Visual Responses in the Absence of Visual Stimuli

As previously mentioned, mirror-box therapy has proven effective in alleviating phantom limb sensations. This therapy was based on the idea that vision can dictate proprioception (2). By allowing patients to perceive two intact limbs (the mirror allows a projection of the intact limb onto the amputated limb), patients are able to correct their perceived sensation.  To reinforce this relationship between vision and proprioception, researchers recently examined whether visual responses associated with self-generated movements could be evoked in the complete absence of light (1).

Generally the visual pathway is as follows: photons hit the retina of the eyes, striking the rods and cones, synapses with bipolar, horizontal, ganglion, and amacrine cells; the action potential propagates through the optic nerve, through the thalamus, and to the primary visual cortex where the stimulus is interpreted.  It is also important to mention that when our eyes are not tracking anything in particular they engage in jerky movements, called saccades. In contrast, while tracking a particular object in our visual field our eyes make smooth movements, referred to as smooth pursuit. Smooth pursuit is only possible while tracking something tangible – mental imagery doesn’t elicit this response! (1)  Based on this information, in the complete absence of light - the stimulus, we should not be able to generate visual responses nor be able to make smooth pursuit movements.  However, our brains are amazing! Recent research has shown that many people are able to have retinal responses to a moving target in the absence of light.

So you might also be wondering how researchers would test this. Initially, all participants gave self-reports about whether they could see their arm, or a researcher’s arm, moving in front of their eyes with a light-proof blindfold on.  To confirm the presence of smooth pursuit in total darkness, researchers then strapped one of these bad boys on the participants’ heads.

http://neurosciencenews.com/neuropsychology-dark-vision-movement-561/seeing-in-dark/

As before, participants were asked to move their arm in front of their eyes.  Researchers also waved their arms in front of the participants’ eyes.  In a large percentage of participants, smooth pursuit movements, consistent with object tracking in adequate light, were recorded (1). This effect was amplified in participants with varying types of synesthesia (1). Although the mechanism is not entirely clear, this reinforces the idea that the brain is able to use body movement to stimulate a visual response in the complete absence of visual input.

References:

1.  Dieter KC, Hu B, Knill DC, Blake R, Tadin D. Kinesthesis can make an invisible hand

Visible. Psychological Science. Oct. 2013. 10: 1-10.

2. Ramachandran, V. S., & Rogers-Ramachandran, D. Synaesthesia in phantom limbs induced with mirrors. Proceedings of the Royal Society B: Biological Sciences. 1996. 263: 377–386.