Aside from the neurobiological work done in Tritonia relatively few studies have attempted to investigate the neural mechanisms that underlie magnetic orientation behavior. Some initial progress has been reported with electrophysiological approaches in birds (Semm and Demaine, 1986
; Beason and Semm, 1987; Semm and Beason, 1990; Semm and Schneider, 1991), mole rats (Marthold et al., 1997; Nemec et al., 2001), and fish (Walker et al., 1997). Nevertheless, much of what is known or assumed about the neural basis of magnetic orientation behavior has been inferred from behavioral experiments. The reliance on behavioral studies is understandable, given that primary magnetoreceptors have not yet been identified with certainty in any animal. Yet behavioral experiments can ultimately provide only limited insight into the myriad complex and often non-intuitive ways in which nervous systems detect sensory information, process it, and integrate it with other neural input to generate motor responses.
In parallel with behavioral experiments, several theoretical models of magnetoreception have been proposed to explain how animals might detect magnetic fields (Kalmijn, 1978; Kirschvink and Gould, 1981; Schulten and Windemuth, 1986; Ritz et al., 2000, 2002). These models have been invaluable in guiding investigations of possible transduction mechanisms. At the same time, receptor mechanisms (whatever they may be) represent only one small part of the neural processes that comprise magnetoreception. Surprisingly, none of the models proposed so far have considered the crucial role that higher-order processing typically plays in circuits involving sensory information. In all other sensory systems, signal processing results in significant alterations in the neural information that is passed along at each step of a circuit (Kandel et al., 1997). As a result, the neural activity that actually reaches neurons responsible for initiating or modulating behavior often bears little resemblance to the activity of receptor cells. Moreover, higher-order processing often acts as a filter that discards aspects of the sensory world that are not directly relevant to the task which must be performed.
Whereas higher-order mechanisms in vision and hearing have been studied extensively (Kandel et al., 1997), such mechanisms have received little attention in the context of magnetoreception. Nevertheless, the clear lesson to be drawn from other sensory systems is that filtering and feature extraction are often at least as important as receptor responses in shaping the motor outputs that comprise behavior. Thus, identifying areas of the brain that process magnetic field information, determining the role of centers known to be "responsive" to field stimuli, and studying how motor responses are generated are all as important as identifying receptor cells. Only by expanding studies to include higher order processing can the neural mechanisms that underlie magnetoreception and magnetic orientation behavior be fully understood.
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