Search results
Found 12470 matches for
Non-invasive stimulation in the social brain: the methodological challenges.
Use of non-invasive brain stimulation methods (NIBS) has become a common approach to study social processing in addition to behavioural, imaging and lesion studies. However, research using NIBS to investigate social processing faces challenges. Overcoming these is important to allow valid and reliable interpretation of findings in neurotypical cohorts, but also to allow us to tailor NIBS protocols to atypical groups with social difficulties. In this review, we consider the utility of brain stimulation as a technique to study and modulate social processing. We also discuss challenges that face researchers using NIBS to study social processing in neurotypical adults with a view to highlighting potential solutions. Finally, we discuss additional challenges that face researchers using NIBS to study and modulate social processing in atypical groups. These are important to consider given that NIBS protocols are rarely tailored to atypical groups before use. Instead, many rely on protocols designed for neurotypical adults despite differences in brain function that are likely to impact response to NIBS.
The role of prefrontal cortex in object-in-place learning in monkeys.
Previous ablation studies in monkeys suggest that prefrontal cortex is involved in a wide range of learning and memory tasks. However, monkeys with crossed unilateral lesions of frontal and temporal cortex are unimpaired at concurrent object-reward association learning but are impaired at conditional learning and the implementation of memory-based performance rules. We trained seven monkeys preoperatively on an associative learning task that required them to associate objects embedded in unique complex scenes with reward. Three monkeys then had crossed unilateral lesions of frontal and inferior temporal cortex and the remaining monkeys had bilateral prefrontal cortex ablation. Both groups were severely impaired postoperatively. These results show that both bilateral prefrontal cortex ablation and frontal-temporal disconnection impair associative learning for objects embedded in scenes. The results provide evidence that the function of frontal-temporal interactions in memory is not limited to conditional learning tasks and memory-dependent performance rules. We propose that rapid object-in-place learning requires the interaction of frontal cortex with inferotemporal cortex because visual object and contextual information which is captured over multiple saccades must be processed as a unique complex event that is extended in time. The present results suggest a role for frontal-temporal interaction in the integration of visual information over time.
Long-term visuospatial retention unaffected by fornix transection.
As part of an earlier experiment (Kwok and Buckley, 2009), six macaque monkeys (three with fornix transection and three unoperated controls) were trained postoperatively to discriminate a total of 104 new concurrent visuospatial conditional problems to criterion. Our experiment measured and compared long-term retention of these problems with two separate one-trial postoperative retention tests administered 3 and 15 months, respectively, after acquisition. All animals showed some degree of forgetting of these problems but all remembered above chance levels, even after 15 months. The amount forgotten by each group did not differ significantly at either time point. These results show that long-term retention of visuospatial information is independent of the fornix. Similarities in resistance to forgetting are drawn between fornix-transected macaques and patients with amnesia and the implications for clinical rehabilitation are discussed.
The representation of abstract task rules in the human prefrontal cortex.
We have previously reported sustained activation in the ventral prefrontal cortex while participants prepared to perform 1 of 2 tasks as instructed. But there are studies that have reported activation reflecting task rules elsewhere in prefrontal cortex, and this is true in particular when it was left to the participants to decide which rule to obey. The aim of the present experiment was to use functional magnetic resonance imaging (fMRI) to find whether there was activation in common, irrespective of the way that the task rules were established. On each trial, we presented a word after a variable delay, and participants had to decide either whether the word was abstract or concrete or whether it had 2 syllables. The participants either decided before the delay which task they would perform or were instructed by written cues. Comparing the self-generated with the instructed trials, there was early task set activation during the delay in the middle frontal gyrus. On the other hand, a conjunction analysis revealed sustained activation in the ventral prefrontal and polar cortex for both conditions. We argue that the ventral prefrontal cortex is specialized for handling conditional rules regardless of how the task rules were established.
Monkeys can associate visual stimuli with reward delayed by 1 s even after perirhinal cortex ablation, uncinate fascicle section or amygdalectomy.
In the present experiment monkeys learned concurrent associations of two-dimensional objects (presented on a computer screen) with delayed reward. Hypothetical mechanisms of associative memory, such as long-term potentiation (LTP), required coincidental activation of two population of neurons: one representing the object and the other signalling the reward. In monkeys neurons in area TE of temporal cortex show object-specific activity during object presentation but only fraction of those neurons remain active after stimulus offset. In a delayed reward condition the majority of object-specific neurons in TE cease firing before reward is given and can be detected. In the present study the rate of learning with 1000 ms delay of reward was no slower than learning with immediate reward. This indicates that information about the object is somehow retained across the delay, possibly somewhere outside TE. In the present study we tested that assumption. Area TE projects to the perirhinal cortex and, via uncinate fascicle, to the prefrontal cortex. In our hands, ablations of perirhinal cortex or disconnection of prefrontal cortex from TE (by transection of uncinate fascicle) did not impair learning with delayed reward. Ablation of amygdala, a structure involved in reward-learning, slowed down learning equally with and without delay. We conclude that retaining information about the visually perceived objects across a delay does not exclusively depend upon integrity of perirhinal cortex, or uncinate fascicle, or amygdala. Parallel involvement of those structures remains a possibility and establishment of the role of residual activity of TE neurons requires further neurophysiological investigation.
The role of the perirhinal cortex and hippocampus in learning, memory, and perception.
One traditional and long-held view of medial temporal lobe (MTL) function is that it contains a system of structures that are exclusively involved in memory, and that the extent of memory loss following MTL damage is simply related to the amount of MTL damage sustained. Indeed, human patients with extensive MTL damage are typically profoundly amnesic whereas patients with less extensive brain lesions centred upon the hippocampus typically exhibit only moderately severe anterograde amnesia. Accordingly, the latter observations have elevated the hippocampus to a particularly prominent position within the purported MTL memory system. This article reviews recent lesion studies in macaque monkeys in which the behavioural effects of more highly circumscribed lesions (than those observed to occur in human patients with MTL lesions) to different subregions of the MTL have been examined. These studies have reported new findings that contradict this concept of a MTL memory system. First, the MTL is not exclusively involved in mnemonic processes; some MTL structures, most notably the perirhinal cortex, also contribute to perception. Second, there are some forms of memory, including recognition memory, that are not always affected by selective hippocampal lesions. Third, the data support the idea that regional functional specializations exist within the MTL. For example, the macaque perirhinal cortex appears to be specialized for processing object identity whereas the hippocampus may be specialized for processing spatial and temporal relationships.
Fornix transection selectively impairs fast learning of conditional visuospatial discriminations.
As the fornix has previously been implicated in the rapid learning of associations, we hypothesized that fornix transection in macaques would selectively impair the acquisition of rapidly learned conditional visuospatial discrimination problems. Macaque monkeys learned, postoperatively, three sets of concurrent problems of increasing sizes containing 8, 32, and 64 problems, respectively. Each problem consisted of four identical visual stimuli and animals had to learn which stimulus position was rewarded. The lesioned animals made significantly more errors-to-criterion on the smallest set of problems, consistent with the idea that the most rapidly acquired sets would be more vulnerable to fornical damage. Moreover, during the early stages of acquisition across all three sets, fornix transection selectively impeded monkeys' abilities to eliminate nonperseverative errors in correction trials, consistent with an inability to monitor or correct erroneous spatial responses made further back in time than the last trial. Both one-trial learning and an errorless learning (facilitation of performance) were observed in control and fornix lesioned animals but neither were fornix-dependent and overcoming the deleterious effect upon subsequent learning of having made prior errors was also unaffected by fornix transection. The data indicate that the fornix is not important for all forms of new learning; rather it is selectively concerned with the relatively rapid acquisition of spatial and temporal relationships between stimuli and responses.
Perirhinal cortical contributions to object perception.
The traditional theory of the medial temporal lobe (MTL) memory system asserts that the primate MTL (hippocampus, perirhinal, entorhinal and parahippocampal cortices) is exclusively involved in consolidating declarative memories. However, several recent reports have directly challenged this dogma by arguing that MTL structures also contribute to perception. Controversy remains as many of the behavioural tasks used have confounded memory with perception. We review the evidence here and highlight new studies in humans and macaques that indicate a perceptual role for MTL in the absence of such confounds. We argue that the challenge to MTL memory system theory is substantiated and that the implications are considerable, namely that most psychologists and neuroscientists have held a fundamentally flawed view of how memory is implemented in the brain.
Conflict-induced behavioural adjustment: a clue to the executive functions of the prefrontal cortex.
The behavioural adjustment that follows the experience of conflict has been extensively studied in humans, leading to influential models of executive-control adjustment. Recent studies have revealed striking similarities in conflict-induced behavioural adjustment between humans and monkeys, indicating that monkeys can provide a model to study the underlying neural substrates and mechanisms of such behaviour. These studies have advanced our knowledge about the role of different prefrontal brain regions, including the anterior cingulate cortex (ACC) and the dorsolateral prefrontal cortex (DLPFC), in executive-control adjustment and suggest a pivotal role for the DLPFC in the dynamic tuning of executive control and, consequently, in behavioural adaptation to changing environments.
Learning and transfer of object-reward associations and the role of the perirhinal cortex.
Perirhinal cortex ablation has previously been shown only to impair new postoperative object discrimination learning with large stimulus set sizes (> or = 40 problems). In this study, 3 cynomolgus monkeys (Macaca fascicularis) with bilateral perirhinal cortex ablations were impaired relative to 3 normal controls on concurrent discrimination learning tasks with only 10 problems with the objects presented in different orientations in each trial to increase the demands placed on object identification. This supports the hypothesis that perirhinal cortex damage impairs the ability to identify multiple individual objects. Fewer errors were made to digitized images of objects than toward real objects. Both groups subsequently transferred specific object-reward associations from real objects to digitized images of the respective objects and vice versa, providing evidence that cynomolgus monkeys can recognize photographic representations of objects with experience.
Functional double dissociation between two inferior temporal cortical areas: perirhinal cortex versus middle temporal gyrus.
There is both anatomic and cytoarchitectural evidence for dorsal-ventral subdivisions of the inferior temporal cortex. Despite this, there has been only limited evidence of corresponding functional subdivisions and no evidence that two adjacent cortical areas within the inferior temporal cortex, namely area TE and the perirhinal cortex, have distinctly different roles in vision and memory. We assessed the color discrimination abilities of cynomolgus monkeys with either bilateral ablation of the perirhinal cortex or bilateral ablation of the middle temporal gyrus. The stimuli were isoluminant colored squares presented on a touch screen. In each trial the subject had to learn to discriminate and select the correct choice (green) from among a maximum of eight other foils, each varying in either hue or saturation. Relative to unoperated controls, monkeys with middle temporal gyrus lesions were severely impaired in the color discrimination task, whereas monkeys with perirhinal lesions were unimpaired on this task. We also assessed the visual recognition abilities, as measured by a basic delayed nonmatching-to-sample task with trial-unique objects presented in a Wisconsin General Test Apparatus, of rhesus monkeys with bilateral middle temporal gyrus lesions. We then tested the monkeys' postoperative performance on a delayed nonmatching-to-sample task with delays and extended list lengths. The results from this experiment were compared with those from two other groups of rhesus monkeys, an unoperated control group and a group with bilateral perirhinal cortex lesions, both of which had performed the identical tasks in a previous experiment. Relative to unoperated controls, monkeys with perirhinal cortex lesions were severely impaired both in relearning the basic delayed nonmatching-to-sample task and on the postoperative performance test. In contrast, monkeys with middle temporal gyrus lesions were only mildly affected in relearning the basic nonmatching task and were unimpaired on the postoperative performance test. Thus our data demonstrate a clear functional double dissociation between the perirhinal cortex and the middle temporal gyrus. This result gives strong support to the hypothesis that the perirhinal cortex and the adjacent area TE have distinctly different roles in visual learning and memory.
Is top-down control from prefrontal cortex necessary for visual categorization?
The brain mechanisms underlying visual object categorization remain unclear. In this issue of Neuron, Minamimoto and colleagues introduce a novel task that associates each category with a different incentive value, and they demonstrate that it can be learned within a single session even after ablation of the lateral prefrontal cortex.
Fornix transection impairs exploration but not locomotion in ambulatory macaque monkeys.
Prompted by the theoretical prediction that damage to the hippocampal system should abolish exploratory behavior, the present study examined exploratory movements in control monkeys (CON) and monkeys with transection of the fornix (FNX), a major input/output pathway of the hippocampus. CON and FNX monkeys were introduced to a novel octagonal chamber for six daily sessions, each lasting 20 min. Both groups visited, punctuated by stops, the majority of the floor space of the environment in each of the sessions. The exploratory movements of CON and FNX groups were not significantly different on most of the measures taken over six consecutive days. These measures included cumulative distance traveled, number and duration of stops, traveling patterns, and proportion of time spent in each of 12 designated zones of floor space. The high degree of similarity in behavior between CON and FNX groups suggests that an intact hippocampal system is not necessary for the display of normal exploratory movement per se. On the other hand, the CON and FNX groups did behave differently on two measures. First, the CON group exhibited a decrement in distance traversed over consecutive epochs within the first test session, whereas FNX animals did not. Second, on those days in which the chamber was made visually asymmetrical, the CON animals tended to show a predilection for spending proportionally more time within one particular quadrant of the chamber. These observations are consistent with the idea that interrupting normal hippocampal system function by means of fornix transection is detrimental to learning about the spatial layout of environments. We therefore suggest that while monkeys with fornix transection still display intact locomotor and exploratory behavior patterns, their new learning of visuospatial context is impaired.