Top-down modulation of hemispheric differences and spatial attention effects for verbal and non-verbal stimuli

The human brain exhibits lateralization, with language preferentially processed in the left hemisphere, and facial recognition and spatial attention stronger in the right. The balance of hemispheric engagement is also influenced by directed spatial attention, but interactions between these factors are poorly understood. Our studies investigated the role of directed spatial attention and stimulus meaningfulness at modulating hemispheric biases in the recognition of words and faces. Four online studies employed a divided visual field paradigm and a modified "Posner task" to direct spatial attention in two tasks: lexical decision and face detection. Our findings revealed expected hemispheric dominance and performance enhancement with valid spatial cueing. Attentional cueing effects were more salient for meaningful stimuli (words, upright faces) but strongly attenuated for pseudowords and inverted faces. Similarly, hemispheric lateralization effects varied with stimulus type: the left-hemisphere advantage was stronger for valid word stimuli, both with (Expt.1) and without spatial cueing (Expt.3), while right-hemisphere advantage for faces trended larger for upright faces without spatial cueing (Expt.4). These findings indicate that both hemispheric lateralization and spatial cueing involve a top-down process, enhancing stimulus recognition through specialization and directed attention, providing insights into the mechanisms supporting lateralized perceptual performance in healthy adults.<b>Public Significance Statement</b>We show that the brain's ability to recognize words and faces is driven by a top-down process that involves directing our attention to enhance recognition, such that attention benefits recognition of real words and faces, but not rejection of made-up words and inverted faces. Likewise, the brain's hemispheric advantages (left hemisphere specialized for words, right hemisphere for faces) are larger for recognition of real stimuli vs. rejection of false ones. These findings clarify that hemispheric advantages, whether innate (e.g., left hemisphere language superiority) or "on-the-fly" (directing attention to one side of space) apply mainly to real recognizable stimuli, and therefore involve a top-down processing mechanism.