Contrast dependent changes in Monkey V1 gamma frequency undermine its reliability in binding/control
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1
HHMI & Harvard medical school , United States
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2
Harvard Medical School, United States
Electrical signals recorded from the brain reveal oscillatory behavior of the neural population. Oscillations in a frequency band between 30 and 80 Hz, called the gamma band, have been suggested to play a functional role in cortical processing such as feature binding, forming dynamic communication channels across cortical areas, or providing a temporal framework for the firing of neurons so that information could be coded in the timing of spikes relative to the ongoing gamma cycle. These hypothesized functional roles require that the neuronal assemblies processing the features of the same stimulus oscillate at the same frequency. However, the frequency of the gamma rhythm depends on simple stimulus manipulations such as size, velocity, spatial frequency and cross-orientation suppression. For stimuli with features that vary in space and time, it remains unclear whether the induced gamma rhythms in different neural assemblies that process that stimulus are stable and reliable enough to support binding, communication or coding. We tested whether increasing the stimulus contrast, which increases the level of cortical excitation, affects the frequency of the gamma rhythm in the primary visual cortex (V1) of two awake behaving rhesus monkeys. Recordings were made from a chronic array of 96 electrodes (Blackrock Systems) implanted in V1 (right hemisphere). While the monkeys fixated within a 1∘ window and attended to a stimulus in the opposite hemifield, we presented a static Gabor stimulus at different contrasts on the receptive fields of the neurons recorded from the microelectrodes. We found that the peak frequency of the gamma rhythm increased monotonically with stimulus contrast, from ~38 Hz at 25%contrast to ~53 Hz at 100%contrast. Changes in stimulus contrast over time caused fast and reliable gamma frequency modulation. Further, a large Gabor stimulus, whose contrast varied across space, generated gamma rhythms at significantly different frequencies in the neuronal assemblies separated by as little as 0.2o (~400 μm in cortex). Gamma oscillation frequency decreased with increasing distance between the recording microelectrode and the stimulus center, which was well accounted for by the reduction in stimulus contrast with distance. These results suggest that gamma rhythms are generated by highly localized networks that can rapidly track the incoming excitation. In addition to having a variable peak frequency, gamma rhythms were invariably weak, on average less than a few percent of the total signal power; far weaker than the stimulus-evoked transient during the first 100 ms. The weakness and varying center frequency of gamma oscillations suggest that they are poor candidates for either a control signal or information channel. Instead, our findings are consistent with the idea that gamma rhythms are a resonant phenomenon arising from the interaction between local excitation and inhibition. Several fundamental cortical mechanisms such as divisive normalization, adaptation and gain control rely on excitatory-inhibitory interactions; thus it is not surprising that gamma rhythm is also present and is modulated during a variety of cognitive tasks such as attention, working memory or cortico-spinal interactions. Thus, it could be an important neural signature of specific cortical processes.
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:
Ray
S and
Maunsell
JH
(2010). Contrast dependent changes in Monkey V1 gamma frequency undermine its reliability in binding/control.
Front. Neurosci.
Conference Abstract:
Computational and Systems Neuroscience 2010.
doi: 10.3389/conf.fnins.2010.03.00323
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Received:
08 Mar 2010;
Published Online:
08 Mar 2010.
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Correspondence:
Supratim Ray, HHMI & Harvard medical school, Boston, United States, supratim_ray@hms.harvard.edu