Light +Sound = Less Entrainment?

[brewmasher]

Have a look at this study and tell me what you think. It seems to conclude that light stimulation alone is the most effective form of entrainment, followed by sound alone. It claims when you use both light and sound together the effects diminish.

[Robert Austin]

It's unfortunate that the graphics are missing from this paper, which would help clarify Jon's mention that there was a 'trend' towards lower amplitude with bimodal stimulation--the term 'trend' is used when a result is not statistically significant, but looks like it might become so if a larger pool of subjects were to be used. As I recall, though the audio was sinusoidally modulated, they were pulses rather than binaural beats.

It should be noted that, though combined stimulation resulted in slightly lower evoked amplitude, the audio stimulation caused activation in the auditory cortex and associated structures, which light stimulation alone normally cannot. So dual mode stimulation would activate more of the cortex and some subcortical structures than either modality alone.

The EEG was measured at Cz, or the top of the head--directly over the sensory motor cortex, and I suspect that the variations in amplitude for single vs both sensory inputs may be due to interference between signals due to varying propagation path lengths--that is, the number of synapses along the visual pathways is larger than the audio pathway, which could result in something like phase cancellation in certain regions.

Jon Frederick has some other relevant publications, especially his PhD dissertation, which I've seen on the internet and may still be available, which deals with this topic and has a very detailed review of the relevant literature at the time he was working on it.

--Robert

[neuroasis]

Also, it seems that the subjects were put through very short sessions spaced far apart so the novelty of the experience would have to be factored to draw trend conclusions. In this particular study it looks like this was part of the design.

Longer term use of AVS should produce more consistent entrainment? To some degree it is a skill that needs to be acquired.

[neuroasis]

Attached is a study where subjects were given a total of 25 AVS sessions each of 20 minutes in length.

They do note a 'spreading' effect:

"The highest increase was observed in occipital parts as visual cortex is located just there. It is apparent that from these regions the specific rhythm spread as far as to frontal areas of the cortex. Without focusing on mechanism of spreading (generally synaptic or volume conductance), its attenuation is notable in the figure. In frontal region average reaction was
attenuated from 4 to 7-times."

[brewmasher]

It's unfortunate that the graphics are missing from this paper, which would help clarify Jon's mention that there was a 'trend' towards lower amplitude with bimodal stimulation--the term 'trend' is used when a result is not statistically significant, but looks like it might become so if a larger pool of subjects were to be used. As I recall, though the audio was sinusoidally modulated, they were pulses rather than binaural beats.

It should be noted that, though combined stimulation resulted in slightly lower evoked amplitude, the audio stimulation caused activation in the auditory cortex and associated structures, which light stimulation alone normally cannot. So dual mode stimulation would activate more of the cortex and some subcortical structures than either modality alone.

The EEG was measured at Cz, or the top of the head--directly over the sensory motor cortex, and I suspect that the variations in amplitude for single vs both sensory inputs may be due to interference between signals due to varying propagation path lengths--that is, the number of synapses along the visual pathways is larger than the audio pathway, which could result in something like phase cancellation in certain regions.

Jon Frederick has some other relevant publications, especially his PhD dissertation, which I've seen on the internet and may still be available, which deals with this topic and has a very detailed review of the relevant literature at the time he was working on it.

--Robert

Thanks for your thoughts Robert. More research needs to be done on the subject before I make any conclusions.

[Robert Austin]

I'll need to dig into my files a bit to find the specific papers/references, but I recall reading a German study of L&S in the late 90s which also showed some attenuation of eeg amplitude during light plus sound in phase vs. one or the other. I think the same study also showed that left/right visual stimulation resulted in lower amplitude overall signals. Question is, how important is the degree of amplitude change vs. intended goal, over time?

Individual responses to stimulation do seem to vary fairly widely, and sometimes in a seemingly paradoxical manner (for example, some people show lowered alpha amplitude in response to alpha stimulation, instead of the expected amplitude increase).

I'll post more on this topic, once I locate the correct box of documents in storage... :-)

Robert