Theta-burst transcranial magnetic stimulation to the prefrontal or parietal cortex does not impair metacognitive visual awareness

Abstract

Neuroimaging studies commonly associate dorsolateral prefrontal cortex (DLPFC) and posterior parietal cortex with conscious perception. However, such studies only investigate correlation, rather than causation. In addition, many studies conflate objective performance with subjective awareness. In an influential recent paper, Rounis and colleagues addressed these issues by showing that continuous theta burst transcranial magnetic stimulation (cTBS) applied to the DLPFC impaired metacognitive (subjective) awareness for a perceptual task, while objective performance was kept constant. We attempted to replicate this finding, with minor modifications, including an active cTBS control site. Using a between-subjects design for both DLPFC and posterior parietal cortices, we found no evidence of a cTBS-induced metacognitive impairment. In a second experiment, we devised a highly rigorous within-subjects cTBS design for DLPFC, but again failed to find any evidence of metacognitive impairment. One crucial difference between our results and the Rounis study is our strict exclusion of data deemed unsuitable for a signal detection theory analysis. Indeed, when we included this unstable data, a significant, though invalid, metacognitive impairment was found. These results cast doubt on previous findings relating metacognitive awareness to DLPFC, and inform the current debate concerning whether or not prefrontal regions are preferentially implicated in conscious perception.

Fig 1. Experimental design.

Experimental design was identical to Rounis and colleagues [19], apart from exceptions described in methods. Most notably, confidence in choice was used instead of visibility to determine metacognitive judgement. Participants were presented with either a diamond on the left and square on the right or vice versa, followed by a metacontrast mask. They were then required to make a combined judgement as to the stimulus configuration and their level of confidence in that decision. Adapted from Rounis [19] with permission.

Fig 2. Task performance.

Pre and post-TMS performance measures for the different groups. a) Proportion correct. B) Mean contrast C) Mean confidence D) Reaction Time for correct responses. DLPFC = bilateral DLPFC group, PPC = bilateral posterior parietal cortex group, LEFT = left posterior parietal cortex and DLPFC group, RIGHT = right posterior parietal cortex and DLPFC group. All error bars are SE.

Fig 3. Metacognitive measures.

Pre- and post-TMS metacognitive measures for the different groups. a) meta d’—d’. b) type II d’. c) Accuracy-confidence phi correlation. Group labels as Fig 2. All error bars are SE.

Fig 4. Histogram of distribution of meta d’—d’ values.

Histograms, using 0.4 sized bins, of meta d’—d’ for a) stable data only, per subject experimental block; and b) all data (including unstable). Whereas the stable data is Gaussian, the unstable data is not.

Fig 5. Relationship between IQ and average contrast.

The relationship between Cattell Culture Fair IQ score and average contrast. Each blue diamond represents a single participant’s average score for both experimental blocks. The black line is a linear best fit of the data. There was a significant negative relationship between IQ and contrast, such that higher IQ participants tended to achieve a more difficult contrast level.