Neurons in the Primate Orbitofrontal Cortex Respond to Fat Texture Independently of Viscosity

University of Oxford, Department of Experimental Psychology, South Parks Road, Oxford OX1 3UD, United Kingdom Submitted 2 April 2003; accepted in final form 16 May 2003 The primate orbitofrontal cortex (OFC) is a site of convergence from primary taste, olfactory, and somatosensory cortical areas. We describe the responses of a population of single neurons in the OFC that respond to orally applied fat (e.g., safflower oil) and to substances with a similar texture but different chemical composition, such as mineral oil (hydrocarbon) and silicone oil [(Si(CH 3 ) 2 O) n ]. These findings provide evidence that the neurons respond to the oral texture of fat, sensed by the somatosensory system. Use of an oral viscosity stimulus consisting of carboxymethyl-cellulose in the range 1–10,000 centipoise (cP) showed that the responses of these fat-sensitive neurons are not related to stimulus viscosity. Thus a textural component independent of viscosity and related to the slick or oily property is being used to activate these oral fat-sensitive neurons. Moreover, a separate population of neurons responds to viscosity (produced, e.g., by the carboxymethyl-cellulose series), but not to fat with the same viscosity. Thus there is a dissociation between texture channels used to sense fat viscosity and non–fat-produced viscosity. Further, free fatty acids such as linoleic acid do not activate these neurons, providing further evidence that the oral fat-sensing mechanism through which these OFC neurons are activated is not gustatory but textural. Most of this population of fat-sensitive neurons receive convergent taste inputs. These results provide evidence about how oral fat is sensed and are relevant to understanding the physiological and pathophysiological processes related to fat intake. Address for reprint requests: E. T. Rolls (E-mail: Edmund.Rolls{at}psy.ox.ac.uk ; web: www.cns.ox.ac.uk ).