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Abstract:
Phase coding - stimulus coding by the timing of spikes with respect to the phase of local oscillations - is an alternative, complementary coding strategy to that of rate coding. One neocortical mechanism for phase coding posits that rhythmic inhibition in the gamma frequency range may interact with stimulus-evoked excitation, producing spikes earlier in an oscillatory cycle for preferred than non-preferred stimuli (Fries et al. 2007). Thus, the enhanced response for preferred stimuli commonly seen in the slowly-evolving rate code may also be coded through differences in spike timing within a single gamma cycle. Theoretically, this would provide a downstream target with a faster readout than would be possible with rate coding. Evidence for phase coding of visual stimuli was demonstrated recently in V1 of the anesthetized macaque (Montemurro et al. 2008), but only for lower frequencies (<12 Hz). Another study of spike-field phase coding in the secondary somatosensory cortex of the awake monkey also failed to find phase coding in the gamma frequency range (Ray et al. 2008).
To address the generality and frequency-dependence of phase coding, we tested whether phase coding would be observed in an object-selective brain region in the awake macaque. Two monkeys passively viewed images of faces, clip-art objects, and computer-generated 'greebles' during broadband recordings from the upper bank superior temporal sulcus (STS; N=13 sessions). For all stimulus-responsive single units, trials were grouped according to the stimulus category presented and spiking was compared to the phase of the frequency components of the local field potential on that trial. For the majority of these cells (N=15), the phase at which firing occurred differed across stimulus categories. The category-selective phase differences were most common in two frequency bands: below 20Hz and in the gamma range (60-80Hz). The phase differences were not sustained throughout the image presentation, but rather were limited to roughly the first 200ms following stimulus onset, with no difference in the time course across frequencies.
These results suggest that the visual category displayed can be extracted from the oscillatory phase when firing occurs. This holds not only for primary cortical areas known for their precise spike timing, but also for cells in association cortex, such as the upper-bank of the STS. Unlike previous studies, we found evidence of phase coding in the gamma frequency range, suggesting that there may be a regional specificity to the coding strategies used. The superior temporal sulcus receives highly-processed signals from multiple modalities, via projections from widespread cortical areas. As such, cells in STS may be less strictly driven by any given sensory input than are cells in early sensory cortical areas. Timing with respect to an internal gamma 'clock' may be one means by which such association areas maintain precise codes, as has been demonstrated previously for other cortical association areas such as the hippocampus (e.g., Buzsaki Chrobak 1995). The phase coding observed in STS may indicate one role for intrinsic rhythms in the coding of extrinsic - or stimulus-driven - inputs.
