Delayed recall of fear extinction in rats with lesions of ventral medial prefrontal cortex

Extinction of auditory fear conditioning is thought to form a new memory. We previously found that rats with vmPFC lesions could extinguish fear to the tone within a session, but showed no recall of extinction 24 h later. One interpretation is that the vmPFC is the sole storage site of extinction memory. However, it is also possible that lesioned rats were unable to retrieve extinction memory stored in other structures. To determine if a latent extinction memory could be retrieved with additional training, we repeated the experiment but added an additional 5 d of extinction reminder trials. Replicating our previous findings, vmPFC-lesioned rats extinguished normally on day 1, but showed no recall of extinction on day 2. Over the next 5 d, however, lesioned rats showed significant savings in their rate of re-extinction. Thus, the vmPFC is not the only site where extinction memory is stored. Nevertheless, lesioned rats receiving only two extinction trials per day required twice as many days to initiate extinction as controls. Although recall of extinction is possible without the vmPFC, it is significantly delayed. We suggest that the vmPFC accelerates extinction by permitting access to recently learned extinction trials, thereby maximizing behavioral flexibility.     

Social and emotional functioning following bilateral and unilateral neurosurgical prefrontal cortex lesions

Alterations in emotional and social functioning such as impaired ability to recognize emotions in others, a lack of empathy and poor insight have commonly been reported following prefrontal cortex damage. This study sought to investigate the subtleties of such difficulties in 34 individuals with discrete unilateral and bilateral neurosurgical lesions encroaching on the orbitofrontal, medial, and dorsolateral regions of the prefrontal cortex. A specifically devised self‐ and informant report measure, the social‐emotional questionnaire was used to examine five factors of functioning: emotion recognition; empathy; social conformity; antisocial behaviour; and sociability. There were some specific significant differences between the clinical and control groups' informant‐ratings in certain domains of social and emotional functioning. Individuals with damage involving the orbitofrontal region were reported to display elevated levels of antisocial behaviour. Individuals with bilateral orbitofrontal lesions were rated as showing significantly reduced social and emotional functioning in comparison with individuals with unilateral lesions and controls. In addition, individuals with bilateral lesions had significantly less insight overall regarding their social‐emotional abilities. The right unilateral lesion group showed significantly less insight into their abilities to recognize emotion in others in comparison with the left unilateral group. In conclusion, these results suggest that specific social‐emotional and insight deficits may form separate constellations of impairment. The findings also indicate that marked changes in social and emotional functioning are more likely following bilateral damage, and unilateral lesions do not inevitably lead to impairments.     

Role of pituitary-adrenocortical system in mediating avoidance behavior of rats with septal lesions.

Three experiments examined the hypothesis that the effects of septal lesions and systemic injections of scopolamine on avoidance acquisition could be attributed to the effects of either of these treatments on adrenocorticotropic hormone (ACTH) secretion. Septal lesions and scopolamine facilitated 2-way conditioned avoidance response acquisition, and the lesions retarded passive avoidance acquisition. However, neither the injections of dexamethasone, a synthetic glucocorticoid which inhibited ACTH secretion as did septal lesions, nor injections of ACTH which mimicked the facilitatory effects of scopolamine on basal ACTH secretion, affected avoidance in these paradigms. Thus, the main hypothesis was not supported. The finding that scopolamine did not affect passive avoidance indicates that a cholinergic system may not be involved in mediating the suppressive effects of punishment.     

Hierarchical models of behavior and prefrontal function

The recognition of hierarchical structure in human behavior was one of the founding insights of the cognitive revolution. Despite decades of research, however, the computational mechanisms underlying hierarchically organized behavior are still not fully understood. Recent findings from behavioral and neuroscientific research have fueled a resurgence of interest in the problem, inspiring a new generation of computational models. In addition to developing some classic proposals, these models also break fresh ground, teasing apart different forms of hierarchical structure, placing a new focus on the issue of learning and addressing recent findings concerning the representation of behavioral hierarchies within the prefrontal cortex. In addition to offering explanations for some key aspects of behavior and functional neuroanatomy, the latest models also pose new questions for empirical research.     

Orbital prefrontal cortex and guidance of instrumental behavior of rats by visuospatial stimuli predicting reward magnitude

The orbital prefrontal cortex (OPFC) is part of a circuitry mediating the perception of reward and the initiation of adaptive behavioral responses. We investigated whether the OPFC is involved in guidance of the speed of instrumental behavior by visuospatial stimuli predictive of different reward magnitudes. Unoperated rats, sham-lesioned rats, and rats with bilateral lesions of the OPFC by N-methyl-D-aspartate (NMDA) were trained in a visuospatial discrimination task. The task required a lever press on the illuminated lever of two available to obtain a food reward. Different reward magnitudes were permanently assigned to lever presses to respective sides of the operant chamber; that is, responses to one lever (e.g., the left one) were always rewarded with one pellet and responses to the other lever with five pellets. On each trial, the position of the illuminated lever was pseudorandomly determined in advance. Results revealed that OPFC lesions did not impair acquisition of the task, as the speed of conditioned responses was significantly shorter with expectancy of a high reward magnitude. In addition, during reversal, shift and reshift of lever position–reward magnitude contingencies and under extinction conditions, performance of the OPFC-lesioned and control groups did not differ. It is concluded that the OPFC in rats might not be critical for adapting behavioral responses to changes of stimulus–reward magnitude contingencies signaled by visuospatial cues.     

Persistent depression of rearing behavior in rats after extensive septal lesions.

Extensive septal lesions produced a persistent (76 days) reduction in rearing behavior which was not directly attributable to altered deprivation states, emotionality, activity levels, or competing behaviors and which occurred in familiar as well as various novel testing environments. Changes in visual, olfactory, or conspecific stimuli ameliorated or had no effect on the lesion effect. The results indicate that the septal area is importantly involved in normal rearing behavior and that lesion-induced alterations in responsiveness to some forms of stimulation may play a modulatory role in reduced rearing. It is also suggested that the septal area may constitute a portion of a neural system responsible for the initiation of this form of response.