The role of spatial frequency channels in letter identification
The role of spatial frequency channels in letter identification
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DOI：
10.1016/s0042-6989(02)00045-7
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发表时间：
2002-04-01
期刊：
影响因子：
1.8
通讯作者：
Palomares, M
中科院分区：
文献类型：
--
作者：
Majaj, NJ;Pelli, DG;Palomares, M
今天，我们如何看到是由物理光学和视网膜转导，其次是功能检测，在皮层中，通过一个平行的独立空间频率选择通道银行解释。假设观察者使用最适合手头任务的通道。我们目前的研究结果需要对这个框架进行修改：观察者不能自由选择他们使用的通道。我们使用临界带掩蔽来表征介导宽带信号识别的通道：各种字体的字母（Sloan，Bookman，Kunstler，Yung），字母（罗马和中文）和大小（0.1- 55度）。我们还测试了正弦波和方波光栅。掩蔽总是显示一个单一的通道，1.6 +/- 0.7倍频程宽，中心频率取决于字母大小和字母表。我们将字母表的笔画频率定义为一个字母的平均行数除以字母宽度。对于尖锐边缘（即宽带）信号，我们发现，笔划频率完全决定信道频率，独立于字母，字体和大小。此外，即使观察者有多个通道，他们总是使用相同的通道，即使在数百次试验之后，无论噪声是低通，高通还是全通。这表明观察者通过一个单一的通道识别字母，该通道由信号自下而上选择，而不是由观察者自上而下选择。我们认为形状在所有尺寸下都将被类似地处理。带限信号比宽带信号更符合这一期望。在这里，我们通过信道频率来表征处理。对于正弦波光栅，如所预期的，信道频率等于正弦波频率f（信道）= f。对于带通滤波的字母，当尺寸变化且频带（c/letter）固定时，通道频率与中心频率f（channel）成比例，与f（center）成比例（对数-对数斜率1），但当频带变化且尺寸固定时，通道频率小于与中心频率f（channel）成比例，与f（center）成比例（2/3）（对数-对数斜率2/3）。最后，我们的主要结果是，对于边缘尖锐（即宽带）的字母和方波，通道频率仅取决于笔画频率，f（通道）/c/deg =（f（笔画）/10 c/deg）（2/3），对数-对数斜率为2/3。因此，大字母（和粗方波）由它们的边缘识别，小字母（和细方波）由它们的粗笔画识别。(C)2002年由Elsevier Science Ltd.出版
How we see is today explained by physical optics and retinal transduction, followed by feature detection, in the cortex, by a bank of parallel independent spatial-frequency-selective channels. It is assumed that the observer uses whichever channels are best for the task at hand. Our current results demand a revision of this framework: Observers are not free to choose which channels they use. We used critical-band masking to characterize the channels mediating identification of broadband signals: letters in a wide range of fonts (Sloan, Bookman, Kunstler, Yung), alphabets (Roman and Chinese), and sizes (0.1-55degrees). We also tested sinewave and squarewave gratings. Masking always revealed a single channel, 1.6 +/- 0.7 octaves wide, with a center frequency that depends on letter size and alphabet. We define an alphabet's stroke frequency as the average number of lines crossed by a slice through a letter, divided by the letter width. For sharp-edged (i.e. broadband) signals, we find that stroke frequency completely determines channel frequency, independent of alphabet, font, and size. Moreover, even though observers have multiple channels, they always use the same channel for the same signals, even after hundreds of trials, regardless of whether the noise is low-pass, high-pass, or all-pass. This shows that observers identify letters through a single channel that is selected bottom-up, by the signal, not top-down by the observer. We thought shape would be processed similarly at all sizes. Bandlimited signals conform more to this expectation than do broadband signals. Here, we characterize processing by channel frequency. For sinewave gratings, as expected, channel frequency equals sinewave frequency f(channel) = f. For bandpass-filtered letters, channel frequency is proportional to center frequency f(channel) proportional to f(center) (log-log slope 1) when size is varied and the band (c/letter) is fixed, but channel frequency is less than proportional to center frequency f(channel) proportional to f(center)(2/3) (log-log slope 2/3) when the band is varied and size is fixed. Finally, our main result, for sharp-edged (i.e. broadband) letters and squarewaves, channel frequency depends solely on stroke frequency, f(channel)/c/deg = (f(stroke)/10 c/deg)(2/3) , with a log-log slope of 2/3. Thus, large letters (and coarse squarewaves) are identified by their edges small letters (and fine squarewaves) are identified by their gross strokes. (C) 2002 Published by Elsevier Science Ltd.