In their study, the level of response suppression in the LGN was found to be similar to the level found in the retina, confirming previous observations that the characteristics of extra-classical inhibitory effects in the retina are similar to those in LGN (Solomon et al., 2006). Like in the LGN (Solomon et al., 2002), only retinal ganglion M cells, and not P, have an extra-classical surround present, with greater suppression at higher contrasts. This surround must be from ECRF activity
and not CRF activity because it was found to occur in response to stimuli that had not elicited a response in the CRF (Solomon et al., 2006). Another study concluded that ECI may originate in the retina because contrast adaptation in the LGN was not tuned to orientation, spatial frequency, Antidiabetic Compound Library supplier or temporal frequency, which would not be expected if the suppression originated in the visual cortex (Camp et al., 2009). While there are convincing TSA HDAC datasheet arguments for both LGN interneurons and retinal ganglion cells
as ECRF sources, there may be also as-yet unobserved influences from cortico-thalamic feedback. Most studies have been performed with an anesthetized preparation, with therefore reduced levels of cortical activity (Haider et al., 2013, Lamme et al., 1998 and Niell and Stryker, 2010) thereby presumably reducing the level of cortico-thalamic input and effect. In addition, the timescale of cortical influence on thalamic activity may be longer than what has been investigated, especially for anesthetized preparations (Uhl et al., 1980), or may be evident only in transient stimuli. The effect may alternately be too subtle to have been found easily, or a vital input to LGN may have been
missing, like attention as seen in human fMRI by O’Connor et al. (2002), or other behaviorally driven action, like eye motion as seen in peri-saccadic influences on thalamic activity by Reppas et al. (2002). The current evidence suggests that cortico-thalamic feedback does not contribute to extra-classical suppression but the possibility of an excitatory extra-classical influence remains. The presence of extra-classical suppression was found in geniculocortical afferents Fossariinae of anesthetized primates with a muscimol-inactivated visual cortex (Sceniak et al., 2006). Another study has compared surround suppression observed in anesthetized and alert primates and found that anesthesia does not reduce suppression (Alitto and Usrey, 2008). While Alitto and Usrey made only a qualitative comparison of the two conditions, their results suggest that suppression is actually greater in anesthetized primates. With evidence of excitatory ECRFs in V1 (Fitzpatrick, 2000) the effects of which could be communicated through the cortico-thalamic projection, we might expect to see globally balanced excitation and inhibition from the full-voiced influence of the awake cortex.