Contrast Adaptation In Cat Lateral Geniculate Nucleus And Influence Of Corticothalamic Feedback

It is well known that vision system has essential meanings to precept the outer environment. There are many different information-processing levels in vision system. Among these processing phases, the lateral geniculate nucleus (LGN) play a crucial role. LGN that located in the dorsal thalamus is responsible for receiving the information from retina and sending it to the primary visual cortex. Early electrophysiological studies suggests that response properties of LGN relay neurons are similar to those of retina ganglions. Thus, LGN was viewed as a passive, machine-like relay of information to visual cortex. The conclusion arrearage the researching and understanding of the lateral geniculate nucleus.As for adaptation, early relative studies focus on the striate cortex and indicate that adaptation in only a cortical phenomenon. Further works, however, has shown that adaptation has been found in retina and LGN. Yet so far, we are not able to understand the adaptation effect in LGN completely. For example, separate, parallel information channels are found in the visual pathway from retina to cortex. And these channels have different physiological properties and functions. Each channel handles the special information features. But it is not clear whether separate information channels have same adaptation effect.It is noteworthy that vision system is not a simply feedforward informational pathway. Studies showed that neurons at each stage are modulated by feedback from higher structures and that cortical feedback influences LGN neuron's response features. However, it is not clear whether cortical feedback has effects on adaptation effect of LGN neurons.In order to answer mentioned questions, we use single unit recording technology (in vivo) to study it. Our data showed that contrast adaptation causes LGN cells responsiveness changed mainly because contrast gain obviously decreases and the adaptation effect is like to those in visual cortex, which means that LGN neurons adjust its processing range according to current prevailing contrast and adaptation in LGN has contributions to cortex. We found that Y cells shows stronger adaption effect than X cells, which suggested that contrast adaptation has different effect on each information channels. To study whether the cortical feedback affects adaption effect in LGN neurons, area 17 and 18 in some cats were irreversibly inactivated by liquid nitrogen. The results showed that LGN neurons still exhibit contrast adaptation when cortex was inhibited, which indicates that contrast adaptation is an intrinsic mechanism of LGN neurons and is not entirely due to cortical feedback. We also found that the results in cats without feedback are not likely to those in normal cat– X cells and Y cells have similar adaptation effects. Compared with data gotten from normal cats, it was shown that cortical feedback has little effects on X cells, but strengthens the adaptation effects in Y cells. So we indicated that cortical feedback has different effects on each information channels.On the whole, our results are helpful to the understanding contrast adaptation in early visual pathway and provides an example to recognize the relationship between information channels and feedback.