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Rapid Categorization of Snakes in the Infant’s Occipital Cortex: Evidence from Fast Periodic Visual Stimulation

Julie Bertels1*, Arnaud Destrebecqz1 and Adélaïde De Heering2
  • 1 Université Libre de Bruxelles, Center for Research in Cognition and Neurosciences (CRCN), Belgium
  • 2 Université Libre de Bruxelles, Center for Research in Cognition and Neurosciences (CRCN), Belgium

Snakes and primates share a long evolutionary existence, with snakes being the first of the major predators of primates (4). Natural selection may have then fostered primates whose detection of snakes allowed a better defensive behavior. In line with this idea, recent studies have shown that humans are remarkable snake detectors: In visual search tasks, adults and children are faster to detect a picture of a snake in an array of flower pictures than vice versa ( 6,9). These findings have been taken as evidence for the existence of a fear module that could have evolved in mammals’ brain to assist them in responding adequately to recurrent survival threat (8). A stronger and more decisive argument for an inborn threat detection system (that would allow a rapid detection of snakes in particular) would come from infants who, as a matter of fact, do not know about the potential dangerousness of snakes. A few recent studies revealed, by examining infants’ looking behaviors when presented with pictures of snakes, enhanced visual detection of snakes and preferential attentional allocation to these stimuli, as well as attentional broadening resulting from the detection of these reptiles (1, 3, 7). These findings may suggest an inborn mechanism for the rapid detection of snakes that most probably rely on the identification of low-level perceptual features, such as their coiled shape (10). We do not know yet whether the detection of these perceptual features allows infants to discriminate snakes from other animals and to generalize across snakes despite their lack of experience with these reptiles. The neural bases of such discrimination and/or generalization also remain unknown. To clarify this issue, we used a fast periodic visual stimulation approach in the context of an oddball paradigm in which we presented base and oddball stimuli at two different periodic frequencies (5,11). Periodic stimulation is known to generate steady-state visual evoked potentials in participants’ brain that have the same fundamental frequency as the driving stimulus. When two frequencies are used, the brain will generate two periodic responses but only if it differentiates between base and oddball stimuli. This approach can therefore provide an implicit, objective, predictive and robust measure of stimulus categorization. It has recently proven its efficacy in infants with complex visual stimuli (2). We recorded scalp electrical brain activity using a 32-channel Biosemi system in 7- to 13-month-old infants (N=15; 10 females). Infants were presented with 20-second sequences of pictures flickering at 6 Hz (i.e., 6 images per second, see Figure 1). A stimulation sequence contained repetitive series of four pictures of various animals in their natural background followed, every fifth stimulus, by a picture of a snake (in the ‘snake’ sequences), or by a picture of a frog (in the ‘frog’ sequences). The oddball stimulation frequency was therefore of 1.2 Hz (6 Hz/5). Within a sequence, all pictures were different and highly variable. They were however equalized in terms of contrast and luminance across the whole set. Snake and frog sequences were presented in alternation. Half of the infants were presented first with a snake sequence, the other half with a frog sequence. Each infant saw as many sequences as possible, with an average of 13 sequences per infant (min = 8, max = 18). EEG trials were averaged separately for each infant and Fourier transformed into the frequency domain. A sharp response was found over the medial occipital lobe exactly at the base stimulation frequency (6 Hz) in snake and frog sequences (averaged signal-to-noise ratio (SNR) at O1 in snake and frog sequences: 5.84 and 5.28, respectively, see Figure 2, left panel), demonstrating successful synchronization of the infants’ brain to the visual stimulation. Crucially, there was a distinct response at the oddball frequency (1.2 Hz) in snake sequences, more pronounced at the left occipital electrode O1 (averaged SNR at O1: 1.63; see Figure 2, right panel). No significant response was observed at the oddball stimulation frequency in frog sequences. These results reveal infant’s particular brain sensitivity to snakes. Specifically, they show that infants discriminate snakes from other animal images, and that they generalize across snake images despite their perceptual variance. We did not find similar results with frogs. We would therefore suggest that infants’ brain considers snakes as belonging to a specific animal category although they do not have any experience or knowledge about these reptiles. Interestingly, the localization of the effect on the scalp suggests that it is driven by low-level rather than high-level visual areas. In infants, snake categorization might therefore be more perceptual than semantic, namely based on the detection of salient perceptual features shared by snakes such as their coiled shape. These findings go along with the idea that humans have an inborn predisposition to detect snakes rapidly, based on their perceptual features (11). Résumé en Français Au cours de l’évolution, les êtres humains auraient développé une prédisposition innée à détecter les serpents particulièrement rapidement, ceux-ci étant un de nos plus anciens prédateurs. En accord avec cette idée, notre étude montre, chez des nourrissons de moins d’un an (sans expérience ni connaissance de la dangerosité potentielle de ces animaux), des réponses cérébrales liées à la présentation d’images de serpents, mais pas de grenouilles. La localisation de ces réponses suggère que les nourrissons détectent les traits perceptifs saillants des serpents (comme leur forme enroulée). Samenvatting in het Nederlands: In de loop van de evolutie zou de mens een natuurlijke aanleg ontwikkeld hebben om bijzonder snel slangen, onze oudste roofdieren, op te merken. Uitgaande van dit idee, toont onze studie aan dat bij zuigelingen van minder dan één jaar (die dus noch kennis noch ervaring kunnen hebben betreffende het gevaar dat deze dieren vormen) reacties in de hersenen voorkomen wanneer tussen afbeeldingen van andere dieren afbeeldingen van slangen getoond worden, en helemaal niet bij het tonen van afbeeldingen van kikkers. De lokalisatie van deze reacties suggereert dat de baby’s de opvallende kenmerken van de slangen (zoals het opgerold zijn) herkennen.

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Figure 2
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References

1. Bertels, J., Bayard, C., Floccia, C., & Destrebecqz, A. (submitted). Rapid detection of snakes modulates spatial orienting in infancy.
2. de Heering, A., & Rossion, B. (2015). Rapid Categorization of Natural Face Images in the Infant Right Hemisphere. eLife, doi: http://dx.doi.org/10.7554/eLife.06564.
3. DeLoache, J. S., & LoBue, V. (2009). The narrow fellow in the grass: Human infants associate snakes and fear. Developmental Science, 12(1), 201-207.
4. Isbell, L. A. (2009). The fruit, the tree, and the serpent: Why we see so well. Harvard University Press. Cambridge, MA: Harvard University Press.
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6. LoBue, V., & DeLoache, J. S. (2008). Detecting the snake in the grass: Attention to fear-relevant stimuli by adults and young children. Psychological Science, 19(3), 284-289.
7. LoBue, V., & DeLoache, J. S. (2010). Superior detection of threat-relevant stimuli in infancy. Developmental Science, 13(1), 221-228.
8. Öhman, A., & Mineka, S. (2003). The Malicious Serpent: Snakes as a prototypical stimulus for an evolved module of fear. Current Directions in Psychological Science, 12(1), 5-9.
9. Öhman, A., Flykt, A., & Esteves, F. (2001). Emotion drives attention: Detecting the snake in the grass. Journal of Experimental Psychology: General, 130(3), 466-478.
10. Rakison, D. H., & Derringer, J. (2008). Do infants possess an evolved spider-detection mechanism? Cognition, 107(1), 381-393.
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Keywords: Infancy, Snakes, SSVEPs, oddball paradigm, Categorization, threat detection

Conference: 6th Belgian Brain Congress, MONS, Belgium, 8 Oct - 8 Oct, 2016.

Presentation Type: Poster Presentation

Topic: Brain and brain diseases: between heredity and environment

Citation: Bertels J, Destrebecqz A and De Heering A (2016). Rapid Categorization of Snakes in the Infant’s Occipital Cortex: Evidence from Fast Periodic Visual Stimulation. Conference Abstract: 6th Belgian Brain Congress. doi: 10.3389/conf.fnagi.2016.03.00030

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Received: 29 Jun 2016; Published Online: 04 Jul 2016.

* Correspondence: Dr. Julie Bertels, Université Libre de Bruxelles, Center for Research in Cognition and Neurosciences (CRCN), Brussels, Belgium, julie.bertels@ulb.be