Dynamic Response Characteristics Of Neurons Of Cat Primary Visual Cortex Area 17 And Area 21a Of The Feedback Projection Of Information Processing In Area 17 Modulation Impact

Few researches have been done on how the mammalian visual cortex encodes the disappearance of visual stimuli. Here we study the dynamic response of 88 area 17 neurons in cats, from both the stimulus onset and offset. A reliable offset transient response to stationary grating could be seen in 62.5% of the neurons (55 in 88, 31 simple, 24 complex), showing a similar preferred orientation, orientation bias, cumulative detecability, peak response amplitude and latency with that of the onset transient response. However, this offset response, was significantly more dependent on the stimulus duration than the onset transient response, indicating that it needs longer time for the neurocircuit to coactivate in generating offset transient response. Simple and complex cells behaved differently in offset transient response. Complex cells exhibited a stronger linear relationship between the onset and offset response peak latency than simple cells, a greater offset peak response amplitude compared with that of the onset response, and were independent of grating spatial phase both in the onset and offset response in contrast to the simple cells. The linear fading-grating test demonstrated that it was the luminance change in receptive field subregions that generated offset transient response. Visual system of human and some other mammalians are physiologically or psychologically more sensitive to vertical and horizontal visual stimuli than to those obliquely oriented (usually referred to as the 'oblique effect'). Although the neurobiological basis of this bias is not fully understood, one possibility is that more neural machinery is devoted to processing vertical and horizontal contours than to processing oblique ones. The over representation of vertical and horizontal orientations in an orientation map has been found in area 17 as well as area 21a in cat's visual cortex which receives input from and sends output to area 17. The over representation was greater in area 21a than in area 17. It is interesting to investigate whether feedback influence from area 21a could affect the unequal representation in area 17. Using a combination of intrinsic signal optic imaging and pharmacological techniques, we found for the first time that on average, increasing activity of area 21a enhanced the difference of over representation in area 17 from 4.9% to 18.8%, near the level found in area 21a (23.2%), while decreasing activity of area 21a reduced the over representation. However, the basic pattern of orientation map sampled before and during drug application maintained unchanged, and the preferred orientation of most pixels in the orientation map changed little between control and recovery, control and glutamate microinjection, as well as control and GABA microinjection. These findings imply that positive feedback modulation from area 21a alters the preferred orientation of a large number of neurons in area 17, although in a small degree of switch, thus making the global distribution of preferred orientation changed.The finding suggests that feedback from higher-order cortex can recruit more neural machinery in lower-order cortex in order to process certain kind of information, thus making an important contribution to higher level of visual function.