TABLE 4.

Estimates of retinogeniculate divergence: number of geniculate cells sharing a common retinal afferent

Parameter	YC	YA	X
Cell density c, relay cells/mm3	10,000	7,400	14,800
Volume of retinal axonal arbor v, × 10−3 mm−3	2.9	8.7	2.4
Probability of tightly correlated firing p, RF of same type, sign, and overlap ≥ 20%	0.74	0.40	0.31
Number of geniculate cells sharing a retinal afferent, D	12	20	10

We calculated the number of relay cells sharing a retinal afferent (D) based on three parameters: cell density (c), volume of the retinal axonal arbors within each layer of dLGN (v), and probability of tightly correlated firing (p). The cell densities were calculated (Madarasz et al. 1978) by assuming 1) 25% of dLGN neurons are interneurons (LeVay and Ferster 1977, 1979); 2) 22% of neurons in C layers of dLGN are Y cells (Wilson et al. 1976); 3) the volume of layer C occupies about one third of the volume of the C layers (Bickford et al. 1998); and 4) the X/Y ratio in layer A is around 2, to match the sampling bias in the probability measurements. The volume of retinal axonal arbors were retrieved from the results of Sur et al. (1987), assuming that the volume of the Y axonal arbor is one third smaller in layer C than in layer A. The probability of tightly correlated firing was taken from our data (Table 1).