Dissociating hippocampal and basal ganglia contributions to category learning using stimulus novelty and subjective judgments. Seger, Dennison, Lopez-Paniagua, Peterson, Roark. Neuroimage 2011.

  1. “We identified factors leading to hippocampal and basal ganglia recruitment during categorization learning.”
  2. In the experiment there were alternating trial and error catergory learning, interspersed with a subjective judgement task
    1. In the subjective task, subjects categorized the stimulus, but instead of receiving feedback they also recorded the basis of their response with one of 4 options:
      1. Remember: <response is based on?> “Conscious episodic memory of previous trials.”
      2. Know-Automatic: “Automatic, rapid response accompanied by conscious awareness of category membership.”
      3. Know-Intuition: “A ‘gut feeling’ without fully conscious knowledge of category membership.”
      4. Guess: 
  3. “Categorization overall recruited both the basal ganglia and posterior hippocampus.”
    1. Use of basal ganglia showed up when making both types of “know-” based decisions
    2. Posterior hippocampus showed up with remember judgements
    3. <This distinction is a little unclear, as later they say> “First, we used subjects’ subjective judgments to dissociate trials performed on the basis of memory (and found to recruit the hippocampus), from trials performed in a subjectively automatic or intuitive way (found to recruit the basal ganglia).”
    4. <But if you know something how is it not based on memory – the language seems sloppy to me>
  4. “[Analysis shows] the putamen exerting directed influence on the posterior hippocampus, which in turn exerted directed influence on the posterior caudate nucleus.”
  5. “Our results indicate that subjective measures may be effective in dissociating basal ganglia from hippocampal dependent learning, and that the basal ganglia are involved in both conscious and unconscious learning. They also indicate a dissociation within the hippocampus, in which the anterior regions are sensitive to novelty, and the posterior regions are involved in memory based categorization learning.”
  6. Commonly accepted that basal is important for categorization. “… they are particularly important for feedback-based categorization, in which subjects learn via trial and error.”
  7. The evidence of the role of hippocampus in categorization is less strong, although results show that anterior and posterior portions may be playing different roles
    1. Anterior hippocampus activates at the beginning of training and then goes below baseline levels, and may be related to initial processing, perhaps encoding, of novel stimulus
  8. Posterior hippocampus more related to tasks with deterministic but complex visual stimulus, memory based
  9. The experiment here is designed with these believed functional differences in mind
  10. In some tasks (probabilistic categorization) basal ganglia and hippocampal activity is competitive (one goes up while the other down).  In other tasks (spatial learning) they are used at the same time.  Unclear if they regulate each other or if something else is regulating both of them
  11. The basal ganglia and hippocampus are both targets of dopamine projections from the midbrain (ventral tegmental area and substantia nigra). The dopamine signal reflects the valence of the feedback received, and the degree to which the feedback was or was not expected (often referred to as prediction error; …). Dopamine is crucial for synaptic plasticity within the striatum; long term potentiation only occurs in the presence of dopamine (…).”
  12. Granger causality mapping was used to examine interactions between basal ganglia, hippocampus, and dopaminergic midbrain
  13. In the experiment “Subjects make a Remember response if they have a full episodic memory of the stimulus including features such as the time and place at which they first encountered it. A  Know response is used when subjects believe that they have encountered the stimulus before, but their memory is not accompanied by any episodic details.”
  14. Within Know trials, if the basal ganglia are limited to relatively implicit forms of learning (in the sense of not being fully verbalizable; …), than [then?] basal ganglia recruitment should be greatest for Know-Intuition trials. Alternatively, if the basal ganglia are recruited across both implicit and explicit learning, then they may be recruited by both Know-Automatic and Know-Intuition.
  15. In the experiment there were 2 categories, where fractal images were randomly assigned to a category.  Images would be introduced during the task (novel stimulus which is part of the hypothesized function of anterior hippocampus), so that some were seen more than others. Subjects alternated between:
    1. Standard trial and error tasks: presented with a stimulus, give a response, and then indication as to whether response was correct or not
    2. Subjective judgement tasks:  presented with a stimulus, give a response along with the basis for the response (Remember, Know-Automatic, Know-Intuition, and Guess), with no feedback given
  16. <Ah this is good> “Although we refer to these responses by these names here, subjects learned to refer to each response with a neutral letter: B (Remember), A (Know-Automatic), C (Know-Intuition), and D (Guess). These neutral letter names were used because previous research has shown that subjects vary in their preexisting ideas of what is meant by “Remember” and “Know” and we wanted to avoid biasing that might be entailed by using these words as a shorthand (…)”
  17. <Basically skipping “Materials and Methods” and “Results” section – will try to get interesting results from the discussion>
  18. <The paper jumps from section 3 to section 5 whoops>
  19. The results reveal complex patterns of interaction between hippocampus and basal ganglia subserving learning, with the basal ganglia associated with two different kinds of ‘knowing’, and the hippocampus with memory based categorization decisions.”
  20. As said before, results show anterior hippocampus dealing with novel stimuli and posterior hippocampus more with categorization learning and memory
  21. Basal ganglia activated during categorization (this is consistent with previous results)
    1. Additionally, they are associated with ‘knowing’ the answer in both forms of knowing tested (automatic: a rapid high confidence evaluation; intuition: uncertain intuitive decision)
  22. The basal ganglia have long been thought to be associated with conscious deliberate thought and those conlcusions are supported here, although there is also a link to implicit thought
  23. We found basal ganglia recruitment for both kinds of Know judgments. In recognition memory studies, Know judgments are thought to reflect familiarity based memory processing, in contrast with full recollection expressed in Remember judgments. This raises the question of whether the basal ganglia contribute to familiarity-based memory.”  It seems to be related to that as well.
  24. One theory associates anterior hippocampal activity with novelty processing and more posterior regions with recollection (…). This theory is clearly consistent with our reported results finding that the anterior hippocampus was sensitive to stimulus novelty and the posterior to memory based categorization. An alternative view is that the anterior hippocampus is associated with relational encoding (Chua et al., 2007) and flexible use of relational information, whereas posterior regions are more concerned with reinstatement of the context within which the stimulus was originally encountered (…). This theory is also compatible with our results.”
  25. We found a clear interaction between dorsal striatal regions, in which the putamen exerts influence on the anterior caudate, which in turn exerts influence on the posterior caudate… This pattern largely follows the order in which we expect the different corticostriatal loops to be of primary importance within categorization trials: stimulus categorization and motor response selection occur first, which should require the visual and motor loops (including the putamen), then are followed by feedback processing, which requires the executive loop through the anterior caudate nucleus. One puzzling pattern in our data is that the anterior caudate exerts influence on the posterior caudate, whereas the reverse should be true because visual processing recruiting the posterior caudate preceeds feedback processing recruiting the anterior caudate.”
  26. The posterior hippocampus received directed influence from the putamen, and exerted directed influence on the anterior and posterior caudate. This indicates it may be playing an intermediate role in categorization: processing information about the stimulus and information about the response from putamen, and sending the resulting information to the anterior caudate to be integrated there with feedback.”
  27. As described in the Results section, there was no consistent pattern of directed influence from the anterior hippocampal seed regions. This is consistent with our finding that this region was not clearly associated with categorization, and appeared to be primarily sensitive to stimulus novelty.”
  28. In addition, we found directed influence from the midbrain to both the hippocampus and the striatum, consistent with the known projections of dopaminergic neurons from the ventral tegmental area and substantia nigra. Dopamine neurons code for reward (firing rates are greatest when an unexpected reward is encountered), and provide a teaching signal to enable learning about reward contingencies. In both basal ganglia and hippocampus, dopamine affects learning via NMDA receptors. In the basal ganglia, synaptic plasticity requires the presence of dopamine (…), whereas in the hippocampus encoding occurs in the absence of dopamine, but dopamine increases the persistence of memories (…). Dopamine projections from the midbrain may also play a role in the anterior hippocampal sensitivity to novelty. In addition to reward coding, dopamine neurons increase their firing rates for novel stimuli (…).  Electrophysiological studies (Axmacher et al., 2010) of the hippocampus have found both early (187 ms) and later (482 ms) novelty effects; the former may contribute to initial identification of a stimulus as novel and serve as one of the inputs affecting dopamine neurons, and the latter may be a response to the dopamine signal and underlie stronger memory encoding for novel stimuli (…).”
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