Figure 6. Simulation of age-effects. Stimulation at a single location evokes monomodal distributions of activation with surrounding inhibition.

Due to the sharp interaction in the model of young adults the peak of activation is narrow (a) but broader and enhanced in the model of older adults (f). Inputs from stimulation at two sites (corresponding to the situation during two-point discrimination) at small separation distances fall into the excitatory region, and therefore evoke monomodal distributions (b,g). For larger separation distance, the winner-takes-all inhibition becomes strong enough to cancel one of the incoming inputs, leading to a broader, but still monomodal distribution (c,h), whose amplitude is reduced due to the strong mutual inhibition. The conditions shown in (b,g and in c,h) can be regarded as equivalent to perceiving a single pin instead of two. Note that the model behavior is the same for young and older adults, but differs with respect to the equivalence of the separation distances between the two pins of our test device that were larger for older adults (c,h). When the separation distances between inputs increased further, the activation distributions become bimodal (d,i and e,k), which can be regarded as the equivalence for perceiving two separate pins. For intermediate separation distances, the amplitudes of the double-peaks are altered due to the still existing interaction (d,i). At large separation distances the evoked distributions remain unaffected from the neighboring distributions (e,k). The transition from monomodal to bimodal distributions occur in the model of older adults for larger pin distances (i,k).