Recent Abstracts

  • Michalopoulos K, Spinelli P, Pasternak T. 2015. Neurons in the lateral prefrontal cortex compensate for the asymmetries of their sensory inputs during memory-guided comparisons of visual motion. Presented at the The Society for Neuroscience Annual Conference, Chicago, IL.


    Comparing two visual motion stimuli that occur at different times require processing and storage of the initial stimulus, followed by its retrieval and comparison to the current stimulus. Such tasks demand coordination of processes involving bottom-up sensory information and top-down cognitive signals. Prefrontal cortex (PFC) is likely to play a key role in this coordination since it receives inputs from the motion processing area MT and the activity of its neurons reflects task engagement and participation in sensory maintenance. When motion comparisons involve stimuli presented at the fovea, neurons in the lateral PFC (LPFC) show direction selective (DS) responses indicative of their MT origins and display anticipatory and memory-related activity, the likely components of their cognitive influences. Little is known about LPFC activity during tasks involving motion presented outside the fovea. This question is of interest because of the connectivity between LPFC and the highly retinotopic area MT: while the information about the contralateral motion can be supplied directly by MT in the same hemisphere, the ipsilateral motion represented by MT in the opposite hemisphere, can only reach LPFC indirectly, most likely via callosal connections from the opposite LPFC. We examined how the LPFC neurons represent and utilize motion information that originates in the ipsilateral and contralateral hemifields while monkeys compared directions of two stimuli, S1 and S2, separated by a delay. During S1, responses to the contralateral motion were stronger and preceded ipsilateral responses by ~40ms, an indication of the apparent dominance of direct inputs from the ipsilateral MT. The asymmetry between contralateral and ipsilateral responses during S1 was not reflected in their DS activity, since it was equally robust for both stimulus locations. During the S2, responses to the ipsilateral but not the contralateral stimuli were enhanced, eliminating the dominance of the contralateral signals observed during S1. In addition, the activity of many neurons during and after S2 for stimuli in each of the two hemifields was modulated by the direction of S1, reflecting the difference between current and remembered stimuli and was predictive of the perceptual report. These results demonstrate that the process of memory-guided stimulus comparisons seamlessly incorporates sensory information from both hemispheres. Thus, while the sensory components of the LPFC activity reflect the difference between their direct and indirect origins, it appears that its cognitive components can compensate for the inequalities in their bottom-up sensory signals.

  • Michalopoulos K, Spinelli P and Pasternak T. 2015. Prefrontal neurons represent comparisons of motion directions in the contralateral and the ipsilateral visual fields. Presented at the Vision Science Society Annual Meeting, St. Pete Beach, Florida.


    Neurons in the lateral prefrontal cortex (LPFC) are active when monkeys decide whether two stimuli, S1 and S2, separated by a delay, move in the same or in different directions. Their responses show direction selectivity reminiscent of activity in motion processing area MT, and during S2, their responses are modulated by the remembered direction. A similar modulation, termed comparison effect (CE), has also been observed in area MT. These parallels between the two areas are consistent with their connectivity, although the nature of this connectivity suggests a possibility of asymmetries in the way contralateral and ipsilateral motion is represented in the LPFC during the motion tasks. Specifically, while signals about the contralateral motion reach LPFC directly from MT of the same hemisphere, ipsilateral motion processed by MT in the other hemisphere can only reach the LPFC indirectly via callosal connections from the opposite LPFC.

    We explored the role of direct and indirect motion signals in the generation of the comparison effects by examining responses of LPFC to contralateral and ipsilateral stimuli during S2. The CE was measured by comparing response to identical S2 stimuli on trials when S2 was preceded by S1 moving in the same direction (S-trials) with trials when it was preceded by S1 moving in a different direction (D-trials). ROC analysis revealed two distinct groups of neurons preferring either S-trials or D-trials. Although these CE effects were equally likely to occur for ipsilateral and contralateral stimuli, the signals for the contralateral hemifield appeared 100ms earlier, a likely reflection of direct connectivity with area MT. These results demonstrate that the comparison between the current and the remembered stimulus can be carried out in the LPFC even in the absence of direct inputs from area MT.

  • Ren, P., Ramon, M., Spinelli, P.M., Compte, A., Pasternak, T.. 2014. Evoked and mnemonic representations of spatial information in prefrontal cortex during memory-guided location comparisons. Presented at the The Society for Neuroscience Annual Conference, Washington, DC.


    We previously reported that neurons in lateral prefrontal cortex (LPFC) represent behaviorally relevant speed and direction of visual motion during all stages of memoryguided motion comparison tasks (Hussar & Pasternak, JN 2012, 2013). Here, we examined how spatial location is represented in LPFC during an analogous memoryguided comparison task involving locations of motion stimuli. We analyzed dlPFC spiking activity and power spectra of local field potentials (LFP) during a task in which monkeys compared locations of two moving random-dot stimuli, labeled as S1 and S2, separated by a memory delay.

    Analysis of spiking activity revealed location selective responses during both S1 and S2 and these responses were stronger during S2, most likely reflecting additional demands imposed by the comparison with the remembered location of S1. These observations were mirrored by LFP power in the β band of frequencies (12-30 Hz). β-band power showed significant location selective suppression during both S1 and S2, with stronger effects during S2. During the delay, many individual neurons showed gradual anticipatory activity modulation leading to the S2 onset, a pattern also observed in the β-band power of LFPs. In contrast with this close relationship between β-band power and spiking activity, θ-band LFP power showed task-dependent modulations dissociated from neuronal activity. Indeed, location selective delay activity in single neurons was transient and unrelated to the selectivity recorded during S1, appearing in different neurons at different times, a pattern analogous to that observed during the motion comparison tasks. However, LFP power in the θ-band (5-8 Hz) showed a strong enhancement during the memory period with respect to baseline power, which was location selective and decayed by the end of the delay.

    These results reveal a striking parallelism between the way individual neurons represent sensory and spatial information during comparison tasks, suggesting that spatial location shares with the parameters of visual motion a common, or analogous neural substrate for representing sensory information in LPFC during such tasks. This substrate might be different for evoked and mnemonic representations. Similar dynamics and location selectivity in β-band LFP and spiking activity suggest that evoked representations engage populations with clustered representations of sensory attributes, which oscillate at β frequencies. Mnemonic representations during the delay period, instead, may be supported by more distributed circuit-level representations linked to θ-band dynamics.

  • 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.


    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.

  • 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.


    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.