Suboptimality in human categorization and identification

Categorization and identification decision processes were examined and comp ared in 4 separate experiments. In all tasks, the critical stimulus compone nt was a line that varied across trials in length and orientation, and the optimal decision rules were always complex piecewise quadratic functions. E vidence was found that identification is mediated by separate explicit and implicit systems. In addition, a common type of suboptimality was found in both categorization and identification. In particular, observers apparently approximated the piecewise quadratic functions of the optimal decision rul es with simpler piecewise linear functions. A computational model, which wa s motivated by a recent neuropsychological theory of category learning, suc cessfully accounted for this suboptimal performance in both categorization and identification. The model assigns a key role to the striatum and assume s the observed suboptimality was largely due to massive convergence of visu al cortical cells onto single striatal units.