Marhöfer David J, Bach Michael, Heinrich Sven P
Sektion Funktionelle Sehforschung, Klinik für Augenheilkunde, Albert-Ludwigs-Universität, Killianstr. 5, 79106, Freiburg, Germany.
Fakultät für Biologie, Albert-Ludwigs-Universität, Schänzlestr. 1, 79104, Freiburg, Germany.
Doc Ophthalmol. 2015 Oct;131(2):137-48. doi: 10.1007/s10633-015-9502-1. Epub 2015 May 15.
To assess visual acuity objectively "beyond V1", the P300 event-related potential is a promising candidate and closely associated with conscious perception. However, the P300 can be willfully modulated, a disadvantage for objective visual acuity estimation. Faces are very salient stimuli and difficult to ignore. Here, we present a P300-type paradigm to assess visual acuity with faces.
Gray-scale portraits of the respective subject served as oddball stimuli (probability 1/7), scrambled versions of these as the standard stimuli (probability 6/7). Furthermore, stimuli were spatially high-pass filtered (at 0, 2.2, 4.2 and 8.3 cpd), making them recognizable only with sufficient acuity. Acuity was systematically reduced by dioptric blur, chosen individually to render faces unrecognizable when high-passed at ≥ 4.2 cpd. EEG was recorded from 11 subjects at 32 scalp positions and re-referenced to the average of TP9 and TP10. One of the rare face variants was designated as target, for which a button had to be pressed.
The event-related potential was dominated by the P300 at 300-800 ms. All subjects had a significant (P < 0.05) P300 for 0- to 8.3-cpd filtering. When vision was blurred, the fraction of significant P300 responses to 8.3-cpd filtered faces dropped to 18%, but stayed at 100% for 4.2 cpd. Another component, the vertex positive potential (VPP) at 170 ms, was undetectable in most participants with blur and all levels of filtering, even when the images were recognizable.
The study demonstrates the feasibility of a face-based P300 approach to objectively assess visual acuity. The sensitivity to stimulus degradation was comparable to that of a grating-based approach as previously reported. An unexpected finding was the differing behavior of the P300 and the VPP. The VPP was quite sensitive to high-pass filtering, while the P300 sustained stronger filtering, although for its generation, the faces must also be discriminated from scrambled faces.
为了客观评估“超出V1区域”的视敏度，P300事件相关电位是一个有前景的候选指标，且与意识知觉密切相关。然而，P300可被随意调节，这对客观视敏度估计而言是一个不利因素。面孔是非常突出的刺激物且难以忽视。在此，我们提出一种使用面孔评估视敏度的P300型范式。
各自受试者的灰度面部画像用作奇异性刺激（概率为1/7），这些画像的置乱版本用作标准刺激（概率为6/7）。此外，对刺激进行空间高通滤波（分别为0、2.2、4.2和8.3周/度），使其只有在足够的视敏度下才可识别。通过屈光度模糊系统地降低视敏度，个体选择的屈光度模糊使得在≥4.2周/度进行高通滤波时面孔无法识别。从11名受试者的32个头皮位置记录脑电图，并重新参考TP9和TP10的平均值。将其中一个罕见的面部变体指定为目标，必须按下按钮对其做出反应。
事件相关电位在300 - 800毫秒时以P300为主。所有受试者对0至8.3周/度滤波的刺激均有显著（P < 0.05）的P300。当视力模糊时，对8.3周/度滤波面孔的显著P300反应比例降至18%，但对4.2周/度滤波的面孔仍保持在100%。另一个成分，即170毫秒时的头顶正电位（VPP），在大多数视力模糊且处于所有滤波水平的参与者中均无法检测到，即使图像是可识别的。
该研究证明了基于面孔的P300方法客观评估视敏度的可行性。对刺激退化的敏感性与先前报道的基于光栅的方法相当。一个意外发现是P300和VPP的不同表现。VPP对高通滤波相当敏感，而P300能承受更强的滤波，尽管对于其产生而言，面孔也必须与置乱面孔区分开来。
