Five senses; an incurably abstract intellect; a haphazardly selective memory; a set of preconceptions and assumptions so numerous that I can never examine more than a minority of them never become conscious of them all. How much of total reality can such an apparatus let through?
The current issue of Neuron, features a special review of the sensory systems. Ten articles review the various leading edge aspects of sensory systems neurobiology.
As a general concept, the basic functions of all of the sensory modalities are the same, to respond to a specific peripheral stimulus (smell, taste, sound, light or touch) and translate that stimulus into neuronal signals that can be processed to create an internal representation of the external world. Each modality utilizes a special type of receptor cell that selectively responds to a particular stimulus. There are numerous studies over decades that have examined the dynamics of these cells and their complex interaction with our physiology. This issue of Neuron pulls together the latest science and uncovers some interesting and unusual aspects of our sensory perception.
Taste and Smell
It is all about understanding our interaction with our sensory apparatus and how the brain receives and interprets all the signals that we receive. A number of the reviews tackle various questions or seeming incongruities in our sensory system. For example, taste and smell are intimately intertwined. "While the olfactory system is remarkable for the sheer number of olfactory receptors (over a thousand), the gustatory system appears to be much simpler. Relatively fewer receptor types and even fewer taste modalities (five in humans) are required to encode the sense of taste."
Auditory and Visual
For all our senses, representations of the information of the external sensory world are ultimately encoded in the corresponding sensory cortex. How auditory signals are represented in the auditory cortex is an interesting question. There are many more neurons in the auditory cortex than there are in the auditory nerve, and similarly, in the visual system there many more cells in the visual cortex than in the retina, and a key question has been why this is so? A review by DeWeese and colleagues, explores the possibility that these 'extra' neurons are used to counteract the effects of noise that results from moving the sensory signal along the processing stream and to thereby assure a reliable representation of the peripheral signal.
The visual system hijacking the auditory system - ventriloquism
Looking at how auditory and visual signals are integrated in the context of space perception. It has been observed that vision usually dominates over auditory localization in our perception of space. An example used is ventriloquism, where the voice is perceived to be coming from the puppet's moving mouth rather than the puppeteer. Many experiments have shown that vision is capable of making plastic changes in the processing of auditory spatial information. It is suggested that such visual capture may occur because visual information is usually more reliable and precise and the brain has evolved to integrate information optimally, with a greater weighting toward the more statistically reliable information.
Autism and Cognition
Dakin and Frith examine the data that suggest that defects in visual perception play a role in the cognitive defects associated with autism spectrum disorder (ASD). They evaluate three classes of perceptual phenomena that have repeatedly been associated with ASD: superior processing of fine detail (local structure), either inferior processing of overall/global structure or an ability to ignore disruptive global/contextual information, and impaired motion perception. This review evaluates the quality of the evidence bearing on these three phenomena.
Another review looks at seemingly crossed signals, for example. in synesthesia, a condition in which stimulation of one sensory pathway or system elicits experiences in a second, unstimulated pathway or system. So that individuals with grapheme-color synesthesia, viewing letters or numbers causes them to experience colors.
Some of the articles are complex but interesting reading. Neuron also supplements the current issue with several articles from previous issues that can help highlight various concepts about our sensory system. This collection of articles was chosen to complement the topics and talks presented at this year's Neuron satellite meeting on "Neurons and Sensory Systems" at the Society for Neuroscience meeting which takes place this week (November 10 & 11). The presenters are among the world's leading scientists and researchers including Linda Buck, a 2004 Nobel Prize winner in Physiology or Medicine.
These featured research articles will be freely accessible through the Neuron website until November 23rd.
Volume 48 Issue 3: November 3 , 2005