象限訊息對視覺辨識之影響

EFFECTS OF QUADRANT INFORMATION ON RECOGNIZING VISUAL STIMULI

視覺刺激的傅立業轉換包含振幅頻譜與相仿頻譜。就視覺的辨認而言，相位頻譜的重要性又大於振幅頻譜（Oppenheim & Lim，1981）。然也有研究指出90度以內的相位扭曲並不影響形狀辨識（Caelli & Bevan，1982；Piotrowski & Campbell，1982）。本研究首先在實驗一與實驗二中，以中文字辨識與隨機點立體圖辨識作業，再度確定視覺系統可以容忍的相位扭曲程度至少為90度，甚至可達120度。本研究再將傅立業轉換中的相位頻譜進一步細分出象限頻譜（亦即只有傅立業轉換的實部正負號與虛部正負號的訊息），與限制在[0.π/2]範圍內的「直角相位」頻譜。本文利用頻譜成份對調的方式，企圖說明影像的輪廓訊息與像差訊息可能主要由象限頻譜所攜帶。這些結果不只符合Pollen與Ronner（1981）的想法，也說明容忍差距90度的相位訊息處理機制，可能延伸到V1、V2或更高的皮質區域。亦即初級視覺皮質區的神經細胞可能以90度相位角為區隔基礎，並允許區隔內之刺激相對於原相位角±45度內產生空間位移，仍不影響該神經細胞原來的興奮或抑制反應組型，而在計算上達成抽取邊界訊息、抗拒90度相位扭曲，並保持相位訊息比振幅訊息優勢的目標。

Fourier transform comprises two major components, i.e., amplitude spectra and phase spectra. Phase information seems to be more critical than its amplitude counterpart in preserving visual information (Oppenheim & Lim, 1981). However, previous studies also suggested that phase distortion within 90 degrees does not hamper successful recognition of well-defined forms. Experiments 1 & 2 were designed to test this psychophysical reality. The results turn out to be a confirmation. The admissible phase distortion can even be extended to an extent of 120 degrees. In the present study, phase spectrum is further decomposed into a combination of quadrant spectrum and rectangular phase spectrum (0~π/2). A series of simulations are then conducted on Chinese characters, human faces, and random-dot stereograms. The results indicate the following assertions: (1) Visual contour can be retrieved mainly from the information provided by the quadrant spectrum. (2) Any degree of distortion in the assigned rectangular phase spectrum does not prevent a successful 3D emergence from perceiving random-dot stereograms. However, the distortion of quadrant spectrum will severely eliminate a successful 3D reconstruction in perceiving the stereo pair. This implies that identical phase processing mechanism with a tolerance of distortion within 90 degrees may operate in VI as well as V2 or higher cortical regions. (3) Although Fourier analysis is performed by extracting global properties from the presented stimuli, however, one of its critical component, i.e., quadrant spectrum, is manifesting like a local neuron. We then suggest a plausible explanation to establish the link.