Hypothesis concerning embodied calendars: A case study of number form, color spreading, and taste-color synaesthesia.

We propose a hypothesis concerning the neural basis of the mental 'calendar' we all carry around in our brains, based on observations we made on a 25year old 'projector synaesthete', EA, who displays some novel and instructive features. In addition to her grapheme-color synaesthesia, she has a circular 'calendar line', laid out vividly in front of her in the horizontal plane with December 31st passing through the middle of her chest and other months arranged in clockwise sequence ending with December on her right (July was 3 feet in front of her). Her access to episodic memories felt, subjectively, as if it was facilitated by her calendar line. Intriguingly, if she turned her sideways to look to the right, the calendar remained 'stuck to the chest' - meaning that it was body centered rather than head centered. Even more surprising is how, when she rotated her head rightward, the left portion of her calendar became "fuzzy", and memories of February and March became less accessible; a striking example of embodied cognition - memories gated by information from neural networks representing the activity of neck muscles. We postulate that the human calendar is mediated by connections between the angular gyrus and hippocampal place and time cells-via the inferior longitudinal fasciculus. Other aspects of EA's synaesthesia were also explored. The colors evoked by graphemes spread outside the grapheme itself, but the spread could be blocked by real as well as illusory contours. These interactions might be mediated by cells signaling illusory contours (in V2) and color/texture sensitive cells in (V4). Tastes also evoked colors, but, intriguingly, the colors were experienced inside her mouth rather than out there in the world. We discuss the deeper implications of these findings for understanding the nature of quale, and the manner in which the self anchors itself in space and time.