CONDITIONED taste aversion (CTA) is a form of association learning established when an animal associates a conditioned stimulus (taste) with a subsequent unconditioned stimulus (US, illness). We have studied the relationship between the efficacy of inducing CTA to 0.1% saccharin and c-fos expression in the lower brain stem following administration of 13 different USs in rats. The effective USs were grouped into abdominal irritants, rewarding drugs and emetic agents. Regardless of the properties of USs, good correlation was detected between the strength of CTA and c-fos expression within the area postrema, caudal and intermediate subdivisions of nucleus tractus solitarius and the external lateral subnucleus of the parabrachial nucleus. Only hypertonic saline was exceptional because of the experimental procedure: it induced strong c-fos expression, but was not an effective US for CTA formation. Different dosages of the emetic LiCl induced CTA and c-fos expression in a dose-dependent manner.

Magnetoencephalography (MEG) has recently revealed that the transitions between the parietal operculum (Pop) and the insula (area G) and the ventral end of the central sulcus (cs) were activated with the shortest latency by instrumental gustatory stimulation, which suggests that the location of the primary gustatory area is in these two regions. However, studies using other noninvasive brain-imaging methods such as positron-emission tomography or functional magnetic resonance imaging (fMRI) with manual application of tastants into the mouth have been unable to confirm this. The present study examined cortical activation by repetitive stimulation of the tongue tip with 1 M NaCl with a computer-controlled stimulator and used fMRI to detect it. In individual brains, activations were detected with multiple comparisons (false discovery rate) across the whole brain corrected (threshold at P < 0.05) at both area G and frontal operculum (Fop) in 8 of 11 subjects and at the rolandic operculum (Rop) in 7 subjects. Activations were also found at the ventral end of the cs (n = 3). Group analysis with random-effect models (multiple comparison using familywise error in regions of interest, P < 0.02) revealed activation at area G in both hemispheres and in the Fop, Rop, and ventral end of the cs on the left side. The present study revealed no activation on the gyrus of the external cerebral surface except for the Rop. Taking MEG findings into consideration, the present findings strongly indicate that the primary gustatory area is present at both the transition between the Pop and insula and the Rop including the gray matter within a ventral part of the cs.

We make full use of our sensory system, chiefly the visual system, to recognize objects around us. The visual and auditory systems are extroceptive systems, which can detect an object from a distance. On the other hand, gustatory and somatosentory systems are introceptive systems, which can detect an object only when the object is located near us. The olfactory system has both extroceptive and introceptive characteristics. But the effective distance from objects for theolfactory system is nearer than for the visual system. Thus it is highly adaptive to depend on the visual system to recognize objects in the external world.