Two speed factors of visual recognition independently correlated with fluid intelligence

Abstract

Growing evidence indicates a moderate but significant relationship between processing speed in visuo-cognitive tasks and general intelligence. On the other hand, findings from neuroscience proposed that the primate visual system consists of two major pathways, the ventral pathway for objects recognition and the dorsal pathway for spatial processing and attentive analysis. Previous studies seeking for visuo-cognitive factors of human intelligence indicated a significant correlation between fluid intelligence and the inspection time (IT), an index for a speed of object recognition performed in the ventral pathway. We thus presently examined a possibility that neural processing speed in the dorsal pathway also represented a factor of intelligence. Specifically, we used the mental rotation (MR) task, a popular psychometric measure for mental speed of spatial processing in the dorsal pathway. We found that the speed of MR was significantly correlated with intelligence scores, while it had no correlation with one's IT (recognition speed of visual objects). Our results support the new possibility that intelligence could be explained by two types of mental speed, one related to object recognition (IT) and another for manipulation of mental images (MR).

Figure 1. Example stimuli and data analyses in mental rotation (MR) and inspection time (IT) tasks.

In the MR task (panel A), subjects were instructed to answer whether two images of three-dimensional (3D) objects (each consisted of eleven cubes attached face-to-face, see Methods for details) were the same or different in their 3D structures. We plotted reaction times as a function of angular differences between the two shapes and estimated a slope and intercept with a linear fitting. The smaller slope of the linear function represents a higher speed of mental rotation. In the IT task (panel B), target and mask stimuli were successively presented near a fixation point. Subjects judged whether the left or right vertical line of the target was longer. In the figure above, a correct answer is left. A stimulus onset asynchrony (SOA) between the target and mask started from 80 ms, being changed in a step of 10 ms based on an accuracy of the last two trials. We plotted the accuracy as a function of SOA, estimating a 75% threshold with a linear fitting as an index of the IT. The smaller IT represents a faster speed of object recognition.

Figure 2. The relationship among three psychological measures (APM scores, MR slope, and IT) depicted over a template brain image in Statistical Parametric Mapping (SPM, available online athttp://www.fil.ion.ucl.ac.uk/spm/software/spm8/).

Previous studies have indicated a close relationship between intelligence and a function of the frontal cortex. A speed of object recognition (IT) reflects a function of the ventral pathway (from the occipital to temporal regions), while the rotation of mental images is mainly performed in the dorsal pathway (from the occipital to parietal regions). Our study found that the MR slope (not intercept) and the IT were individually correlated with the APM scores, suggesting that the speed of mental rotation and object recognition reflect different factors of fluid intelligence.