Limits...
Short-term memory trace in rapidly adapting synapses of inferior temporal cortex.

Sugase-Miyamoto Y, Liu Z, Wiener MC, Optican LM, Richmond BJ - PLoS Comput. Biol. (2008)

Bottom Line: We found that a large proportion (80%) of stimulus-selective neurons in area TE of macaque ITCs exhibit a memory effect during the stimulus interval.Neurons in perirhinal cortex did not show this correlation.Simulations of a matched filter model match the experimental results, suggesting that area TE neurons store a synaptic memory trace during short-term visual memory.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Neuropsychology, National Institute of Mental Health, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland, United States of America.

ABSTRACT
Visual short-term memory tasks depend upon both the inferior temporal cortex (ITC) and the prefrontal cortex (PFC). Activity in some neurons persists after the first (sample) stimulus is shown. This delay-period activity has been proposed as an important mechanism for working memory. In ITC neurons, intervening (nonmatching) stimuli wipe out the delay-period activity; hence, the role of ITC in memory must depend upon a different mechanism. Here, we look for a possible mechanism by contrasting memory effects in two architectonically different parts of ITC: area TE and the perirhinal cortex. We found that a large proportion (80%) of stimulus-selective neurons in area TE of macaque ITCs exhibit a memory effect during the stimulus interval. During a sequential delayed matching-to-sample task (DMS), the noise in the neuronal response to the test image was correlated with the noise in the neuronal response to the sample image. Neurons in perirhinal cortex did not show this correlation. These results led us to hypothesize that area TE contributes to short-term memory by acting as a matched filter. When the sample image appears, each TE neuron captures a static copy of its inputs by rapidly adjusting its synaptic weights to match the strength of their individual inputs. Input signals from subsequent images are multiplied by those synaptic weights, thereby computing a measure of the correlation between the past and present inputs. The total activity in area TE is sufficient to quantify the similarity between the two images. This matched filter theory provides an explanation of what is remembered, where the trace is stored, and how comparison is done across time, all without requiring delay period activity. Simulations of a matched filter model match the experimental results, suggesting that area TE neurons store a synaptic memory trace during short-term visual memory.

Show MeSH

Related in: MedlinePlus

Predictions of responses by the deterministic model simulation.(A, B) Predictions of responses for TE neurons with inputs from encoder stage estimates (Equations 1–10) (A), and by the model for perirhinal neurons (Equation 3 was applied) (B). Left column shows predictions of sample responses compared to the actual sample responses. Right column shows predictions for nonmatch responses. Each colored dot represents data points for each neuron, and each pattern with a colored outline indicates the mean response versus mean predicted response to the pattern for the neuron. Variance explained is high in all cases, because the cells in both areas are stimulus selective.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2366068&req=5

pcbi-1000073-g005: Predictions of responses by the deterministic model simulation.(A, B) Predictions of responses for TE neurons with inputs from encoder stage estimates (Equations 1–10) (A), and by the model for perirhinal neurons (Equation 3 was applied) (B). Left column shows predictions of sample responses compared to the actual sample responses. Right column shows predictions for nonmatch responses. Each colored dot represents data points for each neuron, and each pattern with a colored outline indicates the mean response versus mean predicted response to the pattern for the neuron. Variance explained is high in all cases, because the cells in both areas are stimulus selective.

Mentions: The encoder output is estimated from the response of the neuron to the match stimulus (Equation 7). Thus, the deterministic model can only be used to predict the responses to sample and nonmatch stimuli (Figure 5). For the population of TE neurons, the correlations between sample response and prediction (R = 0.74) and nonmatch response and prediction (R = 0.73) are significant (Figure 5A; p<0.001; R2 = 0.54 for TE sample predictions, and 0.53 for TE nonmatch predictions). For the population of perirhinal neurons, the correlations are lower, but still significant (Figure 5B; p<0.001, R2 = .31 and 0.39, sample and nonmatch, respectively). This is consistent with our expectations, because both types of neurons showed stimulus selectivity (see Figure 2A). Thus, the scalar matched filter model, with no free parameters and with the simplistic approximation of Equation 7, successfully predicts the responses of the neurons during the DMS task, accounting for a bit more than 50% of the variance in the TE data.


Short-term memory trace in rapidly adapting synapses of inferior temporal cortex.

Sugase-Miyamoto Y, Liu Z, Wiener MC, Optican LM, Richmond BJ - PLoS Comput. Biol. (2008)

Predictions of responses by the deterministic model simulation.(A, B) Predictions of responses for TE neurons with inputs from encoder stage estimates (Equations 1–10) (A), and by the model for perirhinal neurons (Equation 3 was applied) (B). Left column shows predictions of sample responses compared to the actual sample responses. Right column shows predictions for nonmatch responses. Each colored dot represents data points for each neuron, and each pattern with a colored outline indicates the mean response versus mean predicted response to the pattern for the neuron. Variance explained is high in all cases, because the cells in both areas are stimulus selective.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2366068&req=5

pcbi-1000073-g005: Predictions of responses by the deterministic model simulation.(A, B) Predictions of responses for TE neurons with inputs from encoder stage estimates (Equations 1–10) (A), and by the model for perirhinal neurons (Equation 3 was applied) (B). Left column shows predictions of sample responses compared to the actual sample responses. Right column shows predictions for nonmatch responses. Each colored dot represents data points for each neuron, and each pattern with a colored outline indicates the mean response versus mean predicted response to the pattern for the neuron. Variance explained is high in all cases, because the cells in both areas are stimulus selective.
Mentions: The encoder output is estimated from the response of the neuron to the match stimulus (Equation 7). Thus, the deterministic model can only be used to predict the responses to sample and nonmatch stimuli (Figure 5). For the population of TE neurons, the correlations between sample response and prediction (R = 0.74) and nonmatch response and prediction (R = 0.73) are significant (Figure 5A; p<0.001; R2 = 0.54 for TE sample predictions, and 0.53 for TE nonmatch predictions). For the population of perirhinal neurons, the correlations are lower, but still significant (Figure 5B; p<0.001, R2 = .31 and 0.39, sample and nonmatch, respectively). This is consistent with our expectations, because both types of neurons showed stimulus selectivity (see Figure 2A). Thus, the scalar matched filter model, with no free parameters and with the simplistic approximation of Equation 7, successfully predicts the responses of the neurons during the DMS task, accounting for a bit more than 50% of the variance in the TE data.

Bottom Line: We found that a large proportion (80%) of stimulus-selective neurons in area TE of macaque ITCs exhibit a memory effect during the stimulus interval.Neurons in perirhinal cortex did not show this correlation.Simulations of a matched filter model match the experimental results, suggesting that area TE neurons store a synaptic memory trace during short-term visual memory.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Neuropsychology, National Institute of Mental Health, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland, United States of America.

ABSTRACT
Visual short-term memory tasks depend upon both the inferior temporal cortex (ITC) and the prefrontal cortex (PFC). Activity in some neurons persists after the first (sample) stimulus is shown. This delay-period activity has been proposed as an important mechanism for working memory. In ITC neurons, intervening (nonmatching) stimuli wipe out the delay-period activity; hence, the role of ITC in memory must depend upon a different mechanism. Here, we look for a possible mechanism by contrasting memory effects in two architectonically different parts of ITC: area TE and the perirhinal cortex. We found that a large proportion (80%) of stimulus-selective neurons in area TE of macaque ITCs exhibit a memory effect during the stimulus interval. During a sequential delayed matching-to-sample task (DMS), the noise in the neuronal response to the test image was correlated with the noise in the neuronal response to the sample image. Neurons in perirhinal cortex did not show this correlation. These results led us to hypothesize that area TE contributes to short-term memory by acting as a matched filter. When the sample image appears, each TE neuron captures a static copy of its inputs by rapidly adjusting its synaptic weights to match the strength of their individual inputs. Input signals from subsequent images are multiplied by those synaptic weights, thereby computing a measure of the correlation between the past and present inputs. The total activity in area TE is sufficient to quantify the similarity between the two images. This matched filter theory provides an explanation of what is remembered, where the trace is stored, and how comparison is done across time, all without requiring delay period activity. Simulations of a matched filter model match the experimental results, suggesting that area TE neurons store a synaptic memory trace during short-term visual memory.

Show MeSH
Related in: MedlinePlus