Can letter position encoding be modified by visual perceptual elements?

A plethora of studies has revealed that letter position coding is relatively flexible during word recognition (e.g., the transposed-letter [TL] pseudoword CHOLOCATE is frequently misread as CHOCOLATE). A plausible explanation of this phenomenon is that letter identity and location are not perfectly bound as a consequence of the limitations of the visual system. Thus, a complete characterization of letter position coding requires an examination of how letter position coding can be modulated by visual perceptual elements. Here we conducted three lexical decision experiments with TL and replacement-letter pseudowords that manipulated the visual characteristics of the stimuli. In Experiment 1, each syllable was presented either in a different colour or monochromatically (e.g., [Formula: see text] vs. [Formula: see text]) with the transposition occurring across syllables. In Experiment 2, the critical letters had a consistent contrast or not (e.g., [Formula: see text] vs. [Formula: see text]). In Experiment 3, the stimuli were presented either simultaneously or serially, letter by letter (i.e., as occurs in braille reading). Results showed that whereas colouring differently each syllable only produced a small nonsignificant reduction of the TL effect, the other two manipulations—presenting the two critical letters with an altered contrast and presenting the letters one at a time—reduced, but did not eliminate, the magnitude of the TL effect relative to the regular format. Although these findings are consistent with models that postulate an early perceptual locus of the TL effect, the robustness of the TL effect suggests that letter position coding also has an orthographic abstract component.

Reference Repulsion Is Not a Perceptual Illusion

Perceptual decisions are often influenced by contextual factors. For instance, when engaged in a visual discrimination task against a reference boundary, subjective reports about the judged stimulus feature are biased away from the boundary – a phenomenon termed reference repulsion. Until recently, this phenomenon has been thought to reflect a perceptual illusion regarding the appearance of the stimulus, but new evidence suggests that it may rather reflect a post-perceptual decision bias. To shed light on this issue, we examined whether and how orientation judgments affect perceptual appearance. In a first experiment, we confirmed that after judging a grating stimulus against a discrimination boundary, the subsequent reproduction response was indeed repelled from the boundary. To investigate the perceptual nature of this bias, in a second experiment we measured the perceived orientation of the grating stimulus more directly, in comparison to a reference stimulus visible at the same time. Although we did observe a small repulsive bias away from the boundary, this bias was explained by random trial-by-trial fluctuations in sensory representations together with classical stimulus adaptation effects and did not reflect a systematic bias due to the discrimination judgment. Overall, the current study indicates that discrimination judgments do not elicit a perceptual illusion and points towards a post-perceptual locus of reference repulsion.

Forest before illusory trees: illusory contours of local level elements do not influence perceptual global advantage in the hierarchical structure processing

There is a continuing debate in the field of perceptual organization as to whether the locus of global processing is early or late perceptual, as previous studies have yielded contrary results. The conducted behavioural study explored this issue with the paradigm of collating global processing with other process of perceptual organization, namely illusory contours processing. Interaction between these two processes of perceptual organization would indicate that global processing has an early perceptual locus, whereas the lack of such interaction would suggest the late perceptual locus of global processing. In experiment 1, the effect of global dominance was obtained with the use of a compound figure composed of geometrical shapes with real edges. Results showed that the processing of the figure at the global level was faster and that it disrupted the processing of the figures from the local level. In experiment 1b, the compound figure was composed of local geometrical shapes generated with the use of the line-end induced illusory contours (Ehrenstein illusion). Local elements with illusory contours did not affect the processing of the hierarchical structure and the effect of global precedence occurred without any changes. In experiment 2a, a global advantaged effect within a compound figure with local elements with real edges was obtained in the paradigm of attention divided between levels of the hierarchical structure. When illusory contours of local elements of a compound figure were introduced to this paradigm (experiment 2b), this again had no effect on the perceptual global advantage. The results demonstrate the lack of interaction between global processing and illusory contour processing, indicating that the locus of global processing is rather late perceptual.

Letter Identification in Words and Non-Words, with Variation in Visual Angle and Delay of Position Cue

Two experiments are described in which subjects were required to report the name of a single position-cued ‘critical’ letter in a tachistoscopically displayed string of four letters. The stimulus characters were arranged to form three types of letter strings: (i) strings in which the letters did not form words; (ii) words in which contextual constraint of the critical letters was minimised; and (iii) words in which contextual constraint of the critical letters was maximised. The serial position of the letter to be identified in each string was cued at delays of −500, −100, and +500 ms, in experiment 1 and at delays of −510 and +510 ms in experiment 2, and in both experiments one group of subjects responded to letter strings which subtended a horizontal visual angle of 3·95 deg, while a second group responded to strings which subtended 1·02 deg. Correct identifications of critical letters showed that the presentation of words resulted in superior performance. This ‘word superiority effect’ is consistent with earlier findings implying that it has a perceptual locus. For the stimuli which subtended the large visual angle the word advantage was detrimentally affected only when the position of the critical letter to be identified was cued either 500 or 510 ms prior to the display of the letter string.