Visual acuity can be measured in many different ways, including with letters and Vernier offsets. Prior psychophysical work has suggested that the two acuities are strongly linked given that they both depend strongly on retinal eccentricity and both are similarly affected in amblyopia. Here we used high-density EEG recordings to ask whether the underlying neural sources are common as suggested by the psychophysics or distinct. To measure visual acuity for letters, we recorded evoked potentials to 3 Hz alternations between intact and scrambled text comprised of letters of varying size. To measure visual acuity for Vernier offsets, we recorded evoked potentials to 3 Hz alternations between bar gratings with and without a set of Vernier offsets. Both alternation types elicited robust activity at the 3 Hz stimulus frequency that scaled in amplitude with both letter and offset size, starting near threshold. Letter and Vernier offset responses differed in both their scalp topography and temporal dynamics. The earliest evoked responses to letters occurred on lateral occipital visual areas, predominantly over the left hemisphere. Later responses were measured at electrodes over early visual cortex, suggesting that letter structure is first extracted in second-tier extra-striate areas and that responses over early visual areas are due to feedback. Responses to Vernier offsets, by contrast, occurred first at medial occipital electrodes, with responses at later time-points being more broadly distributed—consistent with feedforward pathway mediation. The previously observed commonalities between letter and Vernier acuity may be due to common bottlenecks in early visual cortex but not because the two tasks are subserved by a common network of visual areas.

视力可以通过许多不同的方式测量，包括字母和游标偏移。先前的心理物理学工作表明，这两种视力密切相关，因为它们都强烈依赖于视网膜离心率，并且两者都在弱视中受到类似的影响。在这里，我们使用高密度 EEG 记录来询问潜在的神经源是否如心理物理学所建议的那样常见或不同。为了测量字母的视力，我们记录了由不同大小的字母组成的完整文本和加扰文本之间 3 Hz 交替的诱发电位。为了测量游标偏移的视力，我们记录了带有和不带有一组游标偏移的条形光栅之间 3 Hz 交替的诱发电位。两种交替类型都在 3 Hz 刺激频率下引发了强烈的活动，其幅度随字母和偏移大​​小而变化，从接近阈值开始。Letter 和 Vernier 偏移响应在头皮地形和时间动态方面都不同。对字母的最早诱发反应发生在侧枕部视觉区域，主要是在左半球。后来在早期视觉皮层上的电极上测量了后来的反应，这表明字母结构首先是在第二层的纹外区域提取的，而早期视觉区域的反应是由于反馈。相比之下，对游标偏移的反应首先发生在内侧枕骨电极，稍后时间点的反应分布更广泛——与前馈通路调解一致。