Gaëlle Desbordes - Academia.edu (original) (raw)
Journal Articles by Gaëlle Desbordes
Current Opinion in Psychology, 2019
Self-related processing pertains to the complex construct of 'self', as studied across varied dis... more Self-related processing pertains to the complex construct of 'self', as studied across varied disciplines such as cognitive science, neuroscience, modern psychology (including clinical and behavioral psychology), and Western and Eastern philosophy. On a theoretical level, most contemporary models propose that mindfulness training impacts self-related processes. In this review, the empirical evidence for this hypothesis is examined and discussed. Overall, very few self-related processes have been measured in randomized controlled trials of mindfulness-based interventions to date, and, of those processes that have been measured, only negative self-rumination improved significantly. The data so far remain inconclusive as to whether mindfulness-based interventions have an impact on other self-related processes. Studies are especially needed on more basic levels of self such as embodiment and sense of agency.
In light of a growing interest in contemplative practices such as meditation, the emerging field ... more In light of a growing interest in contemplative practices such as meditation, the emerging field of contemplative science has been challenged to describe and objectively measure how these practices affect health and well-being. While “mindfulness” itself has been proposed as a measurable outcome of contemplative practices, this concept encompasses multiple components, some of which, as we review here, may be better characterized as equanimity. Equanimity can be defined as an even-minded mental state or dispositional tendency toward all experiences or objects, regardless of their origin or their affective valence (pleasant, unpleasant, or neutral). In this article, we propose that equanimity be used as an outcome measure in contemplative research. We first define and discuss the inter-relationship between mindfulness and equanimity from the perspectives of both classical Buddhism and modern psychology and present existing meditation techniques for cultivating equanimity. We then review psychological, physiological, and neuroimaging methods that have been used to assess equanimity either directly or indirectly. In conclusion, we propose that equanimity captures potentially the most important psychological element in the improvement of well-being, and therefore should be a focus in future research studies.
Citation: Desbordes G and Negi LT (2013) A new era for mind studies: training investigators in bo... more Citation: Desbordes G and Negi LT (2013) A new era for mind studies: training investigators in both scientific and contemplative methods of inquiry. Front. Hum. Neurosci. 7:741.
![Research paper thumbnail of Neuropsychology meets Dzogchen: A review of the current science of meditation from a Buddhist perspective [review of the book Mind, Brain and the Path to Happiness: A Guide to Buddhist Mind Training and the Neuroscience of Meditation, by D. Dorjee]](https://attachments.academia-assets.com/37124625/thumbnails/1.jpg)
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Chronic itch, a highly debilitating condition, has received relatively little attention in the ne... more Chronic itch, a highly debilitating condition, has received relatively little attention in the neuroimaging literature. Recent studies suggest that brain regions supporting itch in chronic itch patients encompass sensorimotor and salience networks, and corticostriatal circuits involved in motor preparation for scratching. However, how these different brain areas interact with one another in the context of itch is still unknown. We acquired BOLD fMRI scans in 14 atopic dermatitis patients to investigate resting-state functional connectivity before and after allergen-induced itch exacerbated the clinical itch perception in these patients. A seed-based analysis revealed decreased functional connectivity from baseline resting state to the evoked-itch state between several itch-related brain regions, particularly the insular and cingulate cortices and basal ganglia, where decreased connectivity was significantly correlated with increased levels of perceived itch. In contrast, evoked itch increased connectivity between key nodes of the frontoparietal control network (superior parietal lobule and dorsolateral prefrontal cortex), where higher increase in connectivity was correlated with a lesser increase in perceived itch, suggesting that greater interaction between nodes of this executive attention network serves to limit itch sensation via enhanced top-down regulation. Overall, our results provide the first evidence of itch-dependent changes in functional connectivity across multiple brain regions.
Natural visual stimuli have highly structured spatial and temporal properties which influence the... more Natural visual stimuli have highly structured spatial and temporal properties which influence the way visual information is encoded in the visual pathway. In response to natural scene stimuli, neurons in the lateral geniculate nucleus (LGN) are temporally precise – on a time scale of 10–25 ms – both within single cells and across cells within a population. This time scale, established by non stimulus-driven elements of neuronal firing, is significantly shorter than that of natural scenes, yet is critical for the neural representation of the spatial and temporal structure of the scene. Here, a generalized linear model (GLM) that combines stimulus-driven elements with spike-history dependence associated with intrinsic cellular dynamics is shown to predict the fine timing precision of LGN responses to natural scene stimuli, the corresponding correlation structure across nearby neurons in the population, and the continuous modulation of spike timing precision and latency across neurons. A single model captured the experimentally observed neural response, across different levels of contrasts and different classes of visual stimuli, through interactions between the stimulus correlation structure and the nonlinearity in spike generation and spike history dependence. Given the sensitivity of the thalamocortical synapse to closely timed spikes and the importance of fine timing precision for the faithful representation of natural scenes, the modulation of thalamic population timing over these time scales is likely important for cortical representations of the dynamic natural visual environment.
The timing of spiking activity across neurons is a fundamental aspect of the neural population co... more The timing of spiking activity across neurons is a fundamental aspect of the neural population code. Individual neurons in the retina, thalamus, and cortex can have very precise and repeatable responses but exhibit degraded temporal precision in response to suboptimal stimuli. To investigate the functional implications for neural populations in natural conditions, we recorded in vivo the simultaneous responses, to movies of natural scenes, of multiple thalamic neurons likely converging to a common neuronal target in primary visual cortex. We show that the response of individual neurons is less precise at lower contrast, but that spike timing precision across neurons is relatively insensitive to global changes in visual contrast. Overall, spike timing precision within and across cells is on the order of 10 ms. Since closely timed spikes are more efficient in inducing a spike in downstream cortical neurons, and since fine temporal precision is necessary to represent the more slowly varying natural environment, we argue that preserving relative spike timing at a ∼10-ms resolution is a crucial property of the neural code entering cortex.
During behavior, rats and other rodents use their facial vibrissae to actively explore surfaces t... more During behavior, rats and other rodents use their facial vibrissae to actively explore surfaces through whisking and head/body movement, resulting in complex sensory inputs that vary over a large range of angular velocities and temporal scales. How these complex sensory inputs manifest in the patterns of cortical firing events that ultimately form the perceptual experience is not well understood. Through single-unit cortical recordings of layer 4 neurons in S1 of the anesthetized rat, we systematically quantified the interactions between instantaneous velocity and timing of vibrissa motion, finding a strong interaction between angular velocity and timing of contacts on the tens of milliseconds time scale. From the quantification of these joint tuning properties, a detailed nonlinear encoding model was formulated that was highly predictive of firing probability and timing characteristics of the sparse cortical representation of complex patterned tactile inputs. Within a Bayesian framework, the encoding model was then used to decode tactile patterns under simple transformations of the stimulus along dimensions of velocity and timing, as a demonstration of the lower bound of the idealized perceptual capabilities of the animal.
During visual fixation, small eye movements keep the retinal image continuously in motion. It is ... more During visual fixation, small eye movements keep the retinal image continuously in motion. It is known that neurons in the visual system are sensitive to the spatiotemporal modulations of luminance resulting from this motion. In this study, we examined the influence of fixational eye movements on the statistics of neural activity in the macaque's retina during the brief intersaccadic periods of natural visual fixation. The responses of parvocellular (P) and magnocellular (M) ganglion cells in different regions of the visual field were modeled while their receptive fields scanned natural images following recorded traces of eye movements. Immediately after the onset of fixation, wide ensembles of coactive ganglion cells extended over several degrees of visual angle, both in the central and peripheral regions of the visual field. Following this initial pattern of activity, the covariance between the responses of pairs of P and M cells and the correlation between the responses of pairs of M cells dropped drastically during the course of fixation. Cell responses were completely uncorrelated by the end of a typical 300-ms fixation. This dynamic spatial decorrelation of retinal activity is a robust phenomenon independent of the specifics of the model. We show that it originates from the interaction of three factors: the statistics of natural scenes, the small amplitudes of fixational eye movements, and the temporal sensitivities of ganglion cells. These results support the hypothesis that fixational eye movements, by shaping the statistics of retinal activity, are an integral component of early visual representations.
Although it is known that images tend to disappear when they are stabilized on the retina for ten... more Although it is known that images tend to disappear when they are stabilized on the retina for tens of seconds or minutes, the possible functions of fixational movements during the brief periods of visual fixation that occur during natural viewing remain controversial. Studies that investigated the retinal stabilization of stimuli presented for less than a few seconds have observed neither decrement in contrast sensitivity nor image fading. In this study, we analyzed the effect of retinal stabilization on discriminating the orientation of a low-contrast and noisy small bar that was displayed for either 500 ms or 2 s. The bar was randomly tilted by 45° either clockwise or counterclockwise. For both exposure durations, percentages of correct discrimination were significantly lower under conditions of visual stabilization than in the presence of the normally moving retinal image. These results are consistent with the predictions of recent computational models that simulated neuronal responses in the early visual system during oculomotor activity and support the hypothesis that visual processes deteriorate rapidly in the absence of retinal image motion.
Book Chapters by Gaëlle Desbordes
Meditation practices can yield objective, measurable health benefits, especially in protecting ag... more Meditation practices can yield objective, measurable health benefits, especially in protecting against the deleterious effects of chronic stress. Chronic stress engenders a dysregulation of homeostatic processes and can lead to serious health disorders. The brain is both affected by and centrally involved in the regulation of chronic stress. It has been proposed that meditation practice is a form of mind training that can enhance health by promoting enduring, beneficial changes in the brain and increasing resiliency to stress. In this chapter we review the current body of literature on the short-and long-term effects of meditation practice on the central nervous system and on peripheral systems that are directly or indirectly involved in homeostatic regulation of the brain, including: (i) blood pressure; (ii) immune function; (iii) telomeres; (iv) autonomic regulation of the heart; (v) Hypothalamic-pituitary-adrenal (HPA) axis regulation;
Papers by Gaëlle Desbordes
Journal of Neuroscience, 2012
Thalamic neurons respond to visual scenes by generating synchronous spike trains on the timescale... more Thalamic neurons respond to visual scenes by generating synchronous spike trains on the timescale of 10 -20 ms that are very effective at driving cortical targets. Here we demonstrate that this synchronous activity contains unexpectedly rich information about fundamental properties of visual stimuli. We report that the occurrence of synchronous firing of cat thalamic cells with highly overlapping receptive fields is strongly sensitive to the orientation and the direction of motion of the visual stimulus. We show that this stimulus selectivity is robust, remaining relatively unchanged under different contrasts and temporal frequencies (stimulus velocities). A computational analysis based on an integrate-and-fire model of the direct thalamic input to a layer 4 cortical cell reveals a strong correlation between the degree of thalamic synchrony and the nonlinear relationship between cortical membrane potential and the resultant firing rate. Together, these findings suggest a novel population code in the synchronous firing of neurons in the early visual pathway that could serve as the substrate for establishing cortical representations of the visual scene.
Journal of Neurophysiology, 2010
An understanding of the neural code in a given visual area is often confounded by the immense com... more An understanding of the neural code in a given visual area is often confounded by the immense complexity of visual stimuli combined with the number of possible meaningful patterns that comprise the response spike train. In the lateral geniculate nucleus (LGN), visual stimulation generates spike trains comprised of short spiking episodes ("events") separated by relatively long intervals of silence, which establishes a basis for in-depth analysis of the neural code. By studying this event structure in both artificial and natural visual stimulus contexts and at different contrasts, we are able to describe the dependence of event structure on stimulus class and discern which aspects generalize. We find that the event structure on coarse time scales is robust across stimulus and contrast and can be explained by receptive field processing. However, the relationship between the stimulus and fine-time-scale features of events is less straightforward, partially due to a significant amount of trial-to-trial variability. A new measure called "label information" identifies structural elements of events that can contain Յ30% more information in the context of natural movies compared with what is available from the overall event timing. The first interspike interval of an event most robustly conveys additional information about the stimulus and is somewhat more informative than the event spike count and much more informative than the presence of bursts. Nearly every event is preserved across contrast despite changes in their fine-timescale features, suggesting that-at least on a coarse level-the stimulus selectivity of LGN neurons is contrast invariant. Event-based analysis thus casts previously studied elements of LGN coding such as contrast adaptation and receptive field processing in a new light and leads to broad conclusions about the composition of the LGN neuronal code.
The Journal of Alternative and Complementary Medicine, 2014
ABSTRACT Purpose The hippocampus is an important brain region that atrophies with chronic stress ... more ABSTRACT Purpose The hippocampus is an important brain region that atrophies with chronic stress and aging. Previous studies indicate that hippocampus gray matter concentration is higher in experienced meditation practitioners and may increase after participation in Mindfulness-Based Stress Reduction (MBSR). Here we investigated changes in hippocampal volume in healthy subjects following two different meditation-based interventions: Mindful-Attention Training (MAT) and Cognitively-Based Compassion Training (CBCT) and evaluated whether changes were associated with amount of practice time. Methods As part of a larger randomized controlled trial, healthy adults without prior meditation experience were randomized into 3 arms: MAT, CBCT, or an active control intervention (health education course). Each group met for 2 hours per week for 8 weeks. MAT and CBCT participants kept a log of their meditation practice. We collected high-resolution anatomical brain scans before and after each intervention on a subsample of participants. Longitudinal changes in hippocampal volume were measured using the FreeSurfer longitudinal toolbox, a validated method for computational neuroanatomy with high test-retest reliability. Results There was no main effect of either intervention on hippocampal volume (paired t-test, p > 0.3 in each group). However, in the MAT group, practice time was significantly correlated with increased hippocampal volume (r = 0.82, p < 0.05, N = 13), even after removing one outlier subject with very high practice (r = 0.58, p < 0.05, N = 12). The CBCT group showed no significant effect of practice (r = 0.01, p > 0.9, N = 14). Conclusion Our study indicates that Mindful-Attention Training may promote neuroplasticity in the hippocampus in healthy subjects who engage in regular meditation practice over the course of 8 weeks. These findings confirm and extend similar findings with MBSR. Future research is needed to test these effects in clinical populations with hippocampal volume reduction, such as in major depression. http://online.liebertpub.com/doi/abs/10.1089/acm.2014.5059.abstract
NeuroImage, 2013
Recent advances in brain imaging have improved the measure of neural processes related to percept... more Recent advances in brain imaging have improved the measure of neural processes related to perceptual, cognitive and affective functions, yet the relation between brain activity and subjective experience remains poorly characterized. In part, it is a challenge to obtain reliable accounts of participant's experience in such studies. Here we addressed this limitation by utilizing experienced meditators who are expert in introspection. We tested a novel method to link objective and subjective data, using real-time fMRI (rt-fMRI) to provide participants with feedback of their own brain activity during an ongoing task. We provided real-time feedback during a focused attention task from the posterior cingulate cortex, a hub of the default mode network shown to be activated during mind-wandering and deactivated during meditation. In a first experiment, both meditators and non-meditators reported significant correspondence between the feedback graph and their subjective experience of focused attention and mind-wandering. When instructed to volitionally decrease the feedback graph, meditators, but not non-meditators, showed significant deactivation of the posterior cingulate cortex. We were able to replicate these results in a separate group of meditators using a novel step-wise rt-fMRI discovery protocol in which participants were not provided with prior knowledge of the expected relationship between their experience and the feedback graph (i.e., focused attention versus mind-wandering). These findings support the feasibility of using rt-fMRI to link objective measures of brain activity with reports of ongoing subjective experience in cognitive neuroscience research, and demonstrate the generalization of expertise in introspective awareness to novel contexts.
Current Opinion in Psychology, 2019
Self-related processing pertains to the complex construct of 'self', as studied across varied dis... more Self-related processing pertains to the complex construct of 'self', as studied across varied disciplines such as cognitive science, neuroscience, modern psychology (including clinical and behavioral psychology), and Western and Eastern philosophy. On a theoretical level, most contemporary models propose that mindfulness training impacts self-related processes. In this review, the empirical evidence for this hypothesis is examined and discussed. Overall, very few self-related processes have been measured in randomized controlled trials of mindfulness-based interventions to date, and, of those processes that have been measured, only negative self-rumination improved significantly. The data so far remain inconclusive as to whether mindfulness-based interventions have an impact on other self-related processes. Studies are especially needed on more basic levels of self such as embodiment and sense of agency.
In light of a growing interest in contemplative practices such as meditation, the emerging field ... more In light of a growing interest in contemplative practices such as meditation, the emerging field of contemplative science has been challenged to describe and objectively measure how these practices affect health and well-being. While “mindfulness” itself has been proposed as a measurable outcome of contemplative practices, this concept encompasses multiple components, some of which, as we review here, may be better characterized as equanimity. Equanimity can be defined as an even-minded mental state or dispositional tendency toward all experiences or objects, regardless of their origin or their affective valence (pleasant, unpleasant, or neutral). In this article, we propose that equanimity be used as an outcome measure in contemplative research. We first define and discuss the inter-relationship between mindfulness and equanimity from the perspectives of both classical Buddhism and modern psychology and present existing meditation techniques for cultivating equanimity. We then review psychological, physiological, and neuroimaging methods that have been used to assess equanimity either directly or indirectly. In conclusion, we propose that equanimity captures potentially the most important psychological element in the improvement of well-being, and therefore should be a focus in future research studies.
Citation: Desbordes G and Negi LT (2013) A new era for mind studies: training investigators in bo... more Citation: Desbordes G and Negi LT (2013) A new era for mind studies: training investigators in both scientific and contemplative methods of inquiry. Front. Hum. Neurosci. 7:741.
Chronic itch, a highly debilitating condition, has received relatively little attention in the ne... more Chronic itch, a highly debilitating condition, has received relatively little attention in the neuroimaging literature. Recent studies suggest that brain regions supporting itch in chronic itch patients encompass sensorimotor and salience networks, and corticostriatal circuits involved in motor preparation for scratching. However, how these different brain areas interact with one another in the context of itch is still unknown. We acquired BOLD fMRI scans in 14 atopic dermatitis patients to investigate resting-state functional connectivity before and after allergen-induced itch exacerbated the clinical itch perception in these patients. A seed-based analysis revealed decreased functional connectivity from baseline resting state to the evoked-itch state between several itch-related brain regions, particularly the insular and cingulate cortices and basal ganglia, where decreased connectivity was significantly correlated with increased levels of perceived itch. In contrast, evoked itch increased connectivity between key nodes of the frontoparietal control network (superior parietal lobule and dorsolateral prefrontal cortex), where higher increase in connectivity was correlated with a lesser increase in perceived itch, suggesting that greater interaction between nodes of this executive attention network serves to limit itch sensation via enhanced top-down regulation. Overall, our results provide the first evidence of itch-dependent changes in functional connectivity across multiple brain regions.
Natural visual stimuli have highly structured spatial and temporal properties which influence the... more Natural visual stimuli have highly structured spatial and temporal properties which influence the way visual information is encoded in the visual pathway. In response to natural scene stimuli, neurons in the lateral geniculate nucleus (LGN) are temporally precise – on a time scale of 10–25 ms – both within single cells and across cells within a population. This time scale, established by non stimulus-driven elements of neuronal firing, is significantly shorter than that of natural scenes, yet is critical for the neural representation of the spatial and temporal structure of the scene. Here, a generalized linear model (GLM) that combines stimulus-driven elements with spike-history dependence associated with intrinsic cellular dynamics is shown to predict the fine timing precision of LGN responses to natural scene stimuli, the corresponding correlation structure across nearby neurons in the population, and the continuous modulation of spike timing precision and latency across neurons. A single model captured the experimentally observed neural response, across different levels of contrasts and different classes of visual stimuli, through interactions between the stimulus correlation structure and the nonlinearity in spike generation and spike history dependence. Given the sensitivity of the thalamocortical synapse to closely timed spikes and the importance of fine timing precision for the faithful representation of natural scenes, the modulation of thalamic population timing over these time scales is likely important for cortical representations of the dynamic natural visual environment.
The timing of spiking activity across neurons is a fundamental aspect of the neural population co... more The timing of spiking activity across neurons is a fundamental aspect of the neural population code. Individual neurons in the retina, thalamus, and cortex can have very precise and repeatable responses but exhibit degraded temporal precision in response to suboptimal stimuli. To investigate the functional implications for neural populations in natural conditions, we recorded in vivo the simultaneous responses, to movies of natural scenes, of multiple thalamic neurons likely converging to a common neuronal target in primary visual cortex. We show that the response of individual neurons is less precise at lower contrast, but that spike timing precision across neurons is relatively insensitive to global changes in visual contrast. Overall, spike timing precision within and across cells is on the order of 10 ms. Since closely timed spikes are more efficient in inducing a spike in downstream cortical neurons, and since fine temporal precision is necessary to represent the more slowly varying natural environment, we argue that preserving relative spike timing at a ∼10-ms resolution is a crucial property of the neural code entering cortex.
During behavior, rats and other rodents use their facial vibrissae to actively explore surfaces t... more During behavior, rats and other rodents use their facial vibrissae to actively explore surfaces through whisking and head/body movement, resulting in complex sensory inputs that vary over a large range of angular velocities and temporal scales. How these complex sensory inputs manifest in the patterns of cortical firing events that ultimately form the perceptual experience is not well understood. Through single-unit cortical recordings of layer 4 neurons in S1 of the anesthetized rat, we systematically quantified the interactions between instantaneous velocity and timing of vibrissa motion, finding a strong interaction between angular velocity and timing of contacts on the tens of milliseconds time scale. From the quantification of these joint tuning properties, a detailed nonlinear encoding model was formulated that was highly predictive of firing probability and timing characteristics of the sparse cortical representation of complex patterned tactile inputs. Within a Bayesian framework, the encoding model was then used to decode tactile patterns under simple transformations of the stimulus along dimensions of velocity and timing, as a demonstration of the lower bound of the idealized perceptual capabilities of the animal.
During visual fixation, small eye movements keep the retinal image continuously in motion. It is ... more During visual fixation, small eye movements keep the retinal image continuously in motion. It is known that neurons in the visual system are sensitive to the spatiotemporal modulations of luminance resulting from this motion. In this study, we examined the influence of fixational eye movements on the statistics of neural activity in the macaque's retina during the brief intersaccadic periods of natural visual fixation. The responses of parvocellular (P) and magnocellular (M) ganglion cells in different regions of the visual field were modeled while their receptive fields scanned natural images following recorded traces of eye movements. Immediately after the onset of fixation, wide ensembles of coactive ganglion cells extended over several degrees of visual angle, both in the central and peripheral regions of the visual field. Following this initial pattern of activity, the covariance between the responses of pairs of P and M cells and the correlation between the responses of pairs of M cells dropped drastically during the course of fixation. Cell responses were completely uncorrelated by the end of a typical 300-ms fixation. This dynamic spatial decorrelation of retinal activity is a robust phenomenon independent of the specifics of the model. We show that it originates from the interaction of three factors: the statistics of natural scenes, the small amplitudes of fixational eye movements, and the temporal sensitivities of ganglion cells. These results support the hypothesis that fixational eye movements, by shaping the statistics of retinal activity, are an integral component of early visual representations.
Although it is known that images tend to disappear when they are stabilized on the retina for ten... more Although it is known that images tend to disappear when they are stabilized on the retina for tens of seconds or minutes, the possible functions of fixational movements during the brief periods of visual fixation that occur during natural viewing remain controversial. Studies that investigated the retinal stabilization of stimuli presented for less than a few seconds have observed neither decrement in contrast sensitivity nor image fading. In this study, we analyzed the effect of retinal stabilization on discriminating the orientation of a low-contrast and noisy small bar that was displayed for either 500 ms or 2 s. The bar was randomly tilted by 45° either clockwise or counterclockwise. For both exposure durations, percentages of correct discrimination were significantly lower under conditions of visual stabilization than in the presence of the normally moving retinal image. These results are consistent with the predictions of recent computational models that simulated neuronal responses in the early visual system during oculomotor activity and support the hypothesis that visual processes deteriorate rapidly in the absence of retinal image motion.
Meditation practices can yield objective, measurable health benefits, especially in protecting ag... more Meditation practices can yield objective, measurable health benefits, especially in protecting against the deleterious effects of chronic stress. Chronic stress engenders a dysregulation of homeostatic processes and can lead to serious health disorders. The brain is both affected by and centrally involved in the regulation of chronic stress. It has been proposed that meditation practice is a form of mind training that can enhance health by promoting enduring, beneficial changes in the brain and increasing resiliency to stress. In this chapter we review the current body of literature on the short-and long-term effects of meditation practice on the central nervous system and on peripheral systems that are directly or indirectly involved in homeostatic regulation of the brain, including: (i) blood pressure; (ii) immune function; (iii) telomeres; (iv) autonomic regulation of the heart; (v) Hypothalamic-pituitary-adrenal (HPA) axis regulation;
Journal of Neuroscience, 2012
Thalamic neurons respond to visual scenes by generating synchronous spike trains on the timescale... more Thalamic neurons respond to visual scenes by generating synchronous spike trains on the timescale of 10 -20 ms that are very effective at driving cortical targets. Here we demonstrate that this synchronous activity contains unexpectedly rich information about fundamental properties of visual stimuli. We report that the occurrence of synchronous firing of cat thalamic cells with highly overlapping receptive fields is strongly sensitive to the orientation and the direction of motion of the visual stimulus. We show that this stimulus selectivity is robust, remaining relatively unchanged under different contrasts and temporal frequencies (stimulus velocities). A computational analysis based on an integrate-and-fire model of the direct thalamic input to a layer 4 cortical cell reveals a strong correlation between the degree of thalamic synchrony and the nonlinear relationship between cortical membrane potential and the resultant firing rate. Together, these findings suggest a novel population code in the synchronous firing of neurons in the early visual pathway that could serve as the substrate for establishing cortical representations of the visual scene.
Journal of Neurophysiology, 2010
An understanding of the neural code in a given visual area is often confounded by the immense com... more An understanding of the neural code in a given visual area is often confounded by the immense complexity of visual stimuli combined with the number of possible meaningful patterns that comprise the response spike train. In the lateral geniculate nucleus (LGN), visual stimulation generates spike trains comprised of short spiking episodes ("events") separated by relatively long intervals of silence, which establishes a basis for in-depth analysis of the neural code. By studying this event structure in both artificial and natural visual stimulus contexts and at different contrasts, we are able to describe the dependence of event structure on stimulus class and discern which aspects generalize. We find that the event structure on coarse time scales is robust across stimulus and contrast and can be explained by receptive field processing. However, the relationship between the stimulus and fine-time-scale features of events is less straightforward, partially due to a significant amount of trial-to-trial variability. A new measure called "label information" identifies structural elements of events that can contain Յ30% more information in the context of natural movies compared with what is available from the overall event timing. The first interspike interval of an event most robustly conveys additional information about the stimulus and is somewhat more informative than the event spike count and much more informative than the presence of bursts. Nearly every event is preserved across contrast despite changes in their fine-timescale features, suggesting that-at least on a coarse level-the stimulus selectivity of LGN neurons is contrast invariant. Event-based analysis thus casts previously studied elements of LGN coding such as contrast adaptation and receptive field processing in a new light and leads to broad conclusions about the composition of the LGN neuronal code.
The Journal of Alternative and Complementary Medicine, 2014
ABSTRACT Purpose The hippocampus is an important brain region that atrophies with chronic stress ... more ABSTRACT Purpose The hippocampus is an important brain region that atrophies with chronic stress and aging. Previous studies indicate that hippocampus gray matter concentration is higher in experienced meditation practitioners and may increase after participation in Mindfulness-Based Stress Reduction (MBSR). Here we investigated changes in hippocampal volume in healthy subjects following two different meditation-based interventions: Mindful-Attention Training (MAT) and Cognitively-Based Compassion Training (CBCT) and evaluated whether changes were associated with amount of practice time. Methods As part of a larger randomized controlled trial, healthy adults without prior meditation experience were randomized into 3 arms: MAT, CBCT, or an active control intervention (health education course). Each group met for 2 hours per week for 8 weeks. MAT and CBCT participants kept a log of their meditation practice. We collected high-resolution anatomical brain scans before and after each intervention on a subsample of participants. Longitudinal changes in hippocampal volume were measured using the FreeSurfer longitudinal toolbox, a validated method for computational neuroanatomy with high test-retest reliability. Results There was no main effect of either intervention on hippocampal volume (paired t-test, p > 0.3 in each group). However, in the MAT group, practice time was significantly correlated with increased hippocampal volume (r = 0.82, p < 0.05, N = 13), even after removing one outlier subject with very high practice (r = 0.58, p < 0.05, N = 12). The CBCT group showed no significant effect of practice (r = 0.01, p > 0.9, N = 14). Conclusion Our study indicates that Mindful-Attention Training may promote neuroplasticity in the hippocampus in healthy subjects who engage in regular meditation practice over the course of 8 weeks. These findings confirm and extend similar findings with MBSR. Future research is needed to test these effects in clinical populations with hippocampal volume reduction, such as in major depression. http://online.liebertpub.com/doi/abs/10.1089/acm.2014.5059.abstract
NeuroImage, 2013
Recent advances in brain imaging have improved the measure of neural processes related to percept... more Recent advances in brain imaging have improved the measure of neural processes related to perceptual, cognitive and affective functions, yet the relation between brain activity and subjective experience remains poorly characterized. In part, it is a challenge to obtain reliable accounts of participant's experience in such studies. Here we addressed this limitation by utilizing experienced meditators who are expert in introspection. We tested a novel method to link objective and subjective data, using real-time fMRI (rt-fMRI) to provide participants with feedback of their own brain activity during an ongoing task. We provided real-time feedback during a focused attention task from the posterior cingulate cortex, a hub of the default mode network shown to be activated during mind-wandering and deactivated during meditation. In a first experiment, both meditators and non-meditators reported significant correspondence between the feedback graph and their subjective experience of focused attention and mind-wandering. When instructed to volitionally decrease the feedback graph, meditators, but not non-meditators, showed significant deactivation of the posterior cingulate cortex. We were able to replicate these results in a separate group of meditators using a novel step-wise rt-fMRI discovery protocol in which participants were not provided with prior knowledge of the expected relationship between their experience and the feedback graph (i.e., focused attention versus mind-wandering). These findings support the feasibility of using rt-fMRI to link objective measures of brain activity with reports of ongoing subjective experience in cognitive neuroscience research, and demonstrate the generalization of expertise in introspective awareness to novel contexts.