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Information coding in the rodent prefrontal cortex. II. Ensemble activity in orbitofrontal cortex
Abstract and Figures
1. Neural activity was recorded from the orbitofrontal cortex (OF) of rats performing an eight-odor discrimination task that included predictable associations between particular odor pairs. A modified linear discriminant analysis was employed to characterize the population response in each trial of the task as a point in an N-dimensional activity space with the firing rate of each cell in the population represented on one of the N dimensions. The ability of the ensemble to discriminate among conditions of a variable was reflected in the tendency of population responses to cluster together in this activity space for repetitions of a given condition. We assessed coding of several variables describing the period of odor sampling, focusing on aspects of current, past, and future events reflected in single-neuron firing patterns, in ensembles composed of 22-138 cells active during the period when the rats sampled the discriminative stimulus in each trial. 2. OF ensembles performed well at discriminating variables with relevance to task demands represented in single-neuron firing patterns, specifically the physical attributes and assigned reward contingency of the current odor as well as the expectation of reward in the following trial that could be inferred from the predictable associations between particular pairs of odors. OF ensembles were able to correctly identify the identity and assigned reward contingency of the current odor in up to 52% (chance = 12.5%) and 99% (chance = 50%) of all trials, respectively, such that the observed behavioral performance required a population of 5,364 odor-responsive cells in the case of odor identity and only 40 cells in the case of valence. Expectations regarding upcoming rewards based on both assigned response contingency and associations between particular pairs of odors were correctly classified in up to 67% (chance = 20%) of all trials such that the observed level of behavioral performance required a population of 3,169 cells. 3. Other information represented in the single-neuron firing patterns, such as the identity and reward contingency of the preceding odor and specific odor-odor associations, was poorly encoded by OF ensembles. Thus neural ensembles in OF may represent only some of the information reflected in single-neuron activity. Stable coding of only the most useful and relevant information by the ensemble might emerge from the tuning properties of single neurons under the influence of the task at hand, producing in the well-trained animal the observed pattern of broad and diverse coding by single neurons and selective, task-relevant coding by neural ensembles in OF.
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... Here, we have addressed this question by recording both signals in the lateral orbitofrontal cortex (OFC) of rats during olfactory discrimination learning. Activity in the OFC during olfactory learning has been well-studied in humans, [11][12][13][14] nonhuman primates, 15,16 and rats, [17][18][19][20][21] where it has been shown to signal information about both the sensory properties of odor cues and the rewards they predict. Our single-unit results replicated prior findings, whereas the calcium signal provided only a degraded estimate of the information available in the single-unit spiking, reflecting primarily reward value. ...
... We examined neural correlates related to odor identity and reward prediction during odor sampling post-criterion. Consistent with prior unit recording studies, 16,17,19,20,39 OFC spiking represented information about both the reward prediction of the current odor ( Figure 1E), its identity ( Figure 1G), or both ( Figure 1F). Such single-unit correlates have been characterized previously during odor discrimination learning in rats 17,19 and monkeys. ...
... Single neuron analyses All trial numbers were chosen to allow sufficient sample sizes and statistical power. 17,19,20 Neurons were deemed reward modulated if average activity during the 16 positive versus 16 negative trials immediately following criterion was significantly different as indicated by Friedman test where positive/negative odor identity was treated as a nuisance variable. Neurons were considered odor identity modulated if average activity during the two pairs (eight trials each) of positive or negative exemplars was significantly different as indicated by Rank Sum test. ...
Recording action potentials extracellularly during behavior has led to fundamental discoveries regarding neural function—hippocampal neurons respond to locations in space,¹ motor cortex neurons encode movement direction,² and dopamine neurons signal reward prediction errors³—observations undergirding current theories of cognition,⁴ movement,⁵ and learning.⁶ Recently it has become possible to measure calcium flux, an internal cellular signal related to spiking. The ability to image calcium flux in anatomically⁷,⁸ or genetically⁹ identified neurons can extend our knowledge of neural circuit function by allowing activity to be monitored in specific cell types or projections, or in the same neurons across many days. However, while initial studies were grounded in prior unit recording work, it has become fashionable to assume that calcium is identical to spiking, even though the spike-to-fluorescence transformation is nonlinear, noisy, and unpredictable under real-world conditions.¹⁰ It remains an open question whether calcium provides a high-fidelity representation of single-unit activity in awake, behaving subjects. Here, we have addressed this question by recording both signals in the lateral orbitofrontal cortex (OFC) of rats during olfactory discrimination learning. Activity in the OFC during olfactory learning has been well-studied in humans,¹¹,¹²,¹³,¹⁴ nonhuman primates,¹⁵,¹⁶ and rats,¹⁷,¹⁸,¹⁹,²⁰,²¹ where it has been shown to signal information about both the sensory properties of odor cues and the rewards they predict. Our single-unit results replicated prior findings, whereas the calcium signal provided only a degraded estimate of the information available in the single-unit spiking, reflecting primarily reward value.
... In contrast, WWW rats that remember all episodic information recruit larger brain networks, including the same spatial network and brain regions that are functionally connected and implicated in olfactory memory (piriform, lateral entorhinal, orbitofrontal, prelimbic and infralimbic cortices) (Awad et al., 2015;Barkai and Saar, 2001;Carballo-Marquez et al., 2007;Mouly et al., 2022;Persson et al., 2022;Tronel and Sara, 2003). A comparison of the Where and WWW profiles revealed that complete recollection of remote episodic memory recruits a core network of interconnected brain regions (medio-lateral orbitofrontal, ventral CA1 and basolateral amygdala) (Alvarez and Eichenbaum, 2002;Gottfried et al., 2006;Gottfried and Zelano, 2011;Schoenbaum et al., 2003;Schoenbaum and Eichenbaum, 1995;Yang and Wang, 2017) that are implicated in the valence and emotional processing of information and directly correlated with memory accuracy during recall. This novel result provides new perspectives for exploring the critical role of the emotional brain in old nontraumatic episodic memories and the specific role of odours and their value in maintaining memory of personal events Downes, 2002, 2000;Saive et al., 2014b). ...
... In addition, our results showed that the orbitofrontal cortex hosts certain synaptic plasticity mechanisms that correlate with the precision of remote memory; thus, this region is critical for the retrieval of old and complete episodic memory. This finding is consistent with the major role of the orbitofrontal cortex in the processing and relevance of associative olfactory memory (Alvarez and Eichenbaum, 2002;Gottfried et al., 2006;Gottfried and Zelano, 2011;Lesburgueres et al., 2011;Schoenbaum et al., 2003;Schoenbaum and Eichenbaum, 1995) and the idea that odours and their emotional value are crucial for remote episodic memory Downes, 2002, 2000). The involvement of the hippocampus in remote episodic memories in both Where and WWW rats indicated that their respective memories are still associated with their context, regardless of their content. ...
Memories of life episodes are the heart of individual stories. However, modelling episodic memory is a major challenge in both humans and animals when considering all its characteristics. As a consequence, the mechanisms that underlie the storage of old nontraumatic episodic memories remain enigmatic. Here, using a new task in rodents that models human episodic memory including odour/place/context components and applying advances behavioural and computational analyses, we show that rats form and recollect integrated remote episodic memories of two occasionally encountered complex episodes occurring in their daily life. Similar to humans, the information content and accuracy of memories vary across individuals and depend on the emotional relationship with odours experienced during the very first episode. We used cellular brain imaging and functional connectivity analyses, to find out the engrams of remote episodic memories for the first time. Activated brain networks completely reflect the nature and content of episodic memories, with a larger cortico-hippocampal network when the recollection is complete and with an emotional brain network related to odours that is critical in maintaining accurate and vivid memories. The engrams of remote episodic memories remain highly dynamic since synaptic plasticity processes occur during recall related to memory updates and reinforcement.
... In contrast, trial-based procedures for studying DS-control allow isolation of DSs from other stimuli [14,20,23,24] to identify their unique contribution to drug-seeking behavior. Incorporation of many, intermixed, repeated trials also enables investigation of DS-specific neuronal ensemble activity using in vivo electrophysiology or calcium imaging [25][26][27][28][29][30][31]. Based on these considerations, we developed a trial-based procedure in rats that used DS+ and DS− with a common lever manipulandum and no drug infusion-paired cues [21,32], to isolate the unique effects of the DSs beyond that of previous studies [14,23,24]. ...
... The neural mechanisms of trial-based DS control of drug [14,20,23,24] or non-drug [25][26][27][28][29][30][31] reward seeking have not often been studied. The medial prefrontal cortex (mPFC) has been implicated in drug and non-drug reward seeking [33][34][35][36][37][38]. ...
Persistent susceptibility to cue-induced relapse is a cardinal feature of addiction. Discriminative stimuli (DSs) are one type of drug-associated cue that signal drug availability (DS+) or unavailability (DS−) and control drug seeking prior to relapse. We previously established a trial-based procedure in rats to isolate DSs from context, conditioned stimuli, and other drug-associated cues during cocaine self-administration and demonstrated DS-controlled cocaine seeking up to 300 abstinence days. The behavioral and neural mechanisms underlying trial-based DS-control of drug seeking have rarely been investigated. Here we show that following discrimination training in our trial-based procedure, the DS+ and DS− independently control the expression and suppression of cocaine seeking during abstinence. Using microinjections of GABAA + GABAB receptor agonists (muscimol + baclofen) in medial prefrontal cortex, we report that infralimbic, but not prelimbic, subregion of medial prefrontal cortex is critical to persistent DS-controlled relapse to cocaine seeking after prolonged abstinence, but not DS-guided discriminated cocaine seeking or DS-controlled cocaine self-admininstration. Finally, using ex vivo whole-cell recordings from pyramidal neurons in the medial prefrontal cortex, we demonstrate that the disruption of DS-controlled cocaine seeking following infralimbic cortex microinjections of muscimol+baclofen is likely a result of suppression of synaptic transmission in the region via a presynaptic mechanism of action.
... Furthermore, the medial oPFC subregions MO and VO, considered serving as a link between oPFC and mPFC (Hoover and Vertes 2011), also substantially contribute to PFC top-down projections to VTA, LDTg, and MnR. The medial oPFC is first of all characterized by its rich access to multimodal sensory information (Ongur and Price, 2000), with specific ensembles of oPFC cells responding to the affective (reward) value of an stimulus in rats (Schoenbaum andEichenbaum, 1995, van Duuren et al. 2008) and primates (Rolls 2004). Having in mind these considerations, it is conceivable that prefrontal top-down projections from mPFC and oPFC to state-setting mesopontine neuromodulatory nuclei constitute one of the key tracks underlying possible supervisor (switch operator; Miller and Cohen, 2021) functions of the PFC by constantly adjusting inappropriate behaviors and promoting task and cue relevant ones (Groenewegen and Uylings 2000;Ragozzino et al. 1999 ...
Anatomical and functional evidence suggests that the PFC is fairly unique among all cortical regions, as it not only receives input from, but also robustly projects back to mesopontine monoaminergic and cholinergic cell groups. Thus the PFC is in a position to exert a powerful top-down control over several state setting modulatory transmitter systems that are critically involved in the domains of arousal, motivation, reward/aversion, working memory, mood regulation, and stress processing. Regarding this scenario, the origin of cortical afferents to the ventral tegmental area (VTA), laterodorsal tegmental nucleus (LDTg), and median raphe nucleus (MnR) was here compared, by using the retrograde tracer cholera toxin subunit b (CTb). CTb injections into VTA, LDTg, or MnR produced retrograde labeling in the cortical mantle, which was mostly confined to medial, orbital, and lateral PFC subdivisions, along with rostral and mid-cingulate areas. Remarkably, in all of the three groups, retrograde labeling was densest in layer V pyramidal neurons of the infralimbic, prelimbic, medial orbital and frontal polar cortex. Moreover, a conspicuous lambda-shaped region around the apex of the rostral pole of the nucleus accumbens stood consistently out as heavily labeled. At almost all rostrocaudal levels through the PFC analyzed, retrograde labeling was strongest following injections into MnR and weakest following injections into VTA. Altogether, our findings reveal a broadly similar set of prefrontal afferents to VTA, LDTg, and MnR, further supporting an eminent functional role of the PFC as a controller of all major state setting mesopontine modulatory transmitter systems.
... This analysis is not feasible given the design of the tasks analyzed here, but similar analysis have been done on the neural activity from other cortical regions (e.g. Schoenbaum & Eichenbaum, 1995b, 1995aBernacchia, Seo, Lee, & Wang, 2011;Morcos & Harvey, 2016). ...
Neurons in the hippocampus fire in consistent sequence over the timescale of seconds during the delay period of some memory experiments. For longer timescales, the firing of hippocampal neurons also changes slowly over minutes within experimental sessions. It was thought that these slow dynamics are caused by stochastic drift or a continuous change in the representation of the episode, rather than consistent sequences unfolding over minutes. This paper studies the consistency of contextual drift in three chronic calcium imaging recordings from the hippocampus CA1 region in mice. Computational measures of consistency show reliable sequences within experimental trials at the scale of seconds as one would expect from time cells or place cells during the trial, as well as across experimental trials on the scale of minutes within a recording session. Consistent sequences in the hippocampus are observed over a wide range of time scales, from seconds to minutes. The hippocampal activity could reflect a scale‐invariant spatiotemporal context as suggested by theories of memory from cognitive psychology.
... During olfactory learning, rodents rapidly categorize odors into appetitive or aversive stimuli (Schoenbaum et al. 1998(Schoenbaum et al. , 1999Martin et al. 2004;Chapuis et al. 2009), a process that is underpinned by associative learning (Schoenbaum et al. 1998;Martin et al. 2004;Calu et al. 2007;Roesch et al. 2007;Chapuis et al. 2009). Odor categorization is supported by the orbitofrontal cortex (Schoenbaum and Eichenbaum 1995;Stalnaker et al. 2014;Qu et al. 2016), but this structure, in turn, receives instruction from subcortical and cortical structures that provide key information about the precise nature of the odor. Here, systems involved in reward and aversive information processing, as well as the integration of sensory modalities, are likely to play a role (Schoenbaum et al. 1999;Schoenbaum and Setlow 2003;Chapuis et al. 2009). ...
The olfactory bulb (OB) delivers sensory information to the piriform cortex (PC) and other components of the olfactory system. OB-PC synapses have been reported to express short-lasting forms of synaptic plasticity, whereas long-term potentiation (LTP) of the anterior PC (aPC) occurs predominantly by activating inputs from the prefrontal cortex. This suggests that brain regions outside the olfactory system may contribute to olfactory information processing and storage. Here, we compared functional magnetic resonance imaging BOLD responses triggered during 20 or 100 Hz stimulation of the OB. We detected BOLD signal increases in the anterior olfactory nucleus (AON), PC and entorhinal cortex, nucleus accumbens, dorsal striatum, ventral diagonal band of Broca, prelimbic–infralimbic cortex (PrL-IL), dorsal medial prefrontal cortex, and basolateral amygdala. Significantly stronger BOLD responses occurred in the PrL-IL, PC, and AON during 100 Hz compared with 20 Hz OB stimulation. LTP in the aPC was concomitantly induced by 100 Hz stimulation. Furthermore, 100 Hz stimulation triggered significant nuclear immediate early gene expression in aPC, AON, and PrL-IL. The involvement of the PrL-IL in this process is consistent with its putative involvement in modulating behavioral responses to odor experience. Furthermore, these results indicate that OB-mediated information storage by the aPC is embedded in a connectome that supports valence evaluation.
... When we analyzed the proportion of units contributing to rate changes following choice behavior, we found that PAE mice recruited significantly more choice responsive neurons in both regions, and to both choice types, even when firing rates were reduced. Together, these results suggest that PAE may have subtly altered reward prediction signaling as OFC firing in response to positive reward expectation generally followed patterns described in non-treated rodents and nonhuman primates, but was reduced during criterion performance when associations were well-learned (Thorpe et al., 1983;Schoenbaum and Eichenbaum, 1995;Schoenbaum et al., 2003). Firing following an incorrect response was also altered after PAE in a region specific manner. ...
Deficits in behavioral flexibility are a hallmark of multiple psychiatric, neurological, and substance use disorders. These deficits are often marked by decreased function of the prefrontal cortex (PFC); however, the genesis of such executive deficits remains understudied. Here we report how the most preventable cause of developmental disability, in utero exposure to alcohol, alters cortico-striatal circuit activity leading to impairments in behavioral flexibility in adulthood. We utilized a translational touch-screen task coupled with in vivo electrophysiology in adult mice to examine single unit and coordinated activity of the lateral orbital frontal cortex (OFC) and dorsolateral striatum (DS) during flexible behavior. Prenatal alcohol exposure (PAE) decreased OFC, and increased DS, single unit activity during reversal learning and altered the number of choice responsive neurons in both regions. PAE also decreased coordinated activity within the OFC and DS as measured by oscillatory field activity and altered spike-field coupling. Furthermore, PAE led to sustained connectivity between regions past what was seen in control animals. These findings suggest that PAE causes altered coordination within and between the OFC and DS, promoting maladaptive perseveration. Our model suggests that in optimally functioning mice OFC disengages the DS and updates the newly changed reward contingency, whereas in PAE animals, aberrant and persistent OFC to DS signaling drives behavioral inflexibility during early reversal sessions. Together, these findings demonstrate how developmental exposure alters circuit-level activity leading to behavioral deficits and suggest a critical role for coordination of neural timing during behaviors requiring executive function.
Memory-guided decision making involves long-range coordination across sensory and cognitive brain networks, with key roles for the hippocampus and prefrontal cortex (PFC). In order to investigate the mechanisms of such coordination, we monitored activity in hippocampus (CA1), PFC, and olfactory bulb (OB) in rats performing an odor-place associative memory guided decision task on a T-maze. During odor sampling, the beta (20-30 Hz) and respiratory (7-8 Hz) rhythms (RR) were prominent across the three regions, with beta and RR coherence between all pairs of regions enhanced during the odor-cued decision making period. Beta phase modulation of phase-locked CA1 and PFC neurons during this period was linked to accurate decisions, with a key role of CA1 interneurons in temporal coordination. Single neurons and ensembles in both CA1 and PFC encoded and predicted animals' upcoming choices, with different cell ensembles engaged during decision-making and decision execution on the maze. Our findings indicate that rhythmic coordination within the hippocampal-prefrontal-olfactory bulb network supports utilization of odor cues for memory-guided decision making.
Anatomical and functional evidence suggests that the PFC is fairly unique among all cortical regions, as it not only receives input from, but also robustly projects back to mesopontine monoaminergic and cholinergic cell groups. Thus, the PFC is in position to exert a powerful top-down control over several state-setting modulatory transmitter systems that are critically involved in the domains of arousal, motivation, reward/aversion, working memory, mood regulation, and stress processing. Regarding this scenario, the origin of cortical afferents to the ventral tegmental area (VTA), laterodorsal tegmental nucleus (LDTg), and median raphe nucleus (MnR) was here compared in rats, using the retrograde tracer cholera toxin subunit b (CTb). CTb injections into VTA, LDTg, or MnR produced retrograde labeling in the cortical mantle, which was mostly confined to frontal polar, medial, orbital, and lateral PFC subdivisions, along with anterior- and mid-cingulate areas. Remarkably, in all of the three groups, retrograde labeling was densest in layer V pyramidal neurons of the infralimbic, prelimbic, medial/ventral orbital and frontal polar cortex. Moreover, a lambda-shaped region around the apex of the rostral pole of the nucleus accumbens stood out as heavily labeled, mainly after injections into the lateral VTA and LDTg. In general, retrograde PFC labeling was strongest following injections into MnR and weakest following injections into VTA. Altogether, our findings reveal a fairly similar set of prefrontal afferents to VTA, LDTg, and MnR, further supporting an eminent functional role of the PFC as a controller of major state-setting mesopontine modulatory transmitter systems.
Associative learning restructures the activity of numerous neurons distributed across cortical and subcortical regions. Individual neurons change the rate or timing of spiking patterns in response to environmental stimuli as they become associated with salient outcomes. Recent large–scale activity monitoring in rodents has uncovered that these learning-related changes occur concertedly across groups of neurons within and between brain regions. These changes yield neuronal representations of learned associations in three types of ensemble dynamics: ensemble firing rates, multineuron coactivity, and sequential activity. Here, I review some of the most robust demonstrations of these dynamics in the rodent neocortex and hippocampus and discuss their potential function in memory encoding, consolidation, and retrieval.
The associative cortex of the frontal lobe, commonly termed the prefrontal cortex, undergoes considerable expansion in the course of evolution. It reaches its greatest development in the human brain, constituting nearly one-third of the neocortex. Its phylogenetic expansion outstrips that of other cortical regions and also that of the mediodorsal (MD) nucleus of the thalamus, a nucleus so heavily and distinctly projecting to the prefrontal cortex that the latter is conventionally defined as the cortical territory of MD projection.
A technique is described for making a sturdy, driveable electrode array of ten fine wires. This moveable array is a modification of a stationary electrode [4]. It has a number of notable advantages over other electrodes designed for recording single-unit activity in freely-moving small mammals. With this electrode many single cells can be recorded in each animal, cells can be held for many days, and recording quality is very good.
How does the brain represent objects in the world? A proportion of cells in the temporal cortex of monkeys responds specifically
to objects, such as faces, but the type of coding used by these cells is not known. Population analysis of two sets of such
cells showed that information is carried at the level of the population and that this information relates, in the anterior
inferotemporal cortex, to the physical properties of face stimuli and, in the superior temporal polysensory area, to other
aspects of the faces, such as their familiarity. There was often sufficient information in small populations of neurons to
identify particular faces. These results suggest that representations of complex stimuli in the higher visual areas may take
the form of a sparse population code.
1. We used the Karhunen-Loève (K-L) transform to quantify the temporal distribution of spikes in the responses of lateral geniculate (LGN) neurons. The basis functions of the K-L transform are a set of waveforms called principal components, which are extracted from the data set. The coefficients of the principal components are uncorrelated with each other and can be used to quantify individual responses. The shapes of each of the first three principal components were very similar across neurons. 2. The coefficient of the first principal component was highly correlated with the spike count, but the other coefficients were not. Thus the coefficient of the first principal component reflects the strength of the response, whereas the coefficients of the other principal components reflect aspects of the temporal distribution of spikes in the response that are uncorrelated with the strength of the response. Statistical analysis revealed that the coefficients of up to 10 principal components were driven by the stimuli. Therefore stimuli govern the temporal distribution as well as the number of spikes in the response. 3. Through the application of information theory, we were able to compare the amount of stimulus-related information carried by LGN neurons when two codes were assumed: first, a univariate code based on response strength alone; and second, a multivariate temporal code based on the coefficients of the first three principal components. We found that LGN neurons were able to transmit an average of 1.5 times as much information using the three-component temporal code as they could using the strength code. 4. The stimulus set we used allowed us to calculate the amount of information each neuron could transmit about stimulus luminance, pattern, and contrast. All neurons transmitted the greatest amount of information about stimulus luminance, but they also transmitted significant amounts of information about stimulus pattern. This pattern information was not a reflection of the luminance or contrast of the pixel centered on the receptive field. 5. In addition to measuring the average amount of information each neuron transmitted about all stimuli, we also measured the amount of information each neuron transmitted about the individual stimuli with both the univariate spike count code and the multivariate temporal code. We then compared the amount of information transmitted per stimulus with the magnitudes of the responses to the individual stimuli. We found that the magnitudes of both the univariate and the multivariate responses to individual stimuli were poorly correlated with the information transmitted about the individual stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)
1. A new method of measuring the performance of neurons in sensory discrimination tasks was developed and then applied to single-neuron responses recorded in the auditory nerve of chinchilla and in the striate visual cortex of cat. 2. Most previous methods of measuring discrimination performance have employed decision rules that involve comparing the total counts of action potentials (spikes) produced by two different stimuli. Such measures ignore response pattern and hence may not reflect all the information transmitted by a neuron. The proposed method attempts to measure all (or most) of the transmitted information by constructing descriptive models of the neuron's response to each stimulus in the discrimination experiment; these descriptive models consist of measured probability distributions of the spike counts in small time bins. The measured probability distributions are then used to define an optimal decision rule (an ideal observer) for discriminating the two stimuli. Finally, discrimination performance is measured by applying this decision rule to novel presentations of the same two stimuli. 3. Intensity and temporal-phase discrimination were measured for three neurons in the auditory nerve of chinchilla. The discrimination stimuli were low-frequency pure tones of 70-ms duration. Intensity thresholds were found to be 5-20 dB lower at low intensities using the new pattern method compared with the traditional counting method. The pattern method led to better performance because it utilized both rate and temporal pattern information. Phase discrimination performance using the counting method was at chance because the average spike rate did not change with phase. On the other hand, using the pattern method, phase discrimination thresholds were found to decrease with intensity, often reaching values equivalent to 30-40 microseconds of temporal offset. These thresholds are as good as or better than behavioral thresholds in chinchilla. 4. Contrast and temporal-phase discrimination were measured for three neurons in the striate visual cortex of cat. The discrimination stimuli were drifting sine-wave gratings of 100- to 160-ms duration. Contrast discrimination functions measured by the pattern method and the counting method were found to be essentially identical. Phase discrimination using the counting method was at chance. However, using the pattern method, phase thresholds were found to decrease with contrast, reaching values equivalent to 7 ms of temporal offset for the two simple cells. 5. Our results suggest that temporal response pattern carries substantial information for intensity and phase discrimination in the auditory nerve and for phase discrimination in the striate visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)
Single-unit activity was recorded in the prefrontal cortex of monkeys performing two visual short-term memory tasks: delayed matching-to-sample and delayed response. The animal had to retain a color (sample) in the first task and a positional cue in the second. Cells reacting to the sample or the cue were found throughout the dorsolateral prefrontal cortex, some cells differentiating colors (red and green) and others positions (right and left). About one-half of all cells showed altered discharge during the retention period (delay). In some of them that discharge differed depending on the sample or cue. One cell type showed descending firing frequency in the course of the delay. Another showed the opposite, that is, increasing firing as the delay progressed. Cells with altered delay-period discharge were more common in the cortex of the sulcus principalis and the inferior convexity than in other prefrontal areas. The findings are viewed as reflecting the participation of prefrontal neurons in the temporal structuring of behavior.
Neuronal activity in the dorsolateral and ventral prefrontal cortex was studied during the learning by six monkeys of the performance of a two-color visual discrimination task. The GO/NO-GO task or its reversal consisted of five periods: the Start Lamp Period, Lamp-off Period, Cue Period, Response Period, and ITI Period (Reward and Control Periods). In sum, 271 task-related neurons were studied by means of 545 penetrations. Discharge rates increased or decreased from the rate of ITI or preceding periods of the task. Discharge rate changes were of either the phasic or tonic pattern. During motor learning extending from a poor performance level (50–63% correct), through intermediate (60–83%), to well-trained levels (above 85%), the number of task-related neurons increased in the Cue, Response and ITI Periods, and more task-related neurons showed changes in more than one period. During the learning process, not only do more prefrontal neurons respond phasically to visual stimuli in the Cue and Response Periods or to reward or trial end in ITI, and become activated before lever release, but also they become active tonically in the Cue, Response and ITI Periods.
Neurons of the neocortex differ dramatically in the patterns of action potentials they generate in response to current steps. Regular-spiking cells adapt strongly during maintained stimuli, whereas fast-spiking cells can sustain very high firing frequencies with little or no adaptation. Intrinsically bursting cells generate clusters of spikes (bursts), either singly or repetitively. These physiological distinctions have morphological correlates. RS and IB cells can be either pyramidal neurons or spiny stellate cells, and thus constitute the excitatory cells of the cortex. FS cells are smooth or sparsely spiny non-pyramidal cells, and are likely to be GABAergic inhibitory interneurons. The different firing properties of neurons in neocortex contribute significantly to its network behavior.
Single unit activity was recorded from the dorsolateral prefrontal cortex of two monkeys which were trained on a stimulus-reward association task. The monkeys were trained on a reaction time task overlapped with a classical conditioning paradigm. The sequential events of the task were as follows: (1) lever pressing to start the trial; (2) presentation of a visual cue for 1 s; (3) delay period of 1 s; (4) imperative stimulus presentation; and (5) release of the lever by the animal. The visual cue signaled whether or not a drop of fruit juice would be given (its associative significance) for the animal's release response instead of signaling what response the animal should perform (its behavioral significance). In this task, the animal had to release the lever even on the trial where no juice was given in order to advance to the next trial. A total of 423 units showed activity changes in relation to one or more of the task events, such as the cue presentation, delay, release response and reward delivery. Among 313 units which showed cue-related activity changes, 179 units showed differential activity in relation to the different cues. A majority of them (Type M; n = 120) showed activity changes in relation to whether the cue indicated juice delivery or not, independent of its physical properties. The activity of 13 units (Type P) was related to the physical properties of the stimulus, and the activity of the remaining 46 units (Type MP) appeared to be related to both aspects of the stimulus. Sustained activity changes during the delay period were observed in 68 Type M, in 3 Type P and in 24 Type MP units. The results suggest that the prefrontal cortex plays important roles in the stimulus-reward association and that prefrontal units are involved in higher order information processing, extracting and retaining the "associative significance" of the stimulus independent of its physical properties.