One-dimensional dynamics of associative representations in lateral intraparietal (LIP) area
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1
Harvard Medical School, Department of Neurobiology, United States
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2
Harvard Medical School, United States
We previously showed that the lateral intraparietal area (LIP) flexibly encodes learned associations between multiple types of visual stimuli (Fitzgerald et al, Cosyne 2009). A recent recurrent network model predicts that across a population of LIP neurons, slowly varying patterns of neuronal activity-including spontaneous and delay activity during cognitively demanding tasks-are scaled versions of one another (Ganguli et al., 2008). We tested whether the LIP population activity during memory periods in shape-association and motion-categorization paradigms may be explained by a scaling of the spontaneous activity. Two monkeys performed a delayed paired association task in which they grouped six shapes into three pairs. On each trial a sample shape was presented (650 ms) followed by a delay (1500 ms) and a test shape; the monkeys released a lever if the test shape was that associated with the sample. For many neurons, the spike rate evoked by a particular shape was most similar to the activity elicited by that shape’s learned associate (cells with an influence of pair during sample: 114/161, delay: 117/161, test: 77/161 neurons, ANOVA, p < 0.05). We previously showed that LIP neurons flexibly encode motion direction depending on how those directions are grouped or categorized (Freedman & Assad, 2006). We thus asked whether single neurons reflect associations for both motion and shape stimuli, and we found many neurons modulated by both types of associations (sample: 42/78, delay: 32/78, test 21/78, ANOVA, p < 0.05). For all six shape and six motion stimuli, average delay activity was correlated with spontaneous firing rates across the population of LIP neurons (correlation coefficient: shape: 0.65-0.73, motion: 0.69-0.86 monkey 1; shape: 0.59-0.73, motion: 0.60-0.68 monkey 2, p < 0.01), consistent with the predictions of Ganguli et al. In contrast, during the visual periods, the population activity showed markedly lower correlations (CC, shape: 0.34-0.45, motion: 0.28-0.34 monkey 1; shape: 0.36-0.44, motion: 0.28-0.33 monkey 2). The lower correlation coefficients during visual stimulation argue that the relationship between delay and spontaneous activity was not due to simple differences in excitability among neurons. However, lower correlations during visual stimulation might be a consequence of the higher firing rates during visual stimulation. We thus examined responses from a previous experiment in which monkeys passively viewed moving stimuli that elicited a broad range of sustained responses (Fanini & Assad, 2009). We observed low correlation between these visual responses and spontaneous activity across the range of firing rates (CC = 0.3-0.4), arguing against the alternative excitability explanation. The one-dimensionality of delay activity also predicts that the order of selectivity for pairs of shapes or motion categories should be biased among neurons; this was indeed true for both experiments in at least one animal (chi-square, p<0.01). The biases are not explained by differences in behavior among shape-pairs/motion-categories, such as performance, reaction times, and microsaccades. In conclusion, encoding of associations for multiple stimulus types during memory periods in LIP may arise from a scaling of the population spontaneous activity that is similar for associated stimuli and dissimilar for non-associated stimuli.
Conference:
Computational and Systems Neuroscience 2010, Salt Lake City, UT, United States, 25 Feb - 2 Mar, 2010.
Presentation Type:
Poster Presentation
Topic:
Poster session II
Citation:
Fitzgerald
JK,
Freedman
D,
Fanini
A and
Assad
J
(2010). One-dimensional dynamics of associative representations in lateral intraparietal (LIP) area.
Front. Neurosci.
Conference Abstract:
Computational and Systems Neuroscience 2010.
doi: 10.3389/conf.fnins.2010.03.00296
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Received:
07 Mar 2010;
Published Online:
07 Mar 2010.
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Correspondence:
Jamie K Fitzgerald, Harvard Medical School, Department of Neurobiology, Boston, United States, jkfitzg@fas.harvard.edu