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Cognitive neuroscience

Flutter Discrimination: neural codes, perception, memory and decision making

Key Points

  • Frequency discrimination in the sense of flutter is a useful model to understand how neural codes are related to perception, working memory and decision making. The task can be thought of as a series of steps: encoding a stimulus frequency, maintaining it in memory, encoding a second frequency, comparing it to the frequency in memory, and communicating the result of the comparison to the motor system. Neurophysiological studies have explored the brain regions that participate in each of these steps and how these regions interact to solve the task.

  • Encoding. How is the flutter frequency represented in the nervous system? Does this neural code reflect behavioural responses? Changes in neuronal firing rate as a function of stimulus frequency are evident in several areas (particularly in the primary somatosensory cortex) during the flutter discrimination task. Moreover, several lines of evidence (particularly microstimulation experiments) indicate that these rate variations affect behaviour. By contrast, the high periodicity that flutter elicits does not seem to contribute to frequency discrimination.

  • Memory. The clearest neural correlate of working memory during frequency discrimination is found in the prefrontal cortex, which contains neurons that increase their activity in a frequency-dependent manner during the delay period between the two flutter stimuli. This activity does not seem to be related to the impending motor response. The prefrontal cortex might not be the only structure in which such a mnemonic correlate exists, as neurons with similar activity have been found in the secondary somatosensory cortex and in the medial premotor cortex.

  • Comparison process and decision making. The comparison between the two stimulus frequencies can be simply conceptualized as the difference between them. The firing of some neurons in the secondary somatosensory cortex shows dynamic changes as a function of both frequencies, and evolves to encode the difference between them. Moreover, the firing patterns of these neurons are a good indicator of the actual behavioural response, indicating that this neural activity might be involved in the decision-making process. Similar, but significantly different, dynamic changes have been found in the prefrontal and medial premotor cortices.

  • There is a large overlap between sensory-, mnemonic- and decision-related activity during frequency discrimination. As a result, the comparison between stored and ongoing sensory information seems to take place in a distributed manner, and no single area can be identified as the unique site of decision making. We therefore propose that the motor plan that is established to respond after a discrimination trial already contains two possible outcomes, and that sensory information helps in the selection of one of them. Future studies should rigorously explore this possibility.

Abstract

Recent studies combining psychophysical and neurophysiological experiments in behaving monkeys have provided new insights into how several cortical areas integrate efforts to solve a vibrotactile discrimination task. In particular, these studies have addressed how neural codes are related to perception, working memory and decision making in this model. The primary somatosensory cortex drives higher cortical areas where past and current sensory information are combined, such that a comparison of the two evolves into a behavioural decision. These and other observations in visual tasks indicate that decisions emerge from highly-distributed processes in which the details of a scheduled motor plan are gradually specified by sensory information.

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Figure 1: Flutter discrimination task.
Figure 2: Comparison between S1 activity and psychophysical performance in the flutter discrimination task.
Figure 3: Neuronal responses evoked by the base stimulus in four brain areas during the flutter discrimination task.
Figure 4: Population dynamics in four cortical areas during the flutter discrimination task.
Figure 5: Psychophysical performance in frequency discrimination with mechanical stimuli delivered to the fingertips, and with electrical stimuli delivered directly to primary somatosensory cortex (S1) neurons.
Figure 6: Differential activity in the medial premotor cortex (MPC).

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Acknowledgements

The research of R.R. was partially supported by an International Research Scholars Award from the Howard Hughes Medical Institute, and grants from the Millennium Science Initiative-Consejo Nacional de Ciencia y Tecnología and Dirección General del Personal Académico-Universidad Nacional Autónoma de México. E.S. was supported by startup funds from the Wake Forest University School of Medicine. We appreciate the technical assistance of A. Hernández in preparing the figures.

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FURTHER INFORMATION

Encyclopedia of Life Sciences

learning and memory

neural networks and behaviour

somatosensory system

touch

Glossary

DIFFERENCE LIMEN

In flutter discrimination, the difference limen is a measure of how small an increase in the frequency of a vibrotactile stimulus can be detected when compared to a standard stimulus frequency. A smaller difference limen implies a higher discrimination capacity.

WEBER FRACTION

Weber made the observation that, within a fairly large range, the increase in a stimulus that is just noticeable (ΔI) is a constant proportion of the initial stimulus (I) for any one sense. The proportion ΔI/I is the Weber fraction.

PSYCHOMETRIC CURVE

A plot of the percentage of correct behavioural responses as a function of changes in the properties of the test stimulus.

PHASE LOCKING

The preferential firing of neurons at a certain phase of an amplitude-modulated stimulus.

GRATING

An arrangement of parallel bars. The roughness of a surface may be varied by adjusting the width and spacing of the bars of an embossed grating.

NEUROMETRIC CURVE

 A plot of the percentage of correct behavioural responses that an ideal observer would make on the basis of observing the neuronal responses that are elicited by a given test stimulus.

INFORMATION THEORY

Shannon introduced the term 'mutual information' in a strict mathematical sense within a framework for studying communication channels. Mutual information is a statistic that measures the degree of association between any two quantities or sets of quantities. It is useful because it requires no assumptions about their mathematical form or behaviour, so it is in some sense objective.

INTRACORTICAL MICROSTIMULATION

 A neurophysiological technique that is used to activate a population of neurons within a restricted cortical locus. Pulses of electric current delivered through a microelectrode drive the activation.

TRANSCRANIAL MAGNETIC STIMULATION

(TMS). A non-invasive technique that is based on the application of a time-varying magnetic field near the surface of the head. The magnetic pulse generates electrical currents in the brain that affect the activity of the underlying superficial neurons. Pulses are intense but brief and relatively localized because the magnetic field decreases strongly with distance.

EFFERENCE COPY

A copy of a motor command that is sent back to the central nervous system to inform it of the executed movement.

RANDOM-DOT STIMULUS

A commonly used visual stimulus that consists of dots randomly moving on a screen. The experimenter can vary the coherence of their movement (the fraction of dots that move in the same direction), and the subjects are asked whether they can detect any movement coherence.

SACCADE

A rapid intermittent eye movement that occurs when the eyes fix on one point after another in the visual field.

RECEIVER-OPERATING CHARACTERISTIC INDEX

A measure that allows establishment of the sensitivity and specificity of a given test, enabling us to determine an optimal cut point to distinguish between true and false positives. It is particularly useful when the results of the test are a continuous measure, such as glucose concentration in a blood test.

CENTRAL PATTERN GENERATOR

(CPG). A circuit that produces self-sustaining patterns of behaviour.

REACTION TIME

The period of time between the detection of a stimulus at a sensory receptor and the performance of the appropriate response by the effector organ.

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Romo, R., Salinas, E. Flutter Discrimination: neural codes, perception, memory and decision making. Nat Rev Neurosci 4, 203–218 (2003). https://doi.org/10.1038/nrn1058

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