Recent Abstracts

  • Ren P, Syc S, Spinelli P and Pasternak T. 2013. Spatial specificity of direction selectivity in the dorsolateral prefrontal cortex during direction comparison task. Presented at the The Society for Neuroscience Annual Conference, San Diego, CA.

    Abstract

    As we interact with our environment, the features of objects in the visual scene are not consistently present on the retina and sensory cues used to guide visual behavior are not always available. Thus, active observers are faced with a ubiquitous task of comparing sensory stimuli across time and space. When monkeys compare directions of visual motion of two stimuli presented at the fovea, S1 and S2, separated by a delay, neurons in the dorsolateral prefrontal cortex (DLPFC) show direction selective (DS) responses suggestive of their origins in area MT. In addition, responses during S2, the comparison stage of the task, are often modulated by the direction presented during S1. However, DLPFC neurons respond to motion not only at the fovea but also across the entire visual field, receiving direct bottom-up inputs from neurons in the ipsilateral MT representing contralateral stimuli and indirectly from the opposite MT representing ipsilateral stimuli. We examined whether DLPFC retains the spatial specificity in DS characteristic of its retinotopic inputs by presenting stimuli in the contralateral and ipsilateral hemifields during the direction comparison task. We found that responses to visual motion of many DLPFC neurons changed with location: they were more likely to be DS and this selectivity emerged earlier when stimuli appeared in the contralateral field. Preferred directions of neurons with DS for contralateral and ipsilateral stimuli were strongly correlated, suggesting alignment of direction information arriving in DLPF from MT neurons residing in the opposite hemispheres. Finally, response modulation during S2 also depended on stimulus location, weakening when the preceding S1 appeared in the opposite hemifield, suggesting participation of retinotopically organized cortical regions in the comparison process. Our results show that representation of visual motion in DLPFC is likely to be governed by its direct and indirect connectivity with area MT. The strong correlation between direction preferences in the two hemifields points to a mechanism that may facilitate integration of motion information across the visual field.

  • Hussar, CR, and Pasternak T. 2012. Common rules guide comparisons of speed and direction of motion in the dorsolateral prefrontal cortex.

    Abstract

    When a monkey needs to decide whether motion direction of one stimulus is the same or different as that of another held in working memory, neurons in dorsolateral prefrontal cortex (DLPFC) faithfully represent the motion directions being evaluated and contribute to their comparison. Here, we examined whether DLPFC neurons are more generally involved in other types of sensory comparisons. Such involvement would support the existence of generalized sensory comparison mechanisms within DLPFC, shedding light on top-down influences this region is likely to provide to the upstream sensory neurons during comparison tasks. We recorded activity of individual neurons in the DLPFC while monkeys performed a memory-guided decision task in which the important dimension was the speed of two sequentially presented moving random-dot stimuli. We found that many neurons, both narrow-spiking (NS) putative local interneurons and broad-spiking (BS) putative pyramidal output cells, were speed selective, with tuning reminiscent of that observed in motion processing area MT. Throughout the delay, BS neurons were more active, showing anticipatory rate modulation and transient periods of speed selectivity. During the comparison stimulus, responses of both cell types were modulated by the speed of the first stimulus, and their activity was highly predictive of the animals' behavioral report. These results are similar to those found for comparisons of motion direction, suggesting the existence of generalized neural mechanisms in the DLPFC sub-serving the comparison of sensory signals.

  • Zarella M and Pasternak T. 2012. Trial-to-trial variability of MT neurons reveals the nature of their engagement in a motion discrimination task. Presented at the Computation and Systems Neuroscience (COSYNE) Conference, Salt Lake City, UT.

    Abstract

    We have recently shown that neurons in the motion processing area MT and in the prefrontal cortex (PFC) are actively engaged in all stages of a task in which monkeys compare two directions of motion, S1 and S2, separated by a delay. Neurons in both areas showed direction selective responses, were active during the delay, and showed comparison effects that correlated with perceptual decision. In the PFC, this engagement was also reflected in trial-to-trial variability of spiking activity (Fano Factor, FF) of putative pyramidal neurons, a likely source of top-down influences on MT. The FF tracked consecutive task components and was predictive of the upcoming neuronal events, dropping with stimulus onset, decreasing prior to salient events and flagging neurons participating in sensory comparisons. Here, we report that the variability of spiking activity in MT during the same behavioral task followed a similar pattern. The FF showed a typical rapid drop with stimulus onset, which was present even for stimuli that appeared remotely from the neuron’s receptive field, revealing that even in the absence of overt activity MT neurons were engaged in discrimination. The FF also reflected stimulus identity during several trial components, even in the absence of selective spiking activity. With time in delay, variability of many neurons increased, the pattern opposite to that observed in PFC, suggesting possible interactions between the two areas in preparation for comparison stimulus. Towards the end of the delay, variability of neurons with future comparison effects decreased, an effect analogous but delayed relative to that observed in the PFC, suggesting its possible top-down influences on MT neurons participating in sensory comparisons. Our results demonstrate that the FF provides a sensitive measure of the engagement of MT neurons in motion discrimination tasks and suggest the nature of their interactions with PFC during such tasks.

  • Wimmer, K, Hussar, CR, Pasternak, T, and Compte, A. 2011. Local field potentials recorded in the prefrontal cortex reveal the nature of its engagement in a multi-stage motion discrimination task. Presented at the The Society for Neuroscience Annual Conference, Washington, DC.

    Abstract

    Neurons in the prefrontal cortex (PFC) are active throughout tasks involving comparisons of motion directions across time. We examined local field potentials (LFP) to gain insights into the PFC participation in such tasks from the network perspective. We recorded simultaneously LFPs and single-neuron responses of two monkeys comparing the directions of two moving random-dot patterns, sample and test, separated by a 1500 ms delay. We quantified the temporal fluctuations in the LFP signals by estimating time dependent spectra using multi-taper spectral analysis methods. Spectral analysis revealed that different frequency bands of the LFPs reliably tracked consecutive components of the task. At the onset of each stimulus, the LFP power increased significantly in the gamma range (above 30 Hz). This increase was larger during the test than during the sample, most likely reflecting additional task demands during the comparison phase of the task leading to the perceptual decision. Moreover, the gamma power during the test was modulated by the direction of the preceding sample, an effect indicative of the process of sensory comparisons taking place during the test. Importantly, these effects were absent during a passive fixation task when the animals were not required to perform direction discrimination, suggesting that the observed modulations were task-driven. These gamma-band modulations paralleled previously reported effects in spiking activity. Finally, the beta band oscillations (typically 20 Hz) were suppressed during the sample and began declining again in apparent anticipation of the test, the pattern reminiscent of changes in trial-to-trial variability of individual neurons during the same task. Our results suggest that LFPs along with spiking activity reflect a common network dynamics underlying PFC activity during sensory discrimination tasks.

  • Hussar CR and Pasternak T. 2010. Trial-to-trial Variability of the Putative Pyramidal Prefrontal Neurons Reveals the Nature of their Engagement in a Motion Discrimination Task. Presented at the The Society for Neuroscience Annual Conference, San Diego, CA.

    Abstract

    Neurons in the prefrontal cortex (PFC) are thought to play a key role in cognitive control and to exert top-down influences on sensory cortical regions with which they are reciprocally connected. Recordings from PFC neurons during motion discrimination tasks revealed direction selective (DS) responses, shedding light on the nature of top-down signals these neurons are likely to provide (Hussar & Pasternak, Neuron, 2009; Zaksas & Pasternak, J. NSC, 2006). Recent studies provided evidence that the variability of spiking activity characteristic of cortical neurons may be a source of information about the state of neurons and their participation in sensory and motor tasks. We examined whether the variability of spiking activity of pyramidal neurons in the PFC, a likely source of top-down influences, provides insights into the nature of their involvement in motion discrimination tasks.

    We analyzed trial-to-trial variability of putative pyramidal neurons, identified on the basis of the duration of their action potentials, during a direction discrimination task. In this task, monkeys compared the directions of two random-dot stimuli, sample and test, separated by a delay. As a measure of variability we computed the ratio of spike count variance to mean spike count (Fano Factor, FF). We found that variability of spiking activity tracked consecutive components of the task, dropping with the onset of motion stimuli being discriminated and gradually declining prior to each salient event of the trial: sample, test and response. The decreases in variability during the memory delay and during the test were largely absent during passive fixation when the monkeys were not performing the discrimination, suggesting that the observed modulations were task driven.

    We also found that the decrease in variability during the periods preceding the sample, the test and the response were less likely to be carried by neurons with DS responses, suggesting different functional contributions of DS and non-DS neurons. In addition, we found that neurons with activity that during the test reflected the remembered direction showed a pronounced drop in FF early in the delay, illustrating the predictive nature of neural variability. Our results demonstrate that changes in neural variability provide a sensitive measure of engagement of putative pyramidal PFC neurons in circuits sub-serving sensory discrimination tasks.

  • Hussar CR and Pasternak T. 2010. Memory-related signals in the PFC depend on cell type only in the absence of sensory stimulation. 8 MB Presented at the Computation and Systems Neuroscience (COSYNE) Conference, Salt Lake City, UT.

    Abstract

    Neurons in the prefrontal cortex (PFC) show direction selective responses to behaviorally relevant visual motion (Zaksas & Pasternak, 2006, Hussar & Pasternak, 2009). In this study we examined memory related signals in the PFC during a task where monkeys compared two directions of motion, sample and test, separated by a brief delay. For the analysis of neuronal activity recorded during the sample, the delay and during the comparison test we used spike waveform durations to classify the recorded neurons into narrow-spiking (NS) putative inhibitory interneurons and broad-spiking (BS) putative pyramidal neurons. We found that while responses of both classes of neurons to visual motion used in the task were equally likely to be direction selective, during the memory delay the pattern of activity for the two cell classes was different. BS neurons were significantly more active than NS cells and were more likely to show anticipatory changes in firing rates. Furthermore, BS neurons were also significantly more likely to carry signals reflecting the direction of the preceding sample. These signals were largely transient and appeared in different neurons at different times in the delay, suggesting that the information about the remembered direction is likely to be distributed among PFC neurons. The difference between the two classes of neurons became particularly apparent at the end of the delay when memory-related signals were represented exclusively by BS neurons.

    In contrast, during the comparison phase of the task, responses to the test of NS and BS cells were similar and on trials when test direction matched that of the preceding sample, both cell types showed lower activity. This match suppression, likely to represent the process of sensory comparison, reflected the difference in the direction between sample and test, decreasing with smaller difference between the two stimuli and disappearing when the monkey was not required to perform direction discrimination. Furthermore, responses during the test of both cell classes reflected the upcoming decision, showing significant choice probability towards the end of the response.

    These results reveal important differences in the contribution of the putative inhibitory interneurons and of the pyramidal cells to delayed discrimination tasks. Stimulus-driven activity, likely to represent bottom-up signals arriving from sensory cortex, was similar in both classes of cells, suggesting that both cell types participate in the sensory components of the task. However, in the absence of sensory stimulation, delay activity was dominated by putative pyramidal neurons. Since these neurons are a likely source of top-down projections from the PFC to visual and parietal cortical neurons, they may be a source of anticipatory and stimulus-related delay activity frequently observed in these neurons.

    Supported by NIH grants R01 EY11749, T32 EY07125 & P30 EY01319

  • Hussar CR and Pasternak T. 2010. Memory-related signals in the PFC depend on cell type only in the absence of sensory stimulation. 8 MB Presented at the Computation and Systems Neuroscience (COSYNE) Conference, Salt Lake City, UT.

    Abstract

    Neurons in the prefrontal cortex (PFC) show direction selective responses to behaviorally relevant visual motion (Zaksas & Pasternak, 2006, Hussar & Pasternak, 2009). In this study we examined memory related signals in the PFC during a task where monkeys compared two directions of motion, sample and test, separated by a brief delay. For the analysis of neuronal activity recorded during the sample, the delay and during the comparison test we used spike waveform durations to classify the recorded neurons into narrow-spiking (NS) putative inhibitory interneurons and broad-spiking (BS) putative pyramidal neurons. We found that while responses of both classes of neurons to visual motion used in the task were equally likely to be direction selective, during the memory delay the pattern of activity for the two cell classes was different. BS neurons were significantly more active than NS cells and were more likely to show anticipatory changes in firing rates. Furthermore, BS neurons were also significantly more likely to carry signals reflecting the direction of the preceding sample. These signals were largely transient and appeared in different neurons at different times in the delay, suggesting that the information about the remembered direction is likely to be distributed among PFC neurons. The difference between the two classes of neurons became particularly apparent at the end of the delay when memory-related signals were represented exclusively by BS neurons.

    In contrast, during the comparison phase of the task, responses to the test of NS and BS cells were similar and on trials when test direction matched that of the preceding sample, both cell types showed lower activity. This match suppression, likely to represent the process of sensory comparison, reflected the difference in the direction between sample and test, decreasing with smaller difference between the two stimuli and disappearing when the monkey was not required to perform direction discrimination. Furthermore, responses during the test of both cell classes reflected the upcoming decision, showing significant choice probability towards the end of the response.

    These results reveal important differences in the contribution of the putative inhibitory interneurons and of the pyramidal cells to delayed discrimination tasks. Stimulus-driven activity, likely to represent bottom-up signals arriving from sensory cortex, was similar in both classes of cells, suggesting that both cell types participate in the sensory components of the task. However, in the absence of sensory stimulation, delay activity was dominated by putative pyramidal neurons. Since these neurons are a likely source of top-down projections from the PFC to visual and parietal cortical neurons, they may be a source of anticipatory and stimulus-related delay activity frequently observed in these neurons.

    Supported by NIH grants R01 EY11749, T32 EY07125 & P30 EY01319

  • Lui LL, Mancarella MA, and Pasternak T. 2009. A unilateral PFC lesion affects neuronal activity in area MT during motion discrimination tasks. 7 MB Presented at the Vision Sciences Society (VSS) Conference.

    Abstract

    Neurons in the prearcuate region of prefrontal cortex (PFC) receive inputs from the motion-processing area MT and during motion discrimination exhibit direction selective (DS) responses suggestive of their MT origins (Zaksas & Pasternak, 2006). This region also sends direct top-down projections to MT and the nature of activity in both areas recorded during the same motion task indicates strong functional links between them. We examined the contribution of top-down PFC influences to MT responses and to motion perception while a monkey with a unilateral ibotenic PFC lesion discriminated directions of two sequential stimuli, sample and test, separated by a delay.

    Recordings from the ipsilateral MT revealed abnormalities during all phases of the task. During the sample, a significant decrease in responses and a drop in DS, was accompanied by a transient increase in response variability. During the delay, suppression in activity and a subsequent anticipatory increase in firing rates, common in normal MT, were absent. This was accompanied by abnormally low variability of delay activity, suggesting that the top-down PFC signals may be among contributors to the variability in delay activity in normal MT. During the test, on trials when its direction matched the direction of sample, the normally transient response suppression was more sustained, suggesting PFC involvement in the sensory comparison phase of the task.

    Behavioral testing revealed deficits in direction thresholds at longer delays, indicating a disruption in the maintenance and/or attentional components of the task. These deficits were most dramatic when the task required rapid reallocation of spatial attention. The lesion effects were confined to the contralesional visual field, suggesting a link to retinotopic areas involved in motion processing (eg. MT). Our results demonstrate the importance of PFC influences for normal MT activity during motion discrimination and for normal motion perception.

    Supported by EY11749; NHMRC (Aus) Post-Doc fellowship to L.L. and P30 EY01319 to CVS

Recent Publications