Taste quality and intensity: lessons from the Morrison technique

The nontoxic and nonshock Morrison operant technique was used to evaluate taste quality in rat and marmoset: response to a tastant test solution in pursuit of a pellet reward was dependent on making a choice between two bars that had been linked in discrimination training to qualitatively different stimulus pairs (NaCl versus either HCl, QHCl, or NH(4)Cl). The percentage distribution of bar-press responses to test stimuli showed: (1) stability of quality across 0.069-0.3 M NaCl, 0.003-0.1 M HCl, and 0.0001-0.003 M QHCl; (2) for LiCl, a quality change consistent with human reports of a "sour" to "salty" shift; (3) a suggestion that the "salty-like" quality of NH(4)Cl and NaCl are not perceptually equivalent; (4) NaNO(3) shares NaCl-like, QHCl-like, and NH(4)Cl-like components; (5) CaCl(2), KCl, and MgCl(2) share QHCl-like and NH(4)Cl-like components; and (6) responses to HCl and QHCl were not hedonically driven in the rat. Comparison of rank order correlations of single-unit firing rates to the distribution of bar-press responses for the same test stimulus concentration revealed that (7) no single level of the gustatory pathway exclusively accounts for the operant response distribution pattern to either simple or complex tastants, and (8) discriminations between tastants, one of which may be qualitatively complex, are not necessarily mediated only at levels proximal to the solitary nucleus. Thus, the Morrison discrimination technique effectively yields statements about gustatory quality without use of negative reinforcers and largely uninfluenced by tastant hedonics.