On the relationship between spatial suppression, speed of information processing, and psychometric intelligence

Spatial suppression refers to the increasingly difficult identification of motion direction with increasing size of the moving stimulus. Previous research indicated a close association between stronger spatial suppression and higher psychometric intelligence. Since the measurement of spatial suppression is based on the time needed to correctly identify the motion direction of small vs. large stimuli, the present study aimed to elucidate the unique and shared effects of mental speed, as assessed by reaction times from the Hick task, and spatial suppression on psychometric intelligence. In 177 young adults, neither manifest nor latent variables representing spatial suppression were related to psychometric intelligence. Irrespective of stimulus size, however, individuals with higher intelligence detected motion direction faster than individuals with lower intelligence. While we cannot fully rule out stimulus and apparatus differences as being behind this discrepancy with prior work, our results indicate that the link between spatial suppression and intelligence is at best confined to a specific range of stimulus parameters. The relation between intelligence and speed of motion direction detection (independent from stimulus size) overlapped with the relation between psychometric intelligence and reaction times in the Hick task. This latter result is consistent with the assumption that there is a general (task-independent) speed of information processing, which is robustly related to psychometric intelligence.