Differences in Gamma Frequencies across Visual Cortex Restrict Their Possible Use in Computation

Article in NEURON   by Supratim RAY and by John MAUNSELL at Harv Med School, Boston.

2010

Summary

Neuronal oscillations in the gamma band (30–80 Hz)

have been suggested to play a central role in feature

binding or establishing channels for neural communication.

For these functions, the gamma rhythm

frequency must be consistent across neural assemblies

encoding the features of a stimulus. Here we

test the dependence of gamma frequency on

stimulus contrast in V1 cortex of awake behaving

macaques and show that gamma frequency

increases monotonically with contrast. Changes in

stimulus contrast over time leads to a reliable gamma

frequency modulation on a fast timescale. Further,

large stimuli whose contrast varies across space

generate gamma rhythms at significantly different

frequencies in simultaneously recorded neuronal

assemblies separated by as little as 400 mm, making

the gamma rhythm a poor candidate for binding or

communication, at least in V1. Instead, our results

suggest that the gamma rhythm arises from local

interactions between excitation and inhibition.

Jun 04, 2011 05:18 PM

it has been suggested that gamma (30 to 80 hz) plays a role in feature binding or

in estab channels of communication.  For these functions,

gamma must be consistent across neural assemblies encoding the features of the stimulus.

However, they show that gamma freq incr. monotonically with contrast (and with other stim features);

confirms my suspicion...   gamma is a POOR CANDIDATE for BINDING or COMMUNICATION,

at least in V1.   Instead....  gamma arises from local interactions btwn excitation and inhibition.

(my intuition is that gamma is simply CORRELATED with lots of firing... it is not causal of

consc. or binding)

I did find entire paper online in somebody's journal club collection on the web.

This is an important paper.

INTRO

Neuronal assemblies often exhibit stimulus-induced rhythmic

activity in the gamma range (30–80 Hz), which has been suggested

to play a central role in feature binding (Singer, 1999;

Uhlhaas et al., 2009), to form dynamic communication channels

across cortical areas processing the features of the stimulus

(Fries, 2009; Womelsdorf et al., 2007), or to provide a temporal

framework for the firing of neurons such that information can

be coded in the timing of spikes relative to the ongoing gamma

cycle (Buzsa´ ki and Chrobak, 1995; Fries et al., 2007). These

hypothesized functional roles require that the oscillation

frequency be consistent across neural assemblies processing

the features of a stimulus. However, previous studies have

shown that the frequency of the gamma rhythm depends on

simple stimulus manipulations such as size;

page 891  DISCUSSION

we report 3 new results:  first, gamma freq is highly contrast dependent;

second temporal changes in contrast lead to changes in gamma freq v rapdily.

Third, a stim who contrast varies across space causes adjacent neural

assemblies to osc at difft freqs.

these results suggest that gamma is gener by highly localized networks that can quicly track incoming excitation.

Charact. of gamma in primary visual cortex in monkeys

1)  gamma is absent if no stim is presented

2) first 100 milliseconds (after stim onset) is dominated by sharp transients in the LFP with broad freq range.

gamma becomes noticeable only after abt 100 ms.

3) gamma power is WEAK, only a few percent of total signal power.

4) gamma depends critically on stim properites.

finally  even in a single trial,  gamms can vary in freq across neuraby neural assemblies depending on

vairations in stim props in time and space.

Considering that the latency of visual responses in V1 is abt 30 ms, with the max rate changes btwn 30 and 100 ms....

gamma is unlikely to play a major role in response modulation in this time period.

Further, gamma is weak of absent in many conditions.

page 892  ROLE IN BINDING:

several studies of the realtion btwn gamma synch and perception

HAVE FAILED to find conclus evidence in support of the binding hypothesis.

893 last page:  ROLE in COMUNICATION

"the communic thru coherence" hypothesis:

gamma is gener by networks of inhib interneuorns so that

the gamma assoc IPSPs can modulate the firing probab. of pyramidal neurons.

can also modulate the response gain of neurons by modulating the synch and phase of

this rhythmic inhibition.

... energy of the gamma rhythm is only a few percent of the total signal energy.

Last para of paper:  Gamma as Resonance

our findings support the idea that gamma rhythms are a reonant phenom arising

from the interact of local excit and inhib.

Thus,  altho our results argue against a functional arole of gamma in binding or communic in V1,

it may be an important SIGNATURE of specific types of cortical processing.

Differences in Gamma Frequencies across Visual Cortex Restrict Their Possible Use in Computation

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