Seeing light, color, and geometric patterns when the eyes are closed is a common phenomenon. These internal visuals, which often appear in darkness or just before sleep, are not a product of external light but rather an internally generated signal. The repeating, intricate designs raise the question of why the brain produces such specific and complex images when it is supposedly resting. Exploring the nature of these visuals reveals a deep connection between the brain’s physical structure and the patterns we perceive.
Defining Closed-Eye Visualizations
The scientific term for the perception of light without an actual external light source entering the eye is a phosphene. This phenomenon is a sensation of light that originates from within the visual system itself, rather than from the outside world. Phosphenes can be spontaneous, or they can be induced by various types of stimulation, from mechanical pressure to magnetic fields.
The complex geometric patterns seen within these phosphenes are known as “Form Constants,” a term coined in the 1920s to categorize these recurring shapes. These patterns are not random; they consistently fall into specific categories, such as spirals, tunnels, cobwebs, and lattices like checkerboards or honeycombs. The fact that individuals across different cultures report these same forms suggests a common biological mechanism at their root.
The Origin of Internal Visual Signals
When the eyes are closed in darkness, the visual system does not shut down, but enters a state of spontaneous neural activity. The retina and the visual cortex maintain a continuous electrical potential, meaning neurons continue to fire even without receiving light input. This constant, random electrical “noise” is an intrinsic operation of the brain, demonstrating that the system is never truly quiet.
The absence of a strong external light signal allows this inherent internal activity to become noticeable. The brain interprets this random firing of neurons in the visual pathways as visual information, perceiving the electrical noise of its own system. Studies using functional magnetic resonance imaging (fMRI) have confirmed that spontaneous activity is distinctly present in the primary visual areas even when eyes are closed.
Why the Visual Cortex Generates Geometric Shapes
The transformation of random neural noise into specific geometric shapes is a consequence of the physical architecture of the primary visual cortex (V1). This area of the brain is a highly organized structure that maps the visual field in a specific way known as retinotopic mapping. Neurons whose receptive fields are close together in visual space are similarly grouped together in the cortex.
The mapping between the curved surface of the retina and the relatively flat sheet of the V1 cortex can be mathematically described by a log-polar transformation. This non-linear mapping causes concentric circles in the visual field to be translated into parallel lines within the cortical space. Similarly, patterns like spirals and tunnels in the visual field correspond to simple, straight lines of activity within the V1 cortex.
Therefore, when the spontaneous electrical activity randomly excites the V1 neurons, the activity spreads along the physical pathways of least resistance. Mathematical models suggest that uniform input to this structured cortical space will naturally result in spatially periodic, geometric patterns. The geometric Form Constants we see are essentially the appearance of these simple, straight-line patterns in the V1 cortex translated back into the log-polar coordinates of the visual field.
How Induced Phosphenes Differ
While spontaneous phosphenes originate from internal neural noise, induced phosphenes are triggered by specific external forces acting on the visual system. The most common example is the pressure phosphene, created by rubbing or gently pressing the closed eyes. This mechanical pressure physically deforms retinal cells, activating the light-sensitive photoreceptors.
The mechanical stimulation forces retinal cells to send an electrical signal to the brain, mimicking the signal produced by actual light. Other induced phosphenes, such as those caused by a blow to the head or intense magnetic stimulation, can stimulate the retina or the visual cortex directly. Regardless of the trigger, the resulting signal still travels through the same structured visual pathways, meaning these induced phosphenes also often manifest as the geometric Form Constants.