Daniel Bor

Encoding Strategies Dissociate Prefrontal Activity from Working Memory Demand

It is often proposed that prefrontal cortex is important in organization and control of working memory contents. In some cases, effective reorganization can decrease task difficulty, implying a dissociation between frontal activity and basic memory demand. In a spatial working memory task, we studied the improvement of performance that occurs when materials can be reorganized into higher level groups or chunks. Structured sequences, encouraging reorganization and chunking, were compared with unstructured sequences. Though structured sequences were easier to remember, event-related functional magnetic resonance imaging (fMRI) showed increased activation of lateral frontal cortex, in particular during memory encoding. The results show that, even when memory demand decreases, organization of working memory contents into higher level chunks is associated with increased prefrontal activity.

When thoughts become action: an fMRI paradigm to study volitional brain activity in non-communicative brain injured patients.

The assessment of voluntary behavior in non-communicative brain injured patients is often challenging due to the existence of profound motor impairment. In the absence of a full understanding of the neural correlates of consciousness, even a normal activation in response to passive sensory stimulation cannot be considered as proof of the presence of awareness in these patients. In contrast, predicted activation in response to the instruction to perform a mental imagery task would provide evidence of voluntary task-dependent brain activity, and hence of consciousness, in non-communicative patients. However, no data yet exist to indicate which imagery instructions would yield reliable single subject activation. The aim of the present study was to establish such a paradigm in healthy volunteers. Two exploratory experiments evaluated the reproducibility of individual brain activation elicited by four distinct mental imagery tasks. The two most robust mental imagery tasks were found to be spatial navigation and motor imagery. In a third experiment, where these two tasks were directly compared, differentiation of each task from one another and from rest periods was assessed blindly using a priori criteria and was correct for every volunteer. The spatial navigation and motor imagery tasks described here permit the identification of volitional brain activation at the single subject level, without a motor response. Volunteer as well as patient data [Owen, A.M., Coleman, M.R., Boly, M., Davis, M.H., Laureys, S., Pickard J.D., 2006. Detecting awareness in the vegetative state. Science 313, 1402] strongly suggest that this paradigm may provide a method for assessing the presence of volitional brain activity, and thus of consciousness, in non-communicative brain-injured patients.

Prefrontal cortical involvement in verbal encoding strategies

The lateral prefrontal cortex is critical for the control and organization of information in working memory. In certain situations, effective reorganization can attenuate task difficulty, suggesting a dissociation between lateral prefrontal activity and basic memory demand. In a verbal working memory task, we investigated the enhancement of performance that occurs when incoming information can be reorganized into higher‐level groups or chunks. In the fMRI scanner, volunteers heard and repeated a sequence of digits. Mathematically structured sequences, encouraging ‘chunking’, were compared with unstructured, random sequences. Though structured sequences were easier to remember, fMRI showed increased lateral prefrontal activation for these sequences. Specifically, both the dorsolateral and ventrolateral prefrontal cortices were activated preferentially for the structured sequences during encoding. When visual stimuli that can be chunked using spatial structure are used, similar results are observed. These results demonstrate that cognitively less demanding tasks may elicit greater lateral prefrontal recruitment. Thus, the lateral prefrontal cortex appears to play a general role in strategically recoding information from memory, in order to optimize performance.

Detecting residual cognitive function in persistent vegetative state

Despite converging agreement about the definition of persistent vegetative state, recent reports have raised concerns about the accuracy of diagnosis in some patients, and the extent to which, in a selection of cases, residual cognitive functions may remain undetected. Objective assessment of residual cognitive function can be extremely difficult as motor responses may be minimal, inconsistent, and difficult to document in many patients, or may be undetectable in others because no cognitive output is possible. Here we describe strategies for using H 2 15 O positron emission tomography activation studies to study covert cognitive processing in patients with a clinical diagnosis of persistent vegetative state. Three cases are described in detail. Of these, two exhibited clear and predicted regional cerebral blood flow responses during well-documented activation paradigms (face recognition and speech perception) which have been shown to produce specific, robust and reproducible activation patterns in normal volunteers. Some months after scanning, both patients made a significant recovery. In a third case, blood flow data were acquired during a speech perception task, although methodological difficulties precluded any systematic interpretation of the results. In spite of the multiple logistic and procedural problems involved, these results have major clinical and scientific implications and provide a strong basis for the systematic study of possible residual cognitive function in patients diagnosed as being in a persistent vegetative state.

Multi-voxel coding of stimuli, rules, and responses in human frontoparietal cortex.

In human functional magnetic resonance imaging (fMRI), a characteristic pattern of frontal and parietal activity is produced by many different cognitive demands. Although frontoparietal cortex has been shown to represent a variety of task features in different contexts, little is known about detailed representation of different task features within and across different regions. We used multi-voxel pattern analysis (MVPA) of human fMRI data to assess the representational content of frontoparietal cortex in a simple stimulus-response task. Stimulus-response mapping rule was the most strongly represented task feature, significantly coded in a lateral frontal region surrounding the inferior frontal sulcus, a more ventral region including the anterior insula/frontal operculum, and the intraparietal sulcus. Next strongest was coding of the instruction cue (screen color) indicating which rule should be applied. Coding of individual stimuli and responses was weaker, approaching significance in a subset of regions. In line with recent single unit data, the results show a broad representation of task-relevant information across human frontoparietal cortex, with strong representation of a general rule or cognitive context, and weaker coding of individual stimulus/response instances.

Frontal lobe involvement in spatial span: Converging studies of normal and impaired function

Although monkey lesion studies involving the prefrontal cortex commonly report working memory deficits, and neuroimaging studies consistently show prefrontal involvement in such tasks, patients with damage to this region commonly fail to show any working memory impairment. Such a discrepancy may be due to insensitive testing measures for patients, as well as small, yet critical differences between working memory tasks in imaging and patient studies. The current study utilised a more sensitive measure of spatial working memory spans, based either on structured or unstructured spatial arrays. A PET study in normal subjects confirmed that both variants did indeed activate prefrontal cortex. The same tasks were given to frontal lobe patients and closely matched controls. Patients with large frontal lesions were significantly impaired on this task, with those patients with damage to the right dorsolateral prefrontal cortex appearing particularly impaired. This result demonstrates that prefrontal cortex is necessary for normal working memory, even in simple tasks, such as spatial span. It is suggested, however, that the patient deficit reflects strategic or goal-based dysfunction, rather than storage limitations.

Consciousness and the prefrontal parietal network: insights from attention, working memory, and chunking.

Consciousness has of late become a “hot topic” in neuroscience. Empirical work has centered on identifying potential neural correlates of consciousness (NCCs), with a converging view that the prefrontal parietal network (PPN) is closely associated with this process. Theoretical work has primarily sought to explain how informational properties of this cortical network could account for phenomenal properties of consciousness. However, both empirical and theoretical research has given less focus to the psychological features that may account for the NCCs. The PPN has also been heavily linked with cognitive processes, such as attention. We describe how this literature is under-appreciated in consciousness science, in part due to the increasingly entrenched assumption of a strong dissociation between attention and consciousness. We argue instead that there is more common ground between attention and consciousness than is usually emphasized: although objects can under certain circumstances be attended to in the absence of conscious access, attention as a content selection and boosting mechanism is an important and necessary aspect of consciousness. Like attention, working memory and executive control involve the interlinking of multiple mental objects and have also been closely associated with the PPN. We propose that this set of cognitive functions, in concert with attention, make up the core psychological components of consciousness. One related process, chunking, exploits logical or mnemonic redundancies in a dataset so that it can be recoded and a given task optimized. Chunking has been shown to activate PPN particularly robustly, even compared with other cognitively demanding tasks, such as working memory or mental arithmetic. It is therefore possible that chunking, as a tool to detect useful patterns within an integrated set of intensely processed (attended) information, has a central role to play in consciousness. Following on from this, we suggest that a key evolutionary purpose of consciousness may be to provide innovative solutions to complex or novel problems.

Savant Memory for Digits in a Case of Synaesthesia and Asperger Syndrome is Related to Hyperactivity in the Lateral Prefrontal Cortex

Single case: DT is a savant with exceptional abilities in numerical memory and mathematical calculations. DT also has an elaborate form of synaesthesia for visually presented digits. Further more, DT also has Asperger syndrome (AS). We carried out two preliminary investigations to establish whether these conditions may contribute to his savant abilities. Neuroimaging: In an fMRI digit span study, DT showed hyperactivity in lateral prefrontal cortex when encoding digits, compared with controls. In addition, while controls showed raised lateral prefrontal activation in response to structured (compared to unstructured) sequences of digits, DTs neural activity did not differ between these two conditions. In addition, controls showed a significant performance advantage for structured, compared with unstructured sequences whereas no such pattern was found for DT. We suggest that this performance pattern reflects that DT focuses less on external mathematical structure, since for him all digit sequences have internal structure linked to his synaesthesia. Finally, DT did not activate extra-striate regions normally associated with synaesthesia, suggesting that he has an unusual and more abstract and conceptual form of synaesthesia. This appears to generate structured, highly-chunked content that enhances encoding of digits and aids both recall and calculation. Neuropsychology: People with AS preferentially attend to local features of stimuli. To test this in DT, we administered the Navon task. Relative to controls, DT was faster at finding a target at the local level, and was less distracted by interference from the global level. Discussion: The propensity to focus on local detail, in concert with a form of synaesthesia that provides structure to all digits, may account for DTs exceptional numerical memory and calculation ability. This neural and cognitive pattern needs to be tested in a series of similar cases, and with more constrained control groups, to confirm the significance of this association.

A potential spatial working memory training task to improve both episodic memory and fluid intelligence.

One current challenge in cognitive training is to create a training regime that benefits multiple cognitive domains, including episodic memory, without relying on a large battery of tasks, which can be time-consuming and difficult to learn. By giving careful consideration to the neural correlates underlying episodic and working memory, we devised a computerized working memory training task in which neurologically healthy participants were required to monitor and detect repetitions in two streams of spatial information (spatial location and scene identity) presented simultaneously (i.e. a dual n-back paradigm). Participants’ episodic memory abilities were assessed before and after training using two object and scene recognition memory tasks incorporating memory confidence judgments. Furthermore, to determine the generalizability of the effects of training, we also assessed fluid intelligence using a matrix reasoning task. By examining the difference between pre- and post-training performance (i.e. gain scores), we found that the trainers, compared to non-trainers, exhibited a significant improvement in fluid intelligence after 20 days. Interestingly, pre-training fluid intelligence performance, but not training task improvement, was a significant predictor of post-training fluid intelligence improvement, with lower pre-training fluid intelligence associated with greater post-training gain. Crucially, trainers who improved the most on the training task also showed an improvement in recognition memory as captured by d-prime scores and estimates of recollection and familiarity memory. Training task improvement was a significant predictor of gains in recognition and familiarity memory performance, with greater training improvement leading to more marked gains. In contrast, lower pre-training recollection memory scores, and not training task improvement, led to greater recollection memory performance after training. Our findings demonstrate that practice on a single working memory task can potentially improve aspects of both episodic memory and fluid intelligence, and that an extensive training regime with multiple tasks may not be necessary.

Automated post-hoc noise cancellation tool for audio recordings acquired in an MRI scanner

There are several types of experiment in which it is useful to have subjects speak overtly in a magnetic resonance imaging (MRI) scanner, including those studying the articulatory apparatus and the neural basis of speech production, and fMRI experiments in which speech is used as a response modality. Although it is relatively easy to record sound from the bore, it can be difficult to hear the speech over the very loud acoustic noise from the scanner. This is particularly a problem during echo‐planar imaging, which is usually used for fMRI. We present a post‐hoc sound cancellation algorithm, and describe a Windows‐based tool that implements it. The tool is fast and operates with minimal user intervention. We evaluate cancellation performance in terms of the improvement in signal‐to‐noise ratio, and investigate the effect of the recording medium. A substantial improvement in audibility was obtained. Hum. Brain Mapping 24:299–304, 2005.