Wednesday, December 12, 2007

Using Neuroimaging to Resolve the Psi Debate

Moulton and Kosslyn offer a study in J. Cog. Neurosci attempting to find evidence for the psi effect in brain imaging experiments. Here is their abstract, followed by some edited clips and a figure from the paper:
Parapsychology is the scientific investigation of apparently paranormal mental phenomena (such as telepathy, i.e., "mind reading"), also known as psi. Despite widespread public belief in such phenomena and over 75 years of experimentation, there is no compelling evidence that psi exists. In the present study, functional magnetic resonance imaging (fMRI) was used in an effort to document the existence of psi. If psi exists, it occurs in the brain, and hence, assessing the brain directly should be more sensitive than using indirect behavioral methods (as have been used previously). To increase sensitivity, this experiment was designed to produce positive results if telepathy, clairvoyance (i.e., direct sensing of remote events), or precognition (i.e., knowing future events) exist. Moreover, the study included biologically or emotionally related participants (e.g., twins) and emotional stimuli in an effort to maximize experimental conditions that are purportedly conducive to psi. In spite of these characteristics of the study, psi stimuli and non-psi stimuli evoked indistinguishable neuronal responses—although differences in stimulus arousal values of the same stimuli had the expected effects on patterns of brain activation. These findings are the strongest evidence yet obtained against the existence of paranormal mental phenomena.

In our experiment, participants played one of two roles: "sender" and "receiver." On each trial, sender participants viewed a randomly selected target stimulus from outside the scanner (see Figure), and tried to send this information to the receiver participant by mental means alone. While the senders were doing this, receiver participants completed a simple binary guessing task, and functional magnetic resonance imaging (fMRI) was used to monitor their brain activity. On each trial of the guessing task, the receivers sequentially viewed two stimuli, guessed which one was the stimulus being "sent" (i.e., the psi stimulus), and then saw the psi stimulus a second time. This paradigm allowed us simultaneously to test all three hypothesized mechanisms of psi: telepathy (i.e., "mind reading"), clairvoyance (i.e., direct sensing of remote events), and precognition (i.e., knowing future events). The sender served as the potential telepathic source, the sender's computer monitor served as the potential clairvoyance source, and the second presentation of the psi stimulus served as the potential precognition source.

Figure 1 A schematic of one trial. In this trial for the receiver, the non-psi stimulus appears first and the psi stimulus second. The third stimulus presentation (feedback) in each trial is always the same as the psi stimulus. The sender sees only the psi stimulus for each trial.

The results support the null hypothesis that psi does not exist. The brains of our participants—as a group and individually—reacted to psi and non-psi stimuli in a statistically indistinguishable manner. Given the relatively large number of participants, the use of fixed-effects statistics, the extensive activation elicited separately by both types of stimuli, the subtle psychological effects revealed in the much smaller data set from a single participant, and the non-psi effects we documented on a group level using identical statistical criteria, a lack of statistical power does not reasonably explain our results. Even if the psi effect were very transient, as are many mental events, it should have left a footprint that could be detected by fMRI—as did the other subtle effects we detected. In particular, the large and massively significant activation revealed by our arousal contrast shows that that the psi effect, if it exists, must be substantially smaller than the effect of arousal on brain activity.

But what of the truism that one cannot affirm the null hypothesis? We note that some null results should be taken more seriously than others. ....Consider the possibility of water on Mars. If a set of close-up images of its surface failed to capture frozen lakes, few would accept the nonexistence of Martian water. Yet if a planetwide analysis of its subsurface soil content failed to show telltale signs of water, most would accept the null hypothesis of a Martian desert. Past null results from parapsychology are comparable to scattered snapshots of the surface in that they measure a small sample of outwardly observable variables. The current neuroimaging approach, however, seeks anomalous knowledge at its source, inside the brain, using methods validated by cognitive neuroscience. It is also exhaustive...the study incorporated methodological variables (e.g., biological and emotional relatedness of participants, evocative stimuli) widely considered to facilitate psi by parapsychologists. As such, the current null results do not simply fail to support the psi hypothesis: They offer strong evidence against it. If these results are replicated over a range of participants and situational contexts, the case will become increasingly strong, with as much certainty as is allowed in science, that psi does not exist.

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