Sensitivity to Frequency: A New Model of Hemispheric Asymmetry

"Imagine that the U.S. is preparing for the outbreak of an unusual Asian disease, which is expected to kill 600 people. Two alternative programs to combat the disease have been proposed." The first program will save 200 people. The second program has a 33% probability of saving all 600 people, but a 67% chance that no one will be saved. Which program would you choose?

If you're like most people, you'll pick the first program. However, if these choices had been framed in terms of losses (i.e., 400 people will die in the first program, where the second program has a 33% chance that no one will die, and a 67% chance that 600 people will die), then most people pick the second program.

This change in preference is strange given that the expected outcomes of each program is mathematically equivalent (indeed, it's strange that there's any preference at all). Although originally discovered in 1981 by Tversky & Kahneman, more recent work has begun to explore the neural systems responsible for this frame-related alternation between risk-aversion and risk-seeking behavior.

As reviewed by Gallagher & Dagenbach's in-press article at Brain and Cognition, it appears that these effects can be linked to right-hemispheric processing: the framing effect is more pronounced when the right hemisphere is preferentially stimulated (induced by having subjects tap their left hands and presenting the story only to the left ear). Conversely, the framing effect seemed to disappear when the left hemisphere is preferentially stimulated.

Why should this effect depend on the right hemisphere? The original authors of that study suggest that the right hemisphere is more involved in contextual processing. In contrast, Gallagher & Dagenbach advocate a view of hemispheric lateralization known as the "Double Filtering by Frequency" (DFF) theory, in which the left hemisphere tends to process higher frequencies while the right hemisphere processes relatively lower frequencies.

Although the DFF may sound odd at first, Gallagher & Dagenbach point out that it subsumes several other proposed left/right asymmetries. For example, the increased sensitivity of the right hemisphere to emotional stimuli can be explained by the fact that the emotional quality of words is often apparent in their prosody or audible tone, which is composed of lower-frequency information, while semantic content requires the decipherment of phonemes, which differ from each other primarily in terms of higher frequencies. Likewise, detailed visual information tends to contain higher spatial frequencies, whereas more global elements of visual form are conveyed by lower spatial frequencies. (Note that this roughly maps onto another model of left/right asymmetry: the local/global dichotomy.)

The most compelling evidence for DFF theory, however, is the behavioral data presented by Gallagher & Dagenbach. In their experiment, all 88 right-handed subjects heard the same voice describe the "Asian disease" story at the start of this post, embedded in white noise, followed by two options framed in terms of either gains (for half the subjects) or losses (for the other hald). However, a second distracting voice was also present throughout the study and was reading a nonsense string of words. Critically, this voice contained frequencies either higher or lower than the voice describing the Asian disease problem.

If DFF theory is correct, subjects who heard the Asian disease problem spoken by a voice with a relatively higher frequency should show less framing sensitivity than those who heard the Asian disease problem spoken by the relatively lower-frequency voice. And this is exactly what the authors found.

Gallagher & Dagenbach conclude by suggesting that DFF is an improvement over other theories of cerebral left/right asymmetry because it is computationally specified. In contrast, other theories are merely descriptive. The authors also point out that this result is unique to DFF theory; no other theory of cerebral asymmetry would have necessarily predicted these results.

Related Posts:
Harmony of the Hemispheres: Left/Right Cerebral Assymetry
Functionally Dissociating Right and Left PFC
Neural Noise and Information Theory
Tuned and Pruned: Synaesthesia