Wednesday 15 October 2008

A Delusion or you did Distort Time ?

Demystifying Delusions -

Delusions are pathological beliefs which persist despite clear evidence that they are actually false. They can vary widely in content, but are always characterized by the absolute certainty with which they are held. Such beliefs reflect an abnormality of thought processes; they are often bizarre and completely unrelated to conventional cultural or religious belief systems, or to the level of intelligence of the person suffering from them.

The delusions experienced by psychiatric patients are sometimes categorized according to their theme. For example, schizophrenics often suffer from delusions of control (the belief that an external force is controlling their thoughts or actions), delusions of grandeur (the belief that they are a famous rock star or historical figure) or delusions of persecution (the belief that they are being followed, attacked or conspired against).

Although often associated with psychiatric disorders, delusions can also occur as a symptom of neurodegenerative disorders, and improved diagnostic methods have led to an increase in the identification of brain damage in patients who suffer from them. To date, however, there has not been an all-encompassing theory of how the brain generates delusions. Now though, Orrin Devinsky, a professor of neurology, neurosurgery and psychiatry at New York University, proposes that delusions are generated by a combination of right hemisphere damage and left hemisphere hyperactivity.

In a review published in the journal Neurology, Devinsky examines the neuropathologies underlying two delusional syndromes with the aim of identifying anatomical abnormalities that are common to all four. Specifically, he looks at Capgras syndrome, the delusional belief that close friends or relations are imposters or have identical body doubles with different identities and reduplicative paramnesia (or Capgras for places), in which one believes that a familiar place exists in two locations simultaneously.

These syndromes are related to, and often co-exist with, confabulation (the pathological production of false memories) and anosognosia, a condition in which one fails to recognize, or is unaware of, a neurological deficit such as blindness or paralysis. They also share common mechanisms and pathologies. However, whereas confabulating patients can be convinced that their memories are false, deluded patients hold on to their beliefs firmly.

Devinksy looked at numerous case studies of individuals with these syndromes and, when possible, pinpointed the site of brain damage in each. His analysis showed that the four conditions do indeed share common pathological features. In 69 patients with replicative paramnesia, for example, 52% had incurred damage to the right frontal lobe (as a result of stroke or Alzheimer's Disease), 41% had damage to both, and 7% had damage to the left. Likewise, the case studies of patients with Capgras syndrome showed that they had damage primarily to the right frontal lobe.

The ubiquity of frontal lobe damage in the cases studies supports the hypothesis that these delusions involve impairments in executive function, working memory, decision-making and the abilities to make accurate predictions and to estimate and sequence time. One consequence of damage to the right frontal lobe would therefore be an impairment in the patients' ability to monitor the accuracy of their own cognitive processes.

According to Devinsky's hypothesis, this leads to increased left hemisphere activity - the left hemisphere compensates for the lack of inappropriate inputs from the right, "filling in" the gaps and conjuring a creative and extravagant narrative which leads to false explanations of the patient's experiences. Damage to the right hemisphere may prevent the patient from recognizing his or her cognitive errors, and therefore from changing their false beliefs.

From a different perspective, you could have distorted time to quantify the experience you had, isn't it?

"Where is it, this present? It has melted in our grasp, fled ere we could touch it, gone in the instant of becoming." - William James, 1890

Extremely dangerous, traumatic, or surprising moments are often accompanied by reports that time seemed to "slow down" or "fly by." The perceptual basis of these subjective temporal distortions is unclear, but not for lack of trying: one recent experiment went so far as to drop subjects off a 400 foot tower while testing their ability to decipher a rapidly flashing string of numbers - a test of perceptual processing speed. Unfortunately, it didn't work. Subjects were no better at deciphering these numbers than they were under more mundane circumstances.

Ironically, temporal distortion may be more noticeable in such mundane experiences. A 2004 study by Tse and colleagues reviews evidence that durations are estimated as somewhat longer as more complex stimuli are being presented - as though time is subjectively expanding. In contrast, when subjects must actively attend to those stimuli or perform a secondary task while estimating durations, they tend to estimate those elapsed durations as slightly shorter - as though time is subjectively contracting.

Tse et al. suggested that these effects might emerge in the same way distraction might impair your ability to count the "ticks" from a clock's second hand. According to this "missed temporal cues" hypothesis, duration judgments are accomplished by attending to "temporal units." When attention is fully directed towards these temporal judgments, fewer "temporal units" are missed and duration estimates increase; however, when attention is divided, more of those "temporal units" are missed and so the estimates decrease.

Tse et al also suggest an alternative hypothesis: "attentional boost." According to the "attentional boost" theory, the processing of relatively low probability stimuli may somehow speed information processing of that stimulus, causing more "temporal units" to be counted.

To distinguish between these hypotheses, Tse et al. combined several methods of temporal estimation with the oddball paradigm, in which relatively low-probability stimuli are embedded inside series of more high-frequency stimuli. (For example, in auditory oddball, subjects might hear a string of sounds like "ROOF ROOF ROOF WOOF ROOF." Bizarre, I know.)

In a first experiment, subjects reliably estimated that an oddball visual stimulus was similar in duration to a much more frequent visual stimulus when, in fact, its duration was around half as long (675 msec vs. 1050 msec)!

A second experiment demonstrated that this temporal expansion of "oddballs" occurs only when the more frequent standard stimuli are longer than ~150 msec in duration. In fact, there's a subjective temporal contraction when standard stimuli are around 75 msec in duration - as though attention cannot be allocated to the oddball quickly enough, and the subsequent blank interval or standard stimulus itself undergoes the subjective temporal expansion, instead of the oddball.

An interesting feature of the data is a peculiar "dip" in the degree of temporal distortion measured by these methods when the more frequent standard stimuli last only around 375 msec. Shorter or longer standards cause more temporal distortions. Furthermore, this dip occurs at different times between individuals, and Tse et al. argue that it could reflect what many consider to be the dual mechanisms of attentional reorienting: a transient component and a sustained component.

Subsequent experiments replicated the effect with a variety of stimuli and temporal estimation methods. These temporal distortions (and to some extent the pecular "dip") were apparent when using auditory stimuli and were present with visual stimuli regardless of whether subjects were asked to rate the magnitude of the oddball's duration, to reproduce the duration of the oddball, or to rate whether the oddball lasted longer or shorter than the average of all previous stimuli (all of these are established psychophysics techniques for duration perception).

Tse et al note their results could be an example of the "time order error" (TOE) in which the second of two sequentially judged stimuli is altered along the dimension of judgment. For example, when two weights of large mass are lifted in succession, subjects tend to say the second is heavier. In contrast, when two weights of small mass are lifted one after the other, subjects tend to rate the second as lighter. So, in the case of temporal distortion, subjective duration lengthens if the oddball is presented for longer than 150 msec, but shortens if the oddball is presented for less than 150 msec. They discuss several reasons to doubt this possibility, including that TOE theory seems to contradict several findings in the duration psychophysics literature, as though it might not apply to duration judgments.

There are clear evolutionary advantages to subjective temporal distortion - as Tse et al note, increasing temporal resolution could allow for greater depth of processing and thus more adaptive responding when an organism is endangered or surprised. Although intuitively far-fetched, visual psychophysics has demonstrated that spatial attention can alter the spatial resolution of vision. Thus it seems at least plausible that "temporal attention" could alter the temporal resolution of duration judgments, causing them to contract or expand.

Finally, an interesting question for future research is how these phenomena might interact, given the time-frequency tradeoffs that are inherent to signal processing. For example, increased spatial attention could cause decreased resolution in temporal judgments, and vice versa.


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