A neural correlate of behavioural stimulus intensity discrimination in a mollusc.

Much of molluscan behaviour is stereotyped and therefore more amenable to analysis than the highly complex and variable behaviour of phylogenetically more advanced animals. Several types of molluscan behaviour have been studied, varying from sequential swimming and digging patterns (Dorsett, Willows & Hoyle, 1969; Mellon, 1969; Trueman, 1967) to simple withdrawal responses (Mellon, 1965; Peretz, 1970; Prior, 1970). In most cases tactile stimulation has been employed to elicit the behavioural responses under study. Tactile stimuli applied to the skin of a mollusc usually evoke muscular contractions near the site of stimulation. These responses can be mediated by local reflex pathways. In their natural habitats molluscs receive a rather steady stream of weak tactile stimuli, due to debris such as stirred-up sand and floating weeds. The responses to such stimuli need be no more than siphon closure or local contractions. Thus, responses mediated by local reflexes constitute a fine level of behavioural activity capable of simple defensive manoeuvres such as siphon closure or gill withdrawal. In molluscs sequential behavioural responses to tactile stimulation frequently involve activity mediated by both peripheral and central nervous (CNS) systems. To decide what portion of such a response is due to the CNS and what portion due to peripheral neural elements is often difficult. This difficulty must be dealt with because the failure to include local reflexes in an analysis of a behavioural sequence would constitute the deletion of a component of the neural organization involved. One method of approaching the problem is to first define and analyse the effects of local and central reflexes separately and then study their combined effects on the behaviour. In the siphon closure and withdrawal responses of the surf clam, Spisula, the activity due to local reflexes can be isolated from the activity mediated by the CNS, thus presenting a good opportunity for studying the role of local reflexes in a behavioural sequence. Local reflexes are activated by an intensity of tactile stimulation lower than that necessary to evoke siphon retraction (which is centrally mediated). This simple behavioural discrimination between weak and medium stimulus intensities enables the animal to respond minimally to lesser stimuli and more defensively to threatening stimuli, thereby preventing activation of the complete process of siphon retraction in response to every tactile input. When the stimulus is strong (i.e. a jab