John Assad, Ph.D.

Professor of Neurobiology
  • 617/432-2804

The goal of my lab’s research is to understand how the primate brain controls behavior by integrating external sensory information with internal states, such as attention or motivation. The brain has an astonishing ability to flexibly link sensation and action. The same visual object may trigger different actions depending on external context or the animal’s internal needs, preferences or state of attention.

My lab uses electrophysiological techniques to study the brain computations between sensory input and motor output. We have focused on three important aspects of this processing chain:
1) how visual information is transformed into a behaviorally useful form,
2) how appropriate movements are initiated at precise times, and 
3) how the value of objects or goals in the environment is assigned to guide behavior.

For many of these studies, my lab has focused on the parietal cortex, a part of the brain that acts as a bridge between sensation and action. We have also examined the role of the basal ganglia in movement control, an issue that is relevant for understanding movement disorders such as Parkinson’s disease, and we have studied the orbitofrontal cortex in the context of how the brain represents value.

Why do we do things exactly when we do? What “goes off” in our brains to trigger movements at precise times? Answers to these questions are central to understanding the neural mechanisms of movement control, and may shed light on movement disorders such as Parkinson’s and Huntington’s disease.

This illustration shows responses of neurons in an experiment from our lab designed to investigate mechanisms of movement initiation. Macaques viewed a moving spot of light on a computer screen and performed a movement-timing task. When the spot reached a certain point on the screen the animals released a lever, but the animals had fairly wide latitude in the precise time of when they release the lever. That is, the movements were “self-timed” or “proactive”, not reflexive. The distribution of the times of the lever release (hand movement) were thus quite variable, as shown by the gray Gaussian histograms (spread out over about one second). In each of the three panels, the different colored traces correspond to average neural activity from a pool of neurons, with different colors corresponding to different average times for the lever release. For example, the purple traces correspond to neuronal activity in cases when the animals released the lever very early, while the red when the animals released very late. In the top panel, this subset of parietal neurons (from cortical areas MT and MST) shows very little variability with respect to the time of hand movement. These neurons likely faithfully “encode” the visual motion of the moving spot. In the middle panel, these neurons (from parietal Area 5) show responses that are aligned to the time of movement. These neurons could be involved in driving the arm movement per se. But in the bottom panel, these neurons (from area LIP) show different responses for the different times of the arm movement. When the animal moved his arm very early, the cells activity increased much more steeply than when the animals moved later, almost as if the population activity has to reach a threshold level to trigger the movement. This activity cannot be attributed to either the visual stimulus or the arm movement per se, because these factors were the same for all cases. Rather, the activity may be more “cognitive”, related to internal timing mechanisms or internally driven, proactive movements. These types of movements can be particularly impaired in patients with Parkinson’s disease, so these neural circuits may begin to provide clues to certain movement disorders. [From Maimon G and Assad JA (2006) A parietal signal for the proactive timing of action. Nature Neurosci. 9:948-955]

Why do we do things exactly when we do? What 'goes off' in our brains to trigger movements at precise times? Answers to these questions are central to understanding the neural mechanisms of movement control, and may shed light on movement disorders such as Parkinson’s and Huntington’s disease.

 

The goal of my lab’s research is to understand how the primate brain controls behavior by integrating external sensory information with internal states, such as attention or motivation. The brain has an astonishing ability to flexibly link sensation and action. The same visual object may trigger different actions depending on external context or the animal’s internal needs, preferences or state of attention.

My lab uses electrophysiological techniques to study the brain computations between sensory input and motor output. We have focused on three important aspects of this processing chain:
1) how visual information is transformed into a behaviorally useful form,
2) how appropriate movements are initiated at precise times, and 
3) how the value of objects or goals in the environment is assigned to guide behavior.

For many of these studies, my lab has focused on the parietal cortex, a part of the brain that acts as a bridge between sensation and action. We have also examined the role of the basal ganglia in movement control, an issue that is relevant for understanding movement disorders such as Parkinson’s disease, and we have studied the orbitofrontal cortex in the context of how the brain represents value.

  1. Aghdaee SM, Battelli L, Assad JA. Relative timing: from behaviour to neurons. January 20, 2014. Philosophical transactions of the Royal Society of London. Series B, Biological sciences.

    Link to Abstract
  2. Volcic R, Fantoni C, Caudek C, Assad JA, Domini F. Visuomotor adaptation changes stereoscopic depth perception and tactile discrimination. October 23, 2013. The Journal of neuroscience : the official journal of the Society for Neuroscience.

    Link to Abstract
  3. Fitzgerald JK, Freedman DJ, Fanini A, Bennur S, Gold JI, Assad JA. Biased associative representations in parietal cortex. January 9, 2013. Neuron.

    Link to Abstract
  4. Aghdaee SM, Battelli L, Assad JA. The role of parietal cortex in relative timing January 1, 2013. Phil. Trans. Royal Soc. (In Press).

  5. Fitzgerald JK, Freedman DJ, Assad JA. Generalized associative representations in parietal cortex. July 17, 2011. Nature neuroscience.

    Link to Abstract
  6. Freedman DJ, Assad JA. A proposed common neural mechanism for categorization and perceptual decisions. February 1, 2011. Nature neuroscience.

    Link to Abstract
  7. Herrington TM, Assad JA. Temporal sequence of attentional modulation in the lateral intraparietal area and middle temporal area during rapid covert shifts of attention. March 3, 2010. The Journal of neuroscience : the official journal of the Society for Neuroscience.

    Link to Abstract
  8. Herrington TM, Assad JA. Neural activity in the middle temporal area and lateral intraparietal area during endogenously cued shifts of attention. November 11, 2009. The Journal of neuroscience : the official journal of the Society for Neuroscience.

    Link to Abstract
  9. Maimon G, Assad JA. Beyond Poisson: increased spike-time regularity across primate parietal cortex. May 14, 2009. Neuron.

    Link to Abstract
  10. Herrington TM, Masse NY, Hachmeh KJ, Smith JE, Assad JA, Cook EP. The effect of microsaccades on the correlation between neural activity and behavior in middle temporal, ventral intraparietal, and lateral intraparietal areas. May 6, 2009. The Journal of neuroscience : the official journal of the Society for Neuroscience.

    Link to Abstract
  11. Freedman DJ, Assad JA. Distinct encoding of spatial and nonspatial visual information in parietal cortex. April 29, 2009. The Journal of neuroscience : the official journal of the Society for Neuroscience.

    Link to Abstract
  12. Fanini A, Assad JA. Direction selectivity of neurons in the macaque lateral intraparietal area. November 5, 2008. Journal of neurophysiology.

    Link to Abstract
  13. Montague PR, Assad J. Editorial overview. September 11, 2008. Current opinion in neurobiology.

    Link to Abstract
  14. Padoa-Schioppa C, Assad JA. The representation of economic value in the orbitofrontal cortex is invariant for changes of menu. December 9, 2007. Nature neuroscience.

    Link to Abstract
  15. Freedman DJ, Assad JA. Experience-dependent representation of visual categories in parietal cortex. August 27, 2006. Nature.

    Link to Abstract
  16. Maimon G, Assad JA. A cognitive signal for the proactive timing of action in macaque LIP. June 4, 2006. Nature neuroscience.

    Link to Abstract
  17. Padoa-Schioppa C, Assad JA. Neurons in the orbitofrontal cortex encode economic value. April 23, 2006. Nature.

    Link to Abstract
  18. Maimon G, Assad JA. Parietal area 5 and the initiation of self-timed movements versus simple reactions. March 1, 2006. The Journal of neuroscience : the official journal of the Society for Neuroscience.

    Link to Abstract
  19. Lee IH, Seitz AR, Assad JA. Activity of tonically active neurons in the monkey putamen during initiation and withholding of movement. January 11, 2006. Journal of neurophysiology.

    Link to Abstract
  20. Williams ZM, Elfar JC, Eskandar EN, Toth LJ, Assad JA. Parietal activity and the perceived direction of ambiguous apparent motion. June 1, 2003. Nature neuroscience.

    Link to Abstract
  21. Assad JA. Neural coding of behavioral relevance in parietal cortex. April 1, 2003. Current opinion in neurobiology.

    Link to Abstract
  22. Lee IH, Assad JA. Putaminal activity for simple reactions or self-timed movements. January 15, 2003. Journal of neurophysiology.

    Link to Abstract
  23. Eskandar EN, Assad JA. Distinct nature of directional signals among parietal cortical areas during visual guidance. October 1, 2002. Journal of neurophysiology.

    Link to Abstract
  24. Toth LJ, Assad JA. Dynamic coding of behaviourally relevant stimuli in parietal cortex. January 10, 2002. Nature.

    Link to Abstract
  25. Assad JA. A biased view of attention. April 1, 1999. Neuron.

    Link to Abstract
  26. Assad J. Now you see it: frontal eye field responses to invisible targets. March 1, 1999. Nature neuroscience.

    Link to Abstract
  27. Eskandar EN, Assad JA. Dissociation of visual, motor and predictive signals in parietal cortex during visual guidance. January 1, 1999. Nature neuroscience.

    Link to Abstract
  28. Maunsell JH, Ghose GM, Assad JA, McAdams CJ, Boudreau CE, Noerager BD. Visual response latencies of magnocellular and parvocellular LGN neurons in macaque monkeys. January 1, 1999. Visual neuroscience.

    Link to Abstract
  29. Assad JA, Maunsell JH. Neuronal correlates of inferred motion in primate posterior parietal cortex. February 9, 1995. Nature.

    Link to Abstract
  30. Assad JA, Corey DP. An active motor model for adaptation by vertebrate hair cells. September 1, 1992. The Journal of neuroscience : the official journal of the Society for Neuroscience.

    Link to Abstract
  31. Corey DP, Assad JA. Transduction and adaptation in vertebrate hair cells: correlating structure with function. January 1, 1992. Society of General Physiologists series.

    Link to Abstract
  32. Assad JA, Shepherd GM, Corey DP. Tip-link integrity and mechanical transduction in vertebrate hair cells. December 1, 1991. Neuron.

    Link to Abstract
  33. Hacohen N, Assad JA, Smith WJ, Corey DP. Regulation of tension on hair-cell transduction channels: displacement and calcium dependence. November 1, 1989. The Journal of neuroscience : the official journal of the Society for Neuroscience.

    Link to Abstract
  34. Assad JA, Hacohen N, Corey DP. Voltage dependence of adaptation and active bundle movement in bullfrog saccular hair cells. April 1, 1989. Proceedings of the National Academy of Sciences of the United States of America.

    Link to Abstract
  35. Loretz CA, Assad JA. Urotensin II lowers cytoplasmic free calcium concentration in goby enterocytes: measurements using quin2. December 1, 1986. General and comparative endocrinology.

    Link to Abstract
  36. Full RJ, Herreid CF and Assad JA. Energetics of the exercising wharf crab Sesarma cinereum January 1, 1985. Physiol. Zool..

Harvard Medical School
Dept of Neurobiology, WAB 227
200 Longwood Ave
Boston MA 02115