New Worlds: Arachnophobia changes way spider size is estimated

This study shows how perception of even a basic feature like size is influenced by emotion, and demonstrates how each of us experience the world in a unique and different way.

April 3, 2016 00:38
4 minute read.
Ben Gurion University

Ben Gurion University. (photo credit: WWW.PIKIWIKI.ORG.IL)

A group of Ben-Gurion University researchers reports that spiders seem larger to people who have a of them (arachnophobia) than they do to people who are not afraid of the eight-legged arthropods.

For some with a biblical background, this brings to mind the episode of the spies sent to scout out the Land of Canaan. In the eyes of 10 of the spies the natives were huge and insuperable, while Joshua and Calev, seeing the situation differently, were confident the occupants of the land could be conquered.

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The BGU study began when Dr. Noga Cohen and Dr. Tali Leibovich, then doctoral candidates in Prof. Avishai Henik’s cognitive neuropsychology lab noticed a sider crawling on Leibovich’s desk. As the doctoral student was afraid of spiders, she asked her colleague to “take care” of the spider, but Cohen, on the other hand, couldn’t understand what Leibovich was afraid of. While Cohen insisted the spider was small, her student insisted it was large – even though both saw the same insect.

To understand why, they designed an experiment, the results of which were published recently in the journal Biological Psychology.

They invited female students to fill out a questionnaire that measured fear of spiders and divided the participants into two groups based on their level of fear of spiders. Then the participants were asked to estimate the real-world size of spiders and other animals (presented in pictures) relative to a fly and a rabbit and to report the unpleasantness each picture made them feel.

The results revealed that participants who were afraid of spiders estimated spider size differently than participants who weren’t afraid of spiders. In contrast, the size of other animals, such as wasps and butterflies, were ranked similarly by both groups. Further analyses showed that size estimation was affected by both the level of unpleasantness and how relevant the animal was to the participant (spiders are relevant to those who are afraid of them).

It turns out that both BGU researchers (Cohen and Leibovich) were right.

This study shows how perception of even a basic feature like size is influenced by emotion, and demonstrates how each of us experience the world in a unique and different way.

This study raises additional questions such as: Is it fear that triggers size disturbance, or might size disturbance be what causes fear in the first place? CALCULATING WHISKERS SEND PRECISE DATA TO BRAIN As our sensory organs register objects and structures in the outside world, they are continually engaged in two-way communication with the brain. In research recently published in Nature Neuroscience, Weizmann Institute of Science researchers found that for rats – which use their whiskers to feel out their surroundings at night – clumps of nerve endings called mechanoreceptors located at the base of each whisker act as tiny calculators.

These receptors continuously compute the way the whisker’s base rotates in its socket, expressing it as a fraction of the entire projected rotation of the whisker, so that the brain is continually updated on the way that the whisker’s rotation is being followed through.

Whiskers, like our eyes or fingers, must move to sense the stationary things in their environment. Prof. Ehud Ahissar and his group in the institute’s neurobiology department, have been investigating the rats’ active sensing system for over a decade, applying a method in which the experimenter “does the whisking” for an anesthetized rat. This method enables them to study so-called active sensation, but the whisker’s movement could not mimic that of awake rats – in which sensing and the act of whisking are tightly bound.

In the present study, they combined their method with one developed by Dr. Avner Wallach, at that time a postdoctoral fellow in Ahissar’s lab, whose doctoral research in the Technion had involved work on integrating computers into biological systems. They further developed a closed-loop system in which the rat whisking system and the computer form a sort of “rat-computer hybrid” that recreates the whisking movement and the way it is regulated in awake, freely-moving rats.

The discovery that the mechanoreceptors within the whisker follicle were actually calculating the whisker’s motion phase “online” came as a surprise to the researchers, because knowing the phase implies predictive knowledge of how the whisker motion will develop.

The assumption was that specialized neuronal circuits would perform this calculation using raw data from both the receptor and the brain’s motion-planning circuits.

“On second thought,” said Ahissar, “this work division is sensible. The sensory organs are not merely ‘signal converters.’ Rather, they are broad, inclusive interfaces between organisms and their environments, providing everything the brain needs for making sense out of their signals.”

Next, the researchers would like to know how the sensory organ physically calculates this predictive information. The combined method might, in future research, be used to explore other closed-loop algorithms in the brain.

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