New Research: Retina Wiring Architecture Crucial in Image Processing

Our different senses rely on a complex process known as cellular signal transduction which converts an external stimulus, such as sound or light, to a nerve signal. But the nerve signal doesn’t go straight to the brain. In the case of mammalian vision the massive data stream emanating from the millions of photoreceptor cells undergoes substantial signal processing before the information is sent to the brain. New research is now providing more information about the cellular architecture involved in this intermediate processing stage.

Seeing the light

At the molecular level vision begins with a complex signal transduction cascade. As photons enter your eye they interact with light-sensitive chromophore molecules in the photoreceptor cells. The interaction causes the chromophore to change configuration and this, in turn, influences the large, trans-membrane rhodopsin protein to which the chromophore is attached.

The chromophore photoisomerization is the beginning of a remarkable cascade that causes action potentials to be triggered in the optic nerve. In response to the chromophore photoisomerization, the rhodopsin causes the activation of hundreds of transducin molecules. These, in turn, cause the activation of cGMP phosphodiesterase (by removing its inhibitory subunit), an enzyme that degrades the cyclic nucleotide, cGMP.

A single photon can result in the activation of hundreds of transducins, leading to the degradation of hundreds of thousands of cGMP molecules. cGMP molecules serve to open non selective ion channels in the membrane, so reduction in cGMP concentration serves to close these channels. This means that millions of sodium ions per second are shut out of the cell, causing a voltage change across the membrane. This hyperpolarization of the cell membrane causes a reduction in the release of neurotransmitter, the chemical that interacts with the nearby nerve cell, in the synaptic region of the cell. This reduction in neurotransmitter release ultimately causes an action potential to arise in the nerve cell. The next step is to process these nerve signals.

Image processing

A variety of signal processing, involving different types of cells, takes place downstream of the photoreceptor cells, before the vision information is sent to the brain. For instance, the signals from different photoreceptor cells are processed together to derive image information. In addition to this spatial processing, temporal processing derives motion information.

The amacrine and ganglion cells are important components in this processing stream. But this processing stream is not merely a massively parallel operation, where the signal from each photoreceptor cell is individually and simultaneously processed.

For instance, by the time they reach the ganglion cells, signals from the different photoreceptor cells are mingled. The result is that a given ganglion cell receives information for a circular image area. And different types of ganglion cells are sensitive to different spatial and temporal image patterns within that area.

What the new research discovered is that the signals feeding the ganglion cells sensitive to temporal image patterns (that is, motion) have intricate connection patterns. Specifically, if the ganglion cell detects motion from left to right, then it is connected to dendrites which extend from the amacrine cell in the opposite direction. As the researchers concluded, “Our findings indicate that a structural (wiring) asymmetry contributes to the computation of direction selectivity.”

Meanwhile evolution is left only with its foolish dogma. Evolutionists have long since committed themselves to the insistence that evolution is fact. Like the marching band that took a wrong turn down a dead end alley, they have no graceful exit. This is not going to end pretty.