In this paper, exploiting the tools of linear system theory, we changed from an architectural description of cortical function in which a simple cell's response was a combination of geniculate and intracortical contributions, to a behavioral description in which we related simple cell response directly to the stimulus intensity, through an equivalent feed-forward linear operator. Accordingly, the kernel of such operator yields the resultant simple cell RF profile. Yet, it is not obvious at all that, beyond their appearance, the resultant profiles have the same functional features of those of real cells. In this section, we go deep into the question discussing the physiological meaning of our resultant RFs in the light of what is known on real RF measurement techniques.
Under the condition of linearity, the RF profile assumes a very definite operative functionality that can be used to characterize completely the output cell response. Provided that its discharge threshold is exceeded, simple cell linearly weights the local spatial luminance distribution by its two-dimensional RF profile, consisting of excitatory and inhibitory influences on cell's firing rate. Specifically, the positive values of the RF profile correspond to responses above the spontaneous activity of the cell, whereas the negative values of the RF profile correspond to responses below. In real cases, because simple cells typically have little or no spontaneous activity, it is not possible to quantify inhibition by direct measurements, without artificially raising the maintained discharge of the neuron. Therefore, the measured 2D profile of a simple cell consists of excitatory subregions or subfields defining, as a whole, the cell's excitatory discharge field to complementary stimulations. Such subregions are referred to as sometimes ON and OFF [55], sometimes bright and dark [61], according that the measurements are taken relative to stimulus transition (onset and offset of light) or to two distinct presentations of stimuli of opposite contrast (e.g., dark and bright bars). Mechanisms of spatially opponent inhibition within simple cell subfields enhance contrast sensitivity of the whole field. In addition to the excitatory discharge field, suppressive regions have been often observed. The term suppression is used instead of inhibition, to indicate that such regions appear only against a positive maintained rate, and, thus, they cannot be detected directly by extracellular recordings. Suppressive contributions, as well as antagonism between complementary subregions can be evidenced by bi-impulsive stimulations [108] which cannot be accounted for by a linear analysis, and thus they are beyond the purpose of this work. To overcome measurement limitations, and to obtain RF profiles which are consistent with the linear hypotheses, inhibitory responses are usually estimated by taking the difference between responses to complementary stimuli, assuming that inhibition elicited by a bright stimulus is equivalent to excitation elicited by a dark stimulus and vice versa [26,61,87]. This is a common procedure that allows to obtain a linear estimation of the 2D spatial response profile of the RF that can be directly compared with our results.