Medial geniculate, amygdalar and cingulate cortical training-induced neuronal activity during discriminative avoidance learning in rabbits with auditory cortical lesions

The Neural Correlates of Individual Differences in Reinforcement Learning during Pain Avoidance and Reward Seeking

Organisms learn to gain reward and avoid punishment through action-outcome associations. Reinforcement learning (RL) offers a critical framework to understand individual differences in this associative learning by assessing learning rate, action bias, pavlovian factor (i.e., the extent to which action values are influenced by stimulus values), and subjective impact of outcomes (i.e., motivation to seek reward and avoid punishment). Nevertheless, how these individual-level metrics are represented in the brain remains unclear. The current study leveraged fMRI in healthy humans and a probabilistic learning go/no-go task to characterize the neural correlates involved in learning to seek reward and avoid pain. Behaviorally, participants showed a higher learning rate during pain avoidance relative to reward seeking. Additionally, the subjective impact of outcomes was greater for reward trials and associated with lower response randomness. Our imaging findings showed that individual differences in learning rate and performance accuracy during avoidance learning were positively associated with activities of the dorsal anterior cingulate cortex, midcingulate cortex, and postcentral gyrus. In contrast, the pavlovian factor was represented in the precentral gyrus and superior frontal gyrus (SFG) during pain avoidance and reward seeking, respectively. Individual variation of the subjective impact of outcomes was positively predicted by activation of the left posterior cingulate cortex. Finally, action bias was represented by the supplementary motor area (SMA) and pre-SMA whereas the SFG played a role in restraining this action tendency. Together, these findings highlight for the first time the neural substrates of individual differences in the computational processes during RL.

Dual-Factor Representation of the Environmental Context in the Retrosplenial Cortex

The retrosplenial cortex (RSC) is thought to be involved in a variety of spatial and contextual memory processes. However, we do not know how contextual information might be encoded in the RSC or whether the RSC representations may be distinct from context representations seen in other brain regions such as the hippocampus. We recorded RSC neuronal responses while rats explored different environments and discovered 2 kinds of context representations: one involving a novel rate code in which neurons reliably fire at a higher rate in the preferred context regardless of spatial location, and a second involving context-dependent spatial firing patterns similar to those seen in the hippocampus. This suggests that the RSC employs a unique dual-factor representational mechanism to support contextual memory.

Psychophysical and neural correlates of noised-induced tinnitus in animals: Intra- and inter-auditory and non-auditory brain structure studies

Tinnitus, a ringing in the ear or head without an external sound source, is a prevalent health problem. It is often associated with a number of limbic-associated disorders such as anxiety, sleep disturbance, and emotional distress. Thus, to investigate tinnitus, it is important to consider both auditory and non-auditory brain structures. This paper summarizes the psychophysical, immunocytochemical and electrophysiological evidence found in rats or hamsters with behavioral evidence of tinnitus. Behaviorally, we tested for tinnitus using a conditioned suppression/avoidance paradigm, gap detection acoustic reflex behavioral paradigm, and our newly developed conditioned licking suppression paradigm. Our new tinnitus behavioral paradigm requires relatively short baseline training, examines frequency specification of tinnitus perception, and achieves sensitive tinnitus testing at an individual level. To test for tinnitus-related anxiety and cognitive impairment, we used the elevated plus maze and Morris water maze. Our results showed that not all animals with tinnitus demonstrate anxiety and cognitive impairment. Immunocytochemically, we found that animals with tinnitus manifested increased Fos-like immunoreactivity (FLI) in both auditory and non-auditory structures. The manner in which FLI appeared suggests that lower brainstem structures may be involved in acute tinnitus whereas the midbrain and cortex are involved in more chronic tinnitus. Meanwhile, animals with tinnitus also manifested increased FLI in non-auditory brain structures that are involved in autonomic reactions, stress, arousal and attention. Electrophysiologically, we found that rats with tinnitus developed increased spontaneous firing in the auditory cortex (AC) and amygdala (AMG), as well as intra- and inter-AC and AMG neurosynchrony, which demonstrate that tinnitus may be actively produced and maintained by the interactions between the AC and AMG.

Nicotinic acetylcholine receptors in rat forebrain that bind ¹⁸F-nifene: relating PET imaging, autoradiography, and behavior

Nicotinic acetylcholine receptors (nAChRs) in the brain are important for cognitive function; however, their specific role in relevant brain regions remains unclear. In this study, we used the novel compound ¹⁸F-nifene to examine the distribution of nAChRs in the rat forebrain, and for individual animals related the results to behavioral performance on an auditory-cognitive task. We first show negligible binding of ¹⁸F-nifene in mice lacking the β2 nAChR subunit, consistent with previous findings that ¹⁸F-nifene binds to α4β2* nAChRs. We then examined the distribution of ¹⁸F-nifene in rat using three methods: in vivo PET, ex vivo PET and autoradiography. Generally, ¹⁸F-nifene labeled forebrain regions known to contain nAChRs, and the three methods produced similar relative binding among regions. Importantly, ¹⁸F-nifene also labeled some white matter (myelinated axon) tracts, most prominently in the temporal subcortical region that contains the auditory thalamocortical pathway. Finally, we related ¹⁸F-nifene binding in several forebrain regions to each animal's performance on an auditory-cued, active avoidance task. The strongest correlations with performance after 14 days training were found for ¹⁸F-nifene binding in the temporal subcortical white matter, subiculum, and medial frontal cortex (correlation coefficients, r > 0.8); there was no correlation with binding in the auditory thalamus or auditory cortex. These findings suggest that individual performance is linked to nicotinic functions in specific brain regions, and further support a role for nAChRs in sensory-cognitive function.

Differential involvement of the anterior cingulate and primary sensorimotor cortices in sensory and affective functions of pain

The present study examined the role of neurons in different pain-related functions of the anterior cingulate cortex (ACC) and primary sensorimotor cortex (SmI) by assessing their abilities to code different levels of noxious heat and activity changes evoked by classical fear conditioning involving electric shocks. Multiple single-unit activity was recorded with microwires implanted in the SmI and ACC of each rat. In the first set of experiments, the middle segment of the tail in each rat was irradiated with laser-heat pulses of various intensities. Neuronal responses in both the SmI and ACC increased with the intensity of the laser heat, although there was a significantly higher percentage of intensity-related units in the SmI. Furthermore, the stimulus-response curve of SmI ensemble activity had a steeper slope than that of the ACC. In the second set of experiments, rats were trained and tested on a conditioned fear-potentiated startle task in which a light was paired with an electric shock and, later, the startle response was elicited by a burst of noise in the presence or absence of light. A higher percentage of ACC units changed their neuronal responses to the conditioned stimulus after the light-shock pairing and the average activity change was also significantly stronger. Our results suggest that SmI neurons are better at coding laser-heat intensity than ACC neurons, whereas more ACC neurons are involved in conditioned fear associated with an electric shock than SmI neurons. These data provide evidence for differential contributions of the SmI and ACC to sensory and affective dimensions of pain.

Nicotinic modulation of tone-evoked responses in auditory cortex reflects the strength of prior auditory learning

Nicotinic acetylcholine receptors (nAChRs) contribute to sensory-cognitive function, as demonstrated by evidence that nAChR activation enhances, and nAChR blockade impairs, neural processing of sensory stimuli and sensory-cognitive behavior. To better understand the relationship between nAChR function and behavior, here we compare the strength of nAChR-mediated physiology in individual animals to their prior auditory behavioral performance. Adult rats were trained on an auditory-cued, active avoidance task over 4 days and classified as "good," "intermediate" or "poor" performers based on their initial rate of learning and eventual level of performance. Animals were then anesthetized, and tone-evoked local field potentials (LFPs) recorded in layer 4 of auditory cortex (ACx) before and after a test dose of nicotine (0.7mg/kg, s.c.) or saline. In "good" performers, nicotine enhanced LFP amplitude and decreased response threshold to characteristic frequency (CF) stimuli, yet had opposite effects (decreased amplitude, increased threshold) on responses to spectrally distant stimuli; i.e., cortical receptive fields became more selective for CF stimuli. In contrast, nicotine had little effect on LFP amplitude in "intermediate" or "poor" performing animals. Nicotine did, however, reduce LFP onset latency in all three groups, indicating that all received an effective dose of the drug. Our findings suggest that nicotinic regulation of cortical receptive fields may be a distinguishing feature of the best-performing animals, and may facilitate sensory-related learning by enhancing receptive field selectivity.

On the interdependence of cognition and emotion

Affect and cognition have long been treated as independent entities, but in the current review we suggest that affect and cognition are in fact highly interdependent. We open the article by discussing three classic views for the independence of affect. These are (i) the affective independence hypothesis, that emotion is processed independently from cognition, (ii) the affective primacy hypothesis, that evaluative processing precedes semantic processing, and (iii) the affective automaticity hypothesis, that affectively potent stimuli commandeer attention and evaluation is automatic. We argue that affect is not independent from cognition, that affect is not primary to cognition, nor is affect automatically elicited. The second half of the paper discusses several instances of how affect influences cognition. We review experiments showing affective involvement in perception, semantic activation, and attitude activation. We conclude that one function of affect is to regulate cognitive processing.

Neonatal nicotine exposure impairs nicotinic enhancement of central auditory processing and auditory learning in adult rats

Children of women who smoke cigarettes during pregnancy display cognitive deficits in the auditory-verbal domain. Clinical studies have implicated developmental exposure to nicotine, the main psychoactive ingredient of tobacco, as a probable cause of subsequent auditory deficits. To test for a causal link, we have developed an animal model to determine how neonatal nicotine exposure affects adult auditory function. In adult control rats, nicotine administered systemically (0.7 mg/kg, s.c.) enhanced the sensitivity to sound of neural responses recorded in primary auditory cortex. The effect was strongest in cortical layers 3 and 4, where there is a dense concentration of nicotinic acetylcholine receptors (nAChRs) that has been hypothesized to regulate thalamocortical inputs. In support of the hypothesis, microinjection into layer 4 of the nonspecific nAChR antagonist mecamylamine (10 microM) strongly reduced sound-evoked responses. In contrast to the effects of acute nicotine and mecamylamine in adult control animals, neither drug was as effective in adult animals that had been treated with 5 days of chronic nicotine exposure (CNE) shortly after birth. Neonatal CNE also impaired performance on an auditory-cued active avoidance task, while having little effect on basic auditory or motor functions. Thus, neonatal CNE impairs nicotinic regulation of cortical function, and auditory learning, in the adult. Our results provide evidence that developmental nicotine exposure is responsible for auditory-cognitive deficits in the offspring of women who smoke during pregnancy, and suggest a potential underlying mechanism, namely diminished function of cortical nAChRs.

Semantic Processing Precedes Affect Retrieval: The Neurological Case for Cognitive Primacy in Visual Processing

According to the affective primacy hypothesis, visual stimuli can be evaluated prior to and independent of object identification and semantic analysis (Zajonc, 1980, 2000 ). Our review concludes that the affective primacy hypothesis is, from the available evidence, not likely correct. Although people can react to objects that they cannot consciously identify, such affective reactions are dependent upon prior semantic analysis within the visual cortex. The authors propose that the features of objects must first be integrated, and then the objects themselves must be categorized and identified, all prior to affective analysis. Additionally, the authors offer a preliminary neurological analysis of the mere exposure and affective priming effects that is consistent with the claim that semantic analysis is needed to elicit these effects. In sum, the authors conclude that the brain must know what something is in order to know whether it is good or bad.

Opposite effects of tetanic stimulation of the auditory thalamus or auditory cortex on the acoustic startle reflex in awake rats

The amygdala mediates both emotional learning and fear potentiation of startle. The lateral amygdala nucleus (LA) receives auditory inputs from both the auditory thalamus (medial geniculate nucleus; MGN) and auditory association cortex (AAC), and is critical for auditory fear conditioning. The central amygdala nucleus, which has intra-amygdaloid connections with LA, enhances startle magnitude via midbrain connections to the startle circuits. Tetanic stimulation of either MGN or AAC in vitro or in vivo can induce long-term potentiation in LA. In the present study, behavioural consequences of tetanization of these auditory afferents were investigated in awake rats. The acoustic startle reflex of rats was enhanced by tetanic stimulation of MGN, but suppressed by that of AAC. All the tetanization-induced changes of startle diminished within 24 h. Blockade of GABAB receptors in the LA area reversed the suppressive effect of tetanic stimulation of AAC on startle but did not change the enhancing effect of tetanic stimulation of MGN. Moreover, transient electrical stimulation of MGN enhanced the acoustic startle reflex when it lagged behind acoustic stimulation, but inhibited the acoustic startle reflex when it preceded acoustic stimulation. The results of the present study indicate that MGN and AAC afferents to LA play different roles in emotional modulation of startle, and AAC afferents are more influenced by inhibitory GABAB transmission in LA.

Effects of conditioning during amygdalar inactivation on training-induced neuronal plasticity in the medial geniculate nucleus and cingulate cortex in rabbits (Oryctolagus cuniculus)

This study addressed the amygdala's role in avoidance conditioning in rabbits (Oryctolagus cuniculus). Intra-amygdalar muscimol infusion before 60 or 120 conditioning trials blocked training-induced neuronal activity (TIA) in the medial geniculate (MG) nucleus. One hundred twenty trials with muscimol blocked TIA permanently, during conditioning with muscimol and then later without muscimol; 60 trials with muscimol blocked TIA only when muscimol was present. Cingulate cortical TIA was blocked only when muscimol was present. Behavioral learning did not occur with muscimol, but later learning was facilitated (i.e., savings occurred) in rabbits initially given muscimol plus training. These results define the time period wherein amygdalar processes initiate TIA in the MG nucleus and suggest that distinct forms of amygdalar processes induce TIA in the MG nucleus and cingulate cortex.

Potentiation of amygdaloid and hippocampal auditory-evoked potentials in a discriminatory fear-conditioning task in mice as a function of tone pattern and context

According to the local memory storage hypothesis, information about the tone-shock association in an auditory fear-conditioning paradigm is stored in synapses within the lateral amygdala. Thus, fear-conditioning-induced potentiation of auditory-evoked potentials in response to a conditioned stimulus (CS+, a series of short lasting tones; patterned tone) has been interpreted as an in vivo correlate of amygdaloid synaptic plasticity. Here, we re-examine the specificity of potentiation of auditory-evoked potentials in terms of (i) local confinement to the lateral amygdala, (ii) parameters of CS+ and (iii) influence of context, using a discriminatory fear-conditioning paradigm. Adult male C57BL/6J mice were implanted with recording electrodes aimed at the lateral amygdala, the CA1 region of the hippocampus and the neck muscles for simultaneous recordings of auditory-evoked potentials and startle responses. In a neutral context, auditory-evoked potentials within lateral amygdala and CA1 as well as startle and freezing responses to the CS+ were significantly potentiated following conditioning, as compared with pre-conditioning values and responses to a neutral stimulus (CSn; tone of different frequency). Potentiation was only evident if CS+ was presented as a uniform series but not if presented mixed with CSn. Accordingly, mice failed to show intensified freezing to a patterned tone if a single lasting tone of the same frequency served as CS+. Both CA1 and lateral amygdala auditory-evoked potentials were potentiated in response to CSn if presented in the conditioning context. These findings demonstrate that (i) potentiation of auditory-evoked potentials is not restricted to the lateral amygdala, (ii) both tone frequency and pattern of tone presentation are essential for proper CS+ recognition and (iii) contextual memory leads to a general potentiation of auditory-evoked potentials.

Long-term potentiation in freely moving rats reveals asymmetries in thalamic and cortical inputs to the lateral amygdala

Long-term memory underlying Pavlovian fear conditioning is believed to involve plasticity at sensory input synapses in the lateral nucleus of the amygdala (LA). A useful physiological model for studying synaptic plasticity is long-term potentiation (LTP). LTP in the LA has been studied only in vitro or in anaesthetized rats. Here, we tested whether LTP can be induced in auditory input pathways to the LA in awake rats, and if so, whether it persists over days. In chronically implanted rats, extracellular field potentials evoked in the LA by stimulation of the auditory thalamus and the auditory association cortex, using test simulations and input/output (I/O) curves, were compared in the same animals after tetanization of either pathway alone or after combined tetanization. For both pathways, LTP was input-specific and long lasting. LTP at cortical inputs exhibited the largest change at early time points (24 h) but faded within 3 days. In contrast, LTP at thalamic inputs, though smaller initially than cortical LTP, remained stable until at least 6 days. Comparisons of I/O curves indicated that the two pathways may rely on different mechanisms for the maintenance of LTP and may benefit differently from their coactivation. This is the first report of LTP at sensory inputs to the LA in awake animals. The results reveal important characteristics of synaptic plasticity in neuronal circuits of fear memory that could not have been revealed with in vitro preparations, and suggest a differential role of thalamic and cortical auditory afferents in long-term memory of fear conditioning.

Consideration of a unified model of amygdalar associative functions

This paper considers issues arising from presentations by Barry Everitt and Gorica Petrovich concerning amygdalar involvement in appetitive conditioning. Studies reported by these researchers are beginning to unravel the distribution of associative functions among amygdalar nuclei and their efferent targets. The main issue addressed here concerns how to integrate the information from studies of appetitive conditioning with conclusions from studies of other forms of associative learning, including pavlovian fear conditioning and instrumental conditioning. Most students of this issue will agree that no prima facie integration is readily apparent in current scholarship. Although we do not here provide the definitive integration, we do attempt to take some preliminary, broad-brush steps towards that goal.

Appetitive conditioning-induced plasticity is expressed during paradoxical sleep in the medial geniculate, but not in the lateral amygdala

This study examined whether neurons in the medial division of the medial geniculate (MGm) and the dorsal part of the lateral amygdala (LAd) express learning-induced plasticity in paradoxical sleep (PS) after appetitive conditioning, as they do in PS after fear conditioning. Rats received tone-food pairings in 3 sessions. After each session, the tone was presented at a nonawakening intensity during PS. Multiunit activity was simultaneously recorded in MGm and LAd. During waking, increases in tone-evoked discharges developed in MGm and LAd; however, as training continued, they lessened in LAd, but not in MGm. During PS, conditioned tone responses were expressed in MGm, but not in LAd. Thus, these results demonstrate dissociation of MGm and LAd plasticity. Moreover, compared with fear conditioning results, they suggest that expression of amygdalar plasticity in PS depends on the emotional salience of the stimulus.

Limbic thalamic lesions, appetitively motivated discrimination learning, and training-induced neuronal activity in rabbits

A substantial literature implicates the anterior and mediodorsal (limbic) thalamic nuclei and the reciprocally interconnected areas of cingulate cortex in learning, memory, and attentional processes. Previous studies have shown that limbic thalamic lesions severely impair discriminative avoidance learning and that they block development of training-induced neuronal activity in the cingulate cortex. The present study investigated the possibility that the limbic thalamus and cingulate cortex are involved in reward-based discriminative approach learning, wherein head-extension responses yielding oral contact with a drinking spout that was inserted into the conditioning chamber after a positive conditional stimulus (CS+) were reinforced with a water reward but responses to the spout after a negative conditional stimulus (CS-) were not reinforced. In this task, the rabbits learned primarily to omit their prepotent responses to the spout on CS- trials. Acquisition was severely impaired in rabbits given limbic thalamic lesions before training. As during avoidance learning, posterior cingulate cortical neurons of control rabbits developed learning-related neuronal responses to task-relevant stimuli, but this activity was severely attenuated in rabbits with lesions. These results support a general involvement of the cingulothalamic circuitry in instrumental approach and avoidance learning. The fact that learning consisted of response omission indicated that the cingulothalamic role is not limited to acquisition or production of active behavioral responses, such as locomotion. It is proposed that cingulothalamic neurons mediate associative attention, wherein enhanced neuronal responses to stimuli associated with reinforcement facilitate the selection and production of task-relevant responses.