Abstract
The progression of letter processing through space and time during visual word recognition remains highly controversial -- cf. serial vs. parallel; processing order. The issue was investigated using the method of random temporal sampling (Arguin et al., Sci.Reports 2021, https://rdcu.be/cAp6h). Five-letter words to be read aloud were exposed for 200 ms. On each trial, a distinct random manipulation of signal-to-noise ratio through time was applied independently for each letter position. The Fourier descriptions of the classification images of processing effectiveness according to the time-frequency features of the temporal sampling functions were calculated for each participant (n = 16) and submitted to a classifier (support-vector machine [SVM], leave-one-out [LOO] cross validation). Using only 5% of the features available, the classifier was 100% correct in determining letter position. This indicates highly distinct temporal features of letter processing according to position within the word. Specifically, each letter position was characterized by unique combinations of energy peaks and/or troughs at one or two frequencies in the pattern of processing effectiveness changes through time. Similar analyses were applied to joint visibility functions (i.e. products of temporal sampling functions) to assess the processing of letter conjunctions. Extremely strong signs of parallel processing for all possible letter conjunctions were found, regardless of the number of letters or inter-letter distances involved. An SVM-LOO having to decide (yes/no) whether a particular conjunction includes a specific letter position was 95.5% correct with only 14% of the available features. Again, each letter position within conjunctions was uniquely characterized by its pattern of one or two temporal features, which were very distinct from those characterizing the processing of individual letter positions. These findings thus suggest distinct mechanisms for the recognition of each letter position in the word as well as for the integration of letters across positions, which all operate in parallel.