Abstract
Shifting and focusing attention is a key mechanism, in the extraction of sensory information, for adaptive behavior. Reading is a perfect example: correctly identifying letters and letter positions within a word is fundamental to word recognition that demands precise attentional selection. We measured the effects of spatial cues in a multi-letter processing task, in order to characterize the differential effects of exogenous and endogenous covert spatial attention on various aspects of encoding letter combinations. A string of 6 letters was presented for 120ms and participants (n=22) were asked to report the letter that was at one post-cued position. Attention was manipulated with pre-cues: exogenous (50ms, peripheral, uninformative) and endogenous (150ms, central, informative). On valid trials, the pre- and post-cue sides match; on invalid trials they mismatch, and on neutral trials all stimuli are pre-cued. Letter recognition accuracy showed a significant cue benefit (valid>neutral) for both exogenous and endogenous cues. The cue benefit was significantly larger with endogenous than exogenous cues. Notably, the benefits were greatest at the shortest cue to target onset interval (50ms) for exogenous and longest (600ms) for endogenous cues. To further understand the differential effects of exogenous and endogenous attention, we categorized errors as position swaps (reporting a letter from the wrong position) and misidentification errors (reporting a letter that wasn’t in the display). On neutral trials, observers were 1.6x more likely to make position swaps than misidentification errors. Interestingly, valid endogenous cues reduced position swap errors more than valid exogenous cues, but we found no significant difference in misidentification errors with both cue types. Our results demonstrate that exogenous and endogenous attention have different effects on the encoding of letter combinations. Endogenous, more than exogenous, attention reduces errors by improving position coding and therefore might be a key mechanism for the development of reading ability.