Before drawing conclusions, it is worth considering whether or not our results were specific to the set of stimulus parameters chosen. In our main experiment, the stimuli moved at 5.6°/s and had a diameter of 12°, which is comfortably within the range of speeds and stimulus sizes used in previous studies. Snowden and Kavanagh (
2006) found no difference between older and younger adults' abilities to discriminate the direction of high contrast simple translational motion patterns at this speed but found greater differences between the groups at slower speeds (see also Bidwell, Holzman, & Chen,
2006). It is possible, therefore, that at very slow speeds, greater differences between the older and younger participants' performances might be found. With respect to stimulus size, Mapstone et al. (
2008) found no difference in older adults' heading discrimination with stimuli of different sizes (unless a conflicting pattern was presented in the periphery). However, it should be noted that simply increasing the stimulus size, while maintaining dot density, also increases the number of moving dots (local motion samples) presented. Studies presenting fewer dots (e.g., Billino et al.,
2008) have found qualitatively different results to those presenting greater numbers of dots (e.g., Mapstone et al.,
2008). It is possible that stimulus size, when considered in terms of useable information content, affects older and younger adults differently. Some recent work has suggested, for example, that older adults have reduced surround suppression for moving patterns (Betts, Sekular, & Bennett,
2009; Betts, Taylor, Sekuler, & Bennett,
2005) although this may also be, in part, a function of contrast sensitivity (Aaen-Stockdale et al.,
2009).