Disentangling Temporal and Rate Codes in the Primate Somatosensory Cortex.

Area of Science:

• Neuroscience
• Sensory Coding
• Somatosensory System

Background:

• Precise temporal spiking patterns in tactile neurons encode peripheral sensory information.
• The role of millisecond-scale temporal spiking in neocortical function, particularly the somatosensory cortex (S1), is debated.
• Vibratory frequency information is present in S1 responses via phase locking, but spike timing and rate contributions are confounded.

Purpose of the Study:

• To disentangle the roles of spike timing versus firing rate in coding vibratory frequency within the S1.
• To determine how these neural coding features relate to behavioral performance in frequency discrimination tasks.
• To investigate the influence of amplitude variations on frequency perception and neural coding.

Main Methods:

• Recorded S1 neural responses in rhesus macaques performing a tactile frequency discrimination task.
• Manipulated vibratory frequency and amplitude orthogonally to dissociate their effects.
• Assessed the predictive power of spike timing and firing rate on animal performance and behavioral biases.

Main Results:

• Animals could discriminate frequency, but performance was biased by amplitude variations.
• Rate-based neural representations of frequency were susceptible to amplitude changes, but inconsistently with behavioral biases.
• Timing-based neural representations were informative about frequency and robust to amplitude changes, also inconsistently with behavior.

Conclusions:

• Frequency coding in S1 relies primarily on temporal spiking patterns, not firing rates.
• A model incorporating a temporal code for frequency, modulated by perceived magnitude (amplitude), explains behavioral performance.
• Millisecond-scale temporal patterning, not overall neural activity rate, is the key mechanism for vibratory frequency encoding in S1.