc.