Neural Correlates of Perceptual Learning in a Sensory-motor, but not a Sensory, Cortical Area. Law, Gold. Nature Neuroscience 2008

  1. Seeks to “identify neural mechanisms that underlie perceptual learning in a visual-discrimination task.”
  2. Monkey study
  3. “… for this task, perceptual learning does not appear to involve improvements in how sensory information is represented in the brain, but rather how the sensory representation is interpreted to form the decision that guides behavior.”
  4. Perceptual learning is the improved ability to detect or identify sensory stimuli through training
    1. Our understanding of how this happens, however, is not well understood in terms of what is happening from the perspective of neuroscience
  5. Here they examine two areas, one in the visual cortex, and the other in parietal cortex, to understand what changes occur in those areas
  6. Monkeys were trained to determine direction of motion with a random dot stimulus and to move their eye in the same direction as motion
    1. Neurons that (at least partially) accomplish this task are in area “MT” of the visual cortex.
  7. LIP area of parietal cortex “… has been implicated in a variety of cognitive and visuomotor functions, including attention, intention, reward anticipation, and decision making…” and respond during this decision task.  They also “…can be strongly predictive of the monkey’s saccadic choices… Therefore, area LIP appears to be involved in transforming motion information into a saccadic choice…”
  8. During training, sensitivity to weak motion signals improved, and changes in responses of neurons occurred in LIP but not MT, consistent with how sensory information is used to make decisions (LIP) but not in terms of any change in the representation of the evidence itself (MT).
  9. Recorded individual neurons
  10. “MT responses before and during training showed a pattern of activation similar to that reported in trained monkeys.”
  11. MT responses depend only on motion direction but not viewing time <length of episode?>, and doesn’t change with training (true of both most reactive neurons as well as the population of neurons sampled as well)
    1. The only thing that changed somewhat is reduced variability of responses, but even this was an extremely weak response (5% above chance at 50%).  In terms of this, the most sensitive neurons changed the most over time. “… the systematic change in choice probability probably reflects an increasingly selective readout of activity from MT neurons, particularly those most sensitive to the motion stimulus.”
  12. “…there was little to no influence of the motion stimulus on the LIP responses at the beginning of training, and progressed steadily to more positive values… this result implies that training both established and then shaped the sensory-driven responses in these sensory motor neurons.”
  13. “coherence-specific” pattern of changes in LIP similar to coherence-specific changes in behavior
  14. “Our results suggest that the improvements in behavioral sensitivity result from changes not in the sensory representation, but rather in its interpretation.”
  15. Developed a 3-part model that factors in MT, LIP, and the behavioral data
    1. “A straightforward interpretation of this result is that training does not affect the representation of sensory evidence in MT directly, but effectively scales its output, providing increasingly sensitive evidence to the decision variable in LIP used to guide behavior.”
  16. “Perceptual learning is typically specific to the stimulus attributes used during training, including motion direction in a direction-discrimination task.  This phenomenon helps to distinguish perceptual learning from cognitive or motor changes and has been used to to argue that the underlying changes occur at early stages of of sensory processing.”
  17. “perceptual learning can be specific by virtue of what is learned (in this case, the LIP decision variable) and need not involve changes in the sensory representation (direction-selective responses in area MT).”

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