It is clear, from the advent of more powerful electrophysiological and anatomical methods revealing sensory representations and patterns of connections in the brain, that advanced mammals—such as cats and monkeys—have many sensory representations within separate areas in cortex and nuclei in the thalamus. This chapter discusses the need of the various subdivisions of the sensory system. The basic subdivision of sensory systems is the area or nucleus. Areas and nuclei—as equivalent structures—contain systematic sensory representations, are architectonically distinct, interconnect with a number of other structures in a unique pattern, are distinguished by the response characteristics of the population of neurons, and uniquely contribute to the sensory-perceptual processing of the system. Species differ in number of areas and nuclei, and increases in numbers have occurred independently in many lines of evolution. Advanced mammals have many sensory representations within each major sensory system. These representations are complexly interconnected, and most must participate in the processing of a sensory event within the system. The multitude of subcortical efferent connections of each area suggests that most cortical representations influence behavior, memory, and emotion in a rather direct manner.
This chapter provides an overview of axonal organization in the cat medial superior olivary nucleus. Axons of elongate cells in the medial nucleus of the trapezoid body (MNTB) have a similar brush-like arborization pattern in the dorsomedial periolivary nucleus. Studies have confirmed that the horizontally oriented candelabra like spread of axons of anterior ventral cochlear nucleus spherical cells and the interrelationships of branches of these axons with more than one adjacent, horizontally oriented sheet of cells. These studies also identified four additional Golgi patterns: one of intrinsic collaterals of axons of medial superior olive (MSO) neurons, two of axons of periolivary neurons, and one belonging to axons of unknown origin. Rostrocaudally running axons ramifying in the MSO fiber zones are observed in both neonatal and adult cats. From their location, they probably make contacts primarily on dendrites or on rostrocaudally elongated cell bodies that have a similar orientation and location. The arborization pattern that has been traced to central cell band cells involves branching within a restricted area within the central band that might be associated with one or more somata and proximal dendrites. Further studies have demonstrated that an efficient mechanism for accumulating label after glycogen incubations in a specific population of synaptic terminals in both the MSO and lateral superior olivary nucleus (LSO). The morphological characteristics of these terminals are consistent with the features known to characterize endings of cochlear nucleus projections.
Publisher Summary This chapter presents diverse theories and implications of current research on the auditory temporal integration at threshold. The different theoretical explanations of auditory temporal summation have been concerned mainly with the following issues: the basic nature of the underlying mechanism (probabilistic or deterministic), the locus of integration (peripheral or central), the type of energy integrated (acoustic or neural), the effect of the power spectrum, and the effects of the temporal pattern of stimuli (tones versus periodical pulses). The most frequent stimulus parameters considered in the context of auditory temporal integration are frequency and bandwidth of stimuli, the effects of masking, the mode of stimulation, the phasic characteristics of the stimuli, and the level of stimulation. At threshold, the ear integrates the acoustic energy of a sinusoid signal linearly up to about 250 msec, that is, in this range, a 10-fold increase in duration decreases the signal intensity necessary for threshold by 10 decibel. The influence of different stimulus conditions on auditory temporal summation has proven to be complex. The probabilistic approach may be considered as the converse of the complete summation hypothesis to the extent that it assumes the absence of any physical or physiological summation. The results of recent investigations calls for the design of experiments aimed at a systematic mapping of the relations among different integration and acuity measures utilizing a variety of behavioral tasks, all within a common framework.
This chapter discusses the across-fiber pattern theory of sensory neural coding for the neural representation of the qualitative difference between various tastes stimuli. The distinction between various tastes stimuli is based on individual neuron responses to various stimuli—the labeled-line orientation—in terms of the effects of a given stimulus across the responding population of neurons—the across-fiber pattern orientation. The across-fiber pattern view is that primarily because of the equivocal relationship between the activity in any one neuron and the variety of stimuli that could produce it within a sensory modality, the identity of the stimulus is given in the activity in the population of neurons responding. In the across-fiber pattern theory, the only requirement for adequate stimulus representation is that the activity evoked by each stimulus in the responding population differs in some way from that evoked by every other discriminability different stimulus; the trigger features or best stimuli are of no special concern. Discriminability between stimuli in the across-fiber pattern theory is given by the amount of difference between the evoked patterns of activity; thus, with similarly placed pairs of stimuli, the neural difference (shaded area) between one stimulus (actually, the two stimuli, A and B, applied at the same point) and two stimuli (A′ and B′ separated) is greater with the more narrowly tuned neurons. The principles of the organization of sensory systems might be found to be relevant to other processes such as, sensory, motor, memory, concepts, and movements.
