Dendritic spines serve as preferential sites of excitatory synaptic connections and are pleomorphic. To address the structure-function relationship of the dendritic spines, we used two-photon uncaging of glutamate to allow mapping of functional glutamate receptors at the level of the single synapse. Our analyses of the spines of CA1 pyramidal neurons reveal that AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptors are abundant (up to 150/ spine) in mushroom spines but sparsely distributed in thin spines and filopodia. The latter may be serving as the structural substrates of the silent synapses that have been proposed to play roles in development and plasticity of synaptic transmission. Our data indicate that distribution of functional AMPA receptors is tightly correlated with spine geometry and that receptor activity is independently regulated at the level of single spines.Most excitatory synaptic transmission in the mammalian central nervous system relies on glutamate as a neurotransmitter, and postsynaptic glutamate receptors are central in both the acquisition and maintenance of memory 1, 2 . Substantial biochemical evidence indicates that glutamate receptors in postsynaptic densities (PSDs) are regulated by various protein machineries that link the receptors to the cytoskeleton 3,4,5 and that control the insertion [6][7][8][9][10] and phosphorylation of the receptors 11,12 . Individual dendritic spines have been thought to act as functional compartments of glutamate receptor expression, given that they are physically and thus metabolically separated from the body of the dendrite by the narrow spine neck [13][14][15][16] . Indeed, the Hebbian principle of learning as well as most theories of neuronal networks assume that the strength of synaptic connections is subject to independent control 17 . Moreover, spine geometry has been proposed to be a key determinant of synaptic function and memory in the brain 13, 14, 18-22 . Correspondence to: Haruo Kasai. These various hypotheses have not been tested experimentally, however, because it is not possible to systematically investigate postsynaptic glutamate sensitivities at the level of the individual spine by classical electrophysiological approaches 16,23 . Also, the number of functional AMPA-sensitive glutamate receptors in individual spines has not been estimated directly. Two-photon excitation of caged-glutamate compounds may overcome these difficulties 24 as a result of the inherent three-dimensional resolution of neurotransmitter application associated with this technique. However, caged-glutamate compounds with a cross-section for two-photon absorption, a rate of photolysis, and a stability in aqueous solution sufficient for such studies have not previously been described [25][26][27] . NIH Public AccessWe developed a caged-glutamate compound and microscopic system for two-photon excitation that allowed systematic investigation of functional glutamate receptors at the level of the individual synapse. Our experiment...

Most of our long-term memories of episodes or objects are organized so that we can retrieve them by association. Clinical neuropsychologists assess human memory by the paired-associate learning test, in which a series of paired words or figures is presented and the subject is then asked to retrieve the other pair member associated with each cue. Patients with lesions of the temporal lobe show marked impairment in this test. In our study, we trained monkeys in a pair-association task using a set of computer-generated paired patterns. We found two types of task-related neurons in the anterior temporal cortex. One type selectively responded to both pictures of the paired associates. The other type, which had the strongest response to one picture during the cue presentation, exhibited increasing activity during the delay period when the associate of that picture was used as a cue. These results provide new evidence that single neurons acquire selectivity for visual patterns through associative learning. They also indicate neural mechanisms for storage and retrieval in the long-term memory of paired associates.

Inhibition of an ongoing reaction tendency for adaptation to changing environments is a major function of the human prefrontal cortex. This function has been investigated frequently using the go/no-go task and set-shifting tasks such as the Wisconsin Card Sorting Test (WCST). Studies in humans and monkeys suggest the involvement of the dorsolateral prefrontal cortex in the two task paradigms. However, it remains unknown where in the dorsolateral prefrontal cortex this function is localized, whether a common inhibitory mechanism is used in these task paradigms and how this inhibitory function acts on two different targets, i.e. the go response in the go/no-go task and the cognitive set in the WCST. In the go/no-go task of this study, subjects were instructed to either respond (go trial) or not respond (no-go trial), depending on the cue stimulus presented. The signals of functional MRI (fMRI) related to the inhibitory function should be transient by nature. Thus, we used the temporal resolution of fMRI (event-related fMRI) by which transient signals in go and no-go trials can be analysed separately and compared with each other. We found a focus that showed transient no-go dominant activity in the posterior part of the inferior frontal sulcus in the right hemisphere. This was true irrespective of whether the subjects used their right or left hands. These results suggest that the transient activation in the right inferior prefrontal area is related to the neural mechanism underlying the response inhibition function. Furthermore, this area was found to be overlapped spatially with the area that was activated transiently during cognitive set shifting in the WCST. The transient signals in the go/no-go task peaked 5 s after the transient expression of the inhibitory function, and the transient signals in the WCST peaked 7s after the transient expression, reflecting different durations of neuronal activity in the two inhibitory task paradigms. These results imply that the right inferior prefrontal area is commonly involved in the inhibition of different targets, i.e. the go response during performance of the go/no-go task and the cognitive set during performance of the WCST.

In human long-term memory, ideas and concepts become associated in the learning process. No neuronal correlate for this cognitive function has so far been described, except that memory traces are thought to be localized in the cerebral cortex; the temporal lobe has been assigned as the site for visual experience because electric stimulation of this area results in imagery recall and lesions produce deficits in visual recognition of objects. We previously reported that in the anterior ventral temporal cortex of monkeys, individual neurons have a sustained activity that is highly selective for a few of the 100 coloured fractal patterns used in a visual working-memory task. Here I report the development of this selectivity through repeated trials involving the working memory. The few patterns for which a neuron was conjointly selective were frequently related to each other through stimulus-stimulus association imposed during training. The results indicate that the selectivity acquired by these cells represents a neuronal correlate of the associative long-term memory of pictures.

It has been proposed that visual-memory traces are located in the temporal lobes of the cerebral cortex, as electric stimulation of this area in humans results in recall of imagery. Lesions in this area also affect recognition of an object after a delay in both humans and monkeys, indicating a role in short-term memory of images. Single-unit recordings from the temporal cortex have shown that some neurons continue to fire when one of two or four colours are to be remembered temporarily. But neuronal responses selective to specific complex objects, including hands and faces, cease soon after the offset of stimulus presentation. These results led to the question of whether any of these neurons could serve the memory of complex objects. We report here a group of shape-selective neurons in an anterior ventral part of the temporal cortex of monkeys that exhibited sustained activity during the delay period of a visual short-term memory task. The activity was highly selective for the pictorial information to be memorized and was independent of the physical attributes such as size, orientation, colour or position of the object. These observations show that the delay activity represents the short-term memory of the categorized percept of a picture.

Knowledge or experience is voluntarily recalled from memory by reactivation of the neural representations in the cerebral association cortex. In inferior temporal cortex, which serves as the storehouse of visual long-term memory, activation of mnemonic engrams through electric stimulation results in imagery recall in humans, and neurons can be dynamically activated by the necessity for memory recall in monkeys. Neuropsychological studies and previous split-brain experiments predicted that prefrontal cortex exerts executive control upon inferior temporal cortex in memory retrieval; however, no neuronal correlate of this process has ever been detected. Here we show evidence of the top-down signal from prefrontal cortex. In the absence of bottom-up visual inputs, single inferior temporal neurons were activated by the top-down signal, which conveyed information on semantic categorization imposed by visual stimulus-stimulus association. Behavioural performance was severely impaired with loss of the top-down signal. Control experiments confirmed that the signal was transmitted not through a subcortical but through a fronto-temporal cortical pathway. Thus, feedback projections from prefrontal cortex to the posterior association cortex appear to serve the executive control of voluntary recall.

Inhibition of inappropriate responses is an essential executive function needed for adaptation to changing environments. In stop-signal tasks, which are often used to investigate response inhibition, subjects make "go" responses while they prepare to stop at a suddenly given "stop" signal. However, the preparatory processes ongoing before response inhibition have rarely been investigated, and it remains unclear how the preparation contributes to response inhibition. In the present study, a stop-signal task was designed so that the extent of the preparation could be estimated using behavioral and neuroimaging measures. Specifically, in addition to the conventional go trials where preparation to stop was required ("uncertain-go" trials), another type of go trial was introduced where a stop-signal was never given and such preparation was unnecessary ("certain-go" trials). An index reflecting the "preparation cost" was then calculated by subtracting the reaction times in the certain-go trials from those in the uncertain-go trials. It was revealed that the stop signal reaction time, a common index used to evaluate the efficiency of response inhibition, decreased as the preparation cost increased, indicating greater preparation supports more efficient inhibition. In addition, imaging data showed that response inhibition recruited frontoparietal regions (the contrast "stop vs uncertain-go") and that preparation recruited most of the inhibition-related frontoparietal regions (the contrast "uncertain-go vs certain-go"). It was also revealed that the inhibition-related activity declined as the preparation cost increased. These behavioral and imaging results suggest preparation makes a complementary contribution to response inhibition during performance of a stop-signal task.

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