Learning to identify contrast-defined letters in peripheral vision

Abstract

Performance for identifying luminance-defined letters in peripheral vision improves with training. The purpose of the present study was to examine whether performance for identifying contrast-defined letters also improves with training in peripheral vision, and whether any improvement transfers to luminance-defined letters. Eight observers were trained to identify contrast-defined letters presented singly at 10 degrees eccentricity in the inferior visual field. Before and after training, we measured observers' thresholds for identifying luminance-defined and contrast-defined letters, embedded within a field of white luminance noise (maximum luminance contrast=0, 0.25, and 0.5), at the same eccentric location. Each training session consisted of 10 blocks (100 trials per block) of identifying contrast-defined letters at a background noise contrast of 0.5. Letters (x-height=4.2 degrees) were the 26 lowercase letters of the Times-Roman alphabet. Luminance-defined letters were generated by introducing a luminance difference between the stimulus letter and its mid-gray background. The background noise covered both the letter and its background. Contrast-defined letters were generated by introducing a differential noise contrast between the group of pixels that made up the stimulus letter and the group of pixels that made up the background. Following training, observers showed a significant reduction in threshold for identifying contrast-defined letters (p<0.0001). Averaged across observers and background noise contrasts, the reduction was 25.8%, with the greatest reduction (32%) occurring at the trained background noise contrast. There was virtually no transfer of improvement to luminance-defined letters, or to an untrained letter size (2 x original), or an untrained retinal location (10 degrees superior field). In contrast, learning transferred completely to the untrained contralateral eye. Our results show that training improves performance for identifying contrast-defined letters in peripheral vision. This perceptual learning effect seems to be stimulus-specific, as it shows no transfer to the identification of luminance-defined letters. The complete interocular transfer, and the retinotopic (retinal location) and size specificity of the learning effect are consistent with the properties of neurons in early visual area V2.

Fig. 1

A schematic cartoon illustrating the basic experimental design of the study.

Fig. 2

Samples of luminance-defined (top) and contrast-defined (bottom) letters, embedded at the three background noise contrast levels.

Fig. 3

Contrast threshold for identifying luminance-defined letters (squares) or differential-contrast threshold (ΔC) for identifying contrast-defined letters (circles) are plotted as a function of background noise contrast, and compared before (unfilled symbols) and after (filled symbols) training. Each panel presents data for one observer. Error bars represent ±1 SEM.

Fig. 4

Threshold ratios (post-test/pre-test) are compared for the three background noise contrast levels, and between luminance-defined (left) and contrast-defined (right) letters. Ratios smaller than 1 represent improvements following training. Individual observers’ values are plotted as different gray symbols. Group-average values are represented by black filled circles, with the error bars representing the 95% confidence intervals. The black arrow indicates the training condition (contrast-defined letters at a background noise contrast = 0.5).

Fig. 5

Differential contrast (ΔC) for identifying contrast-defined letters is plotted as a function of training block, for each of the eight observers (gray symbols). The averaged threshold for each session (10 blocks) is represented by the black line. In the few cases where thresholds were not measurable, a value of 1.0 is used to represent the ΔC for that block of trials. Black filled symbols represent the thresholds obtained at pre- and post-tests (they are missing for pre-tests for AC and MK because their thresholds were too high to be measurable).

Fig. 6

Threshold ratios (post-test/pre-test) are compared for the three conditions: untrained letter size (2× original), untrained retinal location (10° in the superior visual field) and the untrained eye. Data for the trained condition are also plotted for comparison. Individual observers’ values are plotted as different gray symbols. Group-average values are represented by black filled circles, with the error bars representing the 95% confidence intervals.