Figure 4. Head orientations and freezing responses of subjects during exposure to object distress.

(a) Estimates of subject head orientations were extracted from freeze-frames of video recordings at 2 time-points with respect to UCS presentation to objects (30 s pre-UCS and 1 s post-UCS). The longitudinal axis of the subject's head, running parallel with the sagittal midline, was referenced at 15° increments. A 0° head orientation was defined by the longitudinal axis of the subject's head forming a right angle with the steel dowels that separated the observation and demonstration compartments. Prior to presentation of the UCS to objects, BALB (left panels; N = 16 mice; age = 7–8 wks) and B6 (right panels; N = 8 mice, age = 7–8 wks) subjects were orientated towards the demonstration compartment (mean head angle = dotted black lines, std. dev. = grey shaded area). Following presentation of the UCS to objects, subject head orientations were more consistently directed at objects (depicted by red dotted lines and pink shading), as indicated by a ≈2-fold reduction in variability of head orientations (Brown-Forsythe ANOVA, P<0.001 for both BALB and B6 subjects). (b) The freezing responses of these subjects were minimal and strain-independent during the 30-s period following presentation of the UCS to objects. Data in panel (b) are presented as the mean±s.e.m.