CARACTERÍSTICAS TÉCNICAS DEL FONDO HABITACIONAL Y DE LA ESTRUCTURA

Functional neuroimaging studies have found correlations between activity in human orbitofrontal cortex (OFC; especially its medial aspect) and subjective ratings of reward value for a diverse range of stimuli including attractive faces. However, it is unclear from these studies whether orbitofrontal cortex activity has a causal inuence on subjective evaluations of reward value, or is merely an epiphenomenon. To address this question, we used a real-time fMRI procedure involving instrumental condition- ing with monetary reward in 13 male subjects who were trained to selectively increase activity in medial OFC, interleaved with binary attractiveness judgments on a set of female faces. Over several sessions subjects showed increased dierential activity in OFC relative to a control condition during which they were instead conditioned to activate the hand area of motor cortex. After controlling for other factors such as whether a trial was rewarded or not, the dierential OFC signal was found to be a sig- nicant predictor of increased attractiveness ratings, while dierential activity in the hand-motor area was not. These results demonstrate that a neurofeedback procedure can be used to condition increased activity in OFC and that by selectively modulat- ing activity levels in medial OFC it is possible to inuence subjective judgments of attractiveness.

Introduction

Activity in medial OFC (mOFC) correlates with subjective ratings of reward value for a diverse range of stimuli, including liquid rewards [28], willingness to pay, [40] and attractive faces [30, 31]. Recent evidence also suggests that mOFC represents the experienced pleasantness of a stimulus, which can be inuenced by factors other than the sensory properties of a stimulus, and the internal state of the subject [41]. While these studies have demonstrated correlations between subjective value and mOFC activity, they have not established whether mOFC has a causal inuence on subjective judgments. Patients with OFC lesions are able to express a range of emotions [185], however they show impairment at relative valuation of sets of options [186]. OFC- lesioned patients are able to express preferences, but their preferences are internally inconsistent, pointing to a causal role for OFC in accessing stimulus value. However, it remains unclear whether elevated mOFC activity causes positive evaluations, or is merely epi-phenomenal. In this study we sought to test whether elevated mOFC activity can positively bias aective judgment of a concurrently presented stimulus.

To elicit reliable increases in mOFC activity on a trial-by-trial basis, we imple- mented a neural conditioning procedure [96, 100], in which subjects were rewarded for elevating mOFC activity upon being presented with a discriminative cue. Similar procedures have been employed in emotional brain regions such as rostral anterior cingulate cortex [100], and insula [108], but as of yet neural conditioning of mOFC has not been reported.

It is unclear what mental strategy is the most eective for elevating activity in the OFC. Based on prior reports of emotional recall activating OFC [187, 188], and the known responses of this region to both primary and abstract reinforcers [33, 189, 109], we instructed subjects that imagining things that they nd personally rewarding would be a good initial strategy.

To test for eects of mOFC activity on aective judgments, subjects were asked to evaluate the attractiveness of a face at the end of each trial. Prior reports have shown that mOFC activity correlates with ratings of facial attractiveness [30, 31].

To control for general eects of elevating regional neural activity for reward, we included a second condition in which subjects were trained to elevate activity in a region of motor cortex related to hand movements, and received rewards contingent on successfully activating that region past a given threshold [160, 96].

Because the mOFC is important for subjective evaluations and decision making, the ability to train reliable increases in mOFC activity has important clinical im- plications for treating patients who show impaired decision-making abilities, such as depressed or addicted individuals. The results of this study also have important implications for understanding the precise inuence of mOFC activity on expressed preferences.

Materials and Methods

Subjects

A total of 18 healthy right-handed male subjects aged 19 to 29 years (mean age 21±

2.3 years) participated in the experiment. All subjects gave informed consent, which was approved by the local research ethics committee. One additional subject did not complete the entire session.

Scan-to-scan motion can have a detrimental impact on learning, as subjects cannot tell whether changes in BOLD response are due to head motion or neural activity. We therefore eliminated subjects who showed large amounts of head motion. We summed the total scan-to-scan motion, estimated during SPM pre-processing (as described below) over all three directions in each session, and eliminated subjects who showed > 30 mm of movement in more than one session. According to this criterion, ve subjects were removed from the imaging analysis. This study included a behavioral response on each trial, as described below; one subject was excluded from the behavioral analysis for making the same response on every trial. This left 13 subjects in the imaging analysis and 12 in the behavioral analysis.