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On the basis of his knowledge of the patterns of evolutionary development of the olfactory system and other forebrain structures, Herrick' proposed that the olfactory cortex served an activating function for "all cortical activities." In recent years, without addressing Herrick's proposal directly, several investigators have provided evidence that is pertinent to it. In an extensive review of this topic, Wenzell concluded that a number of behaviors are altered by a greatly diminished olfactory input and that this effect is not readily explicable in terms of perceptual loss alone. One of the principal themes emerging from this review is the apparent lack of influence on the cognitive or problem-solving aspect of behavior, as compared with the motivational or affective aspect. Regardless of the species being studied, after reducing or removing olfactory input it is typical to find significant changes occurring in such characteristics as reactivity to handling, aggression, and shock-avoidance learning. By contrast, relatively little effect is reported for such tasks as lever pressing with different schedules of food reinforcement or complex visual discriminations reinforced by food.Because so much of this work has been done with rodents, a group of animals for which olfactory signals are of notable importance, it is difficult to determine the degree to which reduced olfactory input represents a loss of specific information rather than a more general effect. For this reason, we have chosen to work exclusively with the pigeon so that the contaminating influence of the cue value of olfaction can be minimized. Although good evidence exists3 that pigeons rely to some extent on olfactory cues for guidance in homing, no other segment of their behavior has as yet been shown to depend directly on odorous information. In addition to this fundamental advantage, the pigeon also presents a distinct anatomical advantage in the ease with which the olfactory nerve fibers can be completely sectioned as compared with the rat, for example, in which such an operation is much more problematic. In a series of reports beginning in 1968,4-6 several types of behavior in the pigeon were shown to be affected by bilateral removal of the olfactory bulbs or bilateral section of the olfactory nerves. No effect on the behaviors in question was observed following the control operation of unilateral removal of a superficial piece of the hyperstriatum dorsale approximately equal in size to that of the olfactory bulbs.

Albino rats were given either habenular lesions or control operations and evaluated postsurgically during one-way active avoidance and passive avoidance training. Rats with habenular lesions, particularly those with damage in the posterior aspects of the habenular complex, were significantly impaired during acquisition of the active avo idance task. No significant differences existed between the two groups on either of two passive avoidance measures. The results are discussed in conjunction with the findings of other research, and hypotheses of either reduced fearfulness or impairment in motor inhibition mechanisms are suggested as frameworks for future research.The functional significance of the habenular nucleus as a major waystation for reciprocal limbic midbrain connections (Herrick, 1948 ;Nauta , 1958Nauta , , 1960 has been emphasized by recent studies finding changes in a variety of avoidance behaviors following habenular lesions. These effects on avoidance behaviors are still controversial, since different investigato rs have cited both deficits and increased efficiency in both active and passive avoidance learning paradigms.Nielson and his associates (Davis, Stevenson , McIver, & Nielson, 1966 ; found that rats with habenular lesions learned a step-down passive avoidance response faster than did controls, but were impaired in the acquisition of a conditioned active avoidance response to shock in a T-maze. On the other hand , Van Hoesen and his coworkers (Van Hoesen, MacDougall, & Mitchell , 1969 ;Wilson, Mitchell, & Van Hoesen, 1972) reported that habenular lesions impaired food-related passive avoidance , while enhancing two-way shuttle box avoidance perform ance. Large medial thalamic lesions, which included damage to the habenula, have been found to severely impair one-way active avoidance acquisition (Delacour, ' 1971 ;Vanderwolf, 1967Vanderwolf, , 1971, although Delacour (1971) stated that lesions limited to the habenular complex did not result in impairment in this task. Nevertheless, Tigner (1972) has reported impairment in one-way avoidance learning after lesions restricted to either the habenular nucleus or the dorsornedial thalamus.Such discrepancies are not uncommon in the behavioral literature for several reasons, including the difficulty of replicating brain damage with the current techniques for making lesions, the changes in apparatus and shock levels from one study to another, and the variability in the responses required of the S in the different behavioral situations. The present study was undertaken with the intention of rrururruzmg these confounds by investigating the effects of habenular lesions in the same animals on two types of avoidance responding , one-way active avoidance and passive avoidance, in the same apparatus. METHOD Subjects and SurgeryThirty-four Holtzman albino rats , approximately 135 days old and weighing 265-330 g at the time of testing, were used as Ss. All the animals were maintained on ad lib Purina Lab Chow and 15 min of water per day . The rat s were assigne...

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