Perceiving the World

As exemplified by sensory illusions, perception is interpretative rather than faithfully representational of the changes in the world. All perceptual pathways involve stimulus transduction, transmission, and modulation before sensory events are coded by the nervous system. The set of stimuli that humans respond to are a subset of the stimuli that elicit reactions across the animal kingdom. The brain processes visual, auditory, mechanical, and vestibular stimuli by breaking stimuli into their sinusoidal components for neuronal processing. The probabilistic response of sensory receptors to stimulation within a receptive field is described. A fundamental property of sensory perception is responsiveness to a wide range of stimulus intensities over several orders of magnitude. Yet, at any one time, the response to a stimulus is proportional to the background level of stimulation. The concept of labeled line sensory transmission is described, and the reality of multimodal integration is revealed through examples.

Open lumina of microampullary electroreceptor organs in the transparent catfish, Kryptopterus bicirrhis (Valenciennes 1840), contain 50 mM potassium: why?

Administration of the cell impermeant fluorescent K+ and Na+ probes potassium-binding fluorescent indicator (PBFI) and sodium-binding fluorescent indicator (SBFI) to the lumina of ampullary electroreceptor organs in the transparent catfish Kryptopterus bicirrhis (Valenciennes 1840), demonstrated an unexpected high concentration of K+ ions: 50 mM. Since the lumina of the ampullary organs are in open contact with the surrounding water, such a high K+ concentration inside the lumen can be maintained only by heavy metabolic transport. The implications of this finding for stimulus transduction in freshwater ampullary electroreceptor cells are discussed.

The Sense of Taste: Neurobiology, Aging, and Medication Effects

The sense of taste is an oral chemical sense in mammals that is involved in the choice of foods. Initial transduction of taste stimuli occurs in taste buds, which are distributed in four discrete fields in the oral cavity. Medications can affect the taste buds and ion channels in taste-bud cell membranes involved in stimulus transduction. The sense of taste gradually declines with aging, with bitter taste most affected. Neural circuits that mediate taste in primates include cranial nerves VII, IX, and X, the solitary nucleus in the brain stem, the ventroposteromedial nucleus of the thalamus, and the insular-opercular cortex. The central taste pathways process taste information about sweet, salty, sour, and bitter stimuli serially and in parallel. Medications associated with "metallic" dysgeusia and taste losses affect the taste system via unknown mechanisms.

The sensory contribution of a single vibrissa's cortical barrel

The sensory contribution of the cortex containing the cortical barrel of the C1 vibrissa was studied in rats using the ablation-behavior method. Three independent experiments were performed, each requiring stimulus transduction by the C1 vibrissa but varying in their perceptual demands. The first required detection of sinusoidal oscillations of the vibrissa generated by an oscillating airstream directed vertically onto the vibrissa tip. The second required detection of a change in rate of the oscillation. The third required the blinded rat to jump a gap in an elevated runway after palpating the far side with its vibrissa. Psychophysical determinations of the single vibrissa system's thresholds before and after ablation of the cortex containing its barrel show that normal sensitivity either for detecting an oscillation or for detecting a change in oscillation frequency are not dependent on either the contralateral or the ipsilateral cortical barrelfield. In contrast to the lack of effect of barrelfield ablation on the spatial and temporal acuity of the vibrissa, the third experiment shows that a rat's ability to collect situation-relevant information with the vibrissa is lost after ablation of the cortex containing its contralateral barrel but not after ablation of the cortex containing its homologous ipsilateral barrel. The results of repeated retesting of an individual rat's ability to make a jump-no jump decision on the basis of vibrissa-transduced information at each stage of a series of successive single-vibrissa removals and unilateral barrelfield ablations show that the loss of the cortex containing the vibrissa's contralateral barrel is tantamount to loss of the vibrissa itself.