Spatiotemporal letter processing in visual word recognition uncovered by perceptual oscillations

Despite decades of intense study, the spatiotemporal processing of letters in visual word recognition has yet to be elucidated, with the debate largely focusing on whether individual letters are processed serially or in parallel. The present study investigated the processing of individual letters and letter combinations through time in visual word recognition using displays where signal-to-noise ratio (SNR) varied randomly throughout a 200 ms exposure duration. In Experiment 1, SNR varied either homogeneously across all letters or independently for each letter position (cf. heterogeneous sampling). Reading accuracy was substantially greater with homogeneous than heterogeneous sampling. Experiment 2 again used heterogeneous sampling and classification images (CIs) were calculated for individual letter positions or conjunctions thereof, reflecting processing efficiency according to time during target exposure. These CIs or their Fourier transforms were passed to a classifier to assess differences in the result patterns across individual letter positions or their conjunctions. Overall, the present results indicate the following: (1) significant parallel letter processing capacity throughout exposure duration; (2) dissociable processing mechanisms for each letter position; and (3) letter position-specific mechanisms for letter conjunctions that are distinct from those for individual letters. The results also provide evidence relevant to the neural code underlying the perceptual mechanisms that were uncovered.