  Personal space (PS) is the distance that people prefer to maintain between
themselves and unfamiliar others. Interpersonal intrusion into a given persons
PS evokes discomfort, and an urge to move further apart. We hypothesized that
in parietal cortex: 1. PS processing involves a previously-described
threat-sensitive brain circuit, and 2. the spatial encoding of distance is
transformed accordingly, from purely sensory to PS-related. These hypotheses
were tested using 7T fMRI at high spatial resolution. In response to images of
different visual stimuli across a range of virtual distances, we found two
categories of distance encoding in functionally corresponding columns within
parietal cortex. First, PD (personal distance) columns responded selectively to
moving and stationary face images presented at virtual distances nearer (but
not further) than each subjects behaviorally-defined PS boundary. In the
majority of PD columns, BOLD response amplitudes increased monotonically and
nonlinearly with increasing virtual face proximity. In the remaining PD
columns, BOLD responses decreased with increasing proximity. These fMRI
response functions appear related to previously-reported variations in
subjective discomfort levels, and physiologic arousal, during intrusion into
but not beyond personal space. Secondly, DD (disparity distance) columns in
parietal cortex responded selectively to disparity-based near or far distances,
in random dot stimuli, like disparity-selective columns described previously in
occipital cortex. In parietal cortex, maps of DD columns were systematically
non-overlapping (interdigitated) with the PD columns. These results suggest
that the transformation of spatial information, from visual to higher-order,
may be computed in multiple small sites, rather than across a larger cortical
gradient, in parietal cortex.
