大鼠對負向對比的視覺偏好

Abstract

人類視覺對於負向對比的偵測能力較正向對比要來的好，這種知覺上的對比極性偏好恰好與初級視覺皮質區的神經反應偏好相符－負向對比的視覺刺激能在人類初級視覺皮質區引起較強烈的神經反應。然而近期的神經電位染劑研究卻在小鼠的初級視覺皮質區發現了相反的情況－正向對比的視覺刺激能在小鼠的初級視覺皮質區引起較強烈的螢光變化。神經電位染劑的研究發現暗示了夜行性嚙齒類動物的視覺可能具有和日行性動物相反的對比極性偏好。為了驗證這個假說，我們訓練大鼠針對視覺刺激進行強迫選擇試驗，透過大鼠在強迫選擇試驗中的作答表現，研究大鼠是否也在行為上表現出對對比極性的偏好。我們意外地發現，在低亮度情境下（20 cd/m2），當對比強度增強時，大鼠對負向對比刺激的偵測狀況較正向對比刺激要來得好。這種對比極性偵測能力的不對稱性同樣也能在學習強迫選擇試驗的過程中發現－大鼠在負向對比刺激和酬賞的連結學習速率明顯的比對正向對比刺激和酬賞的連結學習速率要來得快。整體來說，我們的研究發現夜行性嚙齒類動物在光視覺下同樣會表現出較好的負向對比偵測能力，這暗示了夜行性動物的視覺系統在處理對比極性的機制上應該與日行性動物相類似。而神經電位染劑研究中發現由正向對比刺激引起的強烈螢光變化可能反應了該刺激引發強烈的抑制性局部電場電位。

Humans can detect negative contrast (black on gray) better and more easily than positive contrast (white on gray). The black-over-white preference in visual perception is accordance with the findings that negative-contrast stimuli can evoke larger neuronal responses than positive-contrast stimuli in primary visual cortex (V1). In contrast, strong white-dominant responses were recently found in mouse V1 with the voltage-sensitive dye imaging (VSDI) technique. Based on these findings, it is possible that nocturnal rodents may prefer positive contrast to negative contrast. Here we used a two-alternative forced choice task to test the preference of contrast polarity in behaving rats (Long Evans). We manipulated contrast polarity (positive or negative contrast), contrast intensity and the mean luminance of the screen (high: 50 cd/m2, low: 20 cd/m2) in our experiments. Surprisingly we found that rats could detect the negative-contrast stimuli better than the positive-contrast stimuli, when the contrast intensity became stronger, in the low luminance condition. This asymmetry in detecting contrast polarity also occurred in the learning progress when rats were learning the contrast detection task. Overall, our behavioral results show that nocturnal rodents can detect negative contrast better than positive contrast in photopic vision. The better sensitivity to negative contrast suggests that the visual system of nocturnal rodents also displays black-over-white bias in visual processing. The white-dominant responses in mouse V1 might reflect the stronger inhibitory signals evoked by the positive-contrast stimuli.