The previous page discussed the details of the electrolytic circuits found in animal vision at a detailed circuit level. The main web site of PROCESSES IN BIOLOGICAL VISION has discussed the block diagrams of the visual system. This page will assemble the individual circuits into an end-to-end circuit diagram for a particular path through the overall block diagram. Only a specific path can be presented here (although the technique applies to all of the paths and many of these are presented in detail in the main text. [11.7.2]
The simplest comprehensive circuit path through the visual system follows the signals generated by one photoreceptor cell of each spectral type (in each case, representing an ensemble of individual cells of that type) as they are processed and manipulated until they reach the cortex of the brain. Unfortunately, even this conceptually simple path is complicated by the fact that nearly all of the signal paths pass through an intermediate location in either the thalamus or the mid-brain. At these locations, the projection signals are decoded back to analog form, further processed and then re-encoded for signal projection to their final destination. In the following figures this decoding/re-encoding function will be overlooked in the name of simplicity. The label optic nerve will be interpreted to include:
The optic nerve from the exit of the retina to the thalamus or mid-brain.
Either the functions performed in the thalamus or mid-brain.
Either the optic radiation or the Pulvinar pathway leading to the cortex.
Kelly's Loops and the equivalent structure within the retina will not be discussed here since they only relate to the instantaneous time relationship between individual end-to-end signal paths. Their significance is discussed in detail in the main text. [ XXX ]
The special case of the signals related to the foveola (the very core of the fovea) will also be omitted from discussion here. These signals are treated in a very special manner. They are treated as inputs to a complex set of servomechanism circuit feedback loops. To understand their function requires a more complete presentation than available via a browser. [ XXX ]
The following two figures, when combined with the appropriate values for the parameters of the individual circuit elements, completely describe the signaling performance of the fundamental visual system of animals. The illumination, chromatic, and temporal performance descriptors normally measured in the laboratory can be predicted in detail based on these circuits for all input intensity levels and spectral distributions.
The first figure summarizes all of the circuits related to:
The figure does not address the polarization, N, channel. A polarization channel is treated just like a chrominance channel after the pedicels of the photoreceptor cells. The first figure also omits the details of the Nodes of Ranvier and the initial decoding circuits of the cortex. These details are presented in the second figure.
Frame A displays a typical photoreceptor cell and its connection to each of four potential bipolar cells leading to the individual luminance, R, and chrominance channels, O, P and Q. There are additional connections to the lateral cells that are not shown but will be discussed below. Showing these along with the above connections would demonstrate the unique nature of the pedicels of the photoreceptor cells that were discussed in a page on the histological forms of the neurons.
The reader is referred to the main text for a complete discussion of the unique operational parameters of the adaptation circuit built around the Activa of type AT in this figure. It is the performance of this circuit that determines most of the operational characteristics of the visual system. A separate page of this site discusses the unique morphology, cytology and electrophysiology of the photoreceptor cell.
Frame B illustrates the luminance channel of vision. The luminance signal is created by summing the signals from each of the spectral channels, defined above, at the input to the first bipolar cell. The signal is then passed to a parasol type ganglion cell for encoding prior to transmission over the projection stage to the brain.
Frame C shows the similar circuit for the chrominance channel, and other channels, that relies on differencing between pairs of spectral input signals. Following differencing, the signal is passed to a midget type ganglion cell for encoding prior to transmission over the projection stage of vision.
Frame D shows the character of the appearance, Z, channels of vision. There may be multiple Z channels in a given visual system. These channels originate in the luminance channel following the first lateral matrix at the point labeled Z in frame B. Signals from a great number of individual luminance channels may be summed on each Amercine type lateral cell. These channels are treated much like chrominance channels except they incorporate additional circuit elements to introduce time dispersion between the various signals as they pass through the system. This process generally involves the length of the individual dendritic leads to different R channels in physically smaller versions of Kelly's Loops.
The following figure shows the circuit characteristics of the signal projection stage of vision. The top row represents the luminance channel and the two lower rows represent two difference channels that could be carrying chrominance, polarization or appearance signals. In the simplest case, which includes the human case, they would represent the two independent chrominance channels, P and Q.
The Nodes of Ranvier shown in the figure are all identical. They operate as monopulse signal regenerators, generating a new action potential only following the receipt of a stimulus. Both their input and output connections normally consist of myelinated conduits.
All of the ganglion cells shown on the left are quite similar except for their bias condition. The parasol cell is normally biased so that it does not generate action potentials except in response to a stimulus. The stimulus in the luminance channel is a monopolar analog signal. The resulting output signal consists of action potentials with a temporal spacing that describes the nature of the stimulus.
The midget ganglion cells are normally biased so that they generate a continuous series of equally spaced action potentials in the absence of any stimulus. The stimulus in the difference channels is a bipolar analog signal. The resulting output signal consists of action potentials with a temporal spacing that describes the nature of the stimulus. The temporal spacing of the action potentials can be greater or less than the quiescent spacing.
The stellate cells on the right can be represented by many specific circuit forms in the absence of more definitive laboratory data. The form shown is one of the simplest forms capable of decoding a series of pulses and regenerating a signal equivalent to the original signal applied to the ganglion cells. In the form shown, the output of the upper stellate cell, labeled G1 will be zero in the absence of any stimulation by action potentials and will rise with the frequency of action potentials exciting the circuit.
Although the word frequency appears in the above paragraph, note that it is the pulse spacing that is the information carrier in the signal projection stages of vision. This point is critical to the correct understanding of the luminance channel of the visual system. The frequency of an action potential pulse train can not define the initial occurrence of a pulse in response to a threat to the animal. The text will discuss an alternate form of the above circuit that provides an initial alarm signal as well as a "brightness" signal to the higher cognitive centers of the brain.
The lower two stellate cells receive separate streams of action potentials of equal spacing under quiescent input conditions. The result is a quiescent output voltage, G2 at each of the channels. Each of these voltages will vary independently with the rate of stimulation by action potentials at their input. These channels are not able to transmit an initial pulse reflecting the sudden appearance of a color in the visual field. This is a distinct difference between the differencing channels of vision and the luminance channel. This difference is frequently noted when evaluating the performance of the chromatic system of vision.
All of the Activa shown in this figure are of the same type AG. This is idicative of their similar electrical current carrying capability.
