侧抑制lateralinhibition.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Figure 2.24 Three dark adaptation curves. The red line is the two-stage dark adaptation curve, with an initial cone branch and a later rod branch. The green line is the cone adaptation curve. The black line is the rod adaptation curve. Note that the downward movement of these curves represents an increase in sensitivity. The curves actually begin at the points indicating “light-adapted sensitivity,” but there is a slight delay between the time the lights are turned off and when measurement of the curves begins. (Partial data [pure rod curve] from “Rhodopsin Measurement and Dark Adaptation in a Subject Deficient in cone Vision,” by W. A. H. Ruston, 1961, Journal of Psychology, 156, 193-205. Copyright ? 1961 by the Psychological Society, Cambridge University Press.)  back 錐狀細胞及桿狀細胞的光敏感度曲線(spectral sensitivity curve) 閾值及敏感度 sensitivity=1/threshold rod is most sensitive to light of 500 nm while cone is most sensitive to 560 nm Purkinje shift: 暗適應進行時（如黃昏），不同波長光線的敏感度會因為由錐細胞系統轉為桿細胞系統而有變化，造成知覺亮度的改變，例如，紅花與綠葉，前者變得比較不明豔，但後者變得比較亮 三種cone pigments綜合產生cone spectral sensitivity curve 短波長色素 - 419 nm（最少） 中波長色素 – 531 nm 長波長色素 – 558 nm 一種rod pigment Honeybee absorption spectrum Absorbs ultraviolet light – strongly reflected by flowers 網膜上有五種神經元 垂直連結 桿狀、錐狀細胞→兩極細胞(bipolar cell)→神經節細胞(ganglion cell) 水平連結 水平細胞(horizontal cell) 無軸突細胞(amacrine cell) 神經元匯聚(neural convergence) Figure 3.1(a) 光線先經過其他網膜的神經元，才抵達桿狀及錐狀細胞 怎麼看得清楚呢？ 透明組織 中央錐細胞密度高處較薄 雖然受器細胞有一億二千六百萬個，神經節細胞只有一百萬個 convergence ratio=126:1 桿細胞有一億二千萬個，錐細胞有六百萬個，桿細胞的convergence ratio高於錐細胞?提高桿細胞敏感度  （但個別桿細胞敏感度也高於錐細胞） Fovea的錐細胞多半與神經節細胞有1-1連結?提高錐細胞敏銳度 神經處理如何達成知覺的sensitivity與acuity? Sensitivity -- 匯聚（convergence）或多對一連結增加訊號強度，使敏感度提升 Acuity 一對一連結增加訊號解析度，使敏銳度提升 側抑制（lateral inhibition） 用Limulus作實驗動物 每個小眼連結一個受器（刺激一個小眼引發一個受器反應） 鄰近受器的激發產生抑制作用 側抑制影響亮度知覺—Hermann Grid Figure 3.6, page 50Hermann grid Copyright ? 2002 Wadsworth G