Left Prefrontal Cortex Supports the Recognition of Meaningful Patterns in Ambiguous Stimuli

Abstract

Processing of ambiguous visual stimuli has been associated with an increased activation of the left lateral prefrontal cortex (PFC) in neuroimaging studies. Nevertheless, the functional role of prefrontal activity in this process is not fully understood. In this experiment we asked participants to evaluate ambiguous inkblots from the Rorschach test, while stimulating the left lateral PFC using excitatory anodal transcranial direct current stimulation (tDCS). In addition, visual insight ability was assessed as a control measure requiring visual and conceptual restructuring and convergent thinking rather than divergent idea generation employed to interpret the equivocal Rorschach inkblots. Using a randomized double-blind design, we demonstrated that anodal tDCS increased the number of meaningful patterns recognized in the inkblots but had no significant effect on visual insight. These findings support the role of left lateral PFC in the processing of ambiguous visual information and object recognition. More generally, we discuss that the PFC may be involved in the mechanisms supporting the activation of stored visual and semantic representations in order to compensate for less informative bottom-up inputs and thus facilitate flexible cognition and idea generation.

Keywords: Rorschach test; convergent thinking; divergent thinking; object recognition; prefrontal cortex; semantic memory; tDCS; visual perception.

FIGURE 1

Experimental procedure. The arrow indicates the period during which (sham or active) tDCS was applied.

FIGURE 2

Electrode placement and simulated electric field for the tDCS montage. Anode was centered in between F3-F5 and cathode over Fp2 of the 10-10 international system of EEG electrode placement (A). The estimated electric field intensity induced in the brain is shown in dorsal (B), left lateral (C), and frontal (D) view, based on a biologically plausible computational forward model of the current flow for this montage (for details see section “Materials and Methods”).

FIGURE 3

Performance in the cognitive measures by tDCS group for (A) ROR response fluency: the average number of responses delivered in the post-test when controlling for the pre-test, (B) insight score: the average number of correctly solved insight problems (max = 4 problems), and (C) insight RT: the average time required to solve the problems (max = 180 s). Error bars depict 95% CIs. ** p < 0.01.