Left Eye, Right Eye

There ‘s been a discussion on the MindPlace User Forum about the virtues of alternate-eye flashing, AKA Expand Mode. Early on in the discussion I announced that I found it annoying and that it was more gimmicky than useful. Well, having been lured back into alternate-eye flashing by the iLightz, I’ve gone back to the Sirius with monocolour glasses and I’m sold. The visual effects achieved with the expand mode are spectacularly different to those in the focus mode between 8Hz and 18Hz. Below 8Hz I still find the effect irritating. Above 18Hz the flashes start to merge and the effect diminishes rapidly. My big mistake was to use monocolour glasses with the Proteus on some sessions designed for the standard Proteus Red/Green glasses.

Anyway, enough background. The question that was never quite settled is whether alternate-eye flashing should be at the same rate as both-eye, or whether it should be doubled.

Example – 10Hz flashing both eyes, both eyes see 10Hz; 10Hz flashing alternate eyes, each eye sees 5Hz.

We need to look at how the eyeballs deal with the left and right visual fields, and where the pictures from each eye go in the brain.

Visual Fields, Fig 10.3, Neuroscience; Bear, Connors, Paradiso

Visual Fields, Fig 10.3, Neuroscience; Bear, Connors, Paradiso

As can be seen from the image from my favourite reference book, each eye contributes to each visual field, with the right hand side of each retina feeding the right hemisphere, and the left hand side of each retina feeding the left hemisphere. Ahh, but doesn’t the left side of the brain look after the right side of the body, and vice versa? Remember, the image is laterally inverted by the lens, so the right hand side of each retina is seeing the left hand side of the visual field.

The upshot of this is that 10Hz both-eyes and 10Hz alternating-eyes both deliver exactly the same stimulus rate, but at reduced intensity, as only one eye is contributing with each flash. A flash over the left eye stimulates both hemispheres and a flash over the right eye stimulates both hemispheres – 5Hz over the left eye + 5Hz out of phase over the right eye = 10Hz at the visual cortex.

From here you can have all sorts of fun trying to work out how our depth perception mechanism is affected differently by both- and alternate-eye flashing. It’s hard to resist imagining the different layers of the striate cortex having copies of the flash-images, offsetting the copies and new images to find the overlaps, filling with patterns from our pattern-recognition library, trying to find information in an image that contains no data.

Trying to recognise the mechanisms behind the extraordinary visuals that can arise with AVS is one of my favourite pastimes whilst ‘meditating’. A practice recommended during one phase of my magickal training was to formulate a machine and visualise it in all its motions. Pattern matching in the brain is a fun one to model. I also enjoy motion of the planets and internal combustion engines.


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