Adam Sachs - Academia.edu (original) (raw)

Papers by Adam Sachs

Research paper thumbnail of Decoding decision outcomes from single realizations of lateral prefrontal cortex ensemble activity

Research paper thumbnail of Decoding Saccade Intention From Primate Prefrontal Cortical Local Field Potentials Using Spectral, Spatial, and Temporal Dimensionality Reduction

International Journal of Neural Systems

Most invasive Brain Computer Interfaces (iBCIs) use spike and Local Field Potentials (LFPs) from ... more Most invasive Brain Computer Interfaces (iBCIs) use spike and Local Field Potentials (LFPs) from the motor or parietal cortices to decode movement intentions. It has been debated whether harvesting signals from other brain areas that encode global cognitive variables, such as the allocation of attention and eye movement goals in a variety of spatial reference frames, may improve the outcome of iBCIs. Here, we explore the ability of LFP signals, sampled from the lateral prefrontal cortex (LPFC) of macaque monkeys, to encode eye-movement intention during the pre-movement fixation period of a delayed saccade task. We use spectral dimensionality reduction to examine the spatiotemporal properties of the extracted non-rhythmic broadband activity and explore its usefulness in decoding saccade goals. The dynamics of the broadband signal in low spatial dimensions across the pre-movement fixation period uncovered saccade target separation; its discriminative potential was confirmed using supp...

Research paper thumbnail of Contribution of spike timing in contrast and motion direction coding by single neurons in macaque area MT

Journal of Vision, 2010

We assessed the performance of three spike train metrics, D spike (Victor and Purpura, 1996), the... more We assessed the performance of three spike train metrics, D spike (Victor and Purpura, 1996), the vector product metric of Schreiber et al (2003), and a rate code metric (D count ), on computer simulated spike trains and single unit recordings from area MT of awake ...

Research paper thumbnail of Neuronal ensembles of primate Lateral Prefrontal Cortex encode spatial working memory in different frames of reference

Research paper thumbnail of Structure of Spike Count Correlations Reveals Functional Interactions between Neurons in Dorsolateral Prefrontal Cortex Area 8a of Behaving Primates

Neurons within the primate dorsolateral prefrontal cortex (dlPFC) are clustered in microcolumns a... more Neurons within the primate dorsolateral prefrontal cortex (dlPFC) are clustered in microcolumns according to their visuospatial tuning. One issue that remains poorly investigated is how this anatomical arrangement influences functional interactions between neurons during behavior. To investigate this question we implanted 4 mm×4 mm multielectrode arrays in two macaques' dlPFC area 8a and measured spike count correlations (rsc) between responses of simultaneously recorded neurons when animals maintained stationary gaze. Positive and negative rsc were significantly higher than predicted by chance across a wide range of inter-neuron distances (from 0.4 to 4 mm). Positive rsc were stronger between neurons with receptive fields (RFs) separated by ≤90° of angular distance and progressively decreased as a function of inter-neuron physical distance. Negative rsc were stronger between neurons with RFs separated by >90° and increased as a function of inter-neuron distance. Our results ...

Research paper thumbnail of Improve Subthalamic Nucleus Localization During Deep Brain Stimulation Surgery

Research paper thumbnail of Single trial decoding of visual attention from local field potentials in the primate lateral prefrontal cortex

It has recently been shown that attention can be decoded from the spiking activity of ensembles o... more It has recently been shown that attention can be decoded from the spiking activity of ensembles of neurons in area 8A of the monkey lateral prefrontal cortex (Tremblay et al., 2015). As spikes are believed to reflect the output of a given brain region and local field potentials (LFPs) its input and local activity, we used a decoding approach to investigate the role of area 8A in visual attention. To do so, we used simultaneously recorded LFPs from chronically implanted multi-electrode arrays to decipher the target of visual attention. However, when attempting to decode the information content of an LFP signal, conventional filtering techniques (low-pass at 250 Hz) are insufficient in segregating spikes components from other LPF sources. Indeed, multiple studies have shown that spike waveforms recorded from the same electrode contaminate LFP signals above 80 Hz (Zanos et al., 2012), biasing the decoded information. Using a previously published spike removal algorithm (Zanos et al., 2...

Research paper thumbnail of Behavioral / Cognitive Single-Trial Decoding of Visual Attention from Local Field Potentials in the Primate Lateral Prefrontal Cortex Is Frequency-Dependent X

X Sébastien Tremblay,1,2 Guillaume Doucet,1 Florian Pieper,3 X Adam Sachs,4 and Julio Martinez-Tr... more X Sébastien Tremblay,1,2 Guillaume Doucet,1 Florian Pieper,3 X Adam Sachs,4 and Julio Martinez-Trujillo1,5 1Cognitive Neurophysiology Laboratory, Department of Physiology, McGill University, Montreal, Quebec, QC H3G 1Y6, Canada, 2Integrated Program in Neuroscience, Montreal Neurological Institute, McGill University, Montreal, Quebec, QC H3A 2B4, Canada, 3Institute for Neuro& Pathophysiology, University Medical Center Hamburg-Eppendorf, 20246 Hamburg, Germany, 4Division of Neurosurgery, Department of Surgery, Ottawa Hospital Research Institute, University of Ottawa, Ottawa, Ontario, ON K1Y 4E9, Canada, and 5Robarts Research Institute, Departments of Psychiatry, Physiology and Pharmacology, University of Western Ontario, London, Ontario, ON N6A 5C1, Canada

Research paper thumbnail of Scale-Free Analysis of Intraoperative ECoG During Awake Craniotomy for Glioma

Frontiers in Oncology

BackgroundElectrocorticography (ECoG) has been utilized in many epilepsy cases however, the use o... more BackgroundElectrocorticography (ECoG) has been utilized in many epilepsy cases however, the use of this technique for evaluating electrophysiological changes within tumoral zones is spare. Nonetheless, epileptic activities seem to arise from the neocortex surrounding the gliomas suggesting a link between epileptogenesis and glioma cell infiltration in the peritumoral area. The purpose of this study was to implement novel scale-free measures to assess how cortical physiology is altered by the presence of an invasive brain tumor.MethodsTwelve patients undergoing an awake craniotomy for resection of a supratentorial glioma were included. ECoG data over the main tumor and the exposed surroundings was acquired intra-operatively just prior to tumor resection. Six of the patients presented with seizures and had data acquired both in the awake and anesthetic state. The corresponding anatomical location of each electrode in relation to the macroscopically-detectable tumor was recorded using ...

Research paper thumbnail of Dissociation between eye position and working memory signals during virtual reality tasks in the primate lateral prefrontal cortex

Research paper thumbnail of Small neuronal ensembles of primate lateral prefrontal cortex encode spatial working memory in two reference frames

Research paper thumbnail of Using decoders to understand working memory representations of 3D space in primate prefrontal neuronal ensembles

Research paper thumbnail of A Normalization Circuit Underlying Coding of Spatial Attention in Primate Lateral Prefrontal Cortex

eneuro

Lateral prefrontal cortex (LPFC) neurons signal the allocation of voluntary attention; however, t... more Lateral prefrontal cortex (LPFC) neurons signal the allocation of voluntary attention; however, the neural computations underlying this function remain unknown. To investigate this, we recorded from neuronal ensembles in the LPFC of two Macaca fascicularis performing a visuospatial attention task. LPFC neural responses to a single stimulus were normalized when additional stimuli/distracters appeared across the visual field and were wellcharacterized by an averaging computation. Deploying attention toward an individual stimulus surrounded by distracters shifted neural activity from an averaging regime toward a regime similar to that when the attended stimulus was presented in isolation (winner-take-all; WTA). However, attentional modulation is both qualitatively and quantitatively dependent on a neuron's visuospatial tuning. Our results show that during attentive vision, LPFC neuronal ensemble activity can be robustly read out by downstream areas to generate motor commands, and/or fed back into sensory areas to filter out distracter signals in favor of target signals.

Research paper thumbnail of The Effects of Methylphenidate (Ritalin) on the Neurophysiology of the Monkey Caudal Prefrontal Cortex

eneuro

Methylphenidate (MPH), commonly known as Ritalin, is the most widely prescribed drug worldwide to... more Methylphenidate (MPH), commonly known as Ritalin, is the most widely prescribed drug worldwide to treat patients with attention deficit disorders. Although MPH is thought to modulate catecholamine neurotransmission in the brain, it remains unclear how these neurochemical effects influence neuronal activity and lead to attentional enhancements. Studies in rodents overwhelmingly point to the lateral prefrontal cortex (LPFC) as a main site of action of MPH. To understand the mechanism of action of MPH in a primate brain, we recorded the responses of neuronal populations using chronic multielectrode arrays implanted in the caudal LPFC of two macaque monkeys while the animals performed an attention task (N ϭ 2811 neuronal recordings). Over different recording sessions (N ϭ 55), we orally administered either various doses of MPH or a placebo to the animals. Behavioral analyses revealed positive effects of MPH on task performance at specific doses. However, analyses of individual neurons activity, noise correlations, and neuronal ensemble activity using machine learning algorithms revealed no effects of MPH. Our results suggest that the positive behavioral effects of MPH observed in primates (including humans) may not be mediated by changes in the activity of caudal LPFC neurons. MPH may enhance cognitive performance by modulating neuronal activity in other regions of the attentional network in the primate brain.

Research paper thumbnail of Use of 3D Navigation in Subaxial Cervical Spine Lateral Mass Screw Insertion

Journal of Neurological Surgery Reports

Objective Cervical spine can be stabilized by different techniques. One of the common techniques ... more Objective Cervical spine can be stabilized by different techniques. One of the common techniques used is the lateral mass screws (LMSs), which can be inserted either by freehand techniques or three-dimensional (3D) navigation system. The purpose of this study is to evaluate the difference between the 3D navigation system and the freehand technique for cervical spine LMS placement in terms of complications. Including intraoperative complications (vertebral artery injury [VAI], nerve root injury [NRI], spinal cord injury [SCI], lateral mass fracture [LMF]) and postoperative complications (screw malposition, screw complications). Methods Patients who had LMS fixation for their subaxial cervical spine from January 2014 to April 2015 at the Ottawa Hospital were included. A total of 284 subaxial cervical LMS were inserted in 40 consecutive patients. Surgical indications were cervical myelopathy and fractures. The screws' size was 3.5 mm in diameter and 8 to 16 mm in length. During the...

Research paper thumbnail of Realtime phase-amplitude coupling analysis of micro electrode recorded brain signals

PloS one, 2018

To demonstrate a method to calculate phase amplitude coupling (PAC) quickly and robustly for real... more To demonstrate a method to calculate phase amplitude coupling (PAC) quickly and robustly for realtime applications. We designed and implemented a multirate PAC algorithm with efficient filter bank processing and efficient computation of PAC for many frequency-pair combinations. We tested the developed algorithm for computing PAC on simulated data and on intraoperative neural recording data obtained during deep brain stimulation (DBS) electrode implantation surgery. A combination of parallelized frequency-domain filtering and modulation index for PAC estimation provided robust results that could be calculated in real time on modest computing hardware. The standard methods for calculating PAC can be optimized for quick and robust performance. These results demonstrated that PAC can be extracted in real time and is suitable for neurofeedback applications.

Research paper thumbnail of A Quadrantic Bias in Prefrontal Representation of Visual-Mnemonic Space

Cerebral Cortex

Single neurons in primate dorsolateral prefrontal cortex (dLPFC) are known to encode working memo... more Single neurons in primate dorsolateral prefrontal cortex (dLPFC) are known to encode working memory (WM) representations of visual space. Psychophysical studies have shown that the horizontal and vertical meridians of the visual field can bias spatial information maintained in WM. However, most studies and models have tacitly assumed that dLPFC neurons represent mnemonic space homogenously. The anatomical organization of these representations has also eluded clear parametric description. We investigated these issues by recording from neuronal ensembles in macaque dLPFC with microelectrode arrays while subjects performed an oculomotor delayed-response task. We found that spatial WM representations in macaque dLPFC are biased by the vertical and horizontal meridians of the visual field, dividing mnemonic space into quadrants. This bias is reflected in single neuron firing rates, neuronal ensemble representations, the spike count correlation structure, and eye movement patterns. We also found that dLPFC representations of mnemonic space cluster anatomically in a nonretinotopic manner that partially reflects the organization of visual space. These results provide an explanation for known WM biases, and reveal novel principles of WM representation in prefrontal neuronal ensembles and across the cortical surface, as well as the need to reconceptualize models of WM to accommodate the observed representational biases.

Research paper thumbnail of Effects of neural ensemble size and composition on the decoding of attention in primate lateral prefrontal cortex

Research paper thumbnail of Heterogeneous effects of neuronal ensemble size, tuning, and correlation structure on the decoding of spatial working memory in dorsolateral prefrontal cortex

Research paper thumbnail of A metric-based analysis of the contribution of spike timing to contrast and motion direction coding by single neurons in macaque area MT

Brain Research, Jan 12, 2011

Spike timing is thought to contribute to the coding of motion direction information by neurons in... more Spike timing is thought to contribute to the coding of motion direction information by neurons in macaque area MT. Here, we examined whether spike timing also contributes to the coding of stimulus contrast. We applied a metric-based approach to spike trains fired by MT neurons in response to stimuli that varied in contrast, or direction. We assessed the performance of three metrics, D spike and D product (containing spike count and timing information), and the spike count metric D count. We analyzed responses elicited during the first 200 msec of stimulus presentation from 205 neurons. For both contrast and direction, the large majority of neurons showed the highest mutual information using D spike , followed by D product , and D count. This was corroborated by the performance of a theoretical observer model at discriminating contrast and direction using the three metrics. Our results demonstrate that spike timing can contribute to contrast coding in MT neurons, and support previous reports of its potential contribution to direction coding. Furthermore, they suggest that a combination of spike count with periodic and non-periodic spike timing information (contained in D spike , but not in D product and D count which are insensitive to spike counts and timing respectively) provides the largest coding advantage in spike trains fired by MT neurons during contrast and direction discrimination.

Research paper thumbnail of Decoding decision outcomes from single realizations of lateral prefrontal cortex ensemble activity

Research paper thumbnail of Decoding Saccade Intention From Primate Prefrontal Cortical Local Field Potentials Using Spectral, Spatial, and Temporal Dimensionality Reduction

International Journal of Neural Systems

Most invasive Brain Computer Interfaces (iBCIs) use spike and Local Field Potentials (LFPs) from ... more Most invasive Brain Computer Interfaces (iBCIs) use spike and Local Field Potentials (LFPs) from the motor or parietal cortices to decode movement intentions. It has been debated whether harvesting signals from other brain areas that encode global cognitive variables, such as the allocation of attention and eye movement goals in a variety of spatial reference frames, may improve the outcome of iBCIs. Here, we explore the ability of LFP signals, sampled from the lateral prefrontal cortex (LPFC) of macaque monkeys, to encode eye-movement intention during the pre-movement fixation period of a delayed saccade task. We use spectral dimensionality reduction to examine the spatiotemporal properties of the extracted non-rhythmic broadband activity and explore its usefulness in decoding saccade goals. The dynamics of the broadband signal in low spatial dimensions across the pre-movement fixation period uncovered saccade target separation; its discriminative potential was confirmed using supp...

Research paper thumbnail of Contribution of spike timing in contrast and motion direction coding by single neurons in macaque area MT

Journal of Vision, 2010

We assessed the performance of three spike train metrics, D spike (Victor and Purpura, 1996), the... more We assessed the performance of three spike train metrics, D spike (Victor and Purpura, 1996), the vector product metric of Schreiber et al (2003), and a rate code metric (D count ), on computer simulated spike trains and single unit recordings from area MT of awake ...

Research paper thumbnail of Neuronal ensembles of primate Lateral Prefrontal Cortex encode spatial working memory in different frames of reference

Research paper thumbnail of Structure of Spike Count Correlations Reveals Functional Interactions between Neurons in Dorsolateral Prefrontal Cortex Area 8a of Behaving Primates

Neurons within the primate dorsolateral prefrontal cortex (dlPFC) are clustered in microcolumns a... more Neurons within the primate dorsolateral prefrontal cortex (dlPFC) are clustered in microcolumns according to their visuospatial tuning. One issue that remains poorly investigated is how this anatomical arrangement influences functional interactions between neurons during behavior. To investigate this question we implanted 4 mm×4 mm multielectrode arrays in two macaques' dlPFC area 8a and measured spike count correlations (rsc) between responses of simultaneously recorded neurons when animals maintained stationary gaze. Positive and negative rsc were significantly higher than predicted by chance across a wide range of inter-neuron distances (from 0.4 to 4 mm). Positive rsc were stronger between neurons with receptive fields (RFs) separated by ≤90° of angular distance and progressively decreased as a function of inter-neuron physical distance. Negative rsc were stronger between neurons with RFs separated by >90° and increased as a function of inter-neuron distance. Our results ...

Research paper thumbnail of Improve Subthalamic Nucleus Localization During Deep Brain Stimulation Surgery

Research paper thumbnail of Single trial decoding of visual attention from local field potentials in the primate lateral prefrontal cortex

It has recently been shown that attention can be decoded from the spiking activity of ensembles o... more It has recently been shown that attention can be decoded from the spiking activity of ensembles of neurons in area 8A of the monkey lateral prefrontal cortex (Tremblay et al., 2015). As spikes are believed to reflect the output of a given brain region and local field potentials (LFPs) its input and local activity, we used a decoding approach to investigate the role of area 8A in visual attention. To do so, we used simultaneously recorded LFPs from chronically implanted multi-electrode arrays to decipher the target of visual attention. However, when attempting to decode the information content of an LFP signal, conventional filtering techniques (low-pass at 250 Hz) are insufficient in segregating spikes components from other LPF sources. Indeed, multiple studies have shown that spike waveforms recorded from the same electrode contaminate LFP signals above 80 Hz (Zanos et al., 2012), biasing the decoded information. Using a previously published spike removal algorithm (Zanos et al., 2...

Research paper thumbnail of Behavioral / Cognitive Single-Trial Decoding of Visual Attention from Local Field Potentials in the Primate Lateral Prefrontal Cortex Is Frequency-Dependent X

X Sébastien Tremblay,1,2 Guillaume Doucet,1 Florian Pieper,3 X Adam Sachs,4 and Julio Martinez-Tr... more X Sébastien Tremblay,1,2 Guillaume Doucet,1 Florian Pieper,3 X Adam Sachs,4 and Julio Martinez-Trujillo1,5 1Cognitive Neurophysiology Laboratory, Department of Physiology, McGill University, Montreal, Quebec, QC H3G 1Y6, Canada, 2Integrated Program in Neuroscience, Montreal Neurological Institute, McGill University, Montreal, Quebec, QC H3A 2B4, Canada, 3Institute for Neuro& Pathophysiology, University Medical Center Hamburg-Eppendorf, 20246 Hamburg, Germany, 4Division of Neurosurgery, Department of Surgery, Ottawa Hospital Research Institute, University of Ottawa, Ottawa, Ontario, ON K1Y 4E9, Canada, and 5Robarts Research Institute, Departments of Psychiatry, Physiology and Pharmacology, University of Western Ontario, London, Ontario, ON N6A 5C1, Canada

Research paper thumbnail of Scale-Free Analysis of Intraoperative ECoG During Awake Craniotomy for Glioma

Frontiers in Oncology

BackgroundElectrocorticography (ECoG) has been utilized in many epilepsy cases however, the use o... more BackgroundElectrocorticography (ECoG) has been utilized in many epilepsy cases however, the use of this technique for evaluating electrophysiological changes within tumoral zones is spare. Nonetheless, epileptic activities seem to arise from the neocortex surrounding the gliomas suggesting a link between epileptogenesis and glioma cell infiltration in the peritumoral area. The purpose of this study was to implement novel scale-free measures to assess how cortical physiology is altered by the presence of an invasive brain tumor.MethodsTwelve patients undergoing an awake craniotomy for resection of a supratentorial glioma were included. ECoG data over the main tumor and the exposed surroundings was acquired intra-operatively just prior to tumor resection. Six of the patients presented with seizures and had data acquired both in the awake and anesthetic state. The corresponding anatomical location of each electrode in relation to the macroscopically-detectable tumor was recorded using ...

Research paper thumbnail of Dissociation between eye position and working memory signals during virtual reality tasks in the primate lateral prefrontal cortex

Research paper thumbnail of Small neuronal ensembles of primate lateral prefrontal cortex encode spatial working memory in two reference frames

Research paper thumbnail of Using decoders to understand working memory representations of 3D space in primate prefrontal neuronal ensembles

Research paper thumbnail of A Normalization Circuit Underlying Coding of Spatial Attention in Primate Lateral Prefrontal Cortex

eneuro

Lateral prefrontal cortex (LPFC) neurons signal the allocation of voluntary attention; however, t... more Lateral prefrontal cortex (LPFC) neurons signal the allocation of voluntary attention; however, the neural computations underlying this function remain unknown. To investigate this, we recorded from neuronal ensembles in the LPFC of two Macaca fascicularis performing a visuospatial attention task. LPFC neural responses to a single stimulus were normalized when additional stimuli/distracters appeared across the visual field and were wellcharacterized by an averaging computation. Deploying attention toward an individual stimulus surrounded by distracters shifted neural activity from an averaging regime toward a regime similar to that when the attended stimulus was presented in isolation (winner-take-all; WTA). However, attentional modulation is both qualitatively and quantitatively dependent on a neuron's visuospatial tuning. Our results show that during attentive vision, LPFC neuronal ensemble activity can be robustly read out by downstream areas to generate motor commands, and/or fed back into sensory areas to filter out distracter signals in favor of target signals.

Research paper thumbnail of The Effects of Methylphenidate (Ritalin) on the Neurophysiology of the Monkey Caudal Prefrontal Cortex

eneuro

Methylphenidate (MPH), commonly known as Ritalin, is the most widely prescribed drug worldwide to... more Methylphenidate (MPH), commonly known as Ritalin, is the most widely prescribed drug worldwide to treat patients with attention deficit disorders. Although MPH is thought to modulate catecholamine neurotransmission in the brain, it remains unclear how these neurochemical effects influence neuronal activity and lead to attentional enhancements. Studies in rodents overwhelmingly point to the lateral prefrontal cortex (LPFC) as a main site of action of MPH. To understand the mechanism of action of MPH in a primate brain, we recorded the responses of neuronal populations using chronic multielectrode arrays implanted in the caudal LPFC of two macaque monkeys while the animals performed an attention task (N ϭ 2811 neuronal recordings). Over different recording sessions (N ϭ 55), we orally administered either various doses of MPH or a placebo to the animals. Behavioral analyses revealed positive effects of MPH on task performance at specific doses. However, analyses of individual neurons activity, noise correlations, and neuronal ensemble activity using machine learning algorithms revealed no effects of MPH. Our results suggest that the positive behavioral effects of MPH observed in primates (including humans) may not be mediated by changes in the activity of caudal LPFC neurons. MPH may enhance cognitive performance by modulating neuronal activity in other regions of the attentional network in the primate brain.

Research paper thumbnail of Use of 3D Navigation in Subaxial Cervical Spine Lateral Mass Screw Insertion

Journal of Neurological Surgery Reports

Objective Cervical spine can be stabilized by different techniques. One of the common techniques ... more Objective Cervical spine can be stabilized by different techniques. One of the common techniques used is the lateral mass screws (LMSs), which can be inserted either by freehand techniques or three-dimensional (3D) navigation system. The purpose of this study is to evaluate the difference between the 3D navigation system and the freehand technique for cervical spine LMS placement in terms of complications. Including intraoperative complications (vertebral artery injury [VAI], nerve root injury [NRI], spinal cord injury [SCI], lateral mass fracture [LMF]) and postoperative complications (screw malposition, screw complications). Methods Patients who had LMS fixation for their subaxial cervical spine from January 2014 to April 2015 at the Ottawa Hospital were included. A total of 284 subaxial cervical LMS were inserted in 40 consecutive patients. Surgical indications were cervical myelopathy and fractures. The screws' size was 3.5 mm in diameter and 8 to 16 mm in length. During the...

Research paper thumbnail of Realtime phase-amplitude coupling analysis of micro electrode recorded brain signals

PloS one, 2018

To demonstrate a method to calculate phase amplitude coupling (PAC) quickly and robustly for real... more To demonstrate a method to calculate phase amplitude coupling (PAC) quickly and robustly for realtime applications. We designed and implemented a multirate PAC algorithm with efficient filter bank processing and efficient computation of PAC for many frequency-pair combinations. We tested the developed algorithm for computing PAC on simulated data and on intraoperative neural recording data obtained during deep brain stimulation (DBS) electrode implantation surgery. A combination of parallelized frequency-domain filtering and modulation index for PAC estimation provided robust results that could be calculated in real time on modest computing hardware. The standard methods for calculating PAC can be optimized for quick and robust performance. These results demonstrated that PAC can be extracted in real time and is suitable for neurofeedback applications.

Research paper thumbnail of A Quadrantic Bias in Prefrontal Representation of Visual-Mnemonic Space

Cerebral Cortex

Single neurons in primate dorsolateral prefrontal cortex (dLPFC) are known to encode working memo... more Single neurons in primate dorsolateral prefrontal cortex (dLPFC) are known to encode working memory (WM) representations of visual space. Psychophysical studies have shown that the horizontal and vertical meridians of the visual field can bias spatial information maintained in WM. However, most studies and models have tacitly assumed that dLPFC neurons represent mnemonic space homogenously. The anatomical organization of these representations has also eluded clear parametric description. We investigated these issues by recording from neuronal ensembles in macaque dLPFC with microelectrode arrays while subjects performed an oculomotor delayed-response task. We found that spatial WM representations in macaque dLPFC are biased by the vertical and horizontal meridians of the visual field, dividing mnemonic space into quadrants. This bias is reflected in single neuron firing rates, neuronal ensemble representations, the spike count correlation structure, and eye movement patterns. We also found that dLPFC representations of mnemonic space cluster anatomically in a nonretinotopic manner that partially reflects the organization of visual space. These results provide an explanation for known WM biases, and reveal novel principles of WM representation in prefrontal neuronal ensembles and across the cortical surface, as well as the need to reconceptualize models of WM to accommodate the observed representational biases.

Research paper thumbnail of Effects of neural ensemble size and composition on the decoding of attention in primate lateral prefrontal cortex

Research paper thumbnail of Heterogeneous effects of neuronal ensemble size, tuning, and correlation structure on the decoding of spatial working memory in dorsolateral prefrontal cortex

Research paper thumbnail of A metric-based analysis of the contribution of spike timing to contrast and motion direction coding by single neurons in macaque area MT

Brain Research, Jan 12, 2011

Spike timing is thought to contribute to the coding of motion direction information by neurons in... more Spike timing is thought to contribute to the coding of motion direction information by neurons in macaque area MT. Here, we examined whether spike timing also contributes to the coding of stimulus contrast. We applied a metric-based approach to spike trains fired by MT neurons in response to stimuli that varied in contrast, or direction. We assessed the performance of three metrics, D spike and D product (containing spike count and timing information), and the spike count metric D count. We analyzed responses elicited during the first 200 msec of stimulus presentation from 205 neurons. For both contrast and direction, the large majority of neurons showed the highest mutual information using D spike , followed by D product , and D count. This was corroborated by the performance of a theoretical observer model at discriminating contrast and direction using the three metrics. Our results demonstrate that spike timing can contribute to contrast coding in MT neurons, and support previous reports of its potential contribution to direction coding. Furthermore, they suggest that a combination of spike count with periodic and non-periodic spike timing information (contained in D spike , but not in D product and D count which are insensitive to spike counts and timing respectively) provides the largest coding advantage in spike trains fired by MT neurons during contrast and direction discrimination.