Abstract
The nature-nurture debate regarding the origin of mental lines is fundamental for cognitive neuroscience. We examined natural-nurture effects on the mental time line, applying three different challenges to the directionality of time representation. We tested (1) patients with left-neglect and healthy participants, who are (2) left-to-right or right-to-left readers/writers, using (3) a lateralized left-right button press or a vocal mode in response to a mental time task, which asks participants to judge whether events have already happened in the past or are still to happen in the future. Using lateralized responses, a spatial-temporal association of response code (STEARC) effect was found, in concordance with the cultural effects. With vocal responses (no lateralization), past and future events showed similar results in both cultures. In patients with neglect, who have a deficit of spatial attention in processing the left side of space, future events were processed more slowly and less accurately than past events in both cultures. Our results indicate the existence of a “natural” disposition to map past and future events along a horizontal mental time line, which is affected by the different ways in which spatial representation of time is introduced.
Highlights
Representation of magnitudes plays a major role in our everyday life
In order to examine the directionality of the mental time line (MTL), we first used here a bimodal experimental settings involving 28 participants who are either L → R or R → L readers/writers responding either for past events with the left hand and for future events with the right hand and vice-versa
The present data support a culturally-influenced organization of the MTL, where past and future are represented from left-to-right in L → R readers and writers, and vice versa in R → L ones
Summary
Representation of magnitudes plays a major role in our everyday life. Numbers, time and even emotions quantified in our brain, are hypothesized to be represented on “mental lines”[1,2,3], spatially relying on a continuous left-to-right orientation axis[2,4,5]. While participants are required to perform an odd–even classification task on numbers from 1 to 9 by pressing a left or right key, responses are faster and more accurate when participants respond with the left key to small numbers and with the right key to large numbers, compared to the opposite instructions[8] This finding supports the existence of a horizontal spatial representation of numbers along the MNL where, in Western cultures, numbers are progressively positioned from left-to-right (L → R), while a reverse SNARC is reported in right-to-left (R → L) readers/writers[8,10]. We challenged the habitual STEARC effect using three different modifications of the left-to-right axis: (1) we tested participants who are either L → R or R → L writers and readers; (2) participants responded either with both hands or by vocal response, and (3) we included participants with or without left neglect
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