According to the model currently in fashion, the cortex gets input into layer IV from sources including, among others, the corresponding 'core' part of the thalamus (cite White or Sherman) and other cortical areas (cite Van Essen 1985), and provides 'ascending' output to other cortical areas in layer III, or to 'other' parts of the thalamus and to motor efferents via layer V. There is also some thalamic input to layer I, supposedly from 'matrix' parts of the thalamus (cite Jones), but also as branches of the same neurons providing the layer IV input (cite Sherman).
In addition to the 'ascending' output mentioned above, however, there is a 'reciprocal' output signal to the same place (same general area of thalamus or cortex) that provided the input. This comes from layer 6 (cite Sherman) and possibly other places (cite Van Essen).
Let's assume that the traditional model is right about the semantic content of the input signal, e.g. the semantic content of a signal from a columns in V1 to antoher area (an output signal from V1's point of view, an input signal from the other area's point of view) is the presence or absence of a visual edge with a certain position and orientation, that the semantic content of higher visual areas is stuff like whether something is at a certain position and moving in a certain direction, or whether a certain object is recognized.
What is the content and function of the reciprocal signal?
First, an important question: does the reciprocal signal project to exactly the same 'functional unit' as the one that generated the input signal? For instance, in cortico-cortico connections between different cortical columns, if column A layer III projects to column B layer IV, then does column B layer VI always project to column A (possibly projecting to other places in addition to column A)?
Some possible functions:
- protocol metadata relating to flow control or error control. Examples: "Could you increase your amplitude?" or "I heard you say, 'I see grandma', is that what you said?" or "The checksum doesn't match the message, please resend" or "Slow down, I'm can't keep up with what you're saying"
- protocol metadata relating to multiplexing/contention or uncontrollable external events which have a bearing on the protocol. Example: "This frequency is experiencing heavy contention, please switch to 22.5 mHz" or "Aaaagh, i'm getting too much input from other sources, cease all transmission for 1 second to prevent excitotoxicity from killing me"
- attention. Example: "OK, wake up and start transmitting now" or "Everyone's paying attention to you now, please boost your amplitude so that your signal drowns out other less important signals". White Cortical Circuits p. 184, referencing White and Hersch, 1982, suggests that perhaps the corticothalamic cells are also primary input cells in cortex receiving thalamic input.
- protocol metadata relating to protocol phase. Example: "OK, i received the header for the current data packet, please proceed to transfer the actual data now"
- a subsemantic synchronization signal. Example: a clock signal or sine wave
- a signal that allows the thalamus to determine the round-trip connection latency to cortex and back. White Cortical Circuits p. 184, referencing White and Hersch, 1982, observes that the corticothalamic cells get a lot of input (potentially driving) from the thalamocortical afferents.
- routing information. Examples: (route advertisement) "By the way, i've got a connection right to columns 29684 of temporal cortex, in case you want to tell them anything" (routing command) "That's interesting, please pass that along to column 39231 of prefrontal cortex"
- a semantic query/selection for which the input on the connection being reciprocated is a response. Example: "do you see Grandma?" or "forget about Grandma for now, do you see a turkey?" or "Please fire if and only if you receive input from your afferent 4 in pattern 'Q'"
- information with the same semantics as the input on the connection being reciprocated, except with a different modality. E.g. "OK, imagine you DO see grandma" (cite)
- information with the same semantics as the input on the connection being reciprocated, with the same modality. E.g. in an iterative estimation algorithm, the prior estimate is fed back to serve as the starting point for the next iteration. Some evidence for this possibility may be found in the quote on Self:notes-books-corticalCircuitsWhite from p. 143, and a paper with a possibly related theory is "Towards a Theory of the Laminar Architecture of Cerebral Cortex: Computational Clues from the Visual System" by Raizada and Grossberg.
- information with different semantics as the input on the connection being reciprocated, but as a response to the information that was sent over the connection being reciprocated. Example: (reward signal) "That's good stuff, keep it coming, reward points +10"
- completely unrelated data traffic (seems unlikely). Example: "Oh hey, the Yankees won last night"